Science

Farewell, Philae! Space Agency Pulls Plug on Humankind’s First Comet-Trotter

Philae. Credit: Ita Mehrotra

Philae. Credit: Ita Mehrotra

Philae, the first probe to land on a comet, finally had its life support pulled out last week. The European Space Agency (ESA), which operated the historic probe, turned off communications for the final time on the July 27, 2016. Philae landed on the comet 67P/Churyumov-Gerasimenko’s nucleus on November 12, 2014, after a journey of ten years in space with the Rosetta spacecraft still in orbit around the comet.

The Rosetta spacecraft and Philae lander were launched on March 2, 2004, with the mission of achieving a soft landing (as opposed to an impact) on a comet and study its composition. While Philae landed on the comet and bounced about precariously before becoming stably attached, Rosetta orbited 67P, picking up communications from Philae and relaying them to Earth.

Philae’s mission had been to land successfully on a comet and analyse its composition and environment. Upon its landing on November 12, 2014, it bounced twice at the rate of 15 inches/s, rising to a height of 1 km on each bounce. (Had the lander bounced at 17 inches/s, it would have escaped the comet’s gravity altogether and drifted away into space!) Philae’s final location on the comet was angled away from the Sun in such a way that the lander could obtain solar power only for 90 minutes every twelve hours. So after performing nearly 80% of tests it’d been tasked with, Philae went silent three days later, on November 15.

Then, June 13, 2015, comet 67P had gotten close enough to the Sun in its path to swing around it and for Philae to exit its hibernation mode. But the position in which it was wedged into the comet made it hard to establish a stable connection with Rosetta, still orbiting the comet. Moreover, scientists could not really consider the option of changing Rosetta’s orbit to a more optimal one, specifically one that would bring it directly above Philae, because it was important to have the orbiter out of the way of the gas and dust being expelled by the comet as it got closer to the star.

The last communication from Philae was on July 9, 2015. And by January, scientists had declared communication with the craft dead. On July 27, 2016, scientists turned off the electrical support for communication aboard Rosetta, making Philae lost to humankind forever.

The final decision to turn comms off was based on figuring out how much power Rosetta would need to keep attempting communications with Philae. As Rosetta begins winding down for its crash into 67P, it needs to conserve power – which has been growing scarcer by the day as the comet moves away from the Sun. “By the end of July 2016, the Rosetta spacecraft will be some 520 million kilometres from the sun, and will start facing a significant loss of power,” said ESA science writer Claudia Mignone. “In order to continue scientific operations over the next two months and to maximise their return, it became necessary to start reducing the power consumed by the non-essential payload components on board.

Philae was named for the obelisk used with the famed Rosetta Stone to translate and understand Egyptian hieroglyphs. The lander weighed a total of nearly 100 kg, with ten scientific instruments on board. The project was a collaboration between fourteen countries (thirteen under ESA and Canada) and cost $1.8 billion. Its most significant findings involved the composition of the comet. Instead of soft accumulated gas and dust, the surface of the comet was, unexpectedly, hard ice, making it impossible for Philae to drill on it, as had been planned. The lander also discovered an uneven terrain as well as the presence of 16 organic compounds. Four of them had been observed on a comet for the first time.

Philae successfully performed all its tests, making the overall mission a success. The orbiter Rosetta is scheduled to crash into 67P on September 30 2016.

Sandhya Ramesh is a science writer focusing on astronomy and earth science.