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Countdown to touchdown

Oli Usher10 November 2014

Comet C-G, seen by Rosetta’s NAVCAM on 6 November 2014. Philae’s landing site is towards the top of the image. Credit: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0

After a decade of travelling around the Solar System, the Rosetta probe is now at its destination: Comet 67-P/Churyumov-Gerasimenko (or Comet C-G to its friends).

The Rosetta mission is made up of two parts which have spent the last decade bolted together: the orbiter, and the lander, known as Philae.

Just after 9am GMT on Wednesday, Philae will separate from the mother ship and begin its descent to the comet’s surface. Around seven hours later, if all goes well, it will touch down on C-G’s rough surface.

This will be the first ever landing on a comet.

The gravitational force between two objects is directly proportional to their masses and the distance between them. Philae, at around 100kg, weighs much the same as a (large) human being, but the comet has a tiny fraction of the Earth’s mass. The pull between them is therefore minuscule – of the order of the gravitational force experienced by an object weighing just one gram on Earth.

Even though Philae will only be approaching Comet C-G at walking pace, the low gravity means it will need to attach itself to the surface with a harpoon to avoid bouncing back into space.

Because of this, the manoeuvre has been compared to a ‘docking’ rather than a ‘landing’.

UCL’s Prof Andrew Coates is a member of the Rosetta Plasma Consortium, which will be monitoring the plasma environment of the comet during Philae’s descent and landing. (He was also closely involved with the design and construction of Rosetta’s scientific payload.) He will be at mission control in Darmstadt on Wednesday as the lander begins its descent.

“The Rosetta orbiter and lander provide unique perspectives on how comets interact with the solar wind and on charged dust from the surface. The historic landing attempt will be a huge opportunity for coordinated observations,” says Prof Coates.

Comet seen over Rosetta's solar array, 14 October 2014. Credit: ESA/Rosetta/Philae/CIVA

Comet seen over Rosetta’s solar array, 14 October 2014, when the comet was around 16km away. Credit: ESA/Rosetta/Philae/CIVA (All rights reserved)

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Rosetta landing site chosen

Oli Usher15 September 2014

Rosetta landing site

The landing site for Philae, the lander component of the Rosetta mission, has been chosen and is marked here with a white cross. Photo credit: ESA

The Rosetta mission, which for the past decade has been on a long and convoluted journey to Comet C-G, has recently reached its destination. It is the only artificial object ever to enter orbit around a comet, and is currently circling around it at an altitude of around 30km. (The cometary nucleus itself is around 4km across.)

Part of Rosetta’s mission is to measure the properties of the plasma (electrically charged gas) that surrounds the comet. To this end, the spacecraft features a suite of five sensors built by the Rosetta Plasma Consortium, a scientific collaboration that includes UCL’s Prof Andrew Coates.

But as well as measuring the plasma around the comet, Rosetta will attempt something never achieved before: it will release a lander that, later this year, will touch down on the comet’s surface. The European Space Agency has today announced the site that the lander, known as Philae, will aim for: a spot known as Site J, pinpointed in the photo above with a white cross. The landing site was chosen as the best compromise between safety (the surface of the comet is uneven in places and could damage the probe) and scientific interest (some parts are more active than others).

Copyright: ESA images are free to use providing they are credited, do not imply endorsement by ESA, do not feature identifiable individuals, and are not used in advertising or promotional materials.

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