A Spacecraft Is About to Make the Closest Comet Rendezvous of All Time

The ESA's Rosetta mission will tell us more about comets than we ever knew before, including how they may have brought water to Earth.

Update August 6: The ESA announced that Rosetta successfully reached its destination this morning as planned, becoming the first spacecraft to rendezvous with a comet.

On Wednesday, a European Space Agency spacecraft will bravely venture where no spacecraft has before, and have a close encounter with a comet. After a six-billion-kilometre journey, comet chaser Rosetta will get up close and personal with Comet 67P/Churyumov–Gerasimenko, with Wednesday marking the last manoeuvre to get it into position so that it's ready to orbit the comet nucleus.

Ahead of the event, I spoke to ESA project scientist Matt Taylor, who admitted that there was naturally some apprehension around this critical step in the mission, which was approved way back in 1993 and launched into space in 2004, but said everything was looking good so far. 

"There are little bumps and wiggles as one would expect from a spacecraft that old, and also it was one of the longest hibernations that has ever been carried out," he said. When Rosetta "woke up" in January after its 31-month slumber everything looked good, and Taylor said that's still the case.

After Rosetta has reached its target, it will study the comet in much greater detail than ever before. Previous missions, like the Giotto spacecraft's dalliance with Comet Halley, were quick fly-bys, giving only a comparatively simple glance. But Rosetta will ride alongside Churyumov-Gerasimenko for over a year, even dropping a lander onto its surface for a hands-on inspection. Taylor explained it will therefore track "how the comet evolves from a reasonably inert object, as it is now, to its fully-fledged cometness."

Rosetta took its best picture yet of the comet on August 1, from a distance of about 1,000 kilometres using its narrow angle camera, the results of which have earned Churyumov-Gerasimenko the nickname of "rubber duck."

The closest picture of the comet so far. Image: ESA

"This is a quantum leap forward in terms of cometary science—that we'll characterise a comet in unprecedented detail by doing all of this," said Taylor. "It's not just a snapshot; we'll see the dynamical evolution of the object and its interaction with the sun."

Exploring a comet in such close-up is particularly cool because it could tell us more about our corner of the universe in general. "In the grand scheme of things we look at comets as being kind of primordial leftovers from the beginning of the solar system," said Taylor. The theory goes that comets were left over after planets were formed, along with other small bodies like asteroids. But they're now way out on the edges of the solar system, in places like the Oort cloud and the Kuiper belt.

That essentially means that material from round about when the planets were formed is "deep frozen" out there in the comets. "It's really like grabbing hold of a time capsule that gives you a window to the past, looking at how the solar system formed."

Thanks to Rosetta's instruments, they'll look at what the comet is made of, among many other measurements. It will no doubt contain some ice, which is of particular interest owing to the popular (but not universally accepted) idea that comets may have brought water to the Earth way back when there were no oceans on our home planet. Taylor explained that they can compare the water in the comet to that on the Earth by looking at the particular isotopes it contains.

Back in 2011, ESA researchers claimed to have found the first comet with water containing a similar ratio of deuterium to hydrogen as that in the Earth's oceans. Comet 103P/Hartley 2 was in the Jupiter family of comets; so is Churyumov-Gerasimenko.

Taylor goes even further, suggesting that if comets could have been a water delivery mechanism to the Earth, they may also have delivered some of the fundamental "building blocks" behind DNA and proteins; complex molecules that could explain more about how our Earth ended up as it is today.

However, the first temperature measurement made by Rosetta rules out that the comet could be entirely covered in ice. With a surface temperature of -70 degrees Celsius,   the UK Space Agency said it must instead have a mainly dusty surface. But there's definitely water there—back in June it was steaming off two glasses' worth of vapour a day.

On Wednesday, Rosetta will be slowed down so that it's in step with the comet, though still at a distance of about 100km away. In a blog post, ESA's Daniel Scuka explains it will be travelling at around 55,000 km per hour, but will have a relative velocity to the comet of just one metre per second. It'll then start mapping the comet, both to gather data about it, and to start preparing where it should drop its lander, called Philae, which will be the next major stage of the mission in November.

While there will no doubt be many crossed fingers this week, most of the action is pre-set, which is particularly necessary considering Rosetta's now so far away it would take somewhere around 20 minutes to get a signal to it—and the same again to get one back. 

"But every time you fire a thruster or every time you do anything on the spacecraft, you're always nervous that everything's going to go according to specification," Taylor conceded.