How the Rosetta space probe will land on a comet

Sky at Night presenter Dr Maggie Aderin-Pocock talks us through today's extraordinary space mission


As a comet approaches the Sun, it boils. Every hour, tons and tons of ice from the surface sublimates into gas and drifts into space. This is what forms a comet’s tail, water you could be drinking right now. A comet might even have brought life to Earth. And now we’re going to land on one.


On Wednesday 12 November, the space probe Rosetta will drop the lander Philae towards the surface of Comet Churyumov–Gerasimenko, which was discovered while searching photographs for another comet back in 1969. “We call it 67P,” says Dr Maggie Aderin-Pocock, presenter of The Sky at Night. “I need a good run-up to say the full name.”

Rosetta entered orbit around 67P this August, and this Sunday, a special edition of The Sky at Night will cover Philae’s perilous final descent, as well as retrace Rosetta’s incredible ten-year journey: 6 billion kilometres spent picking up speed by looping around Mars and the Earth over and over again, all to catch a chunk of ice travelling at 135,000 kilometres per hour. If all goes to plan, the programme will feature the latest photographs from 67P, the reaction from mission control and the first findings beamed back by Philae’s onboard laboratory. And if something goes wrong? “We’ll fill it with tears and gnashing of teeth,” says Dr Aderin-Pocock.

It’s a serious concern, as the international team behind Rosetta has set itself an almost ludicrously difficult task – 67P could conceivably break apart at any time. “We don’t think it will,” says Aderin-Pocock, “but it’s a ball of ice flying towards a heat source.” Yet as someone who started her career as a space scientist, she believes it’s all worth it.

In some ways Rosetta is not just travelling through space, but back in time. Comets are the remnants of the young solar system, a time capsule of the material that formed planets and possibly a record of how we came to be.

“We know life here on Earth is special, and we know that we have a lot of water, which might be responsible for that,” she explains. “But where did that water come from? One idea is that asteroids or comets seeded water when they collided into Earth and left it behind.” It’s enough to make you look twice at your Evian.

And it’s not just water that we might owe to comets. Experts often call them “dirty snowballs”, dirty because, despite their sparkling tails, the surface of 67P is dark due to a crust of “organics”: carbon-bearing compounds that could be the building blocks of life. “It doesn’t look like life as we know it, but it might be the precursor to life,” The Sky at Night presenter puts it, almost quoting Star Trek. “Comets could be the life-bringers in the solar system.”

Despite the expense (€1.4 billion) and the difficulty, Dr Aderin-Pocock believes the point of missions like Rosetta is something fundamental. To paraphrase JFK, we choose to go to 67P not because it’s easy, but because it’s hard. “It’s incredibly inspiring,” she says. “People have been working on this all of their careers, starting in the lab as students, and now grown up with kids.”

It’s worth bearing in mind that Rosetta launched in 2004, and work had been going on for years before that. Remember what your mobile looked like back then? Yet the investigative equipment Philae is carrying remains cutting edge.   But what sometimes gets lost in the talk of ultraviolet spectrographs and escape trajectories is the emotional core of such missions, the sense of discovery and perseverance that leads our brightest minds to attempt the almost impossible. “It’s a real life experience,” Dr Aderin-Pocock argues. “People think of scientists as distant and unemotional, but it’s totally the opposite. When we have something we’re really excited about, the full range of emotions come out. Whether it’s a success or not, it’s a chance to experience that human story.”

And if the building blocks of life did come from a comet like 67P, what could be more human than going back home?

Rosetta: A Sky at Night Special will be broadcast on Sunday 16th November, 9.00pm on BBC4

How to land on a comet

Stage 1

Hitting a moving target is hard enough, but comet 67P is travelling incredibly fast — 135,000 km/h, or 81,000mph. To make it even harder, it’s also covered in cliffs and steep slopes — “It looks like the duck from your bath, but is about the size of Mont Blanc,” says Dr Aderin-Pocock.

Stage 2

With no pilot on board and no thrusters to guide it, Philae, the space probe Rosetta’s lander unit, will fall through the weak gravity for seven hours, carrying a precious cargo of hammers, drills and subsurface radars.
At this point, members of the flight control team, in Darmstadt, Germany, begin to hold their breath.


Stage 3

After hitting the ground, the lander will deploy harpoons and ice screws to stop it and anchor it to the surface like a limpet, otherwise it might bounce off. Then the technical work begins. Among the tools on board Philae is a penetrometer, for testing the comet’s crust, and a mass spectrometer to analyse its findings.