After 4.9 billion miles and plenty of comet science, Rosetta mission ends

The Rosetta mission to catch up with a speeding comet, land a space probe on it and follow it as it flies past the sun has officially come to an end.

Early Friday morning, the European Space Agency's Rosetta orbiter committed operational suicide when it deliberately smashed onto the surface of 67P/Churyumov-Gerasimenko, the mountain-sized comet that has been its constant companion for the last two years.

The crash was slow but still fatal. The mission's lander, Philae, was built with legs, springs and harpoons to help it land on a comet and survive. In contrast, the Rosetta orbiter was designed to fly around 67P, not touch down on the surface.

There were no cameras to record Rosetta's final moments, but ESA engineers said the spacecraft probably bounced a few times given the comet's low gravity. In the process, it probably kicked up a cloud of dark, dry dust before settling in its final resting place on the smaller of the comet's two lobes.

"At the moment of impact, Rosetta will be crushed," flight director Andrea Accomazzo said during the mission's final hours. "It will remain on the comet forever because there is no way to get it off the surface."It was conceived when

During its final descent, Rosetta gathered close-range information about the comet and hastily beamed data back to Earth before its main transmitter shut off for good.

"It's kind of bittersweet," said Paul Weissman, a comet scientist at the Planetary Science Institute in Tucson, Ariz., who worked on Rosetta for 20 years. "You'd like to keep going, but it is also very satisfying. It's been a tremendously successful mission."

The $1 billion mission has been full of suspense.

Its many plot twists began before the spacecraft left Earth, when a faulty rocket postponed the launch by two years and caused mission planners at the European Space Agency to abandon their original comet and select a different one instead.

The new comet, known as 67P, was four times larger than the initial target, and meeting up with it required a longer flight than originally planned. Between March 2004 and January of 2014, Rosetta made three Earth flybys and one close pass by Mars, using the planets' gravity to give it a boost.

Along the way, it imaged two asteroids and endured a hibernation of two years, seven months and 12 days.

Engineers programmed four alarm clocks to wake the spacecraft from its epic slumber. Everything hinged on its ability to boot back up, said Rosetta Flight Director Andrea Accomazzo.

"Either we had a mission, or we had no mission at all," he said.

Accomazzo got the wake-up signal Jan. 20, 2014 — 40 nail-biting minutes late.

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Scientists are fascinated by comets because they believe the mysterious bodies were formed in the earliest days of the solar system, and that frozen into their icy nuclei are the same primordial materials that make up the planets.

As Rosetta closed in on its target, researchers were dazzled by the strange and unexpected shape that gradually came into focus. The comet was roughly 2.5 miles across and had two distinct lobes that resembled a rubber duck with a head, thin neck and bulbous body. In time, Rosetta's instruments revealed a dramatic world of towering cliffs, deep pits and massive boulders.

"It was a big surprise," said Claire Vallat, a scientist at the agency's European Space Astronomy Center.

After officially entering 67P's orbit on Aug. 6, 2014, Rosetta spent several months mapping the surface to find the best spot to send Philae, its 220-pound washing-machine-sized lander. The mission team ultimately selected what the late NASA scientist Claudia Alexander described as "the least-worst option" — a site that got enough sunlight to power Philae's solar panels, and that appeared to have shallower slopes and fewer boulders than other areas.

On Nov. 12, 2014, scientists were once again on the edge of their seats as Philae made a slow, seven-hour descent to the surface of 67P. It was humanity's first attempt to make a soft landing on a comet.

The unprecedented maneuver did not go exactly as planned. The lander's harpoons failed to fire and Philae bounced twice before coming to rest in what remained an unknown location for nearly two years.

It soon became clear that Philae's solar panels would not receive enough sunlight to keep powering the onboard instruments, so it was able to conduct experiments for only 60 hours before shutting down. But mission scientists insist that Philae is no failure.

"Philae sent back quite a bit of information for three days," Weissman said. "We didn't learn everything we wanted to from the lander, but we did learn a lot."

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In the meantime, Rosetta continued to orbit the comet as it made its closest approach to the sun in August 2015. From a safe distance of 186 miles, it watched as 67P became more active, with streams of dust and gas shooting off its surface. The comet's display subsided as it flew further from the sun.

"Rosetta had a major goal in mind, which was to rendezvous with a comet far from the sun and watch it wake up and then let it die down again" said Laurence O'Rourke, a lander systems engineer at ESA. "Overwhelmingly, we have met that goal."

But the drama was not over yet. Less than one month before the mission's end, Rosetta's cameras finally spotted Philae wedged into a dark crack on the comet's surface. Two of its legs were sticking up in the air.

Finding Philae after all that time "was like drinking a bottle of adrenaline," O'Rourke said. "I couldn't sleep for the whole night."

But the final death dive was not made in vain. The lander was pointed toward the Ma'at region, which is home to several intriguing pits that released jets of dust when the comet was closer to the sun and more active.

In addition, researchers have spotted lumpy structures on the pit walls that might be cometessimals, the building blocks of the comet. Scientists are eager to get a closer look.

"The plan is to go as low as possible and transmit as late as possible," Vallat said.