[meteorite-list] Historic Rosetta Mission to End with Crash Into Comet

[Ron Baalke]

http://www.nature.com/news/historic-rosetta-mission-to-end-with-crash-into-comet-1.18713

Historic Rosetta mission to end with crash into comet

There were other options, but super close-up shots on descent will provide 
science bonanza.

Elizabeth Gibney
nature.com
04 November 2015

A year since a probe called Philae made history by touching down on a 
comet, the team that pulled off the feat is plotting a different kind 
of landing. Next September, the European Space Agency will crash Philae's 
mothership Rosetta into the icy dust ball, but as gently as possible.

The dramatic act will bring the mission to an abrupt end - and give Rosetta's 
wealth of sensors and instruments their closest view of the comet yet. 
"The crash landing gives us the best scientific  end-of-mission that we 
can hope for," says Rosetta project scientist Matt Taylor.

The collision will be emotional for the scientists, some of whom have 
worked on the mission since its inception in 1993. "There will be a lot 
of tears," says Taylor.

Launched in 2004, the Rosetta orbiter caught up with the comet 67P/Churyumov-Gerasimenko 
ten years later as the rock was travelling from deep in space towards 
the Sun - and dropped Philae onto the surface a few months later, on 12 
November. Scientists have not heard from Philae since July, and don't 
know if they will do so again, but Rosetta's operations to survey the 
comet from orbit are in full swing. However, the orbiter can't keep up 
this work indefinitely. Funding for the mission runs out in September 
2016 - and by that time 67P/Churyumov-Gerasimenko will be well on its 
way back out into deep space, where the solar-powered orbiter will receive 
too little sunlight to function.

Discussions about what to do with Rosetta when that happens have continued 
for more than a year. Rosetta flight director Andrea Accomazzo says that, 
ideally, Rosetta would hibernate while the comet remains in deep space, 
then be resurrected when 67P again approaches the Sun in 4 or 5 years' 
time. But the cold of deep space would probably damage the craft, Accomazzo 
says; others fear that fuel and other resources would run out. Moreover, 
many of the mission's principal investigators (PIs) began their work more 
than 20 years ago and "there's no point putting an old experiment with 
old PIs into hibernation", jokes Kathrin Altwegg, a planetary scientist 
at the University of Bern.

Crash-landing Rosetta emerged as the preferred option last year, but only 
now are orbiter navigators and operators working out how to go about it. 
Rosetta's closest encounter with the comet so far was from 8 kilometres 
above the surface, when it dispatched Philae. The current thinking sees 
Rosetta spiral down to a similar distance next August before creeping 
ever closer in elliptical orbits and crashing in September, says mission 
manager Patrick Martin - but that could still change.

Although Philae sent back some data during its descent, Rosetta has more 
powerful - and more varied - sensors and instruments. The orbiter will 
also descend much more slowly than Philae did, allowing it to gather more 
data and better pictures. Once it gets to 4 kilometres, for example, Rosetta 
should be able to distinguish between the gases emerging from each of 
the duck-shaped comet's two lobes to determine whether the regions vary 
in composition, says Altwegg, who leads the team behind ROSINA (the Rosetta 
Orbiter Spectrometer for Ion and Neutral Analysis). That could shed light 
on the environments in which each was formed.

Rosetta's cameras will get their best-resolution shots of the comet's 
surface yet - less than 1 centimetre per pixel once the craft is within 
500 metres of the surface, adds Holger Sierks, PI for Rosetta's OSIRIS 
(Optical, Spectroscopic, and Infrared Remote Imaging System). This will 
allow researchers to look at surface properties and link these to comet 
activity that Rosetta has observed from orbit.

Over and out

How far into the descent Rosetta will be able to send data back to mission 
control will depend on whether engineers can design the final trajectory 
such that the craft crashes on the side of the comet that faces Earth. 
Navigating while close to the comet will be difficult because the body's 
gravitational field is uneven, but spacecraft-operations manager Sylvain 
Lodiot hopes that the orbiter will transmit until the very end.

The crash will definitely be a hard stop to the mission, he says, however 
gentle the landing. Designed to manoeuvre in orbit, once Rosetta is on 
the comet's surface it will no longer be able to point its antenna to 
communicate with Earth. Similarly, it will not be able to angle its solar 
array, so it will lose power, says Lodiot. "Once we touch, hit or crash, 
whatever you want to call it, it's game over."

Before then, though, the mission still has much to accomplish. As the 
comet approached the Sun, it heated up, with vaporizing ice causing more 
and more gas and dust to stream from its surface. Rosetta had to retreat 
into a wider orbit to stop the dust from confusing its navigation system. 
But now that the comet is speeding away from the Sun, mission scientists 
are relishing the opportunity to steer Rosetta back in. Priorities will 
then be to get images that would enable comparisons of the comet before 
and after its swing around the Sun, as well as a close-up of the southern 
hemisphere, which was largely in darkness until May and will disappear 
back out of view in March.

Rosetta will also resume listening out for Philae. Given the huge public 
interest in anything to do with the lander, Rosetta's finale will make 
for a fitting end to the story, adds Altwegg. "This way Rosetta gets to 
live happily ever after on the comet with Philae."

Nature 527, 16–17 (05 November 2015)
doi:10.1038/527016a