Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System (after Eris) and the tenth-most-massive body observed directly orbiting the Sun. Originally classified as the ninth planet from the Sun, Pluto was recategorized as a dwarf planet and plutoid due to the discovery that it is one of several large bodies within the newly charted Kuiper belt.[i]
Like other members of the Kuiper belt, Pluto is composed primarily of rock and ice and is relatively small: approximately a fifth the mass of the Earth‘s Moonand a third its volume. It has an eccentric and highly inclined orbit that takes it from 30 to 49 AU (4.4–7.4 billion km) from the Sun. This causes Pluto to periodically come closer to the Sun than Neptune. As of 2011, it is 32.1 AU from the Sun.
From its discovery in 1930 until 2006, Pluto was classified as a planet. In the late 1970s, following the discovery of minor planet 2060 Chiron in the outer Solar System and the recognition of Pluto’s relatively low mass, its status as a major planet began to be questioned. In the late 20th and early 21st century, many objects similar to Pluto were discovered in the outer Solar System, notably the scattered disc object Eris in 2005, which is 27% more massive than Pluto. On August 24, 2006, the International Astronomical Union (IAU) defined what it means to be a “planet” within the Solar System. This definition excluded Pluto as a planet and added it as a member of the new category “dwarf planet” along with Eris and Ceres. After the reclassification, Pluto was added to the list of minor planets and given the number 134340. A number of scientists continue to hold that Pluto should be classified as a planet.
Pluto has four known moons, the largest being Charon discovered in 1978, along with Nix and Hydra, discovered in 2005, and the provisionally namedS/2011 P 1, discovered in 2011. Pluto and Charon are sometimes described as a binary system because the barycenter of their orbits does not lie within either body. However, the IAU has yet to formalise a definition for binary dwarf planets, and as such Charon is officially classified as a moon of Pluto.
In the 1840s, using Newtonian mechanics, Urbain Le Verrier predicted the position of the then-undiscovered planet Neptune after analysing perturbations in the orbit of Uranus. Subsequent observations of Neptune in the late 19th century caused astronomers to speculate that Uranus’ orbit was being disturbed by another planet besides Neptune. In 1906, Percival Lowell, a wealthy Bostonian who had founded the Lowell Observatory in Flagstaff, Arizona in 1894, started an extensive project in search of a possible ninth planet, which he termed “Planet X“. By 1909, Lowell and William H. Pickering had suggested several possible celestial coordinates for such a planet. Lowell and his observatory conducted his search until his death in 1916, but to no avail. Unknown to Lowell, on March 19, 1915, his observatory had captured two faint images of Pluto, but did not recognise them for what they were. Lowell was not the first to unknowingly photograph Pluto. There are sixteen known pre-discoveries, with the oldest being made by the Yerkes Observatory on August 20, 1909.
Due to a ten-year legal battle with Constance Lowell, Percival’s widow, who attempted to wrest the observatory’s million-dollar portion of his legacy for herself, the search for Planet X did not resume until 1929, when its director, Vesto Melvin Slipher, summarily handed the job of locating Planet X to Clyde Tombaugh, a 23-year-old Kansas man who had just arrived at the Lowell Observatory after Slipher had been impressed by a sample of his astronomical drawings.
Tombaugh’s task was to systematically image the night sky in pairs of photographs taken two weeks apart, then examine each pair and determine whether any objects had shifted position. Using a machine called a blink comparator, he rapidly shifted back and forth between views of each of the plates to create the illusion of movement of any objects that had changed position or appearance between photographs. On February 18, 1930, after nearly a year of searching, Tombaugh discovered a possible moving object on photographic plates taken on January 23 and January 29 of that year. A lesser-quality photograph taken on January 21 helped confirm the movement. After the observatory obtained further confirmatory photographs, news of the discovery was telegraphed to theHarvard College Observatory on March 13, 1930.
The discovery made headlines across the globe. The Lowell Observatory, which had the right to name the new object, received over 1,000 suggestions from all over the world, ranging from Atlas to Zymal. Tombaugh urged Slipher to suggest a name for the new object quickly before someone else did. Constance Lowell proposed Zeus, then Percival and finallyConstance. These suggestions were disregarded.
The name Pluto was proposed by Venetia Burney (1918–2009), an eleven-year-old schoolgirl in Oxford, England. Venetia was interested in classical mythology as well as astronomy, and considered the name, a name for the god of the underworld, appropriate for such a presumably dark and cold world. She suggested it in a conversation with her grandfatherFalconer Madan, a former librarian at the University of Oxford‘s Bodleian Library. Madan passed the name to ProfessorHerbert Hall Turner, who then cabled it to colleagues in the United States.
The object was officially named on March 24, 1930. Each member of the Lowell Observatory was allowed to vote on a short-list of three: Minerva (which was already the name for an asteroid), Cronus (which had lost reputation through being proposed by the unpopular astronomer Thomas Jefferson Jackson See), and Pluto. Pluto received every vote. The name was announced on May 1, 1930. Upon the announcement, Madan gave Venetia five pounds (£5) (£234 as of 2011), as a reward.
It has been noted that the first two letters of Pluto are the initials of Percival Lowell, and Pluto’s astronomical symbol () is a monogram constructed from the letters ‘PL’. Pluto’s astrological symbol resembles that of Neptune (), but has a circle in place of the middle prong of the trident ().
The name was soon embraced by wider culture. In 1930, Walt Disney introduced for Mickey Mouse a canine companion, named Pluto apparently in the object’s honour, although Disney animator Ben Sharpsteen could not confirm why the name was given. In 1941, Glenn T. Seaborg named the newly created element plutonium after Pluto, in keeping with the tradition of naming elements after newly discovered planets, following uranium, which was named after Uranus, and neptunium, which was named after Neptune.
In Chinese, Japanese and Korean the name was translated as underworld king star (冥王星), as suggested by Houei Nojiri in 1930. Many other non-European languages use a transliteration of “Pluto” as their name for the object; some Indian languages use a form of Yama, the Guardian of Hell in Hindu mythology, such as the Gujarati Yamdev.
Demise of Planet X
Once found, Pluto’s faintness and lack of a resolvable disc cast doubt on the idea that it was Lowell’s Planet X. Estimates of Pluto’s mass were revised downward throughout the 20th century. In 1978, the discovery of Pluto’s moon Charon allowed the measurement of Pluto’s mass for the first time. Its mass, roughly 0.2% that of the Earth, was far too small to account for the discrepancies in the orbit of Uranus. Subsequent searches for an alternate Planet X, notably by Robert Sutton Harrington, failed. In 1992, Myles Standish used data fromVoyager 2‘s 1989 flyby of Neptune, which had revised the planet’s total mass downward by 0.5%, to recalculate its gravitational effect on Uranus. With the new figures added in, the discrepancies, and with them the need for a Planet X, vanished. Today, the majority of scientists agree that Planet X, as Lowell defined it, does not exist. Lowell had made a prediction of Planet X’s position in 1915 that was fairly close to Pluto’s position at that time; Ernest W. Brown concluded almost immediately that this was a coincidence, a view still held today.
Orbit and rotation
Pluto’s orbital period is 248 Earth years. Its orbital characteristics are substantially different from those of the planets, which follow nearly circular orbits around the Sun close to a flat reference plane called the ecliptic. In contrast, Pluto’s orbit is highly inclined relative to the ecliptic (over 17°) and highly eccentric (elliptical). This high eccentricity means a small region of Pluto’s orbit lies nearer the Sun than Neptune‘s. The Pluto–Charon barycentre came to perihelion on September 5, 1989,[j] and was last closer to the Sun than Neptune between February 7, 1979 and February 11, 1999. Detailed calculations indicate that the previous such occurrence lasted only fourteen years, from July 11, 1735 to September 15, 1749, whereas between April 30, 1483 and July 23, 1503, it had also lasted 20 years. The durations of the relationship vary because of the varying speed of motion of Neptune along its orbit; the relationship between the orbitsthemselves varies much more slowly, as Neptune’s orbit precesses and otherwise evolves.
Although this repeating pattern may suggest a regular structure, in the long term Pluto’s orbit is in fact chaotic. While computer simulations can be used to predict its position for several million years (both forward and backward in time), after intervals longer than the Lyapunov time of 10–20 million years, calculations become speculative: Pluto’s tiny size makes it sensitive to unmeasurably small details of the Solar System, hard-to-predict factors that will gradually disrupt its orbit. Millions of years from now, Pluto may well be at aphelion, at perihelion or anywhere in between, with no way for us to predict which. This does not mean Pluto’s orbit itself is unstable, but its position on that orbit is impossible to determine so far ahead. Several resonances and other dynamical effects keep Pluto’s orbit stable, safe from planetary collision or scattering.
Relationship with Neptune
Orbit of Pluto—polar view. This ‘view from above’ shows how Pluto’s orbit (in red) is less circular than Neptune’s (in blue), and how Pluto is sometimes closer to the Sun than Neptune. The darker halves of both orbits show where they pass below theplane of the ecliptic.
Despite Pluto’s orbit appearing to cross that of Neptune when viewed from directly above, the two objects’ orbits are aligned so that they can never collide or even approach closely. There are several reasons why.
At the simplest level, one can examine the two orbits and see that they do not intersect. When Pluto is closest to the Sun, and hence closest to Neptune’s orbit as viewed from above, it is also the farthest above Neptune’s path. Pluto’s orbit passes about 8 AU above that of Neptune, preventing a collision. Pluto’s ascending and descending nodes, the points at which its orbit crosses the ecliptic, are currently separated from Neptune’s by over 21°.
This alone is not enough to protect Pluto; perturbations from the planets (especially Neptune) could alter aspects of Pluto’s orbit (such as its orbital precession) over millions of years so that a collision could be possible. Some other mechanism or mechanisms must therefore be at work. The most significant of these is that Pluto lies in the 3:2 mean motion resonance with Neptune: for every three of Neptune’s orbits around the Sun, Pluto makes two. The two objects then return to their initial positions and the cycle repeats, each cycle lasting about 500 years. This pattern is configured so that, in each 500-year cycle, the first time Pluto is nearperihelion Neptune is over 50° behind Pluto. By Pluto’s second perihelion, Neptune will have completed a further one and a half of its own orbits, and so will be a similar distance ahead of Pluto. Pluto and Neptune’s minimum separation is over 17 AU. Pluto comes closer to Uranus (11 AU) than it does to Neptune.
The 3:2 resonance between the two bodies is highly stable, and is preserved over millions of years. This prevents their orbits from changing relative to one another; the cycle always repeats in the same way, and so the two bodies can never pass near to each other. Thus, even if Pluto’s orbit were not highly inclined the two bodies could never collide.
Numerical studies have shown that over periods of millions of years, the general nature of the alignment between Pluto and Neptune’s orbits does not change. There are several other resonances and interactions that govern the details of their relative motion, and enhance Pluto’s stability. These arise principally from two additional mechanisms (besides the 3:2 mean motion resonance).
First, Pluto’s argument of perihelion, the angle between the point where it crosses the ecliptic and the point where it is closest to the Sun, librates around 90°. This means that when Pluto is nearest the Sun, it is at its farthest above the plane of the Solar System, preventing encounters with Neptune. This is a direct consequence of the Kozai mechanism, which relates the eccentricity of an orbit to its inclination to a larger perturbing body—in this case Neptune. Relative to Neptune, the amplitude of libration is 38°, and so the angular separation of Pluto’s perihelion to the orbit of Neptune is always greater than 52° (= 90°–38°). The closest such angular separation occurs every 10,000 years.
Second, the longitudes of ascending nodes of the two bodies—the points where they cross the ecliptic—are in near-resonance with the above libration. When the two longitudes are the same—that is, when one could draw a straight line through both nodes and the Sun—Pluto’s perihelion lies exactly at 90°, and it comes closest to the Sun at its peak above Neptune’s orbit. In other words, when Pluto most closely intersects the plane of Neptune’s orbit, it must be at its farthest beyond it. This is known as the 1:1 superresonance, and is controlled by all the Jovian planets.
To understand the nature of the libration, imagine a polar point of view, looking down on the ecliptic from a distant vantage point where the planets orbit counter-clockwise. After passing the ascending node, Pluto is interior to Neptune’s orbit and moving faster, approaching Neptune from behind. The strong gravitational pull between the two causes angular momentum to be transferred to Pluto, at Neptune’s expense. This moves Pluto into a slightly larger orbit, where it travels slightly slower, according to Kepler’s third law. As its orbit changes, this has the gradual effect of changing the pericentre and longitudes of Pluto (and, to a lesser degree, of Neptune). After many such repetitions, Pluto is sufficiently slowed, and Neptune sufficiently speeded up, that Neptune begins to catch Pluto at the opposite side of its orbit (near the opposing node to where we began). The process is then reversed, and Pluto loses angular momentum to Neptune, until Pluto is sufficiently speeded up that it begins to catch Neptune again at the original node. The whole process takes about 20,000 years to complete.
Pluto’s rotation period, its day, is equal to 6.39 Earth days. Like Uranus, Pluto rotates on its “side” on its orbital plane, with an axial tilt of 120°, and so its seasonal variation is extreme; at itssolstices, one-fourth of its surface is in permanent daylight, while another fourth is in permanent darkness.
Appearance and surface
Pluto’s visual apparent magnitude averages 15.1, brightening to 13.65 at perihelion. To see it, a telescope is required; around 30 cm (12 in) aperture being desirable. It looks star-like and without a visible disk even in large telescopes, because its angular diameter is only 0.11″.
The earliest maps of Pluto, made in the late 1980s, were brightness maps created from close observations of eclipses by its largest moon, Charon. Observations were made of the change in the total average brightness of the Pluto–Charon system during the eclipses. For example, eclipsing a bright spot on Pluto makes a bigger total brightness change than eclipsing a dark spot. Computer processing of many such observations can be used to create a brightness map. This method can also track changes in brightness over time.
Current maps have been produced from images from the Hubble Space Telescope (HST), which offers the highest resolution currently available, and show considerably more detail, resolving variations several hundred kilometres across, including polar regions and large bright spots. The maps are produced by complex computer processing, which find the best-fit projected maps for the few pixels of the Hubble images. The two cameras on the HST used for these maps are no longer in service, so these will likely remain the most detailed maps of Pluto until the 2015 flyby of New Horizons.
These maps, together with Pluto’s lightcurve and the periodic variations in its infrared spectra, reveal that Pluto’s surface is remarkably varied, with large changes in both brightness and colour. Pluto is one of the most contrastive bodies in the Solar System, with as much contrast asSaturn‘s moon Iapetus. The colour varies between charcoal black, dark orange and white: Buie et al. term it “significantly less red than Mars and much more similar to the hues seen on Io with a slightly more orange cast”.
Pluto’s surface has changed between 1994 and 2002-3: the northern polar region has brightened and the southern hemisphere darkened. Pluto’s overall redness has also increased substantially between 2000 and 2002. These rapid changes are probably related to seasonal condensation and sublimation of portions of Pluto’s atmosphere, amplified by Pluto’s extreme axial tilt and high orbital eccentricity.
Spectroscopic analysis of Pluto’s surface reveals it to be composed of more than 98 percent nitrogen ice, with traces of methane and carbon monoxide.The face of Pluto oriented toward Charon contains more methane ice, while the opposite face contains more nitrogen and carbon monoxide ice.
Observations by the Hubble Space Telescope place Pluto’s density at between 1.8 and 2.1 g/cm3, suggesting its internal composition consists of roughly 50–70 percent rock and 30–50 percent ice by mass. Because decay of radioactive minerals would eventually heat the ices enough for the rock to separate from them, scientists expect that Pluto’s internal structure is differentiated, with the rocky material having settled into a dense core surrounded by a mantle of ice. The diameter of the core should be around 1,700 km, 70% of Pluto’s diameter. It is possible that such heating continues today, creating a subsurface ocean layer of liquid water some 100 to 180 km thick at the core–mantle boundary. The DLRInstitute of Planetary Research calculated that Pluto’s density-to-radius ratio lies in a transition zone, along with Neptune’s moon Triton, between icy satellites like the mid-sized moons of Uranus and Saturn, and rocky satellites such as Jupiter’s Europa.
Mass and size
Pluto’s mass is 1.31×1022 kg, less than 0.24 percent that of the Earth, while its diameter is 2,306 (+/- 20) km, or roughly 66% that of the Moon. Pluto’s atmosphere complicates determining its true solid size within a certain margin.
Astronomers, assuming Pluto to be Lowell’s Planet X, initially calculated its mass based on its presumed effect on Neptune and Uranus. In 1955 Pluto was calculated to be roughly the mass of the Earth, with further calculations in 1971 bringing the mass down to roughly that of Mars. In 1976, Dale Cruikshank, Carl Pilcher and David Morrison of the University of Hawaiicalculated Pluto’s albedo for the first time, finding that it matched that for methane ice; this meant Pluto had to be exceptionally luminous for its size and therefore could not be more than 1 percent the mass of the Earth. Pluto’s albedo is 1.3–2.0 times greater than that of Earth.
The discovery of Pluto’s satellite Charon in 1978 enabled a determination of the mass of the Pluto–Charon system by application of Newton’s formulation of Kepler’s third law. Once Charon’s gravitational effect was measured, Pluto’s true mass could be determined. Observations of Pluto in occultation with Charon allowed scientists to establish Pluto’s diameter more accurately, while the invention of adaptive optics allowed them to determine its shape more accurately.
Among the objects of the Solar System, Pluto is much less massive than the terrestrial planets, and at less than 0.2 lunar masses, it is also less massive than seven moons: Ganymede, Titan, Callisto, Io, Earth’s Moon, Europa and Triton. Pluto is more than twice the diameter and a dozen times the mass of the dwarf planet Ceres, the largest object in the asteroid belt. It is less massive than the dwarf planet Eris, a trans-Neptunian object discovered in 2005. Given the error bars in the different size estimates, it is currently unknown whether Eris or Pluto has the larger diameter. Both Pluto and Eris are estimated to have solid-body diameters of about 2330 km. Determinations of Pluto’s size are complicated by its atmosphere, and possible hydrocarbon haze.
Pluto’s atmosphere consists of a thin envelope of nitrogen, methane, and carbon monoxide gases, which are derived from the ices of these substances on its surface. Its surface pressure ranges from 6.5 to 24 μbar. Pluto’s elongated orbit is predicted to have a major effect on its atmosphere: as Pluto moves away from the Sun, its atmosphere should gradually freeze out, and fall to the ground. When Pluto is closer to the Sun, the temperature of Pluto’s solid surface increases, causing the ices to sublimate into gas. This creates an anti-greenhouse effect; much as sweatcools the body as it evaporates from the surface of the skin, this sublimation cools the surface of Pluto. Scientists using the Submillimeter Array have recently discovered that Pluto’s temperature is about 43 K (−230 °C), 10 K colder than would otherwise be expected.
The presence of methane, a powerful greenhouse gas, in Pluto’s atmosphere creates a temperature inversion, with average temperatures 36 K warmer 10 km above the surface. The lower atmosphere contains a higher concentration of methane than its upper atmosphere.
The first evidence of Pluto’s atmosphere was found by the Kuiper Airborne Observatory in 1985, from observations of the occultation of a star behind Pluto. When an object with no atmosphere moves in front of a star, the star abruptly disappears; in the case of Pluto, the star dimmed out gradually. From the rate of dimming, the atmospheric pressure was determined to be 0.15 pascal, roughly 1/700,000 that of Earth. The conclusion was confirmed and significantly strengthened by extensive observations of another similar occultation in 1988.
In 2002, another occultation of a star by Pluto was observed and analysed by teams led by Bruno Sicardy of the Paris Observatory, James L. Elliot of MIT, and Jay Pasachoff of Williams College. Surprisingly, the atmospheric pressure was estimated to be 0.3 pascal, even though Pluto was farther from the Sun than in 1988 and thus should have been colder and had a more rarefied atmosphere. One explanation for the discrepancy is that in 1987 the south pole of Pluto came out of shadow for the first time in 120 years, causing extra nitrogen to sublimate from the polar cap. It will take decades for the excess nitrogen to condense out of the atmosphere as it freezes onto the north pole’s now permanently dark ice cap. Spikes in the data from the same study revealed what may be the first evidence of wind in Pluto’s atmosphere. Another stellar occultation was observed by the MIT-Williams College team of James Elliot, Jay Pasachoff, and a Southwest Research Instituteteam led by Leslie Young on June 12, 2006 from sites in Australia.
In October 2006, Dale Cruikshank of NASA/Ames Research Center (a New Horizons co-investigator) and his colleagues announced the spectroscopic discovery of ethane on Pluto’s surface. This ethane is produced from the photolysis or radiolysis (i.e., the chemical conversion driven by sunlight and charged particles) of frozen methane on Pluto’s surface and suspended in its atmosphere.
Eris, formal designation 136199 Eris, is the most massive known dwarf planet[i] in the Solar System and the ninth most massive body known to orbit theSun directly. It is estimated to be approximately 2300–2400 km in diameter, and 27% more massive than Pluto or about 0.27% of the Earth‘s mass.
Eris was discovered in January 2005 by a Palomar Observatory-based team led by Mike Brown, and its identity was verified later that year. It is a trans-Neptunian object (TNO) and a member of a high-eccentricity population known as the scattered disc. It has one known moon, Dysnomia. As of 2011, its distance from the Sun is 96.6 AU, roughly three times that of Pluto. With the exception of some comets, Eris and Dysnomia are currently the most distant known natural objects in the Solar System.[h]
Because Eris appeared to be larger than Pluto, its discoverers and NASA initially described it as the Solar System’s tenth planet. This, along with the prospect of other similarly sized objects being discovered in the future, motivated the International Astronomical Union (IAU) to define the term planet for the first time. Under the IAU definition approved on August 24, 2006, Eris is a “dwarf planet” along with Pluto, Ceres, Haumea and Makemake.
In 2010, preliminary results from observations of a stellar occultation by Eris on November 6 suggested that its diameter may be only 2,326 km, which would make it essentially the same size as Pluto. Given the error bars in the different size estimates, it is currently uncertain whether Eris or Pluto has the larger diameter. Both Pluto and Eris are estimated to have solid-body diameters of about 2330 km.
Eris was discovered by the team of Mike Brown, Chad Trujillo, and David Rabinowitz on January 5, 2005, from images taken on October 21, 2003. The discovery was announced on July 29, 2005, the same day as Makemake and two days after Haumea. The search team had been systematically scanning for large outer solar system bodies for several years, and had been involved in the discovery of several other large TNOs, including 50000 Quaoar,90482 Orcus, and 90377 Sedna.
Routine observations were taken by the team on October 21, 2003, using the 1200 mm Samuel Oschin Schmidt telescope at Mount Palomar Observatory,California, but the image of Eris was not discovered at that point due to its very slow motion across the sky: The team’s automatic image-searching software excluded all objects moving at less than 1.5 arcseconds per hour to reduce the number of false positives returned. When Sedna was discovered, it was moving at 1.75 arcsec/h, and in light of that the team reanalyzed their old data with a lower limit on the angular motion, sorting through the previously excluded images by eye. In January 2005, the re-analysis revealed Eris’ slow motion against the background stars.
Follow-up observations were then carried out to make a preliminary determination of Eris’ orbit, which allowed the object’s distance to be estimated. The team had planned to delay announcing their discovery until further observations allowed more accurate calculations of Eris’ orbit, but brought their announcement forward when the discovery of another large TNO they had been tracking, Haumea, was announced by a different team in Spain.
More observations released in October 2005 revealed that Eris had a moon, later named Dysnomia. Observations of Dysnomia’s orbit permitted scientists to determine the mass of Eris, which in June 2007 they calculated to be 1.66±0.02×1022 kg, 27% greater than Pluto’s.
Eris is classified as a plutoid, that is, a trans-Neptunian object that is also a dwarf planet. Its orbital characteristics more specifically categorize it a scattered disk object (SDO), or a TNO that is believed to have been “scattered” from the Kuiper belt into more distant and unusual orbits following gravitational interactions withNeptune as the Solar System was forming. Although its high orbital inclination is unusual among the known SDOs, theoretical models suggest that objects that were originally near the inner edge of the Kuiper belt were scattered into orbits with higher inclinations than objects from the outer belt. Inner-belt objects are expected to be generally more massive than outer-belt objects, and so astronomers expect to discover more large objects like Eris in high-inclination orbits, which have traditionally been neglected.
Because Eris may be larger than Pluto, it was initially described as the “tenth planet” by NASA and in media reports of its discovery. In response to the uncertainty over its status, and because of ongoing debate over whether Pluto should be classified as a planet, the IAU delegated a group of astronomers to develop a sufficiently precise definition of the term planet to decide the issue. This was announced as the IAU’s Definition of a Planet in the Solar System, adopted on August 24, 2006. At this time, both Eris and Pluto were classified as dwarf planets, a category distinct from the new definition of planet. Brown has since stated his approval of Pluto losing its status as a planet. The IAU subsequently added Eris to its Minor Planet Catalogue, designating it (136199) Eris.
Eris is named after the Greek goddess Eris (Greek Ἔρις), a personification of strife and discord. The name was assigned on September 13, 2006, following an unusually long period in which the object was known by the provisional designation 2003 UB313, which was granted automatically by the IAU under their naming protocols for minor planets. The regular adjectival form of Eris is Eridian.
Due to uncertainty over whether the object would be classified as a planet or a minor planet, as different nomenclature procedures apply to these different classes of objects, the decision on what to name the object had to wait until after the August 24, 2006, IAU ruling. As a result, for a time the object became known to the wider public as Xena.
“Xena” was an informal name used internally by the discovery team. It was inspired by the eponymous heroine of the television series Xena: Warrior Princess. The discovery team had reportedly saved the nickname “Xena” for the first body they discovered that was larger than Pluto. According to Brown,
We chose it since it started with an X (planet “X”), it sounds mythological (OK, so it’s TV mythology, but Pluto is named after a cartoon, right?),[b] and (this part is actually true) we’ve been working to get more female deities out there (i.e. Sedna). Also, at the time, the TV show was still on TV, which shows you how long we’ve been searching!
“We assumed [that] a real name would come out fairly quickly, [but] the process got stalled,” Mike Brown said in interview,
One reporter called me up from the New York Times who happened to have been a friend of mine from college, [and] I was a little less guarded with him than I am with the normal press. He asked me, “What’s the name you guys proposed?” and I said, “Well, I’m not going to tell.” And he said, “Well, what do you guys call it when you’re just talking amongst yourselves?”… As far as I remember this was the only time I told anybody this in the press, and then it got everywhere, which I only sorta felt bad about—I kinda like the name.
Choosing an official name
According to science writer Govert Schilling, Brown initially wanted to call the object “Lila“, after a concept in Hindu mythology that described the cosmos as the outcome of a game played by Brahma. The name was very similar to “Lilah”, the name of Brown’s newborn daughter. Brown was mindful of not making his name public before it had been officially accepted. He had done so with Sedna a year previously, and had been heavily criticised. However, he listed the address of his personal web page announcing the discovery as /~mbrown/planetlila and in the chaos following the controversy over the discovery of Haumea, forgot to change it. Rather than needlessly anger more of his fellow astronomers, he simply said that the webpage had been named for his daughter and dropped “Lila” from consideration.
Brown had also speculated that Persephone, the wife of the god Pluto, would be a good name for the object. The name had been used several times inscience fiction, and was popular with the public, having handily won a poll conducted by New Scientist magazine (“Xena”, despite only being a nickname, came fourth). However, this was not possible once the object was classified as a dwarf planet, because there is already an asteroid with that name, 399 Persephone. Because IAU regulations require a name from creation mythology for objects with orbital stability beyond Neptune’s orbit, the team had also been considering such possibilities.
With the dispute resolved, the discovery team proposed Eris on September 6, 2006. On September 13, 2006 this name was accepted as the official name by the IAU. Brown decided that, as the object had been considered a planet for so long, it deserved a name from Greek and Roman mythology, like the other planets. However, the asteroids had taken the vast majority of Graeco-Roman names. Eris, whom Brown described as his favourite goddess, had fortunately escaped inclusion. The name in part reflects the discord in the astronomical community caused by the debate over the object’s (and Pluto’s) nature.
Eris has an orbital period of 557 years, and as of 2011 lies at 96.6 astronomical units from the Sun, almost its maximum possible distance (its aphelion is 97.5 AU). It came to perihelion between 1698 and 1699, to aphelion around 1977, and will return to perihelion around 2256 to 2258. Eris and its moon are currently the most distant known objects in the Solar System apart from long-period comets and space probes. However, approximately forty known TNOs, most notably 2006 SQ372, 2000 OO67 and Sedna, while currently closer to the Sun than Eris, have greater average orbital distances than Eris’ semimajor axis of 67.7 AU.
The Eridian orbit is highly eccentric, and brings Eris to within 37.9 AU of the Sun, a typical perihelion for scattered objects. This is within the orbit of Pluto, but still safe from direct interaction with Neptune (29.8–30.4 AU). Pluto, on the other hand, like other plutinos, follows a less inclined and less eccentric orbit and, protected by orbital resonance, can cross Neptune’s orbit. It is possible that Eris is in a 17:5 resonance with Neptune, though further observations will be required to check that hypothesis. Unlike the eight planets, whose orbits all lie roughly in the same plane as the Earth’s, Eris’ orbit is highly inclined: It is tilted at an angle of about 44 degrees to the ecliptic. In about 800 years, Eris will be closer to the Sun than Pluto for some time (see the graph at the right).
Eris currently has an apparent magnitude of 18.7, making it bright enough to be detectable to some amateur telescopes. A 200 mm telescope with a CCDcan detect Eris under favourable conditions.[c] The reason it had not been noticed until now is its steep orbital inclination; most searches for large outer Solar System objects concentrate on the ecliptic plane, where most bodies are found.
Eris is now in the constellation Cetus. It was in Sculptor from 1876 until 1929 and Phoenix from roughly 1840 until 1875. In 2036 it will enter Pisces and stay there until 2065, when it will enter Aries.It will then move into the northern sky, entering Perseus in 2128 and Camelopardalis (where it will reach its northernmost declination) in 2173. Because of the high inclination of its orbit, Eris only passes through a few constellations of the traditional Zodiac.
Size, mass and density
In 2005, the diameter of Eris was measured to be 2,397 km, give or take 100 km, using images from the Hubble Space Telescope (HST). The size of an object is determined from its absolute magnitude (H) and thealbedo (the amount of light it reflects). At a distance of 97 AU, an object with a diameter of 3,000 km would have an angular size of 40 milliarcseconds, which is directly measurable with the Hubble Space Telescope. Although resolving such small objects is at the very limit of the telescope’s capabilities,[d] sophisticated image processing techniques such as deconvolution can be used to measure such angular sizes fairly accurately.[e]
This makes Eris around the same size as Pluto, which is about 2,330 km across. It also indicates an albedo of 0.96, higher than that of any other large body in the Solar System except Enceladus. It is speculated that the high albedo is due to the surface ices being replenished because of temperature fluctuations as Eris’s eccentric orbit takes it closer and farther from the Sun.
In 2007, a series of observations of the largest trans-Neptunian objects with the Spitzer Space Telescope gave an estimate of Eris’s diameter of2,600+400
−200 km. The Spitzer and Hubble estimates overlap in the range of 2,400–2,500 km, 4–8% larger than Pluto. However, astronomers now suspect that Eris’s spin axis is pointing toward the sun, at the moment—a possibility that would keep the sunlit hemisphere warmer than average and skew any infrared measurements toward higher values. So the outcome from the 2010 Chile occultation is actually more in line with the Hubble result from 2005.
In November 2010, Eris was the subject of one of the most distant stellar occultations yet achieved from Earth. Preliminary data from this event cast doubt on previous size estimates. The teams announced their final results from the occultation in October, 2011, with an estimated diameter of 2,326+12
−12 km. However, when using data from this event for comparison to Pluto, there is a range of figures available for Pluto’s radius/diameter that can be selected. This is due in part to Pluto’s atmosphere which interferes with making measurements of its solid surface (as opposed to gaseous haze). The mass of Eris can be calculated with much greater precision. Based on the currently accepted value for Dysnomia’s period—15.774 days— Eris is 27 percent more massive than Pluto. If the 2011 occultation results are used, then Eris has a density of 2.52±0.05 g cm−3; substantially denser than Pluto, and thus must be composed largely of rocky materials.
Surface and atmosphere
The discovery team followed up their initial identification of Eris with spectroscopic observations made at the 8 mGemini North Telescope in Hawaii on January 25, 2005. Infrared light from the object revealed the presence ofmethane ice, indicating that the surface may be similar to that of Pluto, which at the time was the only TNO known to have surface methane, and of Neptune’s moon Triton, which also has methane on its surface. Note that no surface details can be resolved from Earth or its orbit with any instrument currently available.
Due to Eris’s distant eccentric orbit, Eridian surface temperature is estimated to vary between about 30 and 56 kelvin (−243 and −217 degrees Celsius).
Unlike the somewhat reddish Pluto and Triton, however, Eris appears almost grey. Pluto’s reddish colour is believed to be due to deposits of tholins on its surface, and where these deposits darken the surface, the lower albedo leads to higher temperatures and the evaporation of methane deposits. In contrast, Eris is far enough away from the Sun that methane can condense onto its surface even where the albedo is low. The condensation of methane uniformly over the surface reduces any albedo contrasts and would cover up any deposits of red tholins.
Even though Eris can be up to three times further from the Sun than Pluto, it approaches close enough that some of the ices on the surface might warm enough to sublime. As methane is highly volatile, its presence shows either that Eris has always resided in the distant reaches of the Solar System where it is cold enough for methane ice to persist, or that the celestial body has an internal source of methane to replenish gas that escapes from its atmosphere. This contrasts with observations of another recently discovered TNO, Haumea, which reveal the presence of water ice but not methane.
Ceres, formally 1 Ceres, is the smallest identified dwarf planet in the Solar System and the only one in the asteroid belt. With a diameter of about 950 km, Ceres is by far the largest and most-massive asteroid, comprising about a third of the mass of the asteroid belt. Discovered on 1 January 1801 by Giuseppe Piazzi, it was the first asteroid to be identified. It is named after Cerēs, the Roman goddess of growing plants, the harvest, and motherly love.
The Cererian surface is probably a mixture of water ice and various hydrated minerals such as carbonates and clays. Ceres appears to bedifferentiated into a rocky core and icy mantle, and may harbour an ocean of liquid water under its surface.
From Earth, the apparent magnitude of Ceres ranges from 6.7 to 9.3, and hence even at its brightest it is still too dim to be seen with the naked eye except under extremely dark skies. On 27 September 2007, NASA launched the Dawn space probe to explore Vesta (2011–2012) and Ceres (2015).
The idea that an undiscovered planet could exist between the orbits of Mars and Jupiter was first suggested by Johann Elert Bode in 1772. His considerations were based on the Titius–Bode law, a now abandoned theory which had been first proposed by Johann Daniel Titius in 1766, observing that there was a regular pattern in the semi-major axes of the known planets marred only by the large gap between Mars and Jupiter. The pattern predicted that the missing planet ought to have a semi-major axis near 2.8 AU. William Herschel‘s discovery of Uranus in 1781 near the predicted distance for the next body beyond Saturn increased faith in the law of Titius and Bode, and in 1800, they sent requests to twenty-four experienced astronomers, asking that they combine their efforts and begin a methodical search for the expected planet. The group was headed by Franz Xaver von Zach, editor of the Monatliche Correspondenz. While they did not discover Ceres, they later found several large asteroids.
One of the astronomers selected for the search was Giuseppe Piazzi at the Academy of Palermo, Sicily. Before receiving his invitation to join the group, Giuseppe Piazzi discovered Ceres on 1 January 1801. He was searching for “the 87th [star] of the Catalogue of the Zodiacal stars of Mr la Caille“, but found that “it was preceded by another”. Instead of a star, Piazzi had found a moving star-like object, which he first thought was a comet. Piazzi observed Ceres a total of 24 times, the final time on 11 February 1801, when illness interrupted his observations. He announced his discovery on 24 January 1801 in letters to only two fellow astronomers, his compatriot Barnaba Oriani of Milan and Bode of Berlin. He reported it as a comet but “since its movement is so slow and rather uniform, it has occurred to me several times that it might be something better than a comet”. In April, Piazzi sent his complete observations to Oriani, Bode, andJérôme Lalande in Paris. The information was published in the September 1801 issue of the Monatliche Correspondenz.
By this time, the apparent position of Ceres had changed (mostly due to the Earth’s orbital motion), and was too close to the Sun’s glare for other astronomers to confirm Piazzi’s observations. Toward the end of the year, Ceres should have been visible again, but after such a long time it was difficult to predict its exact position. To recover Ceres, Carl Friedrich Gauss, then 24 years old, developed an efficient method of orbit determination. He set himself the task of determining a Keplerian motion from three complete observations (time, right ascension, declination).[clarification needed] In only a few weeks, he predicted the path of Ceres and sent his results to von Zach. On 31 December 1801, von Zach and Heinrich W. M. Olbers found Ceres near the predicted position and thus recovered it.
The early observers were only able to calculate the size of Ceres to within about an order of magnitude. Herschel underestimated its size as 260 km in 1802, while in 1811 Johann Hieronymus Schröteroverestimated it as 2,613 km.
Piazzi originally suggested the name Cerere Ferdinandea for his discovery, after both the mythological figure Ceres (Roman goddess of agriculture, ItalianCerere) and King Ferdinand III of Sicily. “Ferdinandea” was not acceptable to other nations of the world and was thus dropped. Ceres was also called Hera for a short time in Germany. In Greece, it is called Δήμητρα (Demeter), after the Greek equivalent of the goddess Ceres; in English, that name is used for the asteroid 1108 Demeter. The adjectival form of the name is Cererian, derived from the Latin genitive Cereris. The oldastronomical symbol of Ceres is a sickle, ⚳ (), similar to Venus‘ symbol ♀, but with a gap in the upper circle; this was later replaced with the numbered disk ①. The element cerium, discovered in 1803, was named after Ceres. In the same year, another element was also initially named after Ceres, but its discoverer changed its name to palladium (after the second asteroid, 2 Pallas) when cerium was named.
The classification of Ceres has changed more than once and has been the subject of some disagreement.Johann Elert Bode believed Ceres to be the “missing planet” he had proposed to exist between Mars andJupiter, at a distance of 419 million km (2.8 AU) from the Sun. Ceres was assigned a planetary symbol, and remained listed as a planet in astronomy books and tables (along with 2 Pallas, 3 Juno and 4 Vesta) for about half a century.
As other objects were discovered in the area it was realised that Ceres represented the first of a class of many similar bodies. In 1802 Sir William Herschel coined the term asteroid (“star-like”) for such bodies, writing “they resemble small stars so much as hardly to be distinguished from them, even by very good telescopes”. As the first such body to be discovered, it was given the designation 1 Ceres under the modern system of asteroid numbering.
The 2006 debate surrounding Pluto and what constitutes a ‘planet’ led to Ceres being considered for reclassification as a planet. A proposal before theInternational Astronomical Union for the definition of a planet would have defined a planet as “a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid-body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet”. Had this resolution been adopted, it would have made Ceres the fifth planet in order from the Sun. It was not accepted, and in its place an alternate definition came into effect as of 24 August 2006, carrying the additional requirement that a “planet” must have “cleared the neighborhood around its orbit”. By this definition, Ceres is not a planet because it does not dominate its orbit, sharing it as it does with the thousands of other asteroids in the asteroid beltand constituting only about a third of the total mass. It is instead now classified as a dwarf planet.
It is sometimes assumed that Ceres has been reclassified as a dwarf planet, and that it is therefore no longer considered an asteroid. For example, a news update at Space.com spoke of “Pallas, the largest asteroid, and Ceres, the dwarf planet formerly classified as an asteroid”, while an IAU question-and-answer posting states, “Ceres is (or now we can say it was) the largest asteroid”, though it then speaks of “other asteroids” crossing Ceres’ path and otherwise implies that Ceres is still one of the asteroids. The Minor Planet Center notes that such bodies may have dual designations.The 2006 IAU decision that classified Ceres as a dwarf planet never addressed whether it is or is not an asteroid, as indeed the IAU has never defined the word ‘asteroid’ at all, preferring the term ‘minor planet‘ until 2006, and ‘small Solar System body‘ and ‘dwarf planet’ after 2006. Lang (2011) comments, “The [IAU has] added a new designation to Ceres, classifying it as a dwarf planet. [...] By [its] definition, Eris, Haumea, Makemake and Pluto, as well as the largest asteroid, 1 Ceres, are all dwarf planets”, and describes it elsewhere as “the dwarf planet–asteroid 1 Ceres”. NASA continues to refer to Ceres as an asteroid, saying in a 2011 press announcement that “Dawn will orbit two of the largest asteroids in the Main Belt”, as do various academic textbooks.
Ceres is the largest object in the asteroid belt, which lies between Mars and Jupiter. The mass of Ceres has been determined by analysis of the influence it exerts on smaller asteroids. Results differ slightly between researchers. The average of the three most precise values as of 2008 is 9.4×1020 kg. With this mass Ceres comprises about a third of the estimated total 3.0 ± 0.2×1021 kg mass of the asteroid belt, which is in turn about 4% of the mass of the Moon. The mass of Ceres is sufficient to give it a nearly spherical shape inhydrostatic equilibrium. In contrast, other large asteroids such as 2 Pallas, 3 Juno, and in particular 10 Hygiea are known to be somewhat irregular in shape.
Ceres’ oblateness is inconsistent with an undifferentiated body, which indicates that it consists of a rocky core overlain with an icy mantle. This 100 km-thick mantle (23%–28% of Ceres by mass; 50% by volume) contains 200 million cubic kilometres of water, which is more than the amount of fresh water on the Earth. This result is supported by the observations made by the Keck telescope in 2002 and by evolutionary modelling. Also, some characteristics of its surface and history (such as its distance from the Sun, which weakened solar radiation enough to allow some fairly low-freezing-point components to be incorporated during its formation), point to the presence of volatile materials in the interior of Ceres.
Alternatively, the shape and dimensions of Ceres may be explained by an interior that is porous and either partially differentiated or completely undifferentiated. The presence of a layer of rock on top of ice would be gravitationally unstable. If any of the rock deposits sank into a layer of differentiated ice, salt deposits would be formed. Such deposits have not been detected. Thus it is possible that Ceres does not contain a large ice shell, but was instead formed from low-density asteroids with an aqueous component. The decay of radioactive isotopes may not have been sufficient to cause differentiation.
The surface composition of Ceres is broadly similar to that of C-type asteroids. Some differences do exist. The ubiquitous features of the Cererian IR spectra are those of hydrated materials, which indicate the presence of significant amounts of water in the interior. Other possible surface constituents include iron-rich clays (cronstedtite) and carbonate minerals (dolomite and siderite), which are common minerals in carbonaceous chondrite meteorites. The spectral features of carbonates and clay are usually absent in the spectra of other C-type asteroids. Sometimes Ceres is classified as a G-type asteroid.
Only a few Cererian surface features have been unambiguously detected. High-resolution ultraviolet Hubble Space Telescope images taken in 1995 showed a dark spot on its surface which was nicknamed “Piazzi” in honour of the discoverer of Ceres. This was thought to be a crater. Later near-infrared images with a higher resolution taken over a whole rotation with the Keck telescope using adaptive optics showed several bright and dark features moving with the dwarf planet’s rotation. Two dark features had circular shapes and are presumably craters; one of them was observed to have a bright central region, while another was identified as the “Piazzi” feature. More recent visible-light Hubble Space Telescope images of a full rotation taken in 2003 and 2004 showed 11 recognizable surface features, the natures of which are currently unknown. One of these features corresponds to the “Piazzi” feature observed earlier.
These last observations also determined that the north pole of Ceres points in the direction of right ascension 19 h 24 min (291°), declination +59°, in theconstellation Draco. This means that Ceres’ axial tilt is very small—about 3°.
There are indications that Ceres may have a weak atmosphere and water frost on the surface. Surface water ice is unstable at distances less than 5 AU from the Sun, so it is expected to sublime if it is exposed directly to solar radiation. Water ice can migrate from the deep layers of Ceres to the surface, but will escape in a very short time. As a result, it is difficult to detect water vaporization. Water escaping from polar regions of Ceres was possibly observed in the early 1990s but this has not been unambiguously demonstrated. It may be possible to detect escaping water from the surroundings of a fresh impact crater or from cracks in the sub-surface layers of Ceres. Ultraviolet observations by the IUE spacecraft detected statistically significant amounts of hydroxide ion near the Cererean north pole, which is a product of water-vapor dissociation by ultraviolet solar radiation.
Potential for extraterrestrial life
While not as actively discussed as a potential home for extraterrestrial life as Mars or Europa, the potential presence of water ice has led some scientists to hypothesize that life may exist there,and that evidence for this could be found in hypothesized ejecta that could have come from Ceres to Earth.
Ceres follows an orbit between Mars and Jupiter, within the asteroid belt, with a period of 4.6 Earth years. The orbit is moderately inclined (i = 10.6° compared to 7° for Mercury and 17° for Pluto) and moderately eccentric (e = 0.08 compared to 0.09 for Mars).
The diagram illustrates the orbits of Ceres (blue) and several planets (white and grey). The segments of orbits below the ecliptic are plotted in darker colours, and the orange plus sign is the Sun’s location. The top left diagram is a polar view that shows the location of Ceres in the gap between Mars and Jupiter. The top right is a close-up demonstrating the locations of the perihelia (q) and aphelia (Q) of Ceres and Mars. The perihelion of Mars is on the opposite side of the Sun from those of Ceres and several of the large main-belt asteroids, including 2 Pallas and 10 Hygiea. The bottom diagram is a side view showing the inclination of the orbit of Ceres compared to the orbits of Mars and Jupiter.
In the past, Ceres had been considered to be a member of an asteroid family.These groupings of asteroids share similar proper orbital elements, which may indicate a common origin through an asteroid collision some time in the past. Ceres was found to have spectral properties different from other members of the family, and so this grouping is now called the Gefion family, named after the next-lowest-numbered family member, 1272 Gefion. Ceres appears to be merely an interloper in its own family, coincidentally having similar orbital elements but not a common origin.
The rotational period of Ceres (the Cererian day) is 9 hours and 4 minutes.
Transits of planets from Ceres
Mercury, Venus, Earth, and Mars can all appear to cross the Sun, or transit it, from a vantage on Ceres. The most common transits are those of Mercury, which usually happen every few years, most recently in 2006 and 2010. The corresponding dates are 1953 and 2051 for Venus, 1814 and 2081 for Earth, and 767 and 2684 for Mars.
Origin and evolution
Ceres is probably a surviving protoplanet (planetary embryo), which formed 4.57 billion years ago in the asteroid belt. While the majority of inner Solar System protoplanets (including all lunar- to Mars-sized bodies) either merged with other protoplanets to form terrestrial planets or were ejected from the Solar System by Jupiter, Ceres is believed to have survived relatively intact. An alternative theory proposes that Ceres formed in the Kuiper Belt and later migrated to the asteroid belt. Another possible protoplanet, Vesta, is less than half the size of Ceres; it suffered a major impact after solidifying, losing ~1% of its mass.
The geological evolution of Ceres was dependent on the heat sources available during and after its formation: friction from planetesimal accretion, and decay of various radionuclides (possibly including short-lived elements like 26Al). These are thought to have been sufficient to allow Ceres to differentiate into a rocky core and icy mantle soon after its formation. This process may have caused resurfacing by water volcanism and tectonics, erasing older geological features. Due to its small size, Ceres would have cooled early in its existence, causing all geological resurfacing processes to cease. Any ice on the surface would have gradually sublimated, leaving behind various hydrated minerals like clays and carbonates.
Today, Ceres appears to be a geologically inactive body, with a surface sculpted only by impacts. The presence of significant amounts of water ice in its composition raises the possibility that Ceres has or had a layer of liquid water in its interior. This hypothetical layer is often called an ocean. If such a layer of liquid water exists, it is believed to be located between the rocky core and ice mantle like that of the theorized ocean on Europa. The existence of an ocean is more likely if dissolved solutes (i.e. salts), ammonia, sulfuric acid or other antifreeze compounds are dissolved in the water.
When Ceres has an opposition near the perihelion, it can reach a visual magnitude of +6.7. This is generally regarded as too dim to be seen with the naked eye, but under exceptional viewing conditions a very sharp-sighted person may be able to see this dwarf planet. Ceres will be at its brightest (6.73) on December 18, 2012. The only other asteroids that can reach a similarly bright magnitude are 4 Vesta, and, during rare oppositions near perihelion, 2 Pallas and 7 Iris. At a conjunction Ceres has a magnitude of around +9.3, which corresponds to the faintest objects visible with 10×50 binoculars. It can thus be seen with binoculars whenever it is above the horizon of a fully dark sky.
Some notable observational milestones for Ceres include:
- An occultation of a star by Ceres observed in Mexico, Florida and across the Caribbean on 13 November 1984.
- Ultraviolet Hubble Space Telescope images with 50 km resolution taken on 25 June 1995.
- Infrared images with 30 km resolution taken with the Keck telescope in 2002 using adaptive optics.
- Visible light images with 30 km resolution (the best to date) taken using Hubble in 2003 and 2004.
To date, no space probe has visited Ceres. Radio signals from spacecraft in orbit around and on the surface of Mars have been used to estimate the mass of Ceres from its perturbations on the motion of Mars.
The unmanned Dawn spacecraft, launched by NASA in 2007, is en route to Ceres. The probe has been orbiting asteroid 4 Vesta since July 15, 2011. After completing one year of explorations there it will continue on to Ceres, arriving in 2015, five months prior to the arrival of New Horizons at Pluto. Dawn will thus be the first mission to study a dwarf planet at close range.
Dawn’s mission profile calls for it to enter orbit around Ceres at an altitude of 5,900 km. The spacecraft will reduce its orbital distance to 1,300 km after five months of study, and then down to 700 km after another five months. The spacecraft instrumentation includes a framing camera, a visual and infraredspectrometer, and a gamma-ray and neutron detector. These instruments will be used to examine the dwarf planet’s shape and elemental composition.