Rosetta mission: Philae makes historic landing on comet
The signal broke a seven-hour wait of agonising intensity and sparked scenes of jubilation at the European Space Agency’s mission control in Darmstadt. The team in charge of the Rosetta mission had achieved what at times seemed an impossible task by landing a robotic spacecraft on a comet for the first time in history.
The safe landing gives scientists their first chance in history to ride a comet and study close up what happens as its activity ramps up as it gets closer to the sun.
The feat marks a profound success for ESA, which launched the Rosetta spacecraft more than 10 years ago from its Kourou spaceport in French Guiana. Since blasting off in March 2004, Rosetta and its lander Philae have travelled more than six billion kilometres to catch up with the comet which orbits the sun at speeds up to 135,000km/h.
The moment came shortly after 1600 GMT when Philae called home. It had fired harpoons to secure itself on the comet and twisted in ice screws fitted to its feet. “We are there, we are sitting on the surface,” said a jubilant Stephan Ulamec, Philae lander manager at the DLR German Space Centre.
Andrea Accomazzo, Rosetta flight operations director, said: “We cannot be happier than we are now.” Touch down for the lander played out 510 million kilometres from Earth on a comet hurtling through space at 18km/s. At so vast a distance, even radio signals travelling at the speed of light take nearly half an hour to travel from Earth to the spacecraft, making realtime control of the landing impossible. Instead, the entire descent was precalculated, uploaded and run automatically.
The £1bn mission, named after the Rosetta stone which enabled scholars to decipher the language of ancient Egypt, aims to unlock the mysteries of comets made from material predating the birth of the solar system. In the data Rosetta and Philae gather, researchers hope to find answers to questions of how the solar system formed and whether comets carried water and even complex organic molecules to planets, preparing the stage for life on Earth.
Landing Philae on the comet’s surface was never going to be easy. When ESA managers got their first closeup of the comet in July, itsunusual rubber duck shape left some fearing that a safe touchdown was impossible. The shape was not the only problem. The comet’s surface was hostile: hills and spectacular jutting cliffs gave way to cratered plains strewn with boulders. If Philae landed on anything other than even ground it could topple over leaving it stranded and defunct.
Rosetta spent weeks flying around the comet to create a surface map from which mission controllers could choose a landing site. They faced a trade-off: the site had to be fairly flat and clear of boulders, but with a good view of the whole comet and plenty of sunlight to charge the lander’s batteries once down. From a shortlist of five potential landing spots, scientists and engineers unanimously voted for a 1 sq km region on the comet’s “head” later named Agilkia.
At the start of the mission, ESA officials assumed the comet would be potato shaped and rated their chances of a successful landing at 75%. After seeing the shape and terrain of their target close up, those odds fell to around 50%, but climbed again as technical staff learned more about the landing site.
The confidence did not last long though. On Tuesday night, hours before Philae had left its mother ship, the chances of a safe landing took another dip. Overnight, a thruster on the lander failed to respond to commands sent from Earth. Engineers tried for hours to correct the fault but to no avail. The malfunction threatened to abort the mission, but at 0235 GMT on Wednesday mission controllers decided to go ahead with the landing regardless.
The nitrogen thruster, facing upwards from the top of the lander, was designed to fire for 60 seconds as Philae touched down to prevent it from bouncing off the comet’s surface where the gravitational pull is several hundred thousand times weaker than on Earth.
For the mission team, the seven-hour descent, during which Philae fell at walking speed towards the comet’s surface, was a nail-biting experience. The lander separated from its mothership at 0835 GMT with confirmation received on Earth at 0903. For the early part of the 20km descent, Philae was expected to be out of contact with Rosetta. Around 1100 GMT the mother ship reacquired a signal from the lander, which duly unfurled its legs and began to take pictures. The first image taken just 50 seconds after separation revealed a look back at the mother ship, one of its 14m-long solar arrays clearly visible, as Philae fell silently to its destination.
One hour from touchdown, Philae was on the right course. Ulamec of the DLR German Space Centre said everything looked “fantastic”. Telemetry from the lander showed that it was rotating, but not enough to cause problems.
From its orbit around the comet, the Rosetta probe will take more high-resolution images and gather information on the body’s density, temperature and chemical make-up. It will also capture dust and gas released in ever more violent jets from the comet’s nucleus as it nears the sun.
Secured on the surface, the lander can drill down 20cm and collect samples of subsurface material for on-the-spot testing. On board Philae are 10 instruments, including one from the Open University named Ptolemy which will bake pieces of comet material and analyse the gases given off to build up a picture of the comet’s composition. Another instrument, the Comet Nucleus Sounding Experiment by Radiowave Transmission (CONSERT), will reveal the comet’s internal structure by passing radio waves through the icy body to Rosetta on the other side.
The Rosetta mission is planned to run until December 2015, but if enough fuel remains in the spacecraft’s tanks, mission controllers may extend its life by six months and give the mother ship more high-risk tasks, such as flying through one of the gas and dust jets streaming from the comet. Philae has initial battery power to last 40 hours but will then switch to rechargeable ones replenished by sunlight.
The lander could continue working until March next year, when the electronics will become too warm to work properly. Even when Philae packs up, it may still cling on to the comet, perhaps for several 6.45-year-long laps around the sun, before enough material erodes from the comet’s surface for the lander to lose its grip.
