Five potential sites have been selected for what will be one of humanity’s most audacious undertakings – landing a spacecraft on a comet. Scientists will command the Rosetta spacecraft to ‘drop’ the washing-machine-sized lander called Philae onto comet 67P/Churyumov-Gerasimenko’s dusty surface on or about November 14.
If mission controllers are feeling anxious, it’s no surprise. Philae must land on the 2.5-mile-wide dirty iceball before heat from the sun makes it hazardous. Compared to a rocky asteroid, a comet’s practically alive with activity. The closer it gets to the sun, the faster its ices vaporize and the more dust the comet releases.
The last thing scientists want is to send the lander into a blizzard of dust and water ice crystals which could pose clear hazards to Philae and its suite of 10 instruments.
During the maneuver, Rosetta and 67P/C-G will be 280 million miles (450 million km) from the sun, far enough (and cold enough) that the rate of vaporization and ‘geysering’ of water vapor from cracks in the comet’s surface will be low.
An early landing also means scientists get a first-hand look at the surface ices and chemistry before solar heating changes the landscape, converting dirty ice into the vapor and dust that will expand the comet’s coma and flow into a tail. Scientists would like to sample and observe these goodies in as pristine a state as possible.
Comets, which have remained frozen in the far corners of the solar system since shortly after their formation, bear news from a distant and ancient era. Studying one in situ is like following a proverbial trail of breadcrumbs back to its beginning.
“The process of selecting a landing site is extremely complex and dynamic; as we get closer to the comet, we will see more and more details, which will influence the final decision on where and when we can land,” said Fred Jansen, Rosetta’s mission manager from the European Space Agency’s Science and Technology Center.
Rosetta’s lander will obtain the first images taken from a comet’s surface and will provide comprehensive analysis of the comet’s possible primordial composition by drilling into the surface. The tool can penetrate up to 10 inches deep.
From an original 10 sites, lettered ‘A’ through ‘J’, the search has been narrowed to five. Three sites (B, I and J) are located on the smaller of the two lobes of the comet and two sites (A and C) are located on the larger lobe. Choosing the right one is a complex process based on several considerations:
* We need a location with at least six hours of daylight during the comet’s 12.4 hour rotation, both for good illumination of the surface for detailed photography and to provide power to Philae’s batteries via solar cells. But not too much sunlight, otherwise the probe could overheat.
* Flat terrain with as few boulders, cliffs and crevasses as possible for safety’s sake. While the boulders in the photos cry out for exploration, they’re too hazardous to approach. Mission controllers prefer safe and (somewhat) boring. Better than than losing the craft.
* An area where the lander can maintain regular communications with the Rosetta mother ship during its descent to the surface and after landing.
“The five chosen sites offer us the best chance to land and study the composition, internal structure and activity of the comet with the ten lander experiments,” said Jean-Pierre Bibring, one of the lead lander scientists.
Uncertainties in navigating the orbiter close to the comet mean that it’s only possible to specify any given landing zone in terms of an ellipse about four-tenths of a square mile (one square km). By September 14, the sites will be accessed and ranked and the best will be selected along with a backup.
For more on site selection, including profiles of each of the five, click HERE.