When you have a $446 million spacecraft funded by public money, there is a considerable amount of pressure to squeeze it for every last drop of science. The Dawn mission has already made spectacularly successful visits to two asteroids, Vesta and Ceres. It is now orbiting around Ceres, having completed a “final” Low-Altitude Mapping Orbit. There’s not more to see if we can’t get any closer, right? That’s what Dawn’s engineering team thought, and so they came up with a plan that carefully used the remaining fuel to get to a third target, asteroid 145 Adeona. They also thought the plan was official enough that it was announced in the Dawn Journal on June 30 only to be quickly removed, followed by an announcement the next day from NASA’s Director of Planetary Science Jim Green that Dawn was to remain at Ceres because “[t]he long-term monitoring of Ceres, particularly as it gets closer to perihelion – the part of its orbit with the shortest distance to the sun — has the potential to provide more significant science discoveries than a flyby of Adeona.”
So what exactly is Dawn waiting for? A big clue comes from one of the very first things noticed as Dawn approached Ceres, a cluster of spots in Occator crater collectively known as Spot 5. The unusual and variable appearance of Spot 5 generated serious scientific contemplation and allegations of alien mining activity. Currently it is believed that Spot 5 consists of sodium bicarbonate and ammonium either as a chloride or a bicarbonate brought forth from within by water which then evaporated or boiled away at some point in the 80 million year history of the host crater. That’s not all though. The variable appearance of Spot 5 has been ascribed to a fog or haze that builds up in the crater and is allegedly pictured in this low-angle view:
NASA’s explanation of this image can be found here: http://photojournal.jpl.nasa.gov/catalog/PIA20181
Water may still be evaporating or subliming from the floor of this crater, and the big idea at NASA appears to be to wait until Ceres gets to the point in its orbit where it is closest to the Sun. Will the extra warmth cause a thicker fog, or something more exciting – a geyser or comet-like jet? I don’t think anyone knows, and it could fizzle, in which case NASA will have blown a chance to visit a third asteroid. The decision to wait and watch is in line with the big hoopla preceding the recent announcement that water plumes may have been spotted at Europa. Where there’s water, there may be life, or at at least some interesting organic molecules, and it’s possible that NASA is leveraging interest in this to justify some of its funding into the future.