Rosetta and Philae (2014): First Landing on a Comet

The Rosetta mission of the European Space Agency (ESA) made history on 12 November 2014, when its small lander Philae touched down on comet 67P/Churyumov-Gerasimenko, the first landing on a comet. The touchdown did not go to plan: Philae bounced and ended up in a shaded crevice. Even so, the lander returned data from the surface, and the Rosetta orbiter studied the comet up close for more than two years, transforming what scientists know about these icy leftovers of the early Solar System.

Background: why study a comet?

Comets are mixtures of ice, dust and rock that formed about 4.6 billion years ago, at the same time as the planets. Because they spend most of their time far from the Sun, they are thought to preserve some of the most primitive material in the Solar System. Some scientists also suspected that comets delivered water and organic molecules to the young Earth.

Earlier probes had only flown past comets. ESA’s Giotto passed close to the nucleus of Halley’s Comet in 1986, the comet whose return Edmond Halley had predicted. ESA approved Rosetta in 1993 as a far more ambitious mission: to match a comet’s orbit, travel alongside it for months and set down a lander on its surface.

The mission takes its name from the Rosetta Stone, the inscription that unlocked Egyptian hieroglyphs. The lander was named after Philae, an island in the Nile whose obelisk helped in the same work and whose temples are described in the article on the Philae Temple of Isis.

The long journey to 67P

Rosetta was meant to fly to comet 46P/Wirtanen, but the failure of an Ariane 5 rocket in December 2002 delayed the launch and a new target had to be chosen. The replacement was 67P/Churyumov-Gerasimenko, discovered in 1969 by the Soviet astronomers Klim Churyumov and Svetlana Gerasimenko. It orbits the Sun about every six and a half years.

Rosetta lifted off on an Ariane 5 from Kourou, French Guiana, on 2 March 2004. To gain enough speed, it used three gravity-assist flybys of Earth (2005, 2007 and 2009) and one of Mars (2007). On the way it photographed two asteroids, Šteins in 2008 and Lutetia in 2010.

Far from the Sun, Rosetta’s solar panels could not generate enough power to run the spacecraft. In June 2011 it was put into hibernation for 31 months. On 20 January 2014 it woke up on schedule, and on 6 August 2014 it arrived at the comet, becoming the first spacecraft to orbit a comet nucleus.

The landing of Philae

Close-up images revealed a surprise. The nucleus of 67P is shaped like a rubber duck, with two lobes joined by a narrow neck, roughly 4 km across at its longest. Its surface was rugged, with cliffs, boulders and pits, which made picking a safe landing site difficult. Mission teams chose a spot on the smaller lobe, later named Agilkia after the island to which the Philae temples were moved.

Philae was a box-shaped lander of about 100 kg, built by a European consortium led by the German Aerospace Center (DLR), with ten scientific instruments. It separated from Rosetta on 12 November 2014 and descended for about seven hours. Because radio signals took more than 28 minutes to reach Earth, the descent was entirely automatic.

Philae touched down at Agilkia at about 15:34 UTC, and controllers in Darmstadt and Cologne received confirmation around half an hour later. But two harpoons that should have fixed it to the ground did not fire, and a cold-gas thruster meant to hold it down also failed. In the comet’s very weak gravity, Philae rebounded and drifted for almost two hours, rising perhaps a kilometre above the surface. After a second, shorter bounce it came to rest at a site later named Abydos, tilted and largely shaded by a cliff.

Science on the surface

Running on its primary battery, Philae worked for about 60 hours and carried out its first planned sequence of measurements. Its instruments took pictures of the surface, sampled the thin gas around it and measured the ground’s hardness and temperature. Its COSAC instrument reported 16 organic compounds, including several never before found on a comet.

On 15 November 2014 the battery ran down and Philae went into hibernation, because its solar panels received too little light in the shadows. As the comet approached the Sun, Philae woke briefly and made intermittent contact between 13 June and 9 July 2015, but no further science was possible. Its exact location remained uncertain until 2 September 2016, when Rosetta’s OSIRIS camera spotted it wedged in a crack.

What Rosetta found

The orbiter was the heart of the mission. It watched 67P as it passed closest to the Sun on 13 August 2015, recording jets of gas and dust, collapsing cliffs and changes across the surface.

Rosetta’s instruments found that water on 67P has about three times the ratio of deuterium (heavy hydrogen) to ordinary hydrogen found in Earth’s oceans. That suggested comets of this type were not the main source of Earth’s water. Rosetta also detected molecular oxygen, a surprise for researchers, and the amino acid glycine along with phosphorus, ingredients associated with life.

On 30 September 2016, with the comet heading away from the Sun and solar power dwindling, controllers steered Rosetta into a slow descent onto the surface. It sent back data and images until the moment of impact.

Timeline

• 1969: Klim Churyumov and Svetlana Gerasimenko discover comet 67P.
• 2004, 2 March: Rosetta is launched on an Ariane 5 from Kourou, French Guiana.
• 2011, June: Rosetta enters 31 months of deep-space hibernation.
• 2014, 20 January: The spacecraft wakes up and contacts Earth.
• 2014, 6 August: Rosetta arrives at comet 67P and begins to orbit it.
• 2014, 12 November: Philae lands, bounces twice and settles at Abydos.
• 2014, 15 November: Philae’s battery runs down and the lander falls silent.
• 2015, 13 August: Comet 67P passes closest to the Sun.
• 2016, 30 September: Rosetta ends its mission with a controlled descent onto the comet.

Why it matters

Rosetta and Philae achieved two firsts: orbiting a comet and landing on one. The bouncing landing showed how hard it is to touch down on a small, unfamiliar world, but the mission’s data changed ideas about how comets formed and whether they supplied Earth’s water. Its images of a dramatic, active comet also caught the public imagination across Europe and beyond.