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How to land on a comet as it soars through space

BY Jenny Marder  August 27, 2014 at 4:53 PM EST
Five candidate sites were identified on Comet 67P/Churyumov-Gerasimenko during the Landing Site Selection Group meeting held 23–24 August 2014. Image by ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Five candidate sites were identified on Comet 67P/Churyumov-Gerasimenko during the Landing Site Selection Group meeting last week. Image by ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Earlier this month, after a 10-year, 4-billion-mile journey, the Rosetta spacecraft entered orbit around the rubber-duck-shaped Comet 67P/Churyumov-Gerasimenko. Now it must land.

This is no helicopter landing. Putting the lander down onto the comet’s surface will require fantastically precise calculations, maneuvering and navigational skills and, once it’s released, six hours of what U.S. Rosetta project manager Art Chmielewski calls “patient stress.”

“The landing is so difficult,” he said. “So, so difficult. It’s definitely one of the hardest things humankind has ever done.” Imagine, he said, grabbing a mosquito by the wings. Except the mosquito is in New York, and you’re working the controls from Los Angeles.

It’s difficult because the comet is soaring through space at 36,000 mph. The spacecraft has to catch up with the comet, fly alongside it at exactly the same speed and then drop a lander the size of a washing machine onto an area just over half a square mile. (For perspective, the width of Central Park between Central Park West and Fifth Avenue is half a mile.) Unlike Earth or Mars, there’s no substantial atmosphere, just a thin layer of gas particles surrounding the comet’s nucleus called its “coma.” And whereas the Mars Curiosity lander plummeted at about 13,000 miles per hour, the Philae lander will float down at a speed closer to 20 centimeters per second, like a piece of paper floating to the ground.

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“It’s all about this moment of release and the precise calculation of where it’s going to drop,” Chmielewski said. “Once you release it, you have no control.”

Rosetta is now cruising at an altitude of roughly 60 miles — that’s the distance from the spacecraft to the surface of the comet. It is close enough that a quarter of the comet fills the full frame of its camera lens. Earlier photos showed the full comet from different angles. Like this:

Rosetta snapped this shot of Comet 67P/Churyumov-Gerasimenko from a distance of 177 miles on Aug. 3. Photo courtesy of the European Space Agency

Rosetta snapped this shot of Comet 67P/Churyumov-Gerasimenko from a distance of 177 miles on Aug. 3. Photo courtesy of the European Space Agency

And this:

Comet 67P/Churyumov-Gerasimenko on Aug. 2. The image was taken by Rosetta’s OSIRIS wide-angle camera from a distance of about 342 miles. Photo courtesy of the European Space Agency

Comet 67P/Churyumov-Gerasimenko on Aug. 2. The image was taken by Rosetta’s OSIRIS wide-angle camera from a distance of about 342 miles. Photo courtesy of the European Space Agency

This week, a team of 50 scientists, representing a range of countries, narrowed the landing site down to five possibilities. Choosing a landing site for a comet isn’t easy either. The site requires a flat terrain and the right amount of daylight for the landing. It must have enough sun to power the equipment’s solar panels. And then there are the conflicting needs of the mission’s team members.

Engineers want a spot that lacks any obstacles — boulders, for example — that might thwart the landing. Scientists, on the other hand, say that’s geologically boring, Chmielewski said. “They say, ‘We want crevasses, we want boulders, we want varied terrain.’ If you try to find a landing site that meets all of them, you get a headache.”

From NASA’s Jet Propulsion Laboratory on the requirements for a landing site:

“For each possible zone, important questions must be asked: Will the lander be able to maintain regular communications with Rosetta? How common are surface hazards such as large boulders, deep crevasses or steep slopes? Is there sufficient illumination for scientific operations and enough sunlight to recharge the lander’s batteries beyond its initial 64-hour lifetime without causing overheating?”

And from the European Space Agency’s Rosetta Mission Twitter feed:

 

The team plans to have each site “assessed and ranked” by Sept. 14. Rosetta’s lander Philae is slated to land in mid-November. Once there, it will dig up dirt, sample the soil, test its constituents and study the depth of dust, along with ice and water. Meanwhile, the orbiter will continue to chase the comet as its orbit nears the sun.

Earlier this month, Hari Sreenivasan talked to Mark McCaughrean, senior scientific advisor of the European Space Agency.

Comets, McCaughrean said, are “treasure chests of material left over from the birth of the solar system.” They contain dust, water and organic materials — “stuff that could be the origin of life.”

The lander’s research will teach us about the ingredients of the comet itself, but may also give clues to the formation of the solar system and to the “initial ingredients that became the sun and the planet and you and me,” Chmielewski said.

“This time,” McCaughrean said, “we’re going to watch this comet as it comes into the inner solar system, heats up, evolves, changes and gets dynamic — there’s going to be so many unexpected surprises.”