The past, the present, and the challenges ahead for Europe's comet chaser

The European Space Agency's Rosetta probe has been chasing Comet 67P/Churyumov-Gerasimenko for 10 years. It is set to become the first mission to rendezvous with a comet, the first to land on a comet, and the first to follow a comet around the Sun.

Ariane liftoff Rosetta was sent into space on board an Ariane 5G rocket in March 2004
Has Rosetta's target changed?

Rosetta was originally due to launch in 2003 aimed at a different, much smaller comet called 46P/Wirtanen.

However, the failure of an Ariane 5 rocket attempting to put a communications satellite into orbit in 2002 meant that the Rosetta mission was delayed until the fault was found. Instead of the tiny 46P/Wirtanen, the mission was re-focussed on the larger 67P/Churyumov-Gerasimenko.

Rosetta key facts

  • Total cost of the mission is said to be 1.3bn euros (£1bn)
  • The probe weighed 3,000kg at lift-off back in 2004, with over half of that made up of propellant
  • It has two 14m-long solar panels to provide electrical power
  • The orbiter carries 11 experiments
  • The lander, Philae, carries nine experiments including a drill to sample beneath the comet's surface

After a 10-year chase through space, Rosetta is now almost upon comet 67P.

The thrust of this mission is to catch up with the comet while it is still in the colder regions of space and to put a lander on the icy body as it begins to warm up on its journey in towards the Sun.

How does a satellite survive a space trip of 6bn km?

The use of advanced solar-panel technology has been key to making sure the spacecraft is able to power itself throughout this massive journey.

Swinging around the Sun five times, the probe has had to cope with extreme heat and cold, sometimes operating in parts of the Solar System where levels of sunlight are only 4% of what we receive on Earth.

solar panels These 14m-long solar panels have kept Rosetta powered up during its 10-year flight

When it has travelled closer to the Sun, Rosetta has been prevented from overheating by special radiators. It has employed multi-layered insulation blankets to fight off the cold.

Controllers also put the probe into a deep hibernation for almost 1,000 days as a way of conserving energy. When Rosetta finally woke up in January this year, the relief on the faces of Esa staff was plain to see.

How will Esa ensure Rosetta doesn't miss the comet?

Comet 67P is hurtling through space at a speedy 55,000km/h (over 15km per second).

Rosetta has been gaining on it, over the last decade, through a series of three gravity-assisted swings around the Earth and one around Mars.

To ensure that Rosetta is now in the right place at the right time, controllers have carried out a series of 10 orbit-correction manoeuvres over the past two months, in which the probe's speed has been significantly reduced relative to the comet. One of these involved firing the probe's thrusters for almost eight hours.

Rosetta's speed relative to the comet was 2,880km/h, but at the time of orbital insertion it will be walking pace, around 3km/h.

The icy target - Comet 67P/Churyumov-Gerasimenko

  • It was first observed in 1969 and named for the scientists who made the discovery, Klim Churyumov and Svetlana Gerasimenko
  • It orbits the Sun once every 6.5 years, travelling between the orbits of Jupiter and the Earth
  • It is approximately 4km (2.5mi) wide
  • The comet appears to be boot-shaped with a thin, brighter neck joining two rougher and darker parts of the nucleus
  • The surface temperature appears to be around -70C, which is warmer than expected

On Wednesday the final thruster burn will take place, putting the probe into a triangular form of powered orbit, 100km above the "ice mountain".

What comes after the rendezvous?

All this is only the first, tricky step.

The ambitious mission will also see Esa attempt to put a tiny lander, called Philae, on the surface of the comet in November. It will carry out the most comprehensive set of tests ever performed on the composition of these ancient objects.

Before sending Philae, Rosetta itself will carry out an analysis of the comet's surface and lower itself into the gravity of 67P, at a separation of about 30km.

Then the carbon-fibre lander, about the size of fridge, will be dropped to the rocky surface from about 1km. It will use harpoons to attach itself. Philae is designed to be able to land on a slope of up to 30 degrees. Esa says the feet of the lander are equipped with large pads to allow it to touch down on a soft surface, but these feet will also try to place screws into the icy surface.

ESA Delight at the moment Esa controllers realised that Rosetta had woken from a long slumber

If the surface is very soft, the lander's feet may sink into it but sinking will eventually be stopped by the bulkiness of the lander's body. In all scenarios, the lander is expected to be able to safely transmit its data.

It will then use a suite of nine instruments, including a special drill, to probe the icy body in unprecedented detail.

What big scientific questions will Rosetta attempt to answer?

Once the probe goes into orbit around the comet it will carry out a range of scientific examinations of the so called "dirty snowball", over the year they will spend in tandem as the comet approaches the Sun.

As the comet gets closer to our star, it will warm up and a stream of dust and water vapour will flow from it. Rosetta will try to find out if there are organic chemicals in the mix, that might have provided the elements necessary to start life on Earth.

lander An artist's impression of the Philae lander, seen from the comet surface with Earth in the background

Rosetta will also try to answer some slightly less profound questions about the origins of comet 67P, and its distinctive rubber duck shape.

Scientists hope that Rosetta will discover if it is, in fact, two comets that came together in a collision.

If the lander, Philae, touches down successfully it will also attempt to analyse the ice on the surface. If it has the same atomic composition as the water that fills Earth's oceans, it will strengthen the view that comets delivered the liquid to our planet.

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