'Seven Hours of Terror': Harpooning a Comet Isn't Easy, But It's About to Happen
Detail of the Rosetta comet, shot on 10/30. Image: ESA


This story is over 5 years old.

'Seven Hours of Terror': Harpooning a Comet Isn't Easy, But It's About to Happen

​Next Wednesday, a spacecraft will make a slow landing onto the surface of a comet for the very first time.
November 7, 2014, 11:30am

Update 12 November: ESA has confirmed that the Philae lander successfully touched down on Comet 67P. The signal confirming the success came in at 16:03 GMT. In a release, Philae Lander Manager Stephan Ulamec said that, __"In the next hours we'll learn exactly where and how we've landed, and we'll start getting as much science as we can from the surface of this fascinating world."

Next Wednesday, a spacecraft will make a slow landing onto the surface of a comet for the very first time. The European Space Agency (ESA) will release the Philae lander from comet-chasing space probe Rosetta on the morning of November 12, and it will make a slow descent to a carefully selected spot on comet 67P/Churyumov–Gerasimenko.

That's if all goes to plan. Matt Taylor, the project scientist on the Rosetta mission, told me in a phone interview that he's expecting an "exponential increase in stress" as the lander makes its slow journey. Where the Curiosity rover had its "seven minutes of terror" as it touched down on Mars, he said, this would be "seven hours of terror."

The landing is the next momentous stage in a mission that's counted off an impressive list of firsts. After "waking up" in January, the Rosetta spacecraft marked a key milestone in August when it rendezvoused with the comet. Next week, all fingers are crossed that the mission will cross off another major achievement. "This is one of the other major firsts we're going to achieve, which is the landing," said Taylor.

Rosetta should release the Philae lander at around 8:35 AM GMT, and confirmation of the landing should reach ESA's ground stations around 4 PM. Taylor told me more about what will happen during that time.

"We have a number of milestones on the way to the deployment of the landing," he explained. "We have to look at whether our orbit determination is good or not, whether the lander is in a situation that we think is good enough—these are the go/no-go scenarios."

By 9 AM, if they have the go-ahead, Philae will be released, and "some of us will sit there twiddling our thumbs, waiting for the lander to descend and get to the surface of the comet," he said.

The action itself isn't as dramatic as you might think. Rosetta is currently gliding alongside the comet at a walking pace, and Philae will get a gentle nudge to descend at a similar rate of around one metre per second, carried to the comet surface by the weak gravitational pull. Taylor said Philae project manager Stephan Ulamec compared the impact to "a bit like walking into a wall: It doesn't particularly hurt or damage you, but you know you've done it."

Meanwhile, the Rosetta orbiter will fly out into a bigger orbit "to make sure it doesn't smack into the comet, or put itself in a situation where that is a possibility."

On Philae's release, its CIVA camera—already known for taking a selfie of Rosetta in orbit—will take a "goodbye image" of Rosetta as the space-travelling duo part ways, and a camera on the bottom of the lander will start taking images.

Taylor said that from about two hours after the release, the team will have telemetry measurements to observe what's going on, until it gets to the crunch point of whether the lander is in place or not.

Serendipitously enough, the landing will be assisted by Philae's other instruments. The CONSERT experiment—one of many on the mission—sends radio waves through the comet to study its nucleus. The signals are perturbed as they go through the comet to be picked up by Rosetta, and those changes will tell the scientists more about the comet's interior.

But Taylor explained that by looking at the Doppler effect of the signals, they'll also be able to track the comet's descent. "We'll be using that experiment as the probe descends to look at the Doppler shift and see what the signal's doing, so you'll get an idea of how the lander's moving," he said.

The Rosetta orbiter will also scan for the lander from its safe distance, and once it's landed a whole load of things will react. Two harpoons will deploy to anchor Philae to the specially selected spot on the comet, which was recently named Agilkia after an island in the River Nile.

But once the lander touches down and its army of Twitter fans breathe a communal sigh of relief, the science is just getting started. "Once we touch down we do the panoramic view to see if Bruce Willis is standing there with his team, and then we'll be doing some of the real sniffing measurements," Taylor said.

Both the descent and landing phase and the first science sequence have a tight time limit: They rely on the lander's primary battery system. "As soon as we have detached the lander from the orbiter, it's all on its own power," said Taylor. "There is a little bit of solar power that'll be regenerating things, but it's basically limited to around 70 hours lifetime on those batteries."

A cycle starts whereby each instrument on Philae gets a turn to do its thing before the long-term science sequence, which uses secondary batteries recharged by solar generators, kicks in. That was, in fact, one main factor in choosing the Agilkia site to land on. It might seem that the flat "belly" on the rubber duck-shaped comet would make for a good spot, but that side only gets around four hours of sunlight a day.


Rosetta has already had a waft of 67P's space stink, but Philae will get right up close and personal. As Taylor described it, "Where the orbiter tastes and sniffs the coma, the outer atmosphere of the comet, which comes from the nucleus, the lander's going to get down there and actually scratch and sniff."

Philae will last until about March, when some of its components will start getting too hot to function. But the work doesn't end there; Rosetta's still going. It'll reach its most active next summer when it'll start losing kilogram upon kilogram of material and form a tail like you imagine a comet to have.

There's a chance it could split in half, which Taylor said would be a "mission changer" and limit some of the science they could do. "Who knows what's going to happen?" he said. "Comets are quite unpredictable, and that's the major technological challenge we've had with the mission, to try to learn to live with something like that."

But Taylor teased that they already have some "groundbreaking" results from the Rosetta mission that are being reviewed, and pointed out that the Rosetta mission has already produced an unrivalled set of data about comets. "We've taken more data than has ever been taken at a comet already, and the resolution is the best ever as well," he said.

Here's hoping Philae makes a safe landing and adds to that collection. A few more awesome space selfies wouldn't go amiss, either.