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Telescopes and space missions

Rosetta scientists land probe on comet for first time

12 Nov 2014 Tushna Commissariat
Touchdown: Philae's landing site as seen on 30 October 2014

Space scientists are celebrating after successfully landing a probe on a comet for the first time. Members of the Rosetta mission of the European Space Agency (ESA) saw its “Philae” module touch down safely on the surface of comet 67P/Churyumov–Gerasimenko at 15:35 GMT, although the signal was received at 16:03 GMT, because Rosetta’s radio signals take more than 28 minutes to reach ESA’s Space Operations Centre in Darmstadt, Germany.

The landing follows a seven-hour journey for Philae, which separated from the main Rosetta craft this morning at 08:35 GMT, while the confirmation of correct separation was received at 09:03 GMT.

Philae’s landing sequence began last night, when the team initiated a set of three critical “Go/NoGo” decisions to check the lander’s various systems and overall health. During one of the checks, it was discovered that the active descent system, which provides a thrust to prevent Philae from bouncing at the moment of touchdown, could not be activated. In a briefing earlier today, Stephan Ulamec, Philae lander manager at the DLR German Aerospace Centre, said the problem lay with the “cold-gas thruster” on top of the lander, which did not appear to be working.

As Philae touched down on the surface, its landing gear would have absorbed the force of landing, while “ice screws” in each of the probe’s three feet and a harpoon system would have have fired, locking it onto the surface. The faulty thruster was supposed to push the lander down to counteract the impulse of the harpoon in the opposite direction. But despite the potential problem at the moment of touchdown, ESA scientists decided to go ahead with the Philae launch, fully relying instead on the harpoons at touchdown – a decision that appears to have been vindicated. As the ESA operations team studied the first signal from the lander, it was unclear whether or not the harpoon fired as first thought. It later emerged that while the harpoon did fire, it is still unclear if it penetrated the ground and the team is now looking at options to “refire” it.

According to Paolo Ferri, the head of mission operations at ESA, Philae’s communication signal with Rosetta dropped before coming back online – an unexpected occurrence as it should have been a steady signal – and is continuing to do so. The Rosetta team will also be looking into this over the next few hours as they try to resolve these issues.

Up close and personal

Today’s landing is the latest success story for Rosetta scientists. In August this year, the craft drew up alongside 67P/Churyumov–Gerasimenko, becoming the first mission to rendezvous with a comet. Since then, it has come to within barely 10 km of the comet, allowing ESA staff to pick the ideal spot on which to land Philae. The chosen site, dubbed “Agilkia”, was selected on 15 September, and sits at the “head” of the comet.

The last three months have also seen a suite of instruments on Rosetta scrutinizing 67P. The surface temperature of the comet has been measured using its Visible and Infrared Thermal Imaging Spectrometer (VIRTIS), revealing that the surface is warmer than expected and made of dusty, porous material. VIRTIS has also detected both water and carbon dioxide in the comet’s coma, obtaining values for their relative abundance.

Using the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) or the “comet sniffer” on-board, scientists have also detected a variety of molecules in the coma of 67P, including water, carbon monoxide, carbon dioxide, methane, as well as trace amounts of nitrogen and sulphur. The team has even managed to ensnare a handful of grains, with its analysis having shown that at least one of them contains sodium and magnesium.

Long and winding road

First launched in 2004, Rosetta has reached its destination after making three gravity-assisted fly-bys of Earth and one of Mars. During the decade-long journey to 67P, it has also observed and studied two asteroids – Šteins and Lutetia. The craft also spent more than 31 months in hibernation from June 2011 to January 2014, during which time Rosetta was so far from the Sun that there was not enough power to keep all but a few key instruments on the craft alive. From May onwards, when Rosetta was still nearly two-million kilometres from the comet, mission scientists began to nudge it into orbit around 67P by slowly adjusting its speed and trajectories.

Comet 67P/Churyumov–Gerasimenko is currently about 511 million km from Earth (halfway between the orbits of Jupiter and Mars) and is hurtling towards the inner solar system at nearly 55,000 km/h. The comet follows a 6.5-year elliptical orbit around the Sun, and Rosetta will accompany it for more than a year of its journey, as the two swing around the Sun and back out towards Jupiter.

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