Throughout human history, comets have been distant, mysterious heavenly bodies. The hunks of rock and ice streak through the sky, streaming bright tails of gas as the sun warms them.
On Wednesday, humankind finally made contact with one. The Rosetta spacecraft defied all odds and dropped its payload, a comet-sniffing probe named Philae, on a cold, speeding target more than 300 million miles from Earth.
Scientists aren’t sure that the probe will be able to anchor itself securely, but for now its systems are operational and responsive. This comes as the climax of a decade-long mission.
Swaddled inside the comet are the secrets of the early solar system, the elements present when the sun was new and the planets were forming. To study a comet up close would be a cosmological dream, a time capsule like scientists have never laid eyes on. The black ice of these comets keep those secrets preserved — but also largely hidden.
This one is named 67P/Churyumov-Gerasimenko, an ancient, puckered comet that fell from a cluster of similar bodies near Neptune many years ago. Now in an elliptical orbit around the sun, it has been getting closer and closer to Earth with each go-around. But to really get an intimate look at the comet, scientists knew they would have to get right on top of it.
Landing a 220-pound probe on a 2.5-mile-wide comet is no small step. The comet hurtles around the sun at a speed of 84,000 mph and doesn’t even have the decency to be smooth and evenly shaped, as comets so often appear to be in images.
Instead, the landing target is pitted and uneven, taking the appearance of a child’s Play-Doh impression of a duck.
Tensions were high in the European Space Agency’s mission control room in Darmstadt, Germany, on Wednesday as the clock ticked closer to the proposed landing window. At 4 a.m. EST, the team watched as Rosetta ejected Philae, giving it a push toward the comet. Without thrusters or an engine, Philae simply dropped toward the piece of the comet that scientists thought it was most likely to land on successfully.
Then, a sigh of relief — Philae and Rosetta made satellite contact with each other. Without that connection, the probe would have been functionally lost.
With that final pre-landing checkpoint met, mission control hunkered down for seven hours of nerve-wracking free-fall, until Philae’s sensors confirmed a landing at 11:03 a.m. EST.
“So we’re there, and Philae is talking to us,” said Stephan Ulamec, lander manager, as cheers erupted around the giddy mission control team.
To give Philae a snowball’s chance of hitting the right spot, Rosetta had to get in perfect orbit with the comet. The Rosetta team has spent the past 10 years doing just that, although Rosetta’s story began years earlier.
In 1986, the center — or nucleus — of Halley’s comet was photographed for the first time by the European spacecraft Giotto. This image was far from clear, but it represented a milestone: Instead of an ethereal smear, this famous comet was seen as a rock speeding past. These heavenly bodies begged to be known, and the European Space Agency greenlighted Rosetta — a mission to land a craft on a comet and study it — in 1993.
Since Rosetta’s launch in 2004, the spacecraft has undertaken a 4 billion-mile journey. It gathered speed by looping around the inner solar system, shooting past Earth three times and Mars once. Its final pass of Earth sent it far from home, into the dark cold of space. For nearly three years, the spacecraft was in hibernation while its solar panels were starved of light. Scientists couldn’t be sure it would wake up on cue, and waking the sleeping beauty was the first of the mission’s many moments of breath holding and finger crossing.
It came alive Jan. 20 and had its first sighting of the comet in March. In August, it arrived.
As tense as the project scientists looked Wednesday, they also looked somewhat resigned. For one thing, there was nothing to be done. The comet and lander were so far from Earth, there was a communication delay of about 28 minutes. Once the clock hit 10:30 a.m. ET, Philae had either succeeded or failed — and humankind could only wait.
They knew that a successful landing would be the icing on the cake of the Rosetta mission. Even without Philae on the surface, the scientists had collected massive amounts of data about the comet and its atmosphere just from having Rosetta in close orbit.
And either way, Rosetta would continue to piggyback on the comet’s orbit for a full year, following it on a trip past the sun.
“The orbiter itself will do a great job of figuring out how the comet works as a machine, and how it responds to the sun,” said Claudia Alexander, a project scientist who oversees NASA’s many contributions to the European mission. “This is a very alien body, this comet. It’s this weird extraterrestrial ice. We want to understand the physics of how a body like this works.”
It also is thought to hold clues to the formation of the solar system. The core of a comet contains molecular building blocks that have not changed in billions of years and provide a glimpse into the origin of the planets.
Even if everything goes well with Philae, it will collect data only until March, when the comet will be too close to the sun.
When mission control saw the data that confirmed a landing, the nervous looks melted into smiles, then hugs and cheers.
“The data collected by Rosetta will provide the scientific community, and the world, with a treasure-trove of data,” said NASA astronaut and administrator John Grunsfeld. “Small bodies in our solar system like comets and asteroids help us understand how the solar system formed and provide opportunities to advance exploration.”
But it wasn’t all good news. A few minutes later, Rosetta scientists announced that Philae hadn’t deployed the harpoons that would anchor it to the comet’s surface. Without this stability, the probe could move into any position, and it has very little chance of righting itself again. The team said it would consider firing the harpoons again and would update when it knows more.
