Oort cloud
Distant planetesimals in the Solar System
The Oort cloud (pronounced or ), sometimes called the Öpik–Oort cloud, is theorized to be a vast cloud of icy planetesimals surrounding the Sun at distances ranging from 2,000 to 200,000 AU (0.03 to 3.2 light-years). The concept of such a cloud was proposed in 1950 by the Dutch astronomer Jan Oort, in whose honor the idea was named. Oort proposed that the bodies in this cloud replenish and keep constant the number of long-period comets entering the inner Solar System—where they are eventually consumed and destroyed during close approaches to the Sun.
The cloud is thought to encompass two regions: a disc-shaped inner Oort cloud aligned with the solar ecliptic (also called its Hills cloud) and a spherical outer Oort cloud enclosing the entire Solar System. Both regions lie well beyond the heliosphere and are in interstellar space. The innermost portion of the Oort cloud is more than a thousand times as distant from the Sun as the Kuiper belt, the scattered disc and the detached objects—three nearer reservoirs of trans-Neptunian objects.
The outer limit of the Oort cloud defines the cosmographic boundary of the Solar System. This area is defined by the Sun's Hill sphere, and hence lies at the interface between solar and galactic gravitational dominion. The outer Oort cloud is only loosely bound to the Solar System and its constituents are easily affected by the gravitational pulls of passing stars, the Milky Way itself and the cloud's own microgravity. These forces served to moderate and render more circular the highly eccentric orbits of material ejected from the inner Solar System during its early phases of development. The circular orbits of material in the Oort disc are largely thanks to this galactic gravitational torquing. By the same token, galactic interference in the motion of Oort bodies occasionally dislodges comets from their orbits within the cloud, sending them into the inner Solar System. Based on their orbits, most but not all of the short-period comets appear to have come from the Oort disc. Other short-period comets may have originated from the far larger spherical cloud.
Astronomers hypothesize that the material presently in the Oort cloud formed much closer to the Sun, in the protoplanetary disc, and was then scattered far into space through the gravitational influence of the giant planets. No direct observation of the Oort cloud is possible with present imaging technology. Nevertheless, the cloud is thought to be the source that replenishes most long-period and Halley-type comets, which are eventually consumed by their close approaches to the Sun after entering the inner Solar System. The cloud may also serve the same function for many of the centaurs and .
Development of theory.
By the early 20th century, astronomers had identified two main types of comets: short-period comets (also called "ecliptic" comets) and long-period comets (also called "nearly isotropic" comets). Ecliptic comets have relatively small orbits aligned near the ecliptic plane and are not found much farther than the Kuiper cliff around 50 AU from the Sun (the orbit of Neptune averages about 30 AU and 177P/Barnard has aphelion around 48 AU). Long-period comets, on the other hand, travel in very large orbits thousands of AU from the Sun and are isotropically distributed. This means long-period comets appear from every direction in the sky, both above and below the ecliptic plane. The origin of these comets was not well understood, and many long-period comets were initially assumed to be on parabolic trajectories, making them one-time visitors to the Sun from interstellar space.
In 1907, Armin Otto Leuschner suggested that many of the comets then thought to have parabolic orbits in fact moved along extremely large elliptical orbits that would return them to the inner Solar System after long intervals during which they were invisible to Earth-based astronomy. In 1932, the Estonian astronomer Ernst Öpik proposed a reservoir of long-period comets in the form of an orbiting cloud at the outermost edge of the Solar System. Dutch astronomer Jan Oort revived this basic idea in 1950 to resolve a paradox about the origin of comets. The following facts are not easily reconcilable with the highly elliptical orbits in which long-period comets are always found:
Oort reasoned that comets with orbits that closely approach the Sun cannot have been doing so since the condensation of the protoplanetary disc, more than 4.5 billion years ago. Hence long-period comets could not have formed in the current orbits in which they are always discovered and must have been held in an outer reservoir for nearly all of their existence.
Oort also studied tables of ephemerides for long-period comets and discovered that there is a curious concentration of long-period comets whose farthest retreat from the Sun (their aphelia) cluster around 20,000 AU. This suggested a reservoir at that distance with a spherical, isotropic distribution. He also proposed that the relatively rare comets with orbits of about 10,000 AU probably went through one or more orbits into the inner Solar System and there had their orbits drawn inward by the gravity of the planets.
Structure and composition.
The Oort cloud is thought to occupy a vast space somewhere between from the Sun to as far out as or even . The region can be subdivided into a spherical outer Oort cloud with a radius of some and a torus-shaped inner Oort cloud with a radius of .
The inner Oort cloud is sometimes known as the Hills cloud, named for Jack G. Hills, who proposed its existence in 1981. Models predict the inner cloud to be the much denser of the two, having tens or hundreds of times as many cometary nuclei as the outer cloud. The Hills cloud is thought to be necessary to explain the continued existence of the Oort cloud after billions of years.
Because it lies at the interface between the dominion of Solar and galactic gravitation, the objects comprising the outer Oort cloud are only weakly bound to the Sun. This in turn allows small perturbations from nearby stars or the Milky Way itself to inject long-period (and possibly Halley-type) comets inside the orbit of Neptune. This process ought to have depleted the sparser, outer cloud and yet long-period comets with orbits well above or below the ecliptic continue to be observed. The Hills cloud is thought to be a secondary reservoir of cometary nuclei and the source of replenishment for the tenuous outer cloud as the latter's numbers are gradually depleted through losses to the inner Solar System.
The outer Oort cloud may have trillions of objects larger than , and billions with diameters of . This corresponds to an absolute magnitude of more than 11. On this analysis, "neighboring" objects in the outer cloud are separated by a significant fraction of 1 AU, tens of millions of kilometres. The outer cloud's total mass is not known, but assuming that Halley's Comet is a suitable proxy for the nuclei composing the outer Oort cloud, their combined mass would be roughly , or five Earth masses.
Formerly the outer cloud was thought to be more massive by two orders of magnitude, containing up to 380 Earth masses,
but improved knowledge of the size distribution of long-period comets has led to lower estimates. No estimates of the mass of the inner Oort cloud have been published as of 2023.
If analyses of comets are representative of the whole, the vast majority of Oort-cloud objects consist of ices such as water, methane, ethane, carbon monoxide and hydrogen cyanide.
However, the discovery of the object 1996 PW, an object whose appearance was consistent with a D-type asteroid in an orbit typical of a long-period comet, prompted theoretical research that suggests that the Oort cloud population consists of roughly one to two percent asteroids. Analysis of the carbon and nitrogen isotope ratios in both the long-period and Jupiter-family comets shows little difference between the two, despite their presumably vastly separate regions of origin. This suggests that both originated from the original protosolar cloud, a conclusion also supported by studies of granular size in Oort-cloud comets and by the recent impact study of Jupiter-family comet Tempel 1.
In 2020 researchers discovered an unexpected spiral-shaped pattern in the inner part of the Oort Cloud. This discovery sheds light on how gravity shapes the outer edges of the Solar System.
Origin.
The Oort cloud is thought to have developed after the formation of planets from the primordial protoplanetary disc approximately 4.6 billion years ago. The most widely accepted hypothesis is that the Oort cloud's objects initially coalesced much closer to the Sun as part of the same process that formed the planets and minor planets. After formation, strong gravitational interactions with young gas giants, such as Jupiter, scattered the objects into extremely wide elliptical or parabolic orbits that were subsequently modified by perturbations from passing stars and giant molecular clouds into long-lived orbits detached from the gas giant region.
Recent research has been cited by NASA hypothesizing that a large number of Oort cloud objects are the product of an exchange of materials between the Sun and its sibling stars as they formed and drifted apart and it is suggested that many—possibly the majority—of Oort cloud objects did not form in close proximity to the Sun. Simulations of the evolution of the Oort cloud from the beginnings of the Solar System to the present suggest that the cloud's mass peaked around 800 million years after formation, as the pace of accretion and collision slowed and depletion began to overtake supply.
Models by Julio Ángel Fernández suggest that the scattered disc, which is the main source for periodic comets in the Solar System, might also be the primary source for Oort cloud objects. According to the models, about half of the objects scattered travel outward toward the Oort cloud, whereas a quarter are shifted inward to Jupiter's orbit, and a quarter are ejected on hyperbolic orbits. The scattered disc might still be supplying the Oort cloud with material. A third of the scattered disc's population is likely to end up in the Oort cloud after 2.5 billion years.
Computer models suggest that collisions of cometary debris during the formation period play a far greater role than was previously thought. According to these models, the number of collisions early in the Solar System's history was so great that most comets were destroyed before they reached the Oort cloud. Therefore, the current cumulative mass of the Oort cloud is far less than was once suspected. The estimated mass of the cloud is only a small part of the 50–100 Earth masses of ejected material.
Gravitational interaction with nearby stars and galactic tides modified cometary orbits to make them more circular. This explains the nearly spherical shape of the outer Oort cloud. On the other hand, the Hills cloud, which is bound more strongly to the Sun, has not acquired a spherical shape. Recent studies have shown that the formation of the Oort cloud is broadly compatible with the hypothesis that the Solar System formed as part of an embedded cluster of 200–400 stars. These early stars likely played a role in the cloud's formation, since the number of close stellar passages within the cluster was much higher than today, leading to far more frequent perturbations.
In June 2010 Harold F. Levison and others suggested on the basis of enhanced computer simulations that the Sun "captured comets from other stars while it was in its birth cluster." Their results imply that "a substantial fraction of the Oort cloud comets, perhaps exceeding 90%, are from the protoplanetary discs of other stars." In July 2020 Amir Siraj and Avi Loeb found that a captured origin for the Oort Cloud in the Sun's birth cluster could address the theoretical tension in explaining the observed ratio of outer Oort cloud to scattered disc objects, and in addition could increase the chances of a captured Planet Nine.
Comets.
Comets are thought to have two separate points of origin in the Solar System. Short-period comets (those with orbits of up to 200 years) are generally accepted to have emerged from either the Kuiper belt or the scattered disc, which are two linked flat discs of icy debris beyond Neptune's orbit at 30 AU and jointly extending out beyond 100 AU. Very long-period comets, such as C/1999 F1 (Catalina), whose orbits last for millions of years, are thought to originate directly from the outer Oort cloud. Other comets modeled to have come directly from the outer Oort cloud include C/2006 P1 (McNaught), C/2010 X1 (Elenin), Comet ISON, C/2013 A1 (Siding Spring), C/2017 K2, and C/2017 T2 (PANSTARRS). The orbits within the Kuiper belt are relatively stable, so very few comets are thought to originate there. The scattered disc, however, is dynamically active and is far more likely to be the place of origin for comets. Comets pass from the scattered disc into the realm of the outer planets, becoming what are known as centaurs. These centaurs are then sent farther inward to become the short-period comets.
There are two main types of short-period comets: Jupiter-family comets (with orbits smaller than 5 AU) and Halley-family comets. Halley-family comets, named after Halley's Comet, are distinct because, even though they are short-period comets, they are thought to come from the Oort Cloud rather than the scattered disc. Based on their orbits, it is suggested they were long-period comets that were captured by the gravity of the giant planets and sent into the inner Solar System. This process may have also created the present orbits of a significant fraction of the Jupiter-family comets, although the majority of such comets are thought to have originated in the scattered disc.
Oort noted that the number of returning comets was far less than his model predicted, and this issue, known as "cometary fading", has yet to be resolved. No dynamical process is known to explain the smaller number of observed comets than Oort estimated. Hypotheses for this discrepancy include the destruction of comets due to tidal stresses, impact or heating; the loss of all volatiles, rendering some comets invisible, or the formation of a non-volatile crust on the surface. Dynamical studies of hypothetical Oort cloud comets have estimated that their occurrence in the outer-planet region would be several times higher than in the inner-planet region. This discrepancy may be due to the gravitational attraction of Jupiter, which acts as a kind of barrier, trapping incoming comets and causing them to collide with it, just as it did with Comet Shoemaker–Levy 9 in 1994. An example of a typical dynamically old comet with an origin in the Oort cloud could be C/2018 F4.
Tidal effects.
Most of the comets seen close to the Sun seem to have reached their current positions through gravitational perturbation of the Oort cloud by the tidal force exerted by the Milky Way. Just as the Moon's tidal force deforms Earth's oceans, causing the tides to rise and fall, the galactic tide also distorts the orbits of bodies in the outer Solar System. In the charted regions of the Solar System, these effects are negligible compared to the gravity of the Sun, but in the outer reaches of the system, the Sun's gravity is weaker and the gradient of the Milky Way's gravitational Galactic Center compresses it along the other two axes; these small perturbations can shift orbits in the Oort cloud to bring objects close to the Sun. The point at which the Sun's gravity concedes its influence to the galactic tide is called the tidal truncation radius. It lies at a radius of 100,000 to 200,000 au, and marks the outer boundary of the Oort cloud.
Some scholars theorize that the galactic tide may have contributed to the formation of the Oort cloud by increasing the perihelia (smallest distances to the Sun) of planetesimals with large aphelia (largest distances to the Sun). The effects of the galactic tide are quite complex, and depend heavily on the behaviour of individual objects within a planetary system. Cumulatively, however, the effect can be quite significant: up to 90% of all comets originating from the Oort cloud may be the result of the galactic tide. Statistical models of the observed orbits of long-period comets argue that the galactic tide is the principal means by which their orbits are perturbed toward the inner Solar System.
Stellar perturbations and stellar companion hypotheses.
Besides the galactic tide, the main trigger for sending comets into the inner Solar System is thought to be interaction between the Sun's Oort cloud and the gravitational fields of nearby stars or giant molecular clouds. The orbit of the Sun through the plane of the Milky Way sometimes brings it in relatively close proximity to other stellar systems. For example, it is hypothesized that 70,000 years ago Scholz's Star passed through the outer Oort cloud (although its low mass and high relative velocity limited its effect). During the next 10 million years the known star with the greatest possibility of perturbing the Oort cloud is Gliese 710. This process could also scatter Oort cloud objects out of the ecliptic plane, potentially also explaining its spherical distribution.
In 1984, physicist Richard A. Muller postulated that the Sun has an as-yet undetected companion, either a brown dwarf or a red dwarf, in an elliptical orbit within the Oort cloud. This object, known as Nemesis, was hypothesized to pass through a portion of the Oort cloud approximately every 26 million years, bombarding the inner Solar System with comets. However, to date no evidence of Nemesis has been found, and many lines of evidence (such as crater counts), have thrown its existence into doubt. Recent scientific analysis no longer supports the idea that extinctions on Earth happen at regular, repeating intervals. Thus, the Nemesis hypothesis is no longer needed to explain current assumptions.
A somewhat similar hypothesis was advanced by astronomer John J. Matese of the University of Louisiana at Lafayette in 2002. He contends that more comets are arriving in the inner Solar System from a particular region of the postulated Oort cloud than can be explained by the galactic tide or stellar perturbations alone, and that the most likely cause would be a Jupiter-mass object in a distant orbit. This hypothetical gas giant was nicknamed Tyche. The WISE mission, an all-sky survey using parallax measurements in order to clarify local star distances, was capable of proving or disproving the Tyche hypothesis. In 2014, NASA announced that the WISE survey had ruled out any object as they had defined it.
Future exploration.
Space probes have yet to reach the area of the Oort cloud. "Voyager 1", the fastest and farthest of the interplanetary space probes currently leaving the Solar System, will reach the Oort cloud in about 300 years and would take about 30,000 years to pass through it. However, around 2025, the radioisotope thermoelectric generators on "Voyager 1" will no longer supply enough power to operate any of its scientific instruments, preventing any further exploration by "Voyager 1." The currently escaping the Solar System have either already stopped functioning ("Pioneer 10", "Pioneer 11") or are predicted to also stop functioning before they reach the Oort cloud ("Voyager 2", "New Horizons").
In the 1980s, there was a concept for a probe that could reach 1,000 AU in 50 years, called "TAU"; among its missions would be to look for the Oort cloud.
In the 2014 Announcement of Opportunity for the Discovery program, an observatory to detect the objects in the Oort cloud (and Kuiper belt) called the "Whipple Mission" was proposed. It would monitor distant stars with a photometer, looking for transits up to 10,000 AU away. The observatory was proposed for halo orbiting around L2 with a suggested 5-year mission. It was also suggested that the Kepler space telescope could have been capable of detecting objects in the Oort cloud.
See also.
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References.
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Explanatory notes.
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Voyager 1
NASA space probe launched in 1977
Voyager 1 is a space probe launched by NASA on September 5, 1977, as part of the Voyager program to study the outer Solar System and the interstellar space beyond the Sun's heliosphere. It was launched 16 days after its twin, "Voyager 2". It communicates through the NASA Deep Space Network (DSN) to receive routine commands and to transmit data to Earth. Real-time distance and velocity data are provided by NASA and JPL. At a distance of from Earth as of  2025[ [update]], it is the most distant human-made object from Earth. The probe made flybys of Jupiter, Saturn, and Saturn's largest moon, Titan. NASA had a choice of either doing a Pluto or Titan flyby; exploration of the moon took priority because it was known to have a substantial atmosphere. "Voyager 1" studied the weather, magnetic fields, and rings of the two gas giants and was the first probe to provide detailed images of their moons.
As part of the Voyager program and like its sister craft "Voyager 2", the spacecraft's extended mission is to locate and study the regions and boundaries of the outer heliosphere and to begin exploring the interstellar medium. "Voyager 1" crossed the heliopause and entered interstellar space on August 25, 2012, making it the first spacecraft to do so. Two years later, "Voyager 1" began experiencing a third wave of coronal mass ejections from the Sun that continued to at least December 15, 2014, further confirming that the probe is in interstellar space.
In 2017, the Voyager team successfully fired the spacecraft's trajectory correction maneuver (TCM) thrusters for the first time since 1980, enabling the mission to be extended by two to three years. "Voyager 1"'s extended mission is expected to continue to return scientific data until at least 2025, with a maximum lifespan of until 2030. Its radioisotope thermoelectric generators (RTGs) may supply enough electric power to return engineering data until 2036.
Mission background.
History.
A 1960s proposal for a Grand Tour to study the outer planets led NASA to begin work on a mission during the early 1970s. Information gathered by the "Pioneer 10" spacecraft helped engineers design "Voyager" to better cope with the intense radiation around Jupiter. Still, shortly before launch, strips of kitchen-grade aluminum foil were applied to certain cables to improve radiation shielding.
Initially, "Voyager 1" was planned as "Mariner 11" of the Mariner program. Due to budget cuts, the mission was reduced to a flyby of Jupiter and Saturn and renamed the Mariner Jupiter-Saturn probes. The name was changed to "Voyager" when the probe designs began to differ substantially from Mariner missions.
Spacecraft components.
"Voyager 1" was built by the Jet Propulsion Laboratory (JPL). It has 16 hydrazine thrusters, three-axis stabilization gyroscopes, and referencing instruments to keep the probe's radio antenna pointed toward Earth. Collectively, these instruments are part of the Attitude and Articulation Control Subsystem (AACS), along with redundant units of most instruments and eight backup thrusters. The spacecraft also included 11 scientific instruments to study celestial objects such as planets as it travels through space.
Communication system.
The radio communication system of "Voyager 1" was designed to be used up to and beyond the limits of the Solar System. It has a diameter high-gain Cassegrain antenna to send and receive radio waves via the three Deep Space Network stations on the Earth. The spacecraft normally transmits data to Earth over Deep Space Network Channel 18, using a frequency of either 2.3 GHz or 8.4 GHz, while signals from Earth to Voyager are transmitted at 2.1 GHz.
When "Voyager 1" is unable to communicate with the Earth, its digital tape recorder (DTR) can record about 67 kilobytes of data for later transmission. As of 2023[ [update]], signals from "Voyager 1" take more than 22 hours to reach Earth.
Power.
"Voyager 1" has three radioisotope thermoelectric generators (RTGs) mounted on a boom. Each MHW-RTG contains 24 pressed plutonium-238 oxide spheres. The RTGs generated about 470 W of electric power at the time of launch, with the remainder being dissipated as waste heat. The power output of the RTGs declines over time due to the 87.7-year half-life of the fuel and degradation of the thermocouples, but they will continue to support some of its operations until at least 2025.
Computers.
Unlike "Voyager's" other instruments, the operation of the cameras for visible light is not autonomous, but is controlled by an imaging parameter table contained in one of the digital computers, the Flight Data Subsystem (FDS). Since the 1990s, most space probes have been equipped with completely autonomous cameras.
The computer command subsystem (CCS) controls the cameras. The CCS contains fixed computer programs, such as command decoding, fault-detection and fault-correction routines, antenna pointing routines, and spacecraft sequencing routines. This computer is an improved version of the one that was used in the 1970s Viking orbiters.
The Attitude and Articulation Control Subsystem (AACS) controls the spacecraft orientation (its attitude). It keeps the high-gain antenna pointing towards the Earth, controls attitude changes, and points the scan platform. The custom-built AACS systems on both Voyagers are the same.
Mission profile.
Launch and trajectory.
The "Voyager 1" probe was launched on September 5, 1977, from Launch Complex 41 at the Cape Canaveral Air Force Station, aboard a Titan IIIE launch vehicle. The "Voyager 2" probe had been launched two weeks earlier, on August 20, 1977. Despite being launched later, "Voyager 1" reached both Jupiter and Saturn sooner, following a shorter trajectory.
"Voyager 1"'s launch almost failed because Titan's LR-91 second stage shut down prematurely, leaving of propellant unburned. Recognizing the deficiency, the Centaur stage's on-board computers ordered a burn that was far longer than planned in order to compensate. Centaur extended its own burn and was able to give "Voyager 1" the additional velocity it needed. At cutoff, the Centaur was only 3.4 seconds from propellant exhaustion. If the same failure had occurred during "Voyager 2"'s launch a few weeks earlier, the Centaur would have run out of propellant before the probe reached the correct trajectory. Jupiter was in a more favorable position vis-à-vis Earth during the launch of "Voyager 1" than during the launch of "Voyager 2".
"Voyager 1"'s initial orbit had an aphelion of , just a little short of Saturn's orbit of . "Voyager 2"'s initial orbit had an aphelion of , well short of Saturn's orbit.
Flyby of Jupiter.
"Voyager 1" began photographing Jupiter in January 1979. Its closest approach to Jupiter was on March 5, 1979, at a distance of about from the planet's center. Because of the greater photographic resolution allowed by a closer approach, most observations of the moons, rings, magnetic fields, and the radiation belt environment of the Jovian system were made during the 48-hour period that bracketed the closest approach. Voyager 1 finished photographing the Jovian system in April 1979.
The discovery of ongoing volcanic activity on the moon Io was probably the greatest surprise. It was the first time active volcanoes had been seen on another body in the Solar System. It appears that activity on Io affects the entire Jovian system. Io appears to be the primary source of matter that pervades the Jovian magnetosphere – the region of space that surrounds the planet influenced by the planet's strong magnetic field. Sulfur, oxygen, and sodium, apparently erupted by Io's volcanoes and sputtered off the surface by the impact of high-energy particles, were detected at the outer edge of the magnetosphere of Jupiter.
The two Voyager space probes made a number of important discoveries about Jupiter, its satellites, its radiation belts, and its never-before-seen planetary rings.
Flyby of Saturn.
The gravitational assist trajectories at Jupiter were successfully carried out by both Voyagers, and the two spacecraft went on to visit Saturn and its system of moons and rings. "Voyager 1" encountered Saturn in November 1980, with the closest approach on November 12, 1980, when the space probe came within of Saturn's cloud-tops. The space probe's cameras detected complex structures in the rings of Saturn, and its remote sensing instruments studied the atmospheres of Saturn and its giant moon Titan.
"Voyager 1" found that about seven percent of the volume of Saturn's upper atmosphere is helium (compared with 11 percent of Jupiter's atmosphere), while almost all the rest is hydrogen. Since Saturn's internal helium abundance was expected to be the same as Jupiter's and the Sun's, the lower abundance of helium in the upper atmosphere may imply that the heavier helium may be slowly sinking through Saturn's hydrogen; that might explain the excess heat that Saturn radiates over energy it receives from the Sun. Winds blow at high speeds on Saturn. Near the equator, the Voyagers measured winds about . The wind blows mostly in an easterly direction.
The Voyagers found aurora-like ultraviolet emissions of hydrogen at mid-latitudes in the atmosphere, and auroras at polar latitudes (above 65 degrees). The high-level auroral activity may lead to the formation of complex hydrocarbon molecules that are carried toward the equator. The mid-latitude auroras, which occur only in sunlit regions, remain a puzzle, since bombardment by electrons and ions, known to cause auroras on Earth, occurs primarily at high latitudes. Both Voyagers measured the rotation of Saturn (the length of a day) at 10 hours, 39 minutes, 24 seconds.
"Voyager 1"'s mission included a flyby of Titan, Saturn's largest moon, which had long been known to have an atmosphere. Images taken by "Pioneer 11" in 1979 had indicated the atmosphere was substantial and complex, further increasing interest. The Titan flyby occurred as the spacecraft entered the system to avoid any possibility of damage closer to Saturn compromising observations, and approached to within , passing behind Titan as seen from Earth and the Sun. Voyager's measurement of the atmosphere's effect on sunlight and Earth-based measurement of its effect on the probe's radio signal were used to determine the atmosphere's composition, density, and pressure. Titan's mass was also measured by observing its effect on the probe's trajectory. The thick haze prevented any visual observation of the surface, but the measurement of the atmosphere's composition, temperature, and pressure led to speculation that lakes of liquid hydrocarbons could exist on the surface.
Because observations of Titan were considered vital, the trajectory chosen for "Voyager 1" was designed around the optimum Titan flyby, which took it below the south pole of Saturn and out of the plane of the ecliptic, ending its planetary science mission. Had "Voyager 1" failed or been unable to observe Titan, "Voyager 2"'s trajectory would have been altered to incorporate the Titan flyby, precluding any visit to Uranus and Neptune. The trajectory "Voyager 1" was launched into would not have allowed it to continue on to Uranus and Neptune, but could have been altered to avoid a Titan flyby and travel from Saturn to Pluto, arriving in 1986.
Exit from the heliosphere.
On February 14, 1990, "Voyager 1" took the first "family portrait" of the Solar System as seen from outside, which includes the image of planet Earth known as "Pale Blue Dot". Soon afterward, its cameras were deactivated to conserve energy and computer resources for other equipment. The camera software has been removed from the spacecraft, so it would now be complex to get them working again. Earth-side software and computers for reading the images are also no longer available.
On February 17, 1998, "Voyager 1" reached a distance of from the Sun and overtook "Pioneer 10" as the most distant spacecraft from Earth. Traveling at about , it has the fastest heliocentric recession speed of any spacecraft.
As "Voyager 1" headed for interstellar space, its instruments continued to study the Solar System. Jet Propulsion Laboratory scientists used the plasma wave experiments aboard "Voyager 1" and "2" to look for the heliopause, the boundary at which the solar wind transitions into the interstellar medium. As of 2013[ [update]], the probe was moving with a relative velocity to the Sun of about .
With the velocity the probe is currently maintaining, "Voyager 1" is traveling about per year, or about one light-year per 18,000 years.
Termination shock.
Scientists at the Johns Hopkins University Applied Physics Laboratory believe that "Voyager 1" entered the termination shock in February 2003. This marks the point where the solar wind slows to subsonic speeds. Some other scientists expressed doubt and discussed this in the journal "Nature" of November 6, 2003. The issue would not be resolved until other data became available, since "Voyager 1"'s solar-wind detector ceased functioning in 1990. This failure meant that termination shock detection would have to be inferred from the data from the other instruments on board.
In May 2005, a NASA press release said that the consensus was that "Voyager 1" was then in the heliosheath. In a scientific session at the American Geophysical Union meeting in New Orleans on May 25, 2005, Ed Stone presented evidence that the craft crossed the termination shock in late 2004. This event is estimated to have occurred on December 15, 2004, at a distance of from the Sun.
Heliosheath.
On March 31, 2006, amateur radio operators from AMSAT in Germany tracked and received radio waves from "Voyager 1" using the dish at Bochum with a long integration technique. Retrieved data was checked and verified against data from the Deep Space Network station at Madrid, Spain. This seems to be the first such amateur tracking of "Voyager 1".
It was confirmed on December 13, 2010, that "Voyager 1" had passed the reach of the radial outward flow of the solar wind, as measured by the Low Energy Charged Particle device. It is suspected that solar wind at this distance turns sideways because of interstellar wind pushing against the heliosphere. Since June 2010, detection of solar wind had been consistently at zero, providing conclusive evidence of the event. On this date, the spacecraft was approximately from the Sun.
"Voyager 1" was commanded to change its orientation to measure the sideways motion of the solar wind at that location in space in March 2011 (~33yr 6mo from launch). A test roll done in February had confirmed the spacecraft's ability to maneuver and reorient itself. The course of the spacecraft was not changed. It rotated 70 degrees counterclockwise with respect to Earth to detect the solar wind. This was the first time the spacecraft had done any major maneuvering since the "Family Portrait" photograph of the planets was taken in 1990. After the first roll the spacecraft had no problem in reorienting itself with Alpha Centauri, "Voyager 1"'s guide star, and it resumed sending transmissions back to Earth. "Voyager 1" was expected to enter interstellar space "at any time". "Voyager 2" was still detecting outward flow of solar wind at that point but it was estimated that in the following months or years it would experience the same conditions as "Voyager 1".
The spacecraft was reported at 12.44° declination and 17.163 hours right ascension, and at an ecliptic latitude of 34.9° (the ecliptic latitude changes very slowly), placing it in the constellation Ophiuchus as observed from the Earth on May 21, 2011.
On December 1, 2011, it was announced that "Voyager 1" had detected the first Lyman-alpha radiation originating from the Milky Way galaxy. Lyman-alpha radiation had previously been detected from other galaxies, but because of interference from the Sun, the radiation from the Milky Way was not detectable.
NASA announced on December 5, 2011, that "Voyager 1" had entered a new region referred to as a "cosmic purgatory". Within this stagnation region, charged particles streaming from the Sun slow and turn inward, and the Solar System's magnetic field is doubled in strength as interstellar space appears to be applying pressure. Energetic particles originating in the Solar System decline by nearly half, while the detection of high-energy electrons from outside increases 100-fold. The inner edge of the stagnation region is located approximately 113 AU from the Sun.
Heliopause.
NASA announced in June 2012 that the probe was detecting changes in the environment that were suspected to correlate with arrival at the heliopause. "Voyager 1" had reported a marked increase in its detection of charged particles from interstellar space, which are normally deflected by the solar winds within the heliosphere from the Sun. The craft thus began to enter the interstellar medium at the edge of the Solar System.
"Voyager 1" became the first spacecraft to cross the heliopause in August 2012, then at a distance of from the Sun, although this was not confirmed for another year.
As of September 2012, sunlight took 16.89 hours to get to "Voyager 1" which was at a distance of 121 AU. The apparent magnitude of the Sun from the spacecraft was −16.3 (about 30 times brighter than the full Moon). The spacecraft was traveling at relative to the Sun. At this rate, it would need about 17,565 years to travel a single light-year. To compare, Proxima Centauri, the closest star to the Sun, is about 4.2 light-years () distant. If the spacecraft was traveling in the direction of that star, it would take 73,775 years to reach it. ("Voyager 1" is heading in the direction of the constellation Ophiuchus.)
In late 2012, researchers reported that particle data from the spacecraft suggested that the probe had passed through the heliopause. Measurements from the spacecraft revealed a steady rise since May in collisions with high energy particles (above 70 MeV), which are thought to be cosmic rays emanating from supernova explosions far beyond the Solar System, with a sharp increase in these collisions in late August. At the same time, in late August, there was a dramatic drop in collisions with low-energy particles, which are thought to originate from the Sun.
Ed Roelof, space scientist at Johns Hopkins University and principal investigator for the Low-Energy Charged Particle instrument on the spacecraft, declared that "most scientists involved with "Voyager 1" would agree that [these two criteria] have been sufficiently satisfied". However, the last criterion for officially declaring that "Voyager 1" had crossed the boundary, the expected change in magnetic field direction (from that of the Sun to that of the interstellar field beyond), had not been observed (the field had changed direction by only 2 degrees), which suggested to some that the nature of the edge of the heliosphere had been misjudged.
On December 3, 2012, Voyager project scientist Ed Stone of the California Institute of Technology said, "Voyager has discovered a new region of the heliosphere that we had not realized was there. We're still inside, apparently. But the magnetic field now is connected to the outside. So it's like a highway letting particles in and out." The magnetic field in this region was 10 times more intense than "Voyager 1" encountered before the termination shock. It was expected to be the last barrier before the spacecraft exited the Solar System completely and entered interstellar space.
Interstellar medium.
In March 2013, it was announced that "Voyager 1" might have become the first spacecraft to enter interstellar space, having detected a marked change in the plasma environment on August 25, 2012. However, until September 12, 2013, it was still an open question as to whether the new region was interstellar space or an unknown region of the Solar System. At that time, the former alternative was officially confirmed.
In 2013 "Voyager 1" was exiting the Solar System at a speed of about per year, which is 61,602 km/h, 4.83 times the diameter of Earth (12,742 km) per hour; whereas "Voyager 2" is going slower, leaving the Solar System at per year. Each year, "Voyager 1" increases its lead over "Voyager 2".
"Voyager 1" reached a distance of from the Sun on May 18, 2016. On September 5, 2017, that had increased to about from the Sun, or just over 19 light-hours; at that time, "Voyager 2" was from the Sun.
Its progress can be monitored at NASA's website.
On September 12, 2013, NASA officially confirmed that "Voyager 1" had reached the interstellar medium in August 2012 as previously observed. The generally accepted date of arrival is August 25, 2012 (approximately 10 days before the 35th anniversary of its launch), the date durable changes in the density of energetic particles were first detected. By this point, most space scientists had abandoned the hypothesis that a change in magnetic field direction must accompany a crossing of the heliopause; a new model of the heliopause predicted that no such change would be found.
A key finding that persuaded many scientists that the heliopause had been crossed was an indirect measurement of an 80-fold increase in electron density, based on the frequency of plasma oscillations observed beginning on April 9, 2013, triggered by a solar outburst that had occurred in March 2012 (electron density is expected to be two orders of magnitude higher outside the heliopause than within). Weaker sets of oscillations measured in October and November 2012 provided additional data. An indirect measurement was required because "Voyager 1"'s plasma spectrometer had stopped working in 1980. In September 2013, NASA released recordings of audio transductions of these plasma waves, the first to be measured in interstellar space.
While "Voyager 1" is commonly spoken of as having left the Solar System simultaneously with having left the heliosphere, the two are not the same. The Solar System is usually defined as the vastly larger region of space populated by bodies that orbit the Sun. The craft is presently less than one-seventh the distance to the aphelion of Sedna, and it has not yet entered the Oort cloud, the source region of long-period comets, regarded by astronomers as the outermost zone of the Solar System.
In October 2020, astronomers reported a significant unexpected increase in density in the space beyond the Solar System as detected by the "Voyager 1" and "Voyager 2" space probes. According to the researchers, this implies that "the density gradient is a large-scale feature of the VLISM (very local interstellar medium) in the general direction of the heliospheric nose".
In May 2021, NASA reported on the continuous measurement, for the first time, of the density of material in interstellar space and, as well, the detection of interstellar sounds for the first time.
Communication issues.
In May 2022, NASA reported that "Voyager 1" had begun transmitting "mysterious" and "peculiar" telemetric data to the Deep Space Network (DSN). It confirmed that the operational status of the craft remained unchanged, but that the issue stemmed from the Attitude Articulation and Control System (AACS). NASA's Jet Propulsion Laboratory published a statement on May 18, 2022, that the AACS was functional but sending invalid data. The problem was eventually traced to the AACS sending its telemetry through a computer that had been non-operational for years, resulting in data corruption. In August 2022, NASA transmitted a command to the AACS to use another computer, which resolved the problem. An investigation into what caused the initial switch is underway, though engineers have hypothesized that the AACS had executed a bad command from another onboard computer.
"Voyager 1" began transmitting unreadable data on November 14, 2023. On December 12, 2023, NASA announced that "Voyager 1"'s flight data system was unable to use its telemetry modulation unit, preventing it from transmitting scientific data. On March 24, 2024, NASA announced that they had made significant progress on interpreting the data being received from the spacecraft. Engineers reported in April 2024 that the failure was likely in a memory bank of the Flight Data Subsystem (FDS), one of the three onboard computer systems, probably from being struck by a high-energy particle or that it simply wore out due to age. The FDS was not communicating properly with the telemetry modulation unit (TMU), which began transmitting a repeating sequence of ones and zeros indicating that the system was in a stuck condition. After a reboot of the FDS, communications remained unusable. The probe still received commands from Earth, and was sending a carrier tone indicating it was still operational. Commands sent to alter the modulation of the tone succeeded, confirming that the probe was still responsive. The "Voyager" team began developing a workaround, and on April 20 communication of health and status was restored by rearranging code away from the defective FDS memory chip, three percent of which was corrupted beyond repair. Because the memory is corrupted, the code needed to be relocated, but there were no place for an extra 256 bits; the spacecraft's total memory is only 69.63 kilobytes. To make it work, the engineers deleted unused code, for example the code used to transmit the data from Jupiter, that cannot be used at the current transmission rate. All the data from the "anomaly period" is lost. On May 22, NASA announced that Voyager 1 "resumed returning science data from two of its four instruments", with work towards the others ongoing. On June 13, NASA confirmed that the probe returns data from all four instruments.
In October 2024, the probe turned off its X-band radio transmitter that was used for communications with the DSN. It was caused by the probe's fault protection system that was activated after NASA turned on one of the heaters on October 16. Fault protection system lowered the transmission rate, but the engineers were able to find the signal. Later, on October 19, the transmission stopped; the fault protection system was triggered once again and switched to the S-band transmitter, that was previously used in 1981. NASA reported that the team reactivated the X-band transmitter and then resumed collecting data in mid-November.
Future of the probe.
Remaining lifespan.
In December 2017, NASA successfully fired all four of "Voyager 1"'s trajectory correction maneuver (TCM) thrusters for the first time since 1980. The TCM thrusters were used in the place of a degraded set of jets to help keep the probe's antenna pointed towards Earth. Using the TCM thrusters allowed "Voyager 1" to continue transmitting data to NASA for two to three more years.
Due to the diminishing electrical power available, the Voyager team has had to prioritize which instruments to keep on and which to turn off. Heaters and other spacecraft systems have been turned off one by one as part of power management. The fields and particles instruments that are the most likely to send back key data about the heliosphere and interstellar space have been prioritized to keep operating. Engineers expect the spacecraft to continue operating at least one science instrument until around 2025.
Concerns with the orientation thrusters.
Some thrusters needed to control the attitude of the spacecraft and point its high-gain antenna in the direction of Earth are out of use due to clogging problems in their hydrazine lines. The spacecraft no longer has a backup available for its thruster system and "everything onboard is single-string," according to Suzanne Dodd, Voyager project manager at JPL, in an interview with "Ars Technica". NASA has accordingly decided to modify the spacecraft's computer software in order to reduce the rate at which the hydrazine lines clog. NASA will first deploy the modified software on "Voyager 2", which is less distant from Earth, before deploying it on "Voyager 1".
In September 2024, NASA performed a "thruster swap", switching from a clogged set of thrusters to less clogged ones that had not been used since 2018.
Far future.
Provided "Voyager 1" does not collide with anything and is not retrieved, the "New Horizons" space probe will never pass it, despite being launched from Earth at a higher speed than either Voyager spacecraft. The Voyager spacecraft benefited from multiple planetary flybys to increase its heliocentric velocities, whereas "New Horizons" received only a single such boost, from its Jupiter flyby in 2007. As of 2018[ [update]], "New Horizons" is traveling at about , slower than "Voyager 1", and New Horizons, being closer to the sun, is slowing more rapidly.
"Voyager 1" is expected to reach the theorized Oort cloud in about 300 years and take about 30,000 years to pass through it. Though it is not heading towards any particular star, in about 40,000 years, it will pass within of the star Gliese 445, which is at present in the constellation Camelopardalis and 17.1 light-years from Earth. That star is generally moving towards the Solar System at about . NASA says that "The Voyagers are destined – perhaps eternally – to wander the Milky Way." In 300,000 years, it will pass within less than 1 light-year of the M3V star TYC 3135–52–1.
Golden record.
Both Voyager space probes carry a gold-plated audio-visual disc, a compilation meant to showcase the diversity of life and culture on Earth in the event that either spacecraft is ever found by any extraterrestrial discoverer. The record, made under the direction of a team including Carl Sagan and Timothy Ferris, includes photos of the Earth and its lifeforms, a range of scientific information, spoken greetings from people such as the Secretary-General of the United Nations (Kurt Waldheim) and the President of the United States (Jimmy Carter) and a medley, "Sounds of Earth", that includes the sounds of whales, a baby crying, waves breaking on a shore, and a collection of music spanning different cultures and eras including works by Wolfgang Amadeus Mozart, Blind Willie Johnson, Chuck Berry and Valya Balkanska. Other Eastern and Western classics are included, as well as performances of indigenous and folk music from around the world. The record also contains greetings in 55 different languages. The project aimed to portray the richness of life on Earth and stand as a testament to human creativity and the desire to connect with the cosmos.
References.
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Oort limit
The Oort limit is a theoretical location at the outer limits of the Oort cloud, where the amount of comets and minor planets orbiting the Sun drops drastically, or drops entirely. The exact location of such a limit, if there is such one, is uncertain. About 100 comets, of 3500 known comets,[#endnote_] come more than 5000 AU from the Sun, and a very few come as far as 20,000 AU from the Sun.
So far, it appears that rather than a sudden drop in the amount of comets orbiting the sun at about 50,000 AU, the Oort cloud rather uniformly decreases in size, the further away from the Sun it goes. As current observations indicate, the Oort limit is somewhere around 50,000 AU (0.8 LY) from the Sun.
Notes.
<templatestyles src="Citation/styles.css"/>^ Of these known comets, the majority (>2000) were discovered using the SOHO telescope, and are mostly sungrazing comets from the Kreutz Sungrazers. Of the other comets, about half of them are long-period comets, orbiting several hundred Astronomical Units out or further. Considering this, Oort Cloud comets are fairly common.

Jan Oort
Dutch astronomer (1900–1992)
Jan Hendrik Oort ( or ; 28 April 1900 – 5 November 1992) was a Dutch astronomer who made significant contributions to the understanding of the Milky Way and who was a pioneer in the field of radio astronomy. "The New York Times" called him "one of the century's foremost explorers of the universe"; the European Space Agency website describes him as "one of the greatest astronomers of the 20th century" and states that he "revolutionised astronomy through his ground-breaking discoveries." In 1955, Oort's name appeared in "Life" magazine's list of the 100 most famous living people. He has been described as "putting the Netherlands in the forefront of postwar astronomy".
Oort determined that the Milky Way rotates and overturned the idea that the Sun was at its center. He also postulated the existence of the mysterious invisible dark matter in 1932, which is believed to make up roughly 84.5% of the total mass in the Universe and whose gravitational pull causes "the clustering of stars into galaxies and galaxies into connecting strings of galaxies". He discovered the galactic halo, a group of stars orbiting the Milky Way but outside the main disk. Additionally Oort is responsible for a number of important insights about comets, including the realization that their orbits "implied there was a lot more solar system than the region occupied by the planets."
The Oort cloud, the Oort constants, Oort Limit, an impact crater on Pluto (Oort), and the asteroid 1691 Oort were all named after him.
Early life and education.
Oort was born in Franeker, a small town in the Dutch province of Friesland, on April 28, 1900. He was the second son of Abraham Hermanus Oort, a physician, who died on May 12, 1941, and Ruth Hannah Faber, who was the daughter of Jan Faber and Henrietta Sophia Susanna Schaaii, and who died on November 20, 1957. Both of his parents came from families of clergymen, with his paternal grandfather, a Protestant clergyman with liberal ideas, who "was one of the founders of the more liberal Church in Holland" and who "was one of the three people who made a new translation of the Bible into Dutch." The reference is to Henricus Oort (1836–1927), who was the grandson of a famous Rotterdam preacher and, through his mother, Dina Maria Blom, the grandson of theologian Abraham Hermanus Blom, a "pioneer of modern biblical research". Several of Oort's uncles were pastors, as was his maternal grandfather. "My mother kept up her interests in that, at least in the early years of her marriage", he recalled. "But my father was less interested in Church matters."
In 1903 Oort's parents moved to Oegstgeest, near Leiden, where his father took charge of the Endegeest Psychiatric Clinic. Oort's father, "was a medical director in a sanitorium for nervous illnesses. We lived in the director's house of the sanitorium, in a small forest which was very nice for the children, of course, to grow up in." Oort's younger brother, John, became a professor of plant diseases at the University of Wageningen. In addition to John, Oort had two younger sisters and an elder brother who died of diabetes when he was a student.
Oort attended primary school in Oegstgeest and secondary school in Leiden, and in 1917 went to Groningen University to study physics. He later said that he had become interested in science and astronomy during his high-school years, and conjectured that his interest was stimulated by reading Jules Verne. His one hesitation about studying pure science was the concern that it "might alienate one a bit from people in general", as a result of which "one might not develop the human factor sufficiently." But he overcame this concern and ended up discovering that his later academic positions, which involved considerable administrative responsibilities, afforded a good deal of opportunity for social contact.
Oort chose Groningen partly because a well known astronomer, Jacobus Cornelius Kapteyn, was teaching there, although Oort was unsure whether he wanted to specialize in physics or astronomy. After studying with Kapteyn, Oort decided on astronomy. "It was the personality of Professor Kapteyn which decided me entirely", he later recalled. "He was quite an inspiring teacher and especially his elementary astronomy lectures were fascinating." Oort began working on research with Kapteyn early in his third year. According to Oort one professor at Groningen who had considerable influence on his education was physicist Frits Zernike.
After taking his final exam in 1921, Oort was appointed assistant at Groningen, but in September 1922, he went to the United States to do graduate work at Yale and to serve as an assistant to Frank Schlesinger of the Yale Observatory.
Career.
At Yale, Oort was responsible for making observations with the Observatory's zenith telescope. "I worked on the problem of latitude variation", he later recalled, "which is quite far away from the subjects I had so far been studying." He later considered his experience at Yale useful as he became interested in "problems of fundamental astronomy that [he] felt was capitalized on later, and which certainly influenced [his] future lectures in Leiden." Personally, he "felt somewhat lonesome in Yale", but also said that "some of my very best friends were made in these years in New Haven."
Early discoveries.
In 1924, Oort returned to the Netherlands to work at Leiden University, where he served as a research assistant, becoming Conservator in 1926, Lecturer in 1930, and Professor Extraordinary in 1935. In 1926, he received his doctorate from Groningen with a thesis on the properties of high-velocity stars. The next year, Swedish astronomer Bertil Lindblad proposed that the rate of rotation of stars in the outer part of the galaxy decreased with distance from the galactic core, and Oort, who later said that he believed it was his colleague Willem de Sitter who had first drawn his attention to Lindblad's work, realized that Lindblad was correct and that the truth of his proposition could be demonstrated observationally. Oort provided two formulae that described galactic rotation; the two constants that figured in these formulae are now known as "Oort's constants". Oort "argued that just as the outer planets appear to us to be overtaken and passed by the less distant ones in the solar system, so too with the stars if the Galaxy really rotated", according to the "Oxford Dictionary of Scientists". He "was finally able to calculate, on the basis of the various stellar motions, that the Sun was some 30,000 light-years from the center of the Galaxy and took about 225 million years to complete its orbit. He also showed that stars lying in the outer regions of the galactic disk rotated more slowly than those nearer the center. The Galaxy does not therefore rotate as a uniform whole but exhibits what is known as 'differential rotation'."
These early discoveries by Oort about the Milky Way overthrew the Kapteyn system, named after his mentor, which had envisioned a galaxy that was symmetrical around the Sun. As Oort later noted, "Kapteyn and his co-workers had not realized that the absorption in the galactic plane was as bad as it turned out to be." Until Oort began his work, he later recalled, "the Leiden Observatory had been concentrating entirely on positional astronomy, meridian circle work and some proper motion work. But no astrophysics or anything that looked like that. No structure of the galaxy, no dynamics of the galaxy. There was no one else in Leiden who was interested in these problems in which I was principally interested, so the first years I worked more or less by myself in these projects. De Sitter was interested, but his main line of research was celestial mechanics; at that time the expanding universe had moved away from his direct interest." As the European Space Agency states, Oort "sh[ook] the scientific world by demonstrating that the Milky Way rotates like a giant 'Catherine Wheel'." He showed that all the stars in the galaxy were "travelling independently through space, with those nearer the center rotating much faster than those further away."
This breakthrough made Oort famous in the world of astronomy. In the early 1930s he received job offers from Harvard and Columbia University, but chose to stay at Leiden, although he did spend half of 1932 at the Perkins Observatory, in Delaware, Ohio.
In 1934, Oort became assistant to the director of Leiden Observatory; the next year he became General Secretary of the International Astronomical Union (IAU), a post he held until 1948; in 1937 he was elected to the Royal Academy. In 1939, he spent half a year in the U.S., and became interested in the Crab Nebula, concluding in a paper, written with American astronomer Nicholas Mayall, that it was the result of a supernova explosion.
Nazi invasion of Netherlands.
In 1940, Nazi Germany invaded the Netherlands. Soon after, the occupying regime dismissed all Jewish professors from Leiden University and other universities. "Among the professors who were dismissed", Oort later recalled, "was a very famous … professor of law by the name of Meyers. On the day when he got the letter from the authorities that he could no longer teach his classes, the dean of the faculty of law went into his class … and delivered a speech in which he started by saying, 'I won't talk about his dismissal and I shall leave the people who did this, below us, but will concentrate on the greatness of the man dismissed by our aggressors.'"
This speech (26 November 1940) made such an impression on all his students that on leaving the auditorium they defiantly sang the anthem of the Netherlands and went on strike. Oort was present for the lecture and was greatly impressed. This occasion formed the beginning of the active resistance in Holland. The speech by Rudolph Cleveringa, the dean of the faculty of Law and former graduate student of professor Meijers, was widely circulated during the rest of the war by the resistance groups. Oort was in a little group of professors in Leiden who came together regularly and discussed the problems the university faced in view of the German occupation. Most of the members of this group were put in hostage camps soon after the speech by Cleveringa. Oort refused to collaborate with the occupiers, "and so we went down to live in the country for the rest of the war." Resigning from the Royal Academy, from his professorial post at Leiden, and from his position at the Observatory, Oort took his family to Hulshorst, a quiet village in the province of Gelderland, where they sat out the war. In Hulshorst, he began writing a book on stellar dynamics.
Oort's radio astronomy.
Before the war was over, he initiated, in collaboration with a Utrecht University student, Hendrik van de Hulst, a project that eventually succeeded, in 1951, in detecting the 21-centimeter radio emission from interstellar hydrogen spectral line at radio frequencies. Oort and his colleagues also made the first investigation of the central region of the Galaxy, and discovered that "the 21-centimeter radio emission passed un-absorbed through the gas clouds that had hidden the center from optical observation. They found a huge concentration of mass there, later identified as mainly stars, and also discovered that much of the gas in the region was moving rapidly outward away from the center." In June 1945, after the end of the war, Oort returned to Leiden, took over as director of the Observatory, and became Full Professor of Astronomy. During this immediate postwar period, he led the Dutch group that built radio telescopes at Radio Kootwijk, Dwingeloo, and Westerbork and used the 21-centimeter line to map the Milky Way, including the large-scale spiral structure, the Galactic Center, and gas cloud motions. Oort was helped in this project by the Dutch telecommunications company, PTT, which, he later explained, "had under their care all the radar equipment that was left behind by the Germans on the coast of Holland. This radar equipment consisted in part of reflecting telescopes of 7 1/2 meter aperture... Our radio astronomy was really started with the aid of one of these instruments… it was in Kootwijk that the first map of the Galaxy was made." For a brief period, before the completion of the Jodrell Bank telescope, the Dwingeloo instrument was the largest of its kind on Earth.
It has been written that "Oort was probably the first astronomer to realize the importance" of radio astronomy. "In the days before radio telescopes," one source notes, "Oort was one of the few scientists to realise the potential significance of using radio waves to search the heavens. His theoretical research suggested that vast clouds of hydrogen lingered in the spiral arms of the Galaxy. These molecular clouds, he predicted, were the birthplaces of stars." These predictions were confirmed by measurements made at the new radio observatories at Dwingeloo and Westerbork. Oort later said that "it was Grote Reber's work which first impressed me and convinced me of the unique importance of radio observations for surveying the galaxy." Just before the war, Reber had published a study of galactic radio emissions. Oort later commented, "The work of Grote Reber made it quite clear [radio astronomy] would be a very important tool for investigating the Galaxy, just because it could investigate the whole disc of the galactic system unimpeded by absorption." Oort's work in radio astronomy is credited by colleagues with putting the Netherlands in the forefront of postwar astronomy. Oort also investigated the source of the light from the Crab Nebula, finding that it was polarized, and probably produced by synchrotron radiation, confirming a hypothesis by Iosif Shklovsky.
Comet studies.
Oort went on to study comets, which he formulated a number of revolutionary hypotheses. He hypothesized that the Solar System is surrounded by a massive cloud consisting of billions of comets, many of them "long-period" comets that originate in a cloud far beyond the orbits of Neptune and Pluto. This cloud is now known as the Oort Cloud. He also realized that these external comets, from beyond Pluto, can "become trapped into tighter orbits by Jupiter, and become periodic comets, like Halley's comet." According to one source, "Oort was one of the few people to have seen Comet Halley on two separate apparitions. At the age of 10, he was with his father on the shore at Noordwijk, Netherlands, when he first saw the comet. In 1986, 76 years later, he went up in a plane and was able to see the famous comet once more."
In 1951 Oort and his wife spent several months in Princeton and Pasadena, an interlude that led to a paper by Oort and Lyman Spitzer on the acceleration of interstellar clouds by O-type stars. He went on to study high-velocity clouds. Oort served as director of the Leiden Observatory until 1970. After his retirement, he wrote comprehensive articles on the galactic center and on superclusters and published several papers on the quasar absorption lines, supporting Yakov Zel'dovich's pancake model of the universe. He also continued researching the Milky Way and other galaxies and their distribution until shortly before his death at 92.
One of Oort's strengths, according to one source, was his ability to "translate abstruse mathematical papers into physical terms," as exemplified by his translation of the difficult mathematical terms of Lindblad's theory of differential galactic rotation into a physical model. Similarly, he "derived the existence of the comet cloud on the outskirts of the Solar System from the observations, using the mathematics needed in dynamics, but then deduced the origin of this cloud using general physical arguments and a minimum of mathematics."
Personal life.
In 1927, Oort married Johanna Maria (Mieke) Graadt van Roggen (1906–1993). They had met at a university celebration at Utrecht, where Oort's brother was studying biology at the time. Oort and his wife had two sons, Coenraad (Coen) and Abraham, and a daughter, Marijke. Abraham became a professor of climatology at Princeton University.
According to the website of Leiden University, Oort was very interested in and knowledgeable about art. "[W]hen visiting another country he would always try to take some time off to visit the local museums and exhibitions…and in the fifties served for some years as chairman of the pictorial arts committee of the Leiden Academical Arts Centre, which had among other things the task of organizing expositions".
"Colleagues remembered him as a tall, lean and courtly man with a genial manner," reported his "New York Times" obituary.
Writings.
An incomplete list:
Honours.
Awards
Named after him
Memberships
Upon his death, Nobel Prize winning astrophysicist Subrahmanyan Chandrasekhar remarked, "The great oak of Astronomy has been felled, and we are lost without its shadow."
References.
Notes.
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Voyager program
Ongoing NASA interstellar program
The Voyager program is an American scientific program that employs two interstellar probes, "Voyager 1" and "Voyager 2". They were launched in 1977 to take advantage of a favorable planetary alignment to explore the two gas giants Jupiter and Saturn and potentially also the ice giants, Uranus and Neptune - to fly near them while collecting data for transmission back to Earth. After "Voyager 1" successfully completed its flyby of Saturn and its moon Titan, it was decided to send "Voyager 2" on flybys of Uranus and Neptune.
After the planetary flybys were complete, decisions were made to keep the probes in operation to explore interstellar space and the outer regions of the Solar System. On 25 August 2012, data from "Voyager 1" indicated that it had entered interstellar space. On 5 November 2019, data from "Voyager 2" indicated that it also had entered interstellar space. On 4 November 2019, scientists reported that on 5 November 2018, the "Voyager 2" probe had officially reached the interstellar medium (ISM), a region of outer space beyond the influence of the solar wind, as did "Voyager 1" in 2012. In August 2018, NASA confirmed, based on results by the "New Horizons" spacecraft, the existence of a "hydrogen wall" at the outer edges of the Solar System that was first detected in 1992 by the two Voyager spacecraft.
As of 2024,[ [update]] the Voyagers are still in operation beyond the outer boundary of the heliosphere in interstellar space. "Voyager 1"