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Probe Enters Orbit Around Dwarf Planet Ceres

NASA's Dawn probe arrived at Ceres, becoming the first spacecraft ever to orbit a dwarf planet.

The year of the dwarf planet has begun.

NASA's Dawn probe arrived at Ceres today (March 6) at about 7:39 a.m. EST (1239 GMT), becoming the first spacecraft ever to orbit a dwarf planet. Dawn's observations over the next 16 months should lift the veil on Ceres, which has remained largely mysterious since its 1801 discovery, mission team members say.

"Since its discovery in 1801, Ceres was known as a planet, then an asteroid and later a dwarf planet," Dawn mission director and chief engineer Marc Rayman, who's based at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California, said in a statement. "Now, after a journey of 3.1 billion miles (4.9 billion kilometers) and 7.5 years, Dawn calls Ceres, home." [Read more coverage of Dawn's Ceres arrival]

Photos: Our Dwarf Planet Dreams are Coming into Focus

NASA officials got a signal from Dawn confirming that it's healthy and in orbit at about 8:36 a.m. EST (1336 GMT) today.

The milestone comes just four months ahead of another highly anticipated dwarf-planet encounter: On July 14, NASA's New Horizons probe will zoom through the Pluto system, giving scientists their first good looks at that faraway dwarf planet and its five known moons.

Dawn of the solar system The $473 million Dawn mission launched in September 2007 to study Vesta and Ceres, the two largest objects in the main asteroid belt between Mars and Jupiter. Vesta's diameter is 330 miles (530 kilometers), while Ceres is about 590 miles (950 km) wide.

Both Vesta and Ceres are leftovers from the solar system's early days, planetary building blocks that would likely have kept growing if not for the interfering influence of Jupiter's immense gravitational tug.

The two bodies are "intact protoplanets from the very dawn of the solar system," Dawn Deputy Principal Investigator Carol Raymond, also of JPL, said during a news conference Monday (March 2)." So they're literally fossils that we can investigate to really understand the processes that were going on at that time."

Mysterious Dwarf Planet Ceres May Be Ripe for Life

Dawn orbited Vesta from July 2011 through September 2012, when the probe departed for Ceres. So today's arrival made history in another way as well: Dawn became the first spacecraft ever to orbit two objects beyond the Earth-moon system. [Photos: Asteroid Vesta and NASA's Dawn Spacecraft]

The mission's spaceflight feats are made possible by Dawn's innovative propulsion system, which accelerates xenon ions out the back of the spacecraft. This process generates tiny amounts of thrust; it would take Dawn four days to go from 0 to 60 mph (97 km/h), team members have said.

But Dawn's ion drive is about 10 times more efficient than traditional chemical systems. So the engines can keep firing for weeks, months and years, accelerating Dawn to tremendous speeds.

"With the 1,000 lbs. of xenon propellant that was loaded on board, Dawn has already accomplished more than 24,000 mph of velocity change," Dawn project manager Robert Mase of JPL said during Monday's news conference. "To put that in context: That's more than it takes to get a vehicle from the surface of the Earth up to the International Space Station."

Thanks to ion propulsion, Dawn crept up on Ceres slowly and gradually. The probe eased into orbit today without the need for any harrowing make-or-break maneuvers.

The mysteries of Ceres Ceres is an intriguing world that in many ways looks more like the icy moons of the outer solar system, such as Jupiter's satellite Europa and the Saturn moon Enceladus, than its rocky neighbors in the asteroid belt.

For example, the dwarf planet is thought to consist of 25 to 30 percent water by mass, mostly in the form of ice. Ceres may also once have had (and might even still possess) an ocean of liquid water beneath its surface, as Europa and Enceladus do. Indeed, some researchers think Ceres may be capable of supporting microbial life.

Hunt for Dwarf Planet Ceres' Mysterious Water Begins

"It's really going to be exciting to see what this exotic, alien world looks like," Rayman told Space.com in late January. "We're finally going to learn about this place."

Dawn is not equipped to search for signs of life. But the probe might be able to spot evidence of an underground ocean (if it exists), if it burbles up in places to interact with surface rocks, Rayman said. Measurements of Ceres' surface temperatures, when coupled with models of heat transportation through Ceres, could also shed light on the question of underground liquid water, said Dawn principal investigator Chris Russell of UCLA.

Dawn will also investigate two Ceres mysteries that have cropped up in the past year or so. Mission scientists will try to figure out just what is producing Ceres' mysterious bright spots, and they'll attempt to confirm and characterize a tenuous water-vapor plume spotted recently by researchers using Europe's Herschel Space Observatory.

Overall, Dawn will characterize the dwarf planet in detail, mapping out its surface and determining what Ceres is made of.

"We'll do typical planetary geology, more similar to what we do on Mars than what we did with Vesta," Russell told Space.com.

This work will not start immediately; Dawn will spend the next six weeks spiraling down to its initial science orbit, getting there on April 23. The probe will then begin taking Ceres' measure from an altitude of 8,400 miles (13,500 km). Dawn will study the dwarf planet from a series of increasingly closer-in orbits until the mission ends in June 2016.

Sizing up dwarf planets While Ceres and Pluto are both dwarf planets - a category created by the International Astronomical Union (IAU) in 2006, when it demoted Pluto from a full-fledged planet in a decision that remains controversial today - they're quite different from each other, Russell said.

"Pluto formed differently, formed at a different time and formed out of different materials" than Ceres, he said.

Pluto is also more than twice as large as Ceres and lies more than 14 times farther from the sun than the queen of the asteroid belt does. So the data returned by Dawn and New Horizons will likely not paint a unifying picture of just what it means to be a dwarf planet, Russell said.

"The legacy [of the two missions] is freeing these bodies from arbitrary labels based on their size or their ability to scatter other objects, or whatever the IAU had going through its head," he said. "These bodies are being liberated from classification, and we now can understand them in their own right."

Originally published on Space.com.

Dawn Over Ceres - NASA Probe To Arrive At Dwarf Planet | Images and Visualization Dwarf Planet Ceres Coming Into NASA Probe's View | Video Photos of Pluto and Its Moons Copyright 2015 SPACE.com, a Purch company. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

NASA's Dawn spacecraft took this image of Ceres on March 1, 2015 when it was about 30,000 miles (about 48,000 kilometers) from the dwarf planet. The spacecraft arrived at Ceres on March 6, 2015.

The term "dwarf planet" wasn't defined until the infamous International Astronomical Union (IAU) vote in 2006, but this year, 9 years later, we are beginning to get our first ever close-up views of two of our solar system's most famous dwarf planets: Pluto and Ceres.

VIDEO: Pluto Flyby and Black Holes: Top Space Events for 2015

Currently spiraling in on Ceres, the innermost dwarf planet inhabiting the asteroid belt between the orbits of Mars and Jupiter, NASA's Dawn spacecraft is slowly revealing a cratered and complex world, details of which that have so far eluded even Hubble's powerful vision. Dawn is scheduled to make final orbital insertion around Ceres in March 2015 where it is destined to remain after its fuel runs out as a permanent human-made satellite of Ceres. A comparison image of the Hubble and Dawn views of Ceres is shown above.

ANALYSIS: NASA Spacecraft Ready to Unlock Ceres' Mysteries

But Dawn is just the first dwarf planet encounter of 2015. In July, NASA's New Horizons mission will flyby Pluto and its system of moons, exploring the mysterious Kuiper Belt. Between Hubble's blurry observations of Ceres and Pluto and this year's NASA encounters, many artists' impressions of these enigmatic worlds have guessed at what lies in store for our robotic explorers. But how do they measure up now we are beginning to see Ceres' and Pluto's surfaces?

This artist's impression of Ceres shows NASA's Dawn spacecraft in orbit around the dwarf planet. As opposed to an ice encrusted world, this visualization shows a cratered, moon-like surface.

NEWS: Tantalizing Detail Seen on Mysterious Dwarf Planet

Again with Dawn in view, this artist's impression shows an active Ceres complete with water vapor escaping from a possible sub-surface ocean. Water vapor was detected in the vicinity of Ceres by Hubble, so Dawn will be on the look-out for any trace of geysers venting water.

ANALYSIS: Water Plume 'Unequivocally' Detected at Dwarf Planet Ceres

As seen by Hubble from afar, curious white patches and possible variations in Ceres' surface composition can be seen. However, any detail in these images have so far prevented planetary scientists from fully understanding the dwarf planet's true nature.

NEWS: What is That Mysterious White Blob on Ceres?

But now, as Dawn fast approaches orbital insertion, we're being treated to a bounty of data that shows a possibly ancient, rocky surface. Those curious white patches originally spied by Hubble are also snapping into view -- but what are they? Theories abound, but they may be tentative signs of subsurface water escaping to space and freezing on the surface. These are all signs of cryovolcanism, a dynamic that may dominate dwarf planet surface morphology.

NEWS: Dwarf Planet's Puzzling Landscape Snaps into View

From afar, NASA's Dawn mission is able to watch Ceres rotate, as this series of observations on Feb. 4 shows.

ANALYSIS: Craters Pop as NASA’s Dawn Probe Approaches Ceres

As Dawn gets up-close and personal with Ceres, the drama in the outer solar system is only just beginning to unfold. After 9 years of flying toward Pluto, NASA's New Horizons mission has begun approach preparations for its flyby in July.

VIDEO: Pluto Getting Bigger in New Horizons Probe's Window

From ground-based and Hubble observations, there at tantalizing clues that this frozen world has a surprisingly dynamic surface with a thin atmosphere that changes during Pluto's 248 year orbit around the sun. In this artist's impression of New Horizons flying over Pluto, an atmosphere has been included with cryovolcanos -- the latter of which planetary scientists hope to confirm in July.

Pluto has a system of known moons, the largest of which, Charon, may be considered to be Pluto's binary partner. As Charon orbits Pluto, its powerful gravitational field tugs the dwarf planet off center, a dynamic that New Horizons has observed as it approaches.

ANALYSIS: NASA Probe Captures First Pluto Approach Photos

This artist's conception shows Pluto's moon Charon eclipsing the dwarf planet. Twice every orbit around the sun, each world eclipses the other.

NEWS: Tally-Ho on Targets for New Horizons After Pluto

When Hubble spies on Pluto, it can see the different shades of the dwarf planet's surface rotate. As shown here in these blotchy images, little detail is obvious, but large regions with differing albedo (reflectiveness) may reveal huge craters, vast plains or mountains. But until New Horizons gets close, these regions will remain a mystery.

In this digital illustration rendered from 3-D NASA data of Pluto, an attempt has been made at matching observations with possible surface features.

In July 2014, NASA's New Horizons looked ahead and spied its ultimate goal: Pluto and Charon. Although tiny pinpricks of light, the pair can be seen orbiting one another in a binary dance that shifts Pluto off center. Both masses actually orbit an invisible point in space, above Pluto's surface, known as the Pluto-Charon barycenter. These observations have increased calls for Pluto to be redefined (yet again) as a 'binary planet.'

MORE: Can We Call Pluto and Charon a 'Binary Planet' Yet?

Having spotted Charon months ago, New Horizons is now beginning to see Pluto's wider family of moons pop into view. Shown here are moons Nix (yellow diamond) and Hydra (orange diamond).

MORE: Pluto's Tiny Moons Spied by Incoming NASA Probe

Once NASA's New Horizons mission careens through the Pluto-Charon system, assuming it doesn't hit any debris on its way through, its mission in the Kuiper Belt has only just begun. Hubble is currently being used to identify possible icy targets

after

the spacecraft's Pluto encounter. Shown here is an artist's impression of another dwarf planet, Eris, that was discovered in 2005. Originally thought to be the

tenth

planet of the solar system, its discovery led to the IAU's decision to reclassify these small worlds as dwarf planets, demoting Pluto in the process, leaving us with 8 planets. But as we approach Pluto and begin to understand Ceres, just because they are dwarf planets doesn't mean they're not rich and dynamic places to explore. Our voyage of dwarf planet discovery has only just begun and regardless of our need to classify celestial objects, Pluto and Ceres hold some fascinating clues to planetary formation and solar system evolution.

For more, regularly check on the

NASA Dawn

and

NASA New Horizons

mission web sites.