Dawn: Exploring Vesta and Ceres

When I was young, Ceres and Pluto were the biggest blank spots on the map of the Solar System. Most of the other interesting places had been long since explored, from Mercury all the way out to the moons of Neptune, but when I opened my astronomy books to the two minor planets, I saw only grainy, pixelated images with no discernible features—the very best that Earth-based telescopes could manage. I would look at those blurry discs, and wonder at the secrets they kept…

It only took a decade to find out. There was, of course, Pluto’s grand unveiling in the summer of 2015, but today we will focus on the humbler, lower-profile expedition to Ceres, just four months earlier. It was the culmination of a long voyage out past Mars and through the Asteroid Belt, bringing whole new worlds into the sphere of human knowledge. Undertaking this adventure was Dawn, an intrepid one-ton probe, propelled not by chemical reactions but by the steady glow of solar-electric ion engine.

In 2001 NASA selected Dawn for a Discovery Program slot, alongside the Kepler Space Telescope. It was to launch in 2006, though a difficult development cycle—it was canceled three times—pushed that back to September 2007. Finally it took off aboard a Delta II, with chemical rocket stages propelling it all the way out of Earth’s orbit. Dawn then engaged its main, far gentler propulsion, the ion drive, accelerating for about 270 days to put it on a Mars intercept trajectory.

I won’t dive too much into the workings of ion drives here, but essentially they operate by ionizing a neutral gas (usually xenon) and expelling it at high velocity using electric fields. Per the Tsiolkovsky rocket equation¹, the high exhaust velocity means a higher delta-v—11.5 kilometers per second for Dawn—far exceeding anything chemical rockets can accomplish. The downside is that these engines have very little thrust, so maneuvers are painfully slow. It took Dawn four days to accelerate to a highway cruising speed of sixty miles per hour.

A thruster from the testbed mission *Deep Space 1*. This design would later see use on *Dawn*.

The probe made its scheduled flyby of Mars on February 17, 2009. More than two years later, at about 2.36 times Earth’s distance from the Sun, it began its real work, encountering the minor planet Vesta and entering into a survey orbit in August 2011. This was the very first time any spacecraft had orbited an Asteroid Belt object. Dawn stayed there for about a year, imaging every square kilometer of the surface as scientists back home gleaned what they could from the data.

There were no profound discoveries—no subsurface oceans or alien monoliths to be found—but Vesta nevertheless proved quite informative from a scientific standpoint. It is the second largest asteroid, after Ceres; it has a differentiated interior with an olivine mantle and an iron core. Liquid-cut gullies and pockmarks in the surface suggest the ancient presence of volatiles (possibly water!)². Most importantly, it is a geological window into the early Solar System, a relic of the generation of protoplanets that smashed together to form Earth and Mars.

Vesta! It’s about 570 kilometers from end to end, so nothing to sneeze at.

On September 5, 2012, Dawn broke orbit and began the journey towards its next destination, Ceres. This was to be the grand event. While Vesta is a space potato, albeit a very large one, Ceres is quite spherical in shape and resembles a small planet—it is larger, in fact, than the famous moons Enceladus, Mimas, and Miranda. As Dawn inched closer in the first months of 2015, its cameras snapped successively sharper pictures of a hitherto unseen worldlet. The full spectacle came upon orbital insertion on March 6, 2015. A whole new planetary surface became known to humanity, and there was a corresponding rush to name all the new craters, mountains, and plains that were so suddenly revealed.

This image, taken on January 25, 2015, was the first to exceed the resolution offered by the Hubble Space Telescope.

Like most other Solar System objects, Ceres is a rock, not quite as endlessly fascinating as Mars or Titan. Yet there were certain curiosities that emerged upon exploration. For one, the surface turned out to be comparatively rich in carbon, about twenty percent by mass. Dawn‘s sensors detected tholins in several spots, showing that solar radiation had turned carbon and other elements into more complex organic compounds.

There were also the mysterious bright spots, concentrated primarily at the bottom of Occator Crater. They were probably the most widely reported find of the mission, with media theories ranging from surface ice to alien outposts, and by the end of the year it became clear that the bright areas were salt deposits of some sort. Researchers announced in August 2020 that the phenomenon was caused by brine seeping up from a subsurface reservoir—perhaps indicating a faint possibility of life! The odds are low, given Ceres’ small size, but life does have a way of popping up in the strangest places…

Occator Crater, showing the bright spots. They are about four times brighter than the surrounding regolith.

Dawn remains around Ceres to this day, though major operations ended in June 2017. Despite proposals to fly by a third asteroid, it now enjoys a quiet retirement, and will circle that planetoid for a long time to come. This is a fitting conclusion for a stunningly successful mission. There were some fairly minor technical problems—some of the reaction reaction wheels failed in orbit of Vesta, making it harder to keep the craft oriented—but it was nevertheless a technical triumph, demonstrating that ion drives are superbly effective over long distances and durations. Thanks to this plucky little space probe, propelled by an eerie electric thruster, two more worlds were opened to us; two more of the blank spots on the map were filled in.