Comet chaser prepares for take-off

Rosetta is due to meet Comet Wirtanen in 2011. NASA

Swiss scientists are anxiously awaiting the launch of one of the most daring projects ever planned by the European Space Agency (Esa).

The mission will involve the first attempt to try to land a robot on a comet.

The Rosetta spacecraft, whose payload includes an instrument designed at Bern University, is embarking on a ten-year journey.

It will be the most detailed study of a comet ever undertaken and should reveal much about how the solar system formed.

However, if Rosetta fails to launch before the end of January, it will not be able to reach its destination and the entire mission will have to be redesigned.

Take-off has already been postponed as a result of the investigation into the loss of a new upgraded Ariane-5 rocket, which exploded on its maiden flight last month.

If all goes well, Rosetta could be launched on an older version of the Ariane-5 rocket, from Kourou, French Guiana, on or shortly after January 22.

Gravitational pull

During its trip, the spacecraft has to swing by Mars once and Earth twice, exploiting their gravity to build up the speed it needs to catch Comet Wirtanen.

If it is unable to leave Earth within the narrow launch window, the planets will be in the wrong position for it to reach the comet.

“I wasn’t really nervous until the last Ariane failed. Now I must confess I am a little bit nervous for this launch,” physicist Kathrin Altwegg of Bern University’s space research department told swissinfo.

“We will have to wait 18 months for another try and then it will be a different mission with a different comet. Otherwise, we will have to wait 80 years to have Earth and Mars in place again.”

Comet orbit

The spacecraft is due to rendezvous with comet Wirtanen in 2011, about 770 million kilometres from the Sun.

Rosetta will orbit around the comet, inspecting it thoroughly and dropping a lander onto the comet’s nucleus to study its composition and structure.

Altwegg and her team have dedicated seven years to the design and construction of one of the experiments aboard the craft.

The Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (Rosina) is designed to use mass spectrometers to identify atoms, molecules and ions in the comet’s vapour.

At 35 kilogrammes, Rosina is the heaviest instrument on board apart from the lander.

It was built in collaboration with eight institutes worldwide at a total cost of about SFr60 million ($43.35 million).

Life on Earth

Rosetta’s instruments will study the composition of the dust and gas released from the comet’s nucleus and help determine the role of comets in the origin of life on Earth.

“Rosina consists of three different sensors to analyse the chemical composition of the volatile material of the comet – water and carbon dioxide. But mostly we are interested in the organic material,” explained Altwegg.

Previous studies, notably by Esa’s Giotto spacecraft, have shown that comets contain complex organic molecules – compounds that are rich in carbon, hydrogen, oxygen and nitrogen.

These are the elements which make up nucleic acids and amino acids, the building blocks of life.

Scientists are keen to know whether comets brought water and organic material to Earth.

“We know from Giotto that there is heavy organic material but we have no clue what it is. This has to do with origin of life so we are very interested in it,” said Altwegg.

“There is even a hypothesis that the water on Earth comes from comets, and if we can look very precisely at the water on Comet Wirtanen, we might get a clue as to whether this is true or not.”

Robotic probe

Comet Wirtanen is a large, dirty snowball which orbits the Sun once every five and a half years.

The comet was discovered in 1948 by Carl Wirtanen of the Lick Observatory in California.

Rosetta is due to meet the comet at its farthest point from the Sun around November 2011.

Here in the cold parts of the solar system, where Wirtanen shows no surface activity, Rosetta will try to land a robotic probe on the comet’s icy heart which measures a mere 1.2 kilometres in diameter.

Released from Rosetta about a kilometre from the surface, it is hoped that the lander will hit the nucleus and anchor itself to the ground.

The robot’s miniaturised instruments will examine the materials and texture of the surface as well as taking photographs.

“Nobody’s tried to land on a comet before and little is known about the surface of a comet,” said Altwegg. “We don’t know if it’s hard [like ice] or soft like snow.”

In addition to dropping a lander on the comet’s surface for detailed observations, Rosetta will follow Wirtanen as it hurtles towards the Sun at speeds of up to 135,000 kilometres per hour.

Onboard instruments will analyse the dust and gas particles which emanate from the comet with increasing vigour as the Sun’s rays warm it.

Rosetta will keep orbiting the comet up to the end of the mission in July 2013 when Wirtanen is about 160 million kilometres from the Sun – its closest point.

Previous spacecraft have had only fleeting glimpses of the make-up of comets. In 1986, Esa’s Giotto made the closest approach to the nucleus of Halley’s Comet.

During a brief encounter, most observations were crammed into half an hour, and the nearest approach was 600 kilometres.

Pristine material

Comets are remnants from the birth of the solar system. They contain samples of the materials from which the Earth and the other planets were created 4.6 billion years ago.

Since then, matter on Earth and the other planets has been altered by geological, biological and chemical processes. But matter has retained its primitive form in some asteroids and comets.

“It’s pristine material from the time the solar system was made,” said Swiss mathematician, Walter Flury, who heads the group which works out the trajectory calculations at mission control – the European Space Operations Centre (ESOC) in Darmstadt, Germany.

“Comets are really very important bodies to tell about the creation and evolution of the solar system.”

Radio contact

All of the scientific data collected by the instruments on board the spacecraft are sent to Earth via a radio link. The operations centre remotely controls the spacecraft and its scientific instruments in the same way.

Rosetta will also inspect two asteroids, Otawara and Siwa, on its journey. Like comets, asteroids are micro-planets but they have no dusty tails.

The mission is named after the Rosetta Stone, whose inscriptions helped unravel the secrets of ancient Egypt.

Scientists hope that Rosetta will provide similar insights into the ancient history of space and the planets.

swissinfo, Vincent Landon

Key facts

The Rosetta spacecraft is expected to arrive at Comet Wirtanen in 2011.
Rosetta will orbit the comet, usually keeping 10-20 kilometres from the nucleus.
The spacecraft resembles a large aluminium box about 2.8 x 2.1 x 2.0 metres.
The scientific instruments are mounted on top.
Rosetta and its lander carry 21 experiments.
The spacecraft alone has cost about €600 million (SFr876 million).