How to Land on a Comet: Go South, Approach With Caution

The Comet-probing Rosetta spacecraft will reach Comet 67P/Churyumov-Gerasimenko in 2014, after a total of three Earth swing-bys and one Mars swing-by. CREDIT: ESA

A spacecraft that's been chasing a comet for six years should drop its lander on the ice ball's southern half when it finally catches up, a new study suggests.

The European Space Agency's Rosetta spacecraft has been zooming toward Comet 67P/Churyumov-Gerasimenko since it launched in 2004. It should reach the comet in 2014 and drop a lander named Philae on the surface to take an up-close look.

The new study should help scientists fine-tune Philae's descent onto the comet's surface. [The Greatest Comet Photos]

"Southern sites appear to be both the safest and the most scientifically interesting," study co-author Jeremie Lasue of Los Alamos National Laboratory said in a statement.

Rosetta and Philae are expected to monitor Churyumov-Gerasimenko's evolution over the course of 13 months as the comet approaches the sun, then careens out into deep space again. The comet is about 2.4 miles (4 kilometers) across and completes one orbit around the sun every 6.6 years.

Safety first

The heart of a comet is a porous mixture of dust, water ice and other frozen substances like carbon dioxide and carbon monoxide. As a comet approaches the sun and its nucleus starts to heat up, many of these materials vaporize, forming the comet's coma — a diffuse cloud of dust and gas around the nucleus — and tail.

Lasue and his colleagues developed 3-D computer models that predict the activity of Churyumov-Gerasimenko's nucleus — how heat transfers through it, for example, and how its various ices vaporize.

The study covered the period of Rosetta's initial encounter with the comet in 2014, which should take place just inside the orbit of Jupiter, about 373 million miles (600 million km) from the sun. It ran through the comet's closest approach to the sun in August 2015.

The models show that the comet's southern hemisphere should be the most stable and safest place for Philae to land, researchers said.

During Churyumov-Gerasimenko's approach to the sun, solar heat will boil off huge amounts of the comet's dust, water ice, frozen carbon dioxide and other core materials.

The bulk of this dramatic venting should occur in the comet's northern half when Philae touches down in November 2014, the researchers said.

The models predict that Churyumov-Gerasimenko's northern face will be emitting up to 66 pounds (30 kilograms) of gas and 110 pounds (50 kg) of dust every second — conditions that don't make for a calm, easy landing.

So the comet's southern hemisphere is a safer landing bet, researchers said. And, there's a bonus.

"In addition, due to the orientation of the comet, the southern hemisphere will be protected from extreme temperature variations at the time of delivery," Lasue said.

Comet's easy access

The southern hemisphere of comet Churyumov-Gerasimenko is also a better target for scientific reasons, the researchers said.

The team's computer simulations suggest that the comet's southern region is significantly more eroded than the north.

That's because the comet's south pole has been cooked more by the sun over the years. Churyumov-Gerasimenko's south pole faces the sun pretty much head-on when it makes its closest approaches, researchers said.

The southern face of Churyumov-Gerasimenko should therefore give Philae easier access to material just below the comet's surface, researchers said. The lander will be able to drill down up to 12 inches (30 centimeters) to collect samples of cometary dirt for on-board analysis.

"Philae will not have to drill down far to find those pristine samples," Lasue said.

Philae will be powered by solar cells, so the more intense solar energy on the south side during the comet's approach should also help keep the lander charged up and running, the scientists said.

"From our present results, we've concluded that the southern hemisphere promises the best landing sites," said study co-researcher Maria Cristina De Sanctis of the Istituto Nazionale di Astrofisica's Istituto di Astrofisica Spaziale e Fisica Cosmica in Rome, Italy. "As more data on Churyumov-Gerasimenko becomes available to better quantify our results, we will be able to add to the picture and help prepare for a safe landing for Philae."

The team presented its findings at the European Planetary Science Congress in Rome on Sept. 23.

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