October 23, 2019

For comet scientists, elation and redemption at Philae’s wakeup


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Philae_on_the_comet_Front_view
Artist’s concept of Philae. Credit: ESA

Research teams across Europe spent the last half-year meticulously going through a wish list of experiments for the Philae comet lander without knowing whether they would ever get a chance to execute the tasks.

With Philae now awake after a seven-month slumber, scientists are eager to turn on the lander’s instruments, take fresh images and eventually attempt to sample a block of organic dust and ice perched just out of the probe’s grasp.

Jean-Pierre Bibring, Philae’s chief scientist, said the dishwasher-sized lander can rotate to put the cometary material within reach of its drill. But such movement is risky, and mission planners only want to command the rotation when they are sure Philae is able to charge its battery.

“In the last seven months, we were not just praying for the lander being back,” said Barbara Cozzoni, an engineer at Philae’s control center in Cologne, Germany. “We worked a lot so we have could some operations ready. We will start with what we call low-cost, low-risk (activities).”

Engineers will first have to wrestle control of the lander. Philae’s Rosetta mothership — the centerpiece of Europe’s comet mission — is moving to a new orbit to improve the communications link with the probe on the surface.

Rosetta dropped Philae to the nucleus of comet 67P/Churyumov-Gerasimenko in November, but the landing craft bounced off the comet and settled a half-mile from its intended touchdown zone. Philae ended up lodged against a cliff eclipsing sunlight, and the solar-powered spacecraft ran out of juice 60 hours after landing.

The likely orientation of Rosetta’s lander, Philae, in a visualization of a topographic model of the comet's surface. Credit: ESA/Rosetta/Philae/CNES/FD
The likely orientation of Rosetta’s lander, Philae, in a visualization of a topographic model of the comet’s surface. Credit: ESA/Rosetta/Philae/CNES/FD

Scientists were optimistic the hibernating probe would wake up as comet 67P streaked closer to the sun, and Philae finally radioed Earth through Rosetta on June 13, three months after controllers started listening for new signals from the lander.

The comet is heading for perihelion — the point in its orbit closest to the sun — on Aug. 13. Rosetta and Philae are conducting the most extensive survey ever made of a comet.

“This (comet) is 4 kilometers (2.5 miles) across,” said Mark McCaughrean, a senior science adviser at the European Space Agency, which oversees the Rosetta mission. “It looks like it’s made of rock, but we now know that it has density less than that of water — half the density of water. It’ a mixture of ice and dust and complex molecules — carbon-bearing molecules — and a completely bizarre landscape in which we see boulders sitting on the surface at apparently bizarre angles. We see plains of dust and rivulets of material.”

Pits and fissures on the comet’s nucleus — dormant when the body was far away from the sun in the cold outer solar system — are firing up as the tiny world encounters warmer conditions.

Philae’s science team is hopeful the lander will make measurements from the comet’s surface when it is most active in August and September.

“This extraordinary journey is by no means over,” McCaughrean said. “We are not at a dead object. We are at a living, breathing dragon of a comet, which is coming to life as we come close to the sun.”

Once engineers establish a stable communications link with Philae, the science team will ease the lander back into action.

“We have to improve the connection between Rosetta and the lander,” Bibring said in a press briefing Wednesday at the Paris Air Show. “We are marginal in the duration of the link now. We are in the few tens of seconds to minutes. We want to go to tens of minutes.”

Members of the Rosetta and Philae teams brief reporters on the status of the mission. Credit: ESA–P. Sebirot, 2015
Members of the Rosetta and Philae teams brief reporters at the Paris Air Show on the status of the mission. Credit: ESA–P. Sebirot, 2015

First up for scientists will be to activate the lander’s low-power instruments. At the top of that list are Philae’s magnetometer and sensors to deduce the physical properties — temperature, texture, density and electrical conductivity — of rock and dust at the landing site, according to Philippe Gaudon, the lander’s project manager at CNES, the French space agency.

If ground teams are satisfied — and if Philae’s power status continues to improve — the next step will be to switch on probe’s cameras, potentially beaming back fresh views of the the lander’s treacherous surroundings.

Philae’s descent camera and panoramic imaging system returned photos of the craft’s landing sequence and final resting place before the lander went into hibernation in November.

Gas analyzers mounted on Philae could then be powered up to sniff the dust around the spacecraft in search of organic molecules, and potentially amino acids, the building blocks of life-supporting proteins.

Engineers will also resume a ranging experiment using radio signals sent between Rosetta and Philae. The transmissions serve two purposes: studying the comet’s internal structure and helping scientists pinpoint the lander’s exact location, which remains unconfirmed.

Using signals from Philae’s November landing and high-resolution imagery from Rosetta’s camera, officials say they have narrowed the craft’s location to an ellipse about the size of a football field.

The final instruments to be turned on will be Philae’s X-ray spectrometer — called APXS — positioned on a boom extended to the comet’s surface, along with the lander’s sampling drill — named SD2.

“The last activity should be the most risky,” Gaudon said. “That means to do a second deployment of APXS after rotation of the lander to optimize the distance between the platform and the soil. We should be able, with APXS, to touch the soil, and we should be able, with SD2, to retrieve a sample.”

The tentative plan calls for the sample to be imaged by Philae’s cameras, then dumped into ovens inside the main body of the spacecraft for heating and chemical analysis.

“It’s something we can do maybe not in the next weeks, but certainly in the next months,” Gaudon said.

The first two images from Philae's CIVA panoramic camera after landing on comet 67P on Nov. 12, 2014. Credit: ESA/Rosetta/Philae/CIVA
The first two images from Philae’s CIVA panoramic camera after landing on comet 67P on Nov. 12, 2014. Credit: ESA/Rosetta/Philae/CIVA

The sun is shining on Philae’s body-mounted power-generating solar panels more than twice as long each day than in November, and the comet — which rotates every 12.4 hours — is closer to the sun, further boosting the energy output from the solar cells.

“Because the comet has come closer to the sun, we have gained at least a factor of four in terms of energy on the solar panels,” Gaudon said.

Philae transmitted useful science data from eight of its 10 instruments during the initial phase of the mission in November. The lander ran out of power before it could complete experimental runs with the X-ray spectrometer and drill.

Scientists now have a second chance, a prospect that Bibring and other members of the Philae team said they expected. Nevertheless, scientists could not be sure the lander would ever wake up.

Assuming the lander still has all its functions, researchers have a bounty of discoveries looming just an arm’s length from Philae.

“We know exactly by which angle we should rotate for the drill now to access the surface and even the subsurface,” Bibring said. “That’s part of the game here — that if we wakeup and if we have the capability to go up to where we want to go, which is to have access to this material, we know how do do it … These are the carbon-rich materials. They’re everywhere, and they’re at hand.”

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Follow Stephen Clark on Twitter: @StephenClark1.


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