Dawn marks anniversary of entering Ceres’ orbit
NASA’s Dawn mission has marked the first anniversary of the spacecraft’s entrance into orbit around the dwarf planet Ceres on March 6, 2015. The probe, which used ion propulsion to glide into orbit around Ceres after having circled protoplanet Vesta for a year in 2011–2012, was the first spacecraft to explore a dwarf planet and the first to have orbited two separate celestial objects.
Currently at more than 3.95 AU (367 million miles or 591 million km) from Earth, Dawn is flying at its lowest altitude above Ceres’ surface as part of an effort to map that surface from close range.
Launched in September 2007, the probe visited Vesta and Ceres, both of which have remained largely unchanged since the beginning of the Solar System, with the goal of learning more about the conditions and processes characteristic of the Solar System’s earliest days.
Vesta and Ceres are the two largest objects in the asteroid belt between Mars and Jupiter, but the two worlds evolved differently. Vesta is a dry world, nearly spherical world that appears to have had one side lobbed off in an early impact, whereas Ceres is a wet, round world in hydrostatic equilibrium (squeezed into a round or nearly round shape by its own gravity).
In its first year of orbiting Ceres, Dawn has returned numerous images and data revealing the secrets of the dwarf planet, first discovered in 1801, for the first time.
The images and data, especially a tall mountain named Ahura Mons and mysterious bright spots in Occator Crater, have both thrilled and puzzled scientists.
“Ceres has defied our expectations and surprised us in many ways, thanks to a year’s worth of data from Dawn,” said deputy principal investigator Carol Raymond of NASA’s Jet Propulsion Laboratory in California.
“We are hard at work on the mysteries the spacecraft has presented to us,” she stated.
With an overall average height of 2.5 miles (4 km), Ahura Mons is taller than both Mount Whitney in California and Mount Rainier in Washington.
It was first seen by Dawn as a small, bright surface bump in February 2015 from a distance of 29,000 miles (46,000 km). While initially seeming to be shaped like a pyramid, closer observation in lower orbits revealed it as dome-shaped with walls that are steep but smooth.
Dawn’s most recent images of the mountain, taken from low-altitude orbit, 120 times closer than those captured in February 2015, reveal Ahura Mons to have bright material scattered unevenly across its surface.
Mission principal investigator Chris Russell of the University of California acknowledged none of the scientists expected to find mountains on Ceres, and Ahura Mons’ formation process remains a mystery. Similar smaller but less-defined mountains are becoming visible to Dawn from its low-altitude orbit.
Ceres’ greatest puzzle has been the bright spots on Occator Crater, initially seen by the Hubble Space Telescope as bright surface patches.
Occator Crater, about 50 miles (80 km) wide, is located approximately 420 miles (670 km) northwest of Ahura Mons.
Upon approach to the dwarf planet, Dawn picked up what appeared to be two bright, very reflective spots in the crater. A closer look, which produced higher resolution images, revealed what were at minimum 10 bright spots, with the brightest at the crater’s center.
Speculation as to the composition of the bright material ranged from water ice to iron-depleted clay minerals to salt deposits.
In December 2015, scientists identified the spots as a type of salt or brine containing magnesium sulfate (hexahydrite) possibly being emitted by a liquid interior or subsurface ocean.
Another possibility is that a meteor impacted the site, either removing a layer of material covering the salty ice or heating the site enough to make subsurface salty water rise as a geyser, then evaporate, leaving only the salt.
Mission scientists do not know if the bright spots at Ahura Mons are composed of the same material as those at Occator Crater.
Marc Rayman, Dawn chief engineer and mission director, emphasized the spacecraft will have to complete many low-altitude orbits before Dawn’s camera and other instruments can view the entirety of Ceres’ surface.
The Dawn team will present new information and images from the mission at a March 22 press briefing held during the 47th Lunar and Planetary Science Conference in The Woodlands, Texas.
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.