The European Space Agency’s comet-chasing Rosetta mission is set to launch from Kourou in French Guiana early on Thursday morning.
The 3000-kilogram spacecraft should have launched in January 2003. But ESA officials had to postpone the mission after a beefed-up version of Rosetta’s launch rocket – Ariane 5 – blew up a month before the launch date.
The delay forced the entire mission to be redesigned, with a new target called Comet 67P/Churyumov-Gerasimenko or Chury, which it will reach in 2014. The original target was Comet Wirtanen.
“It’s been a tough year,” ESA science director David Southwood told a news briefing in Kourou. But he says the delay was necessary because “it’s important we have an utterly reliable system to get to space”.
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This is the first mission designed to orbit and land on a comet’s nucleus. Understanding Chury’s composition will give clues to the make-up of the early Solar System as comets are thought to be flying relics of primordial material.
Rosetta is now launching from a generic Ariane-5 that has been tweaked to allow the spacecraft to sail away from Earth, rather than going into orbit around it.
Delayed ignition
The Ariane 5 has three components – a large “main stage” that carries the rocket’s engine and electrical equipment, two solid rocket boosters that provide the bulk of the thrust for lift-off and flank the main stage, and an “upper stage” at the top of the rocket that carries the payload – in this case, Rosetta.
In the 16 previous launches of Ariane 5, the rocket’s upper stage – which steers the payload into the correct orbit – has ignited right after it separated from the main stage.
On 26 February, the main stage will separate as usual about 10 minutes after lift-off – splashing down west of the Galapagos Islands. But the upper stage will then coast for about an hour and 45 minutes – circling the globe once – before igniting.
The upper stage will also rotate, as if being spun on a barbecue, so the Sun’s heat will spread evenly over the spacecraft as it floats freely.
This delayed ignition is “fundamental” for Rosetta to escape Earth’s gravitational grip and begin its 10-year journey, says spacecraft operations manager Paolo Ferri, who will be directing the launch from the European Space Operations Centre in Darmstadt, Germany.
Cone-like tip
The final countdown will begin nearly 12 hours before the launch, which is currently scheduled for 0736 GMT, or 0436 local time on 26 February. In those last hours, the electrical, telemetry, tracking, and command systems will be inspected and the main stage will be filled with liquid oxygen and hydrogen and cooled.
If Rosetta fails to launch on Thursday the launch can be reattempted once per day until 17 March. After that, Comet Chury will have moved too far for Rosetta to rendezvous with it and the mission might be called off.
Seven minutes before launch a synchronized sequence of 1000 commands, managed by the control station and onboard computers, will kick in. Four seconds before the main stage engine ignites, onboard systems take over.
Two pyrotechnic devices will ignite the main engine, and seven seconds later, the two booster rockets will also fire, lifting the rocket off the ground in just a third of a second. The launcher should climb vertically for six seconds after lift-off before it rotates and veers east.
About two minutes into the flight the booster rockets will fall away, and a minute or so later the cone-like tip of the rocket that protects the spacecraft will also be shed. The main stage will be jettisoned 10 minutes after lift-off and then the upper stage will coast with the spacecraft.
Then about 20 minutes after the upper stage ignites – two hours and 13 minutes into the flight, Rosetta will separate from the launcher and begin its interplanetary voyage.
Cube-shaped lander
“This is the most crucial moment of the early mission, when we are expecting the first signal from the spacecraft,” Ferri told New Scientist. Critically, it is also when the two 14-metre-long solar panels are supposed to deploy. These should be released when a heated knife cuts through six cables that keep each panel folded. The craft relies on the solar panels for energy and can run its instruments for only a few hours on battery power.
During its ten-year exodus, Rosetta will fly three times around the Earth and once around Mars – using the gravitational fields of these planets to sling it into the right path for its meeting with Chury.
A few months after the spacecraft reaches its target, it will release a small cube-shaped lander on to the comet’s icy nucleus. The orbiter will then spend two years circling Chury as it heads back towards the Sun.
