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ESA reveals target site for risky comet landing

Posted: September 15, 2014

European scientists have selected an aim point for the landing of Philae, a small probe riding piggyback on the Rosetta spacecraft set for the first-ever descent to a comet's nucleus in November.

Philae's primary landing site will target Site J, the center of which is indicated by the cross in this narrow-angle image from Rosetta's OSIRIS camera. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
The task will not be easy, but officials said Monday they chose the best possible location for the landing on comet 67P/Churyumov-Gerasimenko, an object covered with craggy terrain, rifts and cliffs, and jets of vapor and dust that will become more prominent as the comet dives closer to the sun over the next 11 months.

Scientists say combining data from Rosetta and the Philae lander will provide an unprecedented glimpse of a comet's behavior, yielding insights into the building blocks left over from the formation of the solar system 4.5 billion years ago.

Researchers believe comets may have seeded Earth with water and organic material, depositing the materials necessary for life.

Philae's landing location, known as Site J during lander team's deliberations, is on the small lobe of the comet, which is made up of two distinct segments connected by a narrow neck.

"There are flat areas, but there is also rough terrain," said Stephan Ulamec, head of the Philae lander team at DLR, the German Aerospace Center. "There are some cliffs. There are some boulders, so we have to work a little bit with statistics. It is not a perfectly flat area as we probably would have hoped for a safe landing site."

Landing is set for Nov. 11, following a descent of approximately seven hours once Philae is released from the Rosetta mothership, which arrived at the comet Aug. 6 and has moved within 30 kilometers, or about 18 miles, of 67P/Churyumov-Gerasimenko in the last few weeks.

The lander will be passive and on its own during the free fall to the comet, so engineers on Earth must put the Rosetta spacecraft in the correct position and orientation for separation of Philae.

"We need both Rosetta and Philae in synchronization," said Andrea Accomazzo, flight director for the Rosetta mission at the European Space Operations Center in Darmstadt, Germany. "We need very accurate navigation with Rosetta. This is the most challenging part."

Artist's concept of the Philae lander on the comet. Credit: ESA
The comet's tenuous gravity field makes it difficult to predict Rosetta's orbit around the nucleus, requiring rocket burns to adjust its path.

Thanks to the weak gravity, Ulamec said the 100-kilogram (220-pound) Philae lander will weigh just a gram on the comet. Its mission will last at least two days, but could stretch into early 2015 if the probe's battery can be recharged with solar energy.

Data from the lander will be routed though the Rosetta orbiter and back to Earth, so the mothership must be regularly in view of the landing site, including at the time of touchdown.

After deploying three landing legs fitted with shock absorbers, Philae will hit the comet at about a walking pace, fire a harpoon into the ground to prevent the lander from bouncing off, then screw into rock to firmly attach itself for at least two days of photo-taking, drilling, measurements and other science activities.

Speaking with reporters in a press conference in Paris on Monday, mission officials cautioned the endeavor was fraught with risk, mainly because comet 67P/Churyumov-Gerasimenko is covered in hazards, ranging from unstable, gravely pits to steep slopes and boulders.

"When we designed the lander 15 or 20 years ago, we had no idea of the target body," Ulamec said. "We had no idea of the size, no idea of the gravity, and no idea of the shape or the surface properties. We were even designing it for a different comet. This is completely different than missions that land on the moon or Mars, where you have all this data so you know what you're dealing with."

Armed only with imagery and data collected in the last few months -- once Rosetta was close enough to study the comet in detail -- scientists met last weekend in Toulouse, France, to go over five candidate landing sites.

The decision in favor of Site J as the primary landing site was unanimous, according to a European Space Agency press release. Officials chose a backup site, known as Site C, on the larger lobe of the comet.

Officials brief the media on Philae's landing site. Credit: ESA/S. Corvaja
Fred Jansen, Rosetta's mission manager at ESA, said officials predicted a 70 to 75 percent probability of a successful landing by Philae before the mission's launch in 2004. But that number assumed the comet was a rounded body, not the oddly-shaped world found by Rosetta.

Comet 67P/Churyumov-Gerasimenko's bizarre shape, likened by some to a rubber duck, adds more uncertainty in a smooth landing, officials said. Rosetta returned the first detailed images of the comet in July.

"In the beginning, it was so scary to see this comet as it was," said Jean-Pierre Bibring, a lead scientist on the Philae lander. "It was really unexpected and unpredicted. No one felt we would face such a comet. When we saw it rotating, the first view was we won't be able to reach any place there."

But a closer look revealed more possibilities.

"As soon as we got closer ... suddenly it happened that we might have reachable areas," Bibring said.

There are still many unknowns going into the landing. Ground controllers plan to move Rosetta even closer to the comet through October to get a better look at the landing site.

"For instance, one very important parameter is the surface strength," Ulamec said. "This is still not known. The comet looks more dusty, [and] this is an indication for a rather soft material, but we do not really know. When we designed the lander, our major fear was rebouncing on solid ice. This was one of our major concerns because of our ignorance of what a cometary surface might look like."

It turns out Rosetta has found no evidence of widespread ice on the comet's nucleus.

Even without a successful landing by Philae, Rosetta will continue escorting the comet through the end of 2015, observing its evolution as it passes nearest to the sun next August.

The Rosetta orbiter is the centerpiece of the $1.7 billion mission, supplying most of the data scientists are eager to analyze.

But Philae will provide ground truth for Rosetta's observations.

"We will get to a centimeter resolution, and this will enable us to get closer to the clues of the origin and what were the buidling blocks 4.5 billion years ago," said Holger Sierks, chief scientist for Rosetta's science camera. "It's really a quantum step. It's an historic mission in that sense already now."

Follow Stephen Clark on Twitter: @StephenClark1.