Monday, September 15, 2014

Audacious comet landing site chosen

Europe's Rosetta mission, which aims to land on a comet later this year, has identified what it thinks is the safest place to touch down.

Scientists and engineers have spent weeks studying the 4km-wide 'ice mountain' known as 67P, looking for a location they can place a small contact robot.

They have chosen what they hope is a relatively smooth region on the smaller of the comet's two lobes.

But the team is under no illusions as to how difficult the task will be.

Comet 67P/Churyumov-Gerasimenko, currently sweeping through space some 440 million km from Earth, is highly irregular in shape.

Its surface terrain is marked by deep depressions and towering cliffs.

Even the apparently flat surfaces contain potentially hazardous boulders and fractures.

Avoiding all of these dangers will require a good slice of luck as well as careful planning.

The plan still is to make the landing attempt on 11 November.

The Rosetta probe will despatch its piggybacked Philae robot from a distance of about 10km to 67P.

This spider-like device will then hope to engage the surface at 'walking pace', deploying screws and harpoons in an effort to lock itself down to an object that has very little gravitational attraction.

The European Space Agency (Esa) says it will be a one-shot opportunity. Events will be taking place so far away that real-time radio control will be impossible.

Instead, the process will have to be fully automated with the final commands uploaded to Rosetta and Philae several days in advance.

The choice of landing site follows a weekend of deliberations in Toulouse, France.

Mission team-members gathered to assess the latest imagery downlinked from Rosetta, which has been closely tracking 67P now since early August.

Five potential landing locations were on the table, and these have now been reduced to just two - a primary and a back-up.

Comet 67P/Churyumov-Gerasimenko

  • Named after its 1969 discoverers Klim Churyumov and Svetlana Gerasimenko

  • Referred to as a 'Jupiter class' comet that takes 6.45 years to orbit the Sun

  • Orbit takes it as close as 180 million km from the Sun, and as far as 840 million km

  • The icy core, or nucleus, is about 4km (2.5 mi) across and takes 12.4 hours to rotate

Both will be studied further in the coming weeks before a final go/no-go decision is made in mid-October.

The favoured location is identified for the moment simply by the letter 'J'.

On 67P's smaller lobe, it has good lighting conditions, which for Philae means having some periods of darkness to cool its systems.

The back-up site is situated on the larger of 67P's lobes. Its designation through the selection process has been the letter 'C'.

It hosts a range of surface features, including depressions, cliffs and hills, but - crucially - many smooth plains, also.

More detailed mapping of J and C is ongoing.

This past week, Rosetta manoeuvred into an orbit just 30km from the 67P, enabling its camera system to see details that can be measured on the centimetre scale.

Such information only has a certain usefulness, however, as the 'hands-off' landing can only be targeted with a best precision that will likely run to many tens of metres.

And that error is larger than any of the apparently smooth terrains on the reachable parts of the comet

The whole separation, descent and landing procedure is likely to take several hours.

If Philae gets down successfully into a stable, operable configuration, it would represent a historic first in space exploration.

But Esa cautions that this high-risk venture should really be seen as an 'exciting extra' on the Rosetta mission.

The major objective from the outset has been to catch the comet with the Rosetta probe and to study it from orbit.

This is happening right now. The spacecraft's array of remote-sensing instruments are currently investigating the comet's properties, endeavouring to find out how the object is constructed and from what materials.

'Everything we've discovered at 67P/C-G so far says that we've chosen a fantastic comet to visit,' said Dr Christopher Carr, a principal investigator on the Rosetta Plasma Consortium instruments.

'There's a genuine sense of excitement within the Rosetta community, and we're all looking forward to the year ahead.

'No spacecraft has ever orbited an active comet before, so there's a lot to learn about spacecraft and instrument operations, but we've got a really robust mission carrying some of the best instrumentation possible, and I have to say that the operations teams at the European Space Agency are doing a great job - they are true professionals,' the Imperial College London scientist told BBC News.

But, of course, an in-situ analysis of the surface chemistry would be a huge boon to the mission overall, and this is what Philae aims to provide.

It will carry a drill to pull up comet samples into an onboard laboratory.

And, indeed, any surface information gathered by Philae will provide important 'ground truth' for Rosetta's remote sensing observations.

Irrespective of the outcome on 11 November, Rosetta will continue to follow 67P for at least a year.

The probe will get a grandstand view of the comet as it warms on a swing around the Sun.

67P's ices will vaporise, throwing jets of gas and an immense cloud of dust out into space.