Monday, September 15, 2014

Where to Land? Comet Landing Site Chosen for ESA's Rosetta Spacecraft

Context image showing the location of the primary landing site for Rosetta’s lander Philae.  Site J is located on the head of Comet 67P/Churyumov–Gerasimenko. An inset showing a close up of the landing site is also shown. The inset image was taken by Rosetta’s OSIRIS narrow-angle camera on 20 August 2014 from a distance of about 67 km. The image scale is 1.2 metres/pixel. The background image was taken on 16 August from a distance of about 100 km. The comet nucleus is about 4 km across. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

The dice is cast: Rosetta’s lander Philae will target Site J, an intriguing region on the 'head' of Comet 67P/Churyumov–Gerasimenko. When the Philae lander touches down on 11 November 2014, the comet will have a landing site waiting for it with a varied but not too rugged landscape offering good solar illumination and hardly any steep slopes. In a two-day selection process, the lander team under the leadership of the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) selected landing site 'J' from among five possible candidates. The region on the head of the two-part comet was chosen after careful evaluation of all available data. "As we look at the current close-up images, we see that Comet 67P/Churyumov-Gerasimenko is a beautiful yet very extreme world; it is scientifically exciting, but has a shape that produces a big challenge for landing," says DLR researcher Stephan Ulamec, Project Manager for the Philae lander. "None of the five candidates satisfied 100 percent of the criteria, but landing site 'J' is clearly the best solution."

The decision to select Site J as the primary site was unanimous. The backup, Site C, is located on the ‘body’ of the comet.

“We will make the first ever in situ analysis of a comet at this site, giving us an unparalleled insight into the composition, structure and evolution of a comet,” says Jean-Pierre Bibring, a lead lander scientist and principal investigator of the CIVA instrument at the IAS in Orsay, France.

“Site J in particular offers us the chance to analyse pristine material, characterise the properties of the nucleus, and study the processes that drive its activity.”

It will take around seven hours from the time Philae separates from the ESA Rosetta mother craft until, for the first time ever, a lander will be on the surface of a comet. This amount of time allows the engineers and scientists to ensure that too much electrical energy is not consumed during the descent and so, immediately afterwards, implement a sophisticated plan to operate all 10 instruments.

Close-up of Philae’s primary landing site J, which is located on the ‘head’ of Comet 67P/Churyumov–Gerasimenko. The image was taken by Rosetta’s OSIRIS narrow-angle camera on 20 August 2014 from a distance of about 67 km. The image scale is 1.2 metres/pixel. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
Close-up of Philae’s primary landing site J, which is located on the ‘head’ of Comet 67P/Churyumov–Gerasimenko. The image was taken by Rosetta’s OSIRIS narrow-angle camera on 20 August 2014 from a distance of about 67 km. The image scale is 1.2 metres/pixel. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

During this 'First Science Phase', the lander, which is controlled and monitored from the Lander Control Center (LCC) at DLR Cologne, will be powered by its batteries. Subsequently, an average of seven hours of sunlight per cometary 'day' will allow the batteries to be repeatedly recharged. This is why the team selected a 'sunny' location. The region is scientifically interesting in particular because of the expected activity of the comet as it continues along its path towards the Sun. There are depressions in the vicinity of landing site 'J' that may already be active.

In addition to trajectory calculations and the first results from the instruments on board the Rosetta spacecraft, the planning also incorporates data from, for example, a number of landing tests conducted at DLR Bremen. During the spring of 2013, engineers there used a full-size model to test landings on both hard and soft ground in the LAMA Landing and Mobility Test Facility. Computer simulations have enabled the challenges posed by the various landing sites to be investigated.

A detailed operational timeline will now be prepared to determine the precise approach trajectory of Rosetta in order to deliver Philae to Site J. The landing must take place before mid-November, as the comet is predicted to grow more active as it moves closer to the Sun.

“There’s no time to lose, but now that we’re closer to the comet, continued science and mapping operations will help us improve the analysis of the primary and backup landing sites,” says ESA Rosetta flight director Andrea Accomazzo.

“Of course, we cannot predict the activity of the comet between now and landing, and on landing day itself. A sudden increase in activity could affect the position of Rosetta in its orbit at the moment of deployment and in turn the exact location where Philae will land, and that’s what makes this a risky operation.”

By committing to a landing site and thus enabling more refined trajectory calculations, the engineers at the LCC, among others, can adjust and optimise the sequence for the descent and landing. This will be carried out autonomously by Philae – descending to the comet and preparing for its operations on 67P/Churyumov-Gerasimenko without intervention from Earth. The activities of the flight unit are tested in advance using a ground reference model of the lander in the LCC. The commands for Philae and its ten instruments are sent from the LCC to the European Space Operations Centre (ESOC) in Darmstadt and from there via ground stations to Rosetta and Philae. The telemetry from Philae passes in the opposite direction via ESOC to the LCC.

Context image of Comet 67P/Churyumov–Gerasimenko with the original five candidate landing sites for Rosetta’s lander Philae, and with the backup, Site C, indicated.  The regions are marked on OSIRIS narrow-angle camera images taken on 16 August from a distance of about 100 km. The comet nucleus is about 4 km across.  The sites were assigned a letter from an original preselection of 10 possible sites identified A to J. The lettering scheme does not signify any ranking. On 13 September, Site J was chosen as the primary, with Site C as the backup. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
Context image of Comet 67P/Churyumov–Gerasimenko with the original five candidate landing sites for Rosetta’s lander Philae, and with the backup, Site C, indicated.  The regions are marked on OSIRIS narrow-angle camera images taken on 16 August from a distance of about 100 km. The comet nucleus is about 4 km across.  The sites were assigned a letter from an original preselection of 10 possible sites identified A to J. The lettering scheme does not signify any ranking. On 13 September, Site J was chosen as the primary, with Site C as the backup. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

On 15 October 2014, the landing site selection will be finally confirmed or a decision will be made to switch to the backup site. "Now we have a place to land, so we are preparing for the big day," says Stephan Ulamec.

Once the lander touches down, at the equivalent of walking pace, it will use harpoons and ice screws to fix it onto the surface. It will then make a 360° panoramic image of the landing site to help determine where and in what orientation it has landed.

The initial science phase will then begin, with other instruments analysing the plasma and magnetic environment, and the surface and subsurface temperature. The lander will also drill and collect samples from beneath the surface, delivering them to the onboard laboratory for analysis. The interior structure of the comet will also be explored by sending radio waves through the surface towards Rosetta.

“No one has ever attempted to land on a comet before, so it is a real challenge,” says Fred Jansen, ESA Rosetta mission manager. “The complicated ‘double’ structure of the comet has had a considerable impact on the overall risks related to landing, but they are risks worth taking to have the chance of making the first ever soft landing on a comet.”

The landing date should be confirmed on 26 September after further trajectory analysis.

Rosetta is an ESA mission with contributions from its Member States and NASA. Rosetta’s Philae lander is provided by a consortium led by DLR, MPS, CNES and ASI. Rosetta is the first mission in history to rendezvous with a comet. It will escort the comet as it orbits the Sun and deploys a lander.

Comets are time capsules containing primitive material left over from the epoch when the Sun and its planets formed. By studying the gas, dust and structure of the nucleus and organic materials associated with the comet, via both remote and in situ observations, the Rosetta mission should become the key to unlocking the history and evolution of our Solar System, as well as answering questions regarding the origin of Earth’s water and perhaps even life.

Credit: dlr.deESA

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