Mission to intercept a comet

• In operation
• Launched 2 March 2004
• Due to end in 2015

The European Space Agency (ESA) mission comprises a large orbiter - designed to operate for a decade at large distances from the Sun - and a small lander called ‘Philae’. Each of these carries a large complement of scientific experiments designed to complete the most detailed study of a comet ever attempted. After releasing its lander, Rosetta will spend two years orbiting Comet 67P Churyumov-Gerasimenko as the comet heads towards the Sun.

By studying comets, scientists hope to gain an understanding of what conditions were like as the Solar System formed. Comets also hold clues to the origins of life on Earth. Much of the water on Earth was brought here by comets and it is likely that the complex organic molecules that formed the basis for life also came from cometary debris.

Even though it will not arrive at its destination until 2014, Rosetta has already contributed to our understanding of comets. In July 2005, Rosetta monitored the collision of an ‘impactor’ probe, released from the Deep Impact spacecraft, with the surface of Comet 9p/Tempel 1.

Observations of the plume of debris ejected from the impact suggest that comets are ‘icy dirtballs’ rather than ‘dirty snowballs’ as was previously believed. When the washing machine-sized impactor hit, at around 37,000 km per hour, 4,500 tonnes of water were released but even more dust.

For full details see ESA’s Rosetta mission pages.

Mission facts

• Rosetta is named after the Rosetta stone, which helped scholars decipher hieroglyphics used in ancient Egypt. Just like its namesake, the spacecraft will help to unravel the mysteries of the past.
  
  
• The Rosetta mission involves a number of significant ‘firsts’. It will be the first spacecraft to:
  
  
  • orbit a comet’s nucleus
  • fly alongside a comet as it heads towards the inner Solar System
  • examine from close proximity how a frozen comet is transformed by the warmth of the Sun
  
  
• Rosetta will use the gravitational pull of Earth (three times) and Mars (once) to propel itself towards its final destination. The latest Earth swing-by took place in November 2007. Rosetta will be back in our neighbourhood for its final Earth swing-by in November 2009.
  
  
• Many of the manoeuvres Rosetta will need to perform during its journey are so complex that there is absolutely no room for error for the team controlling the spacecraft.
  
  
• When it arrives in 2014, Rosetta will scan the surface of comet 67P Churyumov-Gerasimenko for a suitable landing spot. The probe will then be released towards the surface.

Technology

The Rosetta mission is made up of an orbiter and a small lander. Between them, they carry a total of 20 scientific instruments.

Orbiter

The orbiter is centred around a large aluminium box (2.8 x 2.1 x 2 m). It contains 11 instruments which will enable scientists to build up a clear picture of the comet's nature:

• ALICE (Ultraviolet Imaging Spectrometer) will analyse gases in the comet and its tail. It will measure how quickly the comet produces water, carbon monoxide and carbon dioxide.
  
  
• CONSERT (Comet Nucleus Sounding Experiment by Radiowave Transmission) will build up a 3-D image of the nucleus and provide information on the internal structure and density of the comet.
  
  
• COSIMA (Cometary Secondary Ion Mass Analyser) will be used to analyse the characteristics of dust grains emitted by the comet. Scientists hope to discover whether the dust contains organic molecules.
  
  
• GIADA (Grain Impact Analyser and Dust Accumulator) will measure how quickly the dust from the comet travels and which direction it is going. A further instrument, MIDAS (Micro-Imaging Dust Analysis System) will also study the dust.
  
  
• OSIRIS (Optical, Spectroscopic and Infrared Remote Imaging System) has cameras which will take high-resolution pictures of the comet's nucleus.
  
  
• ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis) will investigate what makes up the comet's atmosphere.
  
  
• RPC (Rosetta Plasma Consortium) will examine the structure of the comet, monitor its behaviour and study how it interacts with particles in the solar wind – the stream of charged particles coming from the Sun.
  
  
• RSI (Radio Science Investigation) will measure the mass, density and gravity of the nucleus of the comet by monitoring radio signals.
  
  
• VIRTIS (Visible and Infrared Thermal Imaging Spectrometer) will be used to investigate the surface of the comet to find a suitable landing place for the Rosetta probe.

Lander

The box-shaped Philae lander is being carried on the side of the orbiter until it arrives at Comet 67P Churyumov-Gerasimenko.

Once the orbiter is aligned correctly, the lander will be commanded to self-eject from the main spacecraft and unfold its three legs to prepare for a gentle touchdown.

On landing, the legs will absorb most of the kinetic energy to reduce the chance of bouncing. They can also rotate, lift or tilt to return the lander to an upright position.

Immediately after touchdown, a harpoon will be fired to anchor the lander to the ground and prevent it escaping from the comet’s extremely weak gravity.

The lander is designed to last for one week but surface operations may continue for many months.

The lander carries nine experiments and a drilling system to take samples of sub-surface material. One of the instruments, Ptolemy, is built in the UK. Ptolemy will analyse gases that make up the comet. It will also compare water ice found in the comet with major bodies of water on Earth to see if they have any of the same properties.

UK involvement

Rosetta is a mission with significant UK involvement from industry and science.

One of the main challenges for all the companies designing instruments for Rosetta has been to ensure the components remain intact for ten years, while the spacecraft makes its way to the comet, and then work perfectly when it gets there. Not an easy task!

Astrium Limited, based in Stevenage, was the major subcontractor for the Rosetta platform.

AEA Battery Systems Limited provided innovative batteries for the spacecraft and lander. These are smaller, lighter and much more reliable than the traditional nickel-cadmium batteries.

AEA Technology, European Space Tribology Laboratory (ESTL) developed the Micro-Imaging Dust Analysis System (MIDAS).

Technology created by Logica helped to explore some of the issues involved in such a long mission. The company was also involved in the development of the Rosetta on board software.

Polyflex Space Ltd provided tanks to store the helium used by the lander.

STFC Rutherford Appleton Laboratory managed to reduce a lab full of chemistry equipment to enable it to fit into a space the size of a shoebox.

SciSys (Space & Defence) Limited has developed the software systems that will be used to communicate with Rosetta.

Surrey Satellite Technology Limited (SSTL) designed a wheel that will stabilise the probe as it descends and lands on the comet.

VEGA Group plc was involved in many aspects of the Rosetta mission, from the overall design of the spacecraft to the on board software.

UK scientists are involved in ten of the 21 experiments that Rosetta will carry out during its mission.

The Open University is leading the team for the Ptolemy instrument on the lander.

Armagh Observatory will be helping to analyse the results from the OSIRIS instrument.

Imperial College London and University College London’s Mullard Space Science Laboratory (MSSL) supply the team studying the comet's plasma.

Scientists at Oxford University are part of the science team for VISTIS.

Queen Mary College at the University of London will be investigating the results of the CONSORT instrument.