Earlier this year, the Dawn mission made history by arriving at Ceres, the largest known body in the asteroid belt. The probe had previously visited Vesta, another large proto-planet, and was therefore the first manmade object to orbit two extraterrestrial bodies within its lifetime.
Touted as the next generation of NASA exploratory missions (hence the name), the probe used an electric propulsion system involving the expulsion of raw ions rather than traditional propellants to get it to the protoplanets. The ions, positively-charged xenon in this case, were passed through a grid raised to a high voltage, where they were accelerated to incredible velocities. The force produced by the combined motion of all these ions however, is a miniscule 91 millinewtons (0.020 lbf). Dawn is no Tesla; it would take about 4 days to accelerate from 0 to 60mph! To accelerate at dizzying speeds through the cosmos, Dawn has three such engines, though it only uses one at a time. The engines are mounted such that the spacecraft can perform axial adjustments as well as accelerate forwards. Since two of Dawn’s four reaction wheels are nonfunctional, the spacecraft is compensating using an onboard hydrazine engine to control attitude. So in a way, Dawn gets the best of both worlds with propulsion technology.
Dawn arrived at Ceres in March of this year, and both NASA and the general public alike were almost immediately preoccupied with Ceres’ so-called “bright spots.” Could they be the lights of some alien city? A previously unknown civilization carving out an existence in the airless, blank expanse of Ceres? Based on the measurements and photos taken up to this point, scientists say, “No, even better! It’s probably salt!” to the disappointment of Star Wars fans everywhere. This is a conclusion according to Tom McCord, a co-investigator on Dawn. He hypothesizes that volcanic or geologic activity on Ceres can dredge up plumes of water vapor that shoot up and away from the surface. He continues, “What that would do is leave a residual salt deposit, so these bright spots could be salt deposits that accumulated around vents—volcanoes—where the water is coming through.”
The instrumentation on Dawn is the core of the spacecraft: with it, JPL scientists can collect data that allows them to truly characterize Ceres and Vesta for the first time since they were discovered in the 19th century. Instruments include a camera to photograph Ceres and Vesta in three colors and in black and white. There’s also a gamma ray and neutrino detector (GRaND) that allows for the mapping of elemental abundances on the surface of the proto-planets – Oxygen, Magnesium, Aluminum, oh my! The probe collects data to map the inertial and gravitational properties of Ceres and Vesta – things like mass, gravitational field, rotational axis, and so on. Finally, there’s a VIR spectrometer, that measures reflected light off of Ceres and Vesta in certain parts of the electromagnetic spectrum, primarily in the Visible and InfraRed parts. It uses this data to compute information about the composition and chemical properties of whatever it sees.
Along with New Horizon’s arrival at Pluto this year, probes like Dawn represents the progress NASA has made when it comes to exploring new worlds. While private companies like SpaceX or Orbital ATK challenge the notion of the mission to Mars being merely a governmental endeavor, NASA very well may have shown that when it comes to the bleeding edge of exploration, no one has them beat.
-By Arun Nagpal, Publications Co-Chair, SEDS@UM