Spaceflight Insider

Higher orbit will help Dawn address science questions about Ceres

Dawn orbiting Ceres

This artist concept shows NASA’s Dawn spacecraft above dwarf planet Ceres, as seen in images from the mission. Image Credit: JPL

In order to obtain a different view of Ceres’ surface and address science questions regarding the dwarf planet, NASA’s Dawn spacecraft began raising into a higher orbit starting on Sept. 2, 2016, using the spacecraft’s onboard ion engine.

The spacecraft spent the last eight months orbiting Ceres just 240 miles (385 kilometers) above its surface, closer than the International Space Station orbits above Earth’s surface. This vantage point has yielded numerous close-up images as well as a large amount of data about the small world.

HST/Dawn Ceres

Images of Ceres made using combined Dawn and 2005 HST ultraviolet-visible wavelength images. Credit: NASA / JPL-Caltech / UCLA / MPS / DLR / IDA / Philip Stooke / Ian Regan

Dawn is now in its extended mission phase, having completed its initial science goals first at protoplanet Vesta and then at Ceres.

The extended mission, which will involve new studies of Ceres from new vantage points, is still being outlined by the mission team, which is expected to submit a formal report plan to NASA in October 2016.

Moving to a higher orbit will prolong Dawn’s life because communicating with Earth from this position requires less hydrazine than doing so from the lower orbit, where the spacecraft is subject to more of Ceres’ gravitational pull.

Hydrazine is the fuel Dawn uses both to observe Ceres and communicate its findings back to Earth.

The higher orbit will put the probe 910 miles (1,460 km) from the dwarf planet’s surface. While this position is similar to where Dawn orbited a year ago, the spacecraft will this time be oriented differently in terms of the angle between the Sun and its orbital plane. This will give Dawn a new view of Ceres’ surface.

Mission director and chief engineer Marc Rayman credited the spacecraft’s ion propulsion system for making a smooth change in altitude possible.

“Most spacecraft wouldn’t be able to change their orbital altitude so easily,” Rayman said. “But thanks to Dawn’s uniquely capable ion propulsion system, we can maneuver the ship to get the greatest scientific return from the mission.”

Some science questions about Ceres require study from a higher orbit.

The dwarf planet’s only mountain, Ahuna Mons, is a 13,000-foot (4,000-meter) high ice volcano whose base spans 11 miles (18 kilometers).

“Ahuna Mons is the one true mountain on Ceres,” said Dawn science team member David A. Williams of Arizona State University’s School of Earth and Space Exploration. “After studying it closely, we interpret it as a dome raised by cryovolcanism.”

Ceres Science

Volcanic dome Ahuna Mons rises above a foreground impact crater, as seen by NASA’s Dawn spacecraft with no vertical exaggeration. Eruptions of salty, muddy water built the mountain by repeated eruptions, flows, and freezing. Streaks from falls of rocks and debris run down its flanks, while overhead views show fracturing across its summit. Photo Credit: NASA / JPL-Caltech / GSFC

Cryovolcanism occurs in low-temperature environments when molten ice, often mixed with ammonia salts, erupt, flow, and then freeze on the surface.

Other areas observed by Dawn on Ceres also show evidence of cryovolcanism. Although, in their cases, the volcanic activity is not building tall structures but resurfacing the region.

The existence of cryovolcanism on Ceres means the dwarf planet, whose surface averages close to minus 40 degrees Fahrenheit (minus 40 degrees Celsius), has a warm interior that can sustain liquids for extended amounts of time.

“Ceres appears differentiated internally, with a core and a complex crust made of 30 to 40 percent water ice mixed with silicate rock and salts,” said Williams, who is the lead author of a paper on Ceres’ volcanism published in the journal Science.

In April 2018, Ceres will reach its closest point to the Sun, known as perihelion. As a result, surface temperatures are likely to increase, possibly triggering volcanic activity or otherwise changing the surface.

Separate observations with the Hubble Space Telescope have revealed Ceres’ surface to contain sulfur, sulfur dioxide, and graphitized carbon.

Sulfur and sulfur dioxide are volatile at Ceres’ surface temperatures and were likely deposited there recently, possibly having been triggered by geothermal activity. The discovery is the subject of a paper just published in the journal Geophysical Research Letters.

NASA plans for Dawn to orbit Ceres, observe the dwarf planet, and collect data for about another year.



Laurel Kornfeld is an amateur astronomer and freelance writer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science from Swinburne University’s Astronomy Online program. Her writings have been published online in The Atlantic, Astronomy magazine’s guest blog section, the UK Space Conference, the 2009 IAU General Assembly newspaper, The Space Reporter, and newsletters of various astronomy clubs. She is a member of the Cranford, NJ-based Amateur Astronomers, Inc. Especially interested in the outer solar system, Laurel gave a brief presentation at the 2008 Great Planet Debate held at the Johns Hopkins University Applied Physics Lab in Laurel, MD.

Reader Comments

Ms. Kornfeld,

Nice article, and informative. However, I was a bit confused about the surface temperatures reported. You noted that the surface temperature was -104F, or -40C. I thought the two scales were in alignment at precisely -40F/C. Did I miss something, in my ignorance?

Sept. 3, 2016

Dear Mr. Banks,
No, that is our mistake. Apologies, we’ve since corrected the mistake and appreciate you bringing it to our attention.
Sincerely, Jason Rhian – Editor, SpaceFlight Insider

This was a good article. However, it seemed wanting in one regard. It mentions that Dawn’s orbit is being raised to 910 miles and that “Some science questions about Ceres require study from a higher orbit.” However, we’re given no clue as to what those science questions might be, even in the broadest sense. That would have been a really nice detail to have in the article.

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