After 786 days in orbit around comet 67P, and more than 12 years after it was launched from Earth, the European Space Agency's Rosetta probe made its final maneuver this morning: a slow, but terminal, descent onto the comet's dusty surface. Even at such a leisurely pace, many people would call this a crash landing, but, for reasons that will become clear, this is a term that scientists involved in the mission avoided.
Call it an "attitude perturbation"
Instead of "crash landing," we heard about the spacecraft's "final descent" and its "grand finale." We were told that the spacecraft would simply "touch the surface" lading to an "attitude perturbation" before it's "passivated" (or switched off) for good. Some of these are technical terms and some of them are used for drama, but it does all seem a little euphemistic. As if we're telling a sobbing child that Hammy the Hamster has "gone to heaven," rather than admitting that he's residing, in pieces, in the guts of the neighbor's German Shepherd.
Well, there's good reason for this.
For a start, this really was a soft landing. If you remember when Rosetta launched the Philae probe onto 67P, the smaller craft bounced on the comet's surface. This is because 67P is relatively tiny (about the size of a decent mountain) and its gravity is therefore weak — several hundred thousand times weaker than Earth's. Add that to the fact that Rosetta is traveling at the same speed as a dawdling tourist blocking up the sidewalk and you have what is best characterized as a low-energy event.
The design of Rosetta adds to this. Science journalist Emily Lakdawalla pointed out on Twitter that Rosetta's 32-meter-wide solar arrays will take some of the sting out of the landing. They extend like a pair of arms out of the main body of the craft, so when Rosetta hit the comet, it was like a bat or a bird landing hard on the ground, slapping its wings down to soften the impact.
Another reason for avoiding the term "crash landing" is that it implies an accident of some sort, while Rosetta's descent was thoroughly intentional. Comet 67P is headed further and further away from the Sun, and the probe's solar panels are losing power. It had just enough juice for one final move, and it transmitted scientific data all the way down — sending its last image back to Earth just meters from the surface.
Even the target area was chosen carefully, with the landing orchestrated to get the best images possible of a feature of comet geography known as "positive relief features" — meter-sized goose bumps on 67P's surface. It's thought that these might be some of the original lumps of ice that came together to form the comet some 4 billion years ago. If so, they would be some of the very oldest objects known to humanity, dating back to the very formation of our Solar System.
The last reason in the don't-call-it-a-crash-landing thesis is a little more emotional. Data-gathering aside, there's actually no reason to land Rosetta. It's not designed to land on things, and the scientists involved in the mission could easily have let it float away, sending it with the comet or even into space until it could transmit no more. But it would have been a thin, drawn-out sort of ending. Not messy, but lacking color. Anemic.
"Landing it is more a psychological thing."
As the European Space Agency's spacecraft operations manager Andrea Accomazzo told The Guardian: "We could have abandoned it in space or let it bounce off the comet and just switched it off. It wouldn’t have created any problem [...] Landing it is more a psychological thing."
In other words: it's about closure. Some of the scientists involved in Rosetta have been working on the project for nearly 20 years. Planning and designing, waiting and working. Remember the footage from 2014, showing Professor Monica Grady jumping for joy at the news that Philae had landed on the comet? That's what this all means to people! Add together all these elements — the slow landing, the data collection, the psychological weight — and it's clear why this wasn't a crash landing. It really was a grand finale.
Update September 30th, 8:09AM ET: The story has been updated after Rosetta's landing was confirmed.