Ceres is seen from NASA's Dawn spacecraft on March 1, just a few days before the mission achieved orbit around the previously unexplored dwarf planet. The image was taken at a distance of about 30,000 miles (about 48,000 kilometers).
NASA's Dawn spacecraft has become the first mission to achieve orbit around a dwarf planet. The spacecraft was approximately 38,000 miles (61,000 kilometers) from Ceres when it was captured by the dwarf planet’s gravity at about 4:39 a.m. PST (7:39 a.m. EST) Friday.
Mission controllers at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California received a signal from the spacecraft at 5:36 a.m. PST (8:36 a.m. EST) that Dawn was healthy and thrusting with its ion engine, the indicator Dawn had entered orbit as planned.
"Since its discovery in 1801, Ceres was known as a planet, then an asteroid and later a dwarf planet," said Marc Rayman, Dawn chief engineer and mission director at JPL. "Now, after a journey of 3.1 billion miles (4.9 billion kilometers) and 7.5 years, Dawn calls Ceres, home."
In addition to being the first spacecraft to visit a dwarf planet, Dawn also has the distinction of being the first mission to orbit two extraterrestrial targets. From 2011 to 2012, the spacecraft explored the giant asteroid Vesta, delivering new insights and thousands of images from that distant world. Ceres and Vesta are the two most massive residents of our solar system’s main asteroid belt between Mars and Jupiter.
The most recent images received from the spacecraft, taken on March 1 show Ceres as a crescent, mostly in shadow because the spacecraft's trajectory put it on a side of Ceres that faces away from the sun until mid-April. When Dawn emerges from Ceres' dark side, it will deliver ever-sharper images as it spirals to lower orbits around the planet.
"We feel exhilarated," said Chris Russell, principal investigator of the Dawn mission at the University of California, Los Angeles (UCLA). "We have much to do over the next year and a half, but we are now on station with ample reserves, and a robust plan to obtain our science objectives."
Dawn's mission is managed by JPL for NASA's Science Mission Directorate in Washington. Dawn is a project of the directorate's Discovery Program, managed by NASA's Marshall Space Flight Center in Huntsville, Alabama. UCLA is responsible for overall Dawn mission science. Orbital ATK Inc., in Dulles, Virginia, designed and built the spacecraft. The German Aerospace Center, Max Planck Institute for Solar System Research, Italian Space Agency and Italian National Astrophysical Institute are international partners on the mission team.
Im Bann der eisigen Ceres
Framing Camera / Dawn-Sonde
Die Raumsonde Dawn ist in ihrer Umlaufbahn um den Zwergplaneten Ceres angekommen - seit dem 6. März 2015, 13.39 Uhr mitteleuropäischer Zeit, kreist sie um den Himmelskörper. Damit Dawn von Ceres‘ Anziehungskraft in einen Orbit gezogen werden konnte, bremsten die Ionentriebwerke die Raumsonde in 61 000 Kilometern Entfernung von dem Zwergplaneten ab. Während dieses Vorgangs waren keine Aufnahmen mit der Kamera an Bord möglich. "Die insgesamt fast 20 Meter langen Solarpaneele mussten bei diesem Manöver zur Sonne ausgerichtet sein, und daher blickte die Kamera auch nicht in Richtung Ceres", sagt Prof. Ralf Jaumann vom Deutschen Zentrum für Luft- und Raumfahrt (DLR). Der Planetenforscher ist Mitglied im Kamerateam - und wartet gespannt auf die nächsten Bilder, die im April aus 33 000 und 22 000 Kilometern Abstand aufgenommen werden. Doch zunächst verschwindet die Raumsonde Dawn nun bis Mitte April hinter der dunklen, sonnenabgewandten Seite des Zwergplaneten.
Immer näher an die Oberfläche
Siebeneinhalb Jahre nach dem Start und nach fast fünf Milliarden geflogener Kilometer ist Dawn somit an ihrem zweiten Missionsziel angekommen. Ihr erstes Ziel, den Asteroiden Vesta, umkreiste sie von 2011 bis 2012 und lieferte tausende Bilder. Auch ihre Umlaufbahn um Ceres wird nach und nach näher an den Zwergplaneten führen. Ab dem 23. April 2015 wird die Kamera aus nur noch 13 500 Kilometern Entfernung auf die Oberfläche von Ceres blicken und 20 Tage lang den Zwergplaneten aus dieser Höhe erforschen. "Dann können wir schon viel mehr Details erkennen und interpretieren", betont DLR-Planetenforscher Ralf Jaumann. "Und auch die dritte Dimension, das heißt das topographische Relief der Oberfläche, können wir dann zunehmend besser bestimmen." Dann soll auch das erste vollständige dreidimensionale Höhenmodell am DLR-Institut für Planetenforschung entstehen. Bis Ende dieses Jahres wird sich Dawn dann bis auf 375 Kilometer an Ceres annähern. Insgesamt 18 Monate soll die Raumsonde mit insgesamt drei Instrumenten den Zwergplaneten erforschen.
Ceres wurde 1801 entdeckt und als Planet bezeichnet. Später wurde er als Asteroid klassifiziert - um schließlich 2006 in die neu definierte Klasse der Zwergplaneten eingeordnet zu werden. Mit seiner Umlaufbahn um die Sonne, der Kugelform und einem Durchmesser von 950 Kilometern war Ceres bei der Entstehung unseres Sonnensystems auf dem besten Weg, ein Planet zu werden. Doch die Gravitation von Jupiter verhinderte dies sehr wahrscheinlich, und so blieb Ceres in dieser Entwicklung stecken. Das macht Ceres interessant für die Planetenforscher: "Er verkörpert ein Stadium der Planetenentstehung, das uns Aufschluss darüber geben kann, was vor 4,6 Milliarden Jahren passierte", sagt Prof. Ralf Jaumann.
Die Mission DAWN wird vom Jet Propulsion Laboratory (JPL) der amerikanischen Weltraumbehörde NASA geleitet. JPL ist eine Abteilung des California Institute of Technology in Pasadena. Die University of California in Los Angeles ist für den wissenschaftlichen Teil der Mission verantwortlich. Das Kamerasystem an Bord der Raumsonde wurde unter Leitung des Max-Planck-Instituts für Sonnensystemforschung in Göttingen in Zusammenarbeit mit dem Institut für Planetenforschung des Deutschen Zentrums für Luft- und Raumfahrt (DLR) in Berlin und dem Institut für Datentechnik und Kommunikationsnetze in Braunschweig entwickelt und gebaut. Das Kamera-Projekt wird finanziell von der Max-Planck-Gesellschaft, dem DLR und NASA/JPL unterstützt.
Helle Flecken auf Ceres könnten Wasser Vulkane sein
As NASA's Dawn spacecraft pulled into orbit earlier this month around the dwarf planet Ceres in the asteroid belt, it spotted a mysterious bright spot inside a crater. There were suspicions that the spot could be caused by water spewing into space, now fresh views, presented for the first time yesterday, lend weight to the idea.
The pictures show the bright spot is visible even from the side, meaning it probably protrudes above the crater. "What is amazing is you can see this feature while the rim is very likely in front of the line of sight," said Andreas Nathues, who is in charge of the mission's camera, and presented the images yesterday at the Lunar and Planetary Science conference (LPSC) in The Woodlands, Texas. "We believe this could be some kind of outgassing."
Images taken from dusk to dawn on Ceres show that the spot brightens throughout the day and completely disappears at night. This suggests it could be a pocket of ice on the surface that is being heated by the sun and releasing gas, similarly to how a comet behaves. However, Natheus said the team needed higher resolution data to confirm its true nature. This won't come for a while, as Dawn is currently on the dark side of Ceres and won't emerge until mid-April.
Distant observations using the Herschel telescope show Ceres is spitting water from somewhere on its surface, but only Dawn will be able to pinpoint the location. Revealing the origin of Ceres's water could determine whether there is the potential for life beneath its surface, as is thought to be the case on icy moons around Jupiter and Saturn.
But a model of Ceres presented at the LPSC has added a wrinkle by suggesting comet-like behaviour is only possible at the poles of the dwarf planet, not the lower-latitude areas where the bright spot has been seen.
Comet jets and cryo-volcanoes
Timothy Titus of the US Geological Survey in Flagstaff, Arizona, presented a thermal model that examines where on the surface ice could remain stable over the life time of the solar system, rather than boiling away more quickly. If Ceres is acting like a comet, it must have ice patches that can survive for a long time before being heated by the sun as it moves into a warmer part of its orbit.
Titus found that ice could only be stable in regions above 40 degrees latitude. But the plumes spotted by Herschel seemed to come from nearer the equator, which implies they can't be comet-like. "The water ice is just not stable at the latitudes that the plumes are supposedly coming from," Titus says.
Another explanation is cryovolcanism, in which ice and water are forced out of the surface by processes similar to those that drive magma volcanoes on Earth. But according to a second model presented at the LPSC by David O'Brien of the Planetary Science Institute in Tucson, Arizona, Ceres doesn't have enough muscle to drive these eruptions.
Water down deep
The idea is that Ceres has a subsurface ocean covered by an icy shell. As the bottom of the shell freezes, it expands, putting pressure on the ocean and the shell itself. In order to create a cryovolcano, says O'Brien, the water pressure needs to build up enough to launch up through the shell before the ice cracks and relieves the pressure.
We don't know exactly how deep the ice is on Ceres, so O'Brien tried a range of plausible depths. None produced the conditions for spewing cryovolcanoes – the ice always cracked before enough pressure built up. The best case scenario was water reaching about 90 per cent of the way to the surface.
Intriguingly, that means water could potentially reach the surface from a deep crater, where there was less ice to get through – perhaps even from a crater like the one where Dawn saw the bright spot. That doesn't mean there is a cryovolcano producing a massive plume, but it could be just enough to replenish the ice on the surface, countering the instability that Titus discovered.
So Ceres could be producing comet-like emissions in this region, driven by a weak cryovolcano. "It's sort of a midpoint between comets and cryovolcanic icy worlds," says Titus.
There is more than one way to make a cryovolcano, though. Some models suggest the core of Ceres may be heated by radioactive isotopes left over from the dwarf planet's formation. These could provide enough energy for punchier volcanism, perhaps producing larger plumes – and heat would of course be beneficial for any bacteria that may be lurking below the surface. "Any place you've got the potential for liquid water, you've got the potential for life," says Titus. "Ceres could be an extremely exciting astrobiological target."
DAWN-Sonde in ausgezeichneter Form einen Monat nach Ankunft bei Ceres
Artist's concept of Dawn above Ceres around the time it was captured into orbit by the dwarf planet in early March. Since its arrival, the spacecraft turned around to point the blue glow of its ion engine in the opposite direction.
Image Credit: NASA/JPL-Caltech
Since its capture by the gravity of dwarf planet Ceres on March 6, NASA's Dawn spacecraft has performed flawlessly, continuing to thrust with its ion engine as planned. The thrust, combined with Ceres' gravity, is gradually guiding the spacecraft into a circular orbit around the dwarf planet. All of the spacecraft's systems and instruments are in excellent health.
Dawn has been following its planned trajectory on the dark side of Ceres -- the side facing away from the sun -- since early March. After it entered orbit, the spacecraft's momentum carried it to a higher altitude, reaching a maximum of 46,800 miles (75,400 kilometers) on March 18. Today, Dawn is about 26,000 miles (42,000 kilometers) above Ceres, descending toward the first planned science orbit, which will be 8,400 miles (13,500 kilometers) above the surface.
The next optical navigation images of Ceres will be taken on April 10 and April 14, and are expected to be available online after initial analysis by the science team. In the first of these, the dwarf planet will appear as a thin crescent, much like the images taken on March 1, but with about 1.5 times higher resolution. The April 14 images will reveal a slightly larger crescent in even greater detail. Once Dawn settles into the first science orbit on April 23, the spacecraft will begin the intensive prime science campaign.
By early May, images will improve our view of the entire surface, including the mysterious bright spots that have captured the imaginations of scientists and space enthusiasts alike. What these reflections of sunlight represent is still unknown, but closer views should help determine their nature. The regions containing the bright spots will likely not be in view for the April 10 images; it is not yet certain whether they will be in view for the April 14 set.
On May 9, Dawn will complete its first Ceres science phase and begin to spiral down to a lower orbit to observe Ceres from a closer vantage point.
Dawn previously explored the giant asteroid Vesta for 14 months, from 2011 to 2012, capturing detailed images and data about that body.
Dawn's mission is managed by NASA's Jet Propulsion Laboratory, Pasadena, California, for NASA's Science Mission Directorate in Washington. Dawn is a project of the directorate's Discovery Program, managed by NASA's Marshall Space Flight Center in Huntsville, Alabama. UCLA is responsible for overall Dawn mission science. Orbital ATK Inc., in Dulles, Virginia, designed and built the spacecraft.