Rosetta and Comet 67P: The First Comet Landing

The mission in dates

Rosetta took more than ten years to reach its target. It used four gravity assists, one from Mars and three from Earth, to build up speed. Along the way it flew past two asteroids, then spent the years from 2011 to 2014 in hibernation to save power.

• 2004 (March 2): Launch on an Ariane-5 G+ rocket.
• 2005 (March 4): First Earth gravity assist. Mars followed on February 25, 2007, and two more Earth passes on November 13, 2007 and November 13, 2009.
• 2008 (September 5): Flyby of asteroid Steins.
• 2010 (July 10): Flyby of asteroid Lutetia.
• 2011 (June 8): Entered deep-space hibernation. It woke on January 20, 2014.
• 2014 (August 6): Arrival at the comet.
• 2014 (November 12): Philae delivered and landed.
• 2015 (August 13): Closest approach to the Sun, about 186 million km (116 million miles).
• 2016 (September 30): End of mission.

Those two asteroid flybys tie Rosetta to the wider science of small bodies. The guide to how scientists learn what asteroids are made of covers the methods that worked on them.

Meet Comet 67P

Klim Churyumov and Svetlana Gerasimenko discovered the comet in 1969. NASA says Gerasimenko took a photographic plate of another comet on September 11, 1969, and Churyumov found 67P on it on October 22 at the Alma-Ata Observatory in Kazakhstan. The "67P" means it was the 67th periodic comet discovered.

The comet is shaped like two lobes joined together, which ESA calls the body and the head. ESA's figures put the small lobe at about 2.5 by 2.1 by 1.6 km and the large lobe at about 4.1 by 3.5 by 1.6 km. Another ESA page gives slightly different numbers, so treat any single figure as approximate.

One AU, or astronomical unit, is the average Earth-Sun distance, about 150 million km (93 million miles). The density figures mean the nucleus is about half as dense as water. ESA reports a porosity of 70 to 80 percent, which means most of its volume is empty space. An old picture of comets as solid, icy balls does not fit that.

The orbit crosses those of Mars and Jupiter. A close encounter with Jupiter in 1959 set its current perihelion, or closest point to the Sun, at about 1.3 AU, according to NASA. A scientific paper summarized by ESA calls 67P a Jupiter-family comet. Those comets are thought to start in the Kuiper Belt.

The Philae landing

Rosetta released Philae from 22.5 km out, and ESA says the descent used no propulsion or guidance. The landing site, Agilkia, was on the comet's head. Engineers chose it about six weeks after arrival. The plan called for a touchdown at about 1 m per second, with harpoons and ice screws to hold the lander down.

It did not go to plan. ESA says Philae had no downward thruster and the harpoon system did not work. The lander bounced twice after the first touchdown, so it touched the comet three times in all, and it ended up in the shadow of a cliff. Even so, the lander worked for nearly 57 hours, completing its primary science mission. It returned data from instruments including ROLIS, COSAC, Ptolemy, SD2 and CONSERT, and took the first panoramic image from a comet's surface.

Then its power ran down. ESA's report from that time says contact is possible only when enough sunlight reaches the solar panels to wake the lander. Later events are on ESA's Rosetta pages.

What Rosetta found

Rosetta's instruments measured the comet's gas, dust and ice for two years. The results changed several ideas.

Water. ESA says 67P's water is quite different from Earth's water, with the highest ratio of heavy hydrogen (deuterium) to ordinary hydrogen of the 11 comets measured. ESA adds that it also differs from other comets and asteroids. This is one comet, so it does not settle where Earth's water came from.

Gases. NASA lists the first detection of molecular nitrogen and molecular oxygen at a comet, plus the amino acid glycine and phosphorus. ESA reports noble gases as well. Because nitrogen and oxygen can only be trapped at very low temperatures, ESA says the comet formed in a very cold region far from the Sun. Its data point to an age of about 4.6 billion years. Glycine and phosphorus are building blocks of living things, but finding them does not mean the comet carried life.

Dust and surface. ESA reports about four times more dust than gas by mass, averaged over the sunlit surface. NASA's page says the nucleus gives off about twice as much dust as gas. The ratio depends on how and when it is measured. The surface is not uniform either. ESA says the imaged surface falls into 19 regions grouped into five terrain types, and NASA says exposed water ice appears on the surface.

The overview of comet anatomy puts these findings in context.

The end of the mission

Rosetta ended with a planned controlled impact on September 30, 2016. The target was the Ma'at region on the small lobe, a smooth area between two active pits. ESA expected the touchdown inside an ellipse of 700 by 500 meters (about 2,300 by 1,600 feet).

The reasons were practical. The comet was moving away from the Sun, which cut solar power. A month-long solar conjunction, when the Sun sits between Earth and the spacecraft, was about to disrupt communications. The spacecraft was also aging after more than 12 years in space. ESA ruled out hibernation because the power at the comet's farthest point might not keep it warm enough to survive.

The final descent began on the evening of September 29 from about 19 km (12 miles) up. The expected impact speed was about 90 cm per second, roughly walking pace, with data relayed in near real time until the end. Because radio signals needed about 40 minutes to reach Earth, confirmation came after the impact itself.

Common mistakes and myths

"Rosetta landed on the comet." Philae was the lander. The Rosetta orbiter came down only at the end of the mission, in a planned impact.

"Philae landed perfectly." It bounced twice, had no working harpoons, and settled in the shadow of a cliff.

"The mission failed when Philae went quiet." The lander finished its primary science, and the orbiter kept working until 2016.

"Rosetta proved comets brought Earth's oceans." ESA found 67P's water differs from Earth's. That is one comet, and the question stays open.

"A comet is a solid ball of ice." 67P is about half as dense as water, with 70 to 80 percent empty space. For how it compares with rocky bodies, see the guide to comets and asteroids.

Frequently asked questions

When did Philae land on Comet 67P?

On November 12, 2014. Confirmation reached Earth at 16:03 GMT after a seven-hour descent.

Is Rosetta still operating?

No. The mission ended on September 30, 2016, with a controlled impact on the comet.

How big is Comet 67P?

ESA gives about 4.34 by 2.60 by 2.12 km (2.7 by 1.6 by 1.3 miles) for the whole nucleus. It has two lobes, with the larger about 4.1 km long.

What was special about its water?

ESA says it has the highest deuterium-to-hydrogen ratio of the 11 comets measured, and it differs from Earth's water.