The Rosetta spacecraft was scheduled to launch in February 2003 when its rocket rudely exploded while carrying a different satellite. The European Space Agency was stuck with an expensive craft, an approaching launch date, and no ride. It was time for new plan.

Rosetta was scheduled to catch a lift into orbit with the then-newly-designed Ariane 5 in February 2003, when another launch using the rocket went poorly and was remotely detonated to avoid a catastrophic crash. The European Space Agency wasn't about to risk their precious comet-explorer on a rocket before they understood what went wrong, so the launch date was shoved back to allow time for a thorough investigation. Rosetta was going to miss its window for a leisurely trajectory to intercept comet Wirtanen, leaving mission scientists scrambling to come up with a new plan.

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The options were simple:

  1. Stick with comet Wirtanen, and go for more intense orbital maneuvers to catch up to it for the same intercept-date.
  2. Stick with comet Wirtanen, but delay launch by six years to intercept it at a later date.
  3. Redirect to comet Howell, taking a fuel-demanding trajectory.
  4. Redirect to comet Tempel 2, taking a trajectory through the inner solar system.
  5. Redirect to comet Churyumov-Gerasimenko, a larger comet.

In December 2002, mission designers and engineers debated the options. Getting to Wirtanen on the same timeline would require a far bigger and more expensive rocket, axing that plan. Mothballing a delicate piece of expensive equipment for years was too risky, even with careful high-tech storage options. Zipping over to comet Howell would require ridiculous amounts of fuel. Heading to comet Tempel 2 would bring the spacecraft dangerously close to the sun and outside of the designed heat-tolerance specifications. Comet Churyumov-Gerasimenko is larger than Wirtanen, complicating approach and rendezvous using hardware and software designed for a smaller nucleus, but was reachable with a sane rocket size and fuel capacity, and wouldn't bring the spacecraft painfully close to the sun.

Deciding that a complicated landing dance was the best option, the European Space Agency selected Churyumov-Gerasimenko. Figuring out how to get Rosetta from here to there was no easy task, requiring a complex decade-long trajectory using three Earth and one Mars flybys to get the spacecraft to the comet in an efficient manner. Rosetta made her revised launch date in March 2004, catching a ride on the now-approved Ariane 5 rocket. After a music video competition to wake up the spacecraft and its travelling-companion comet-lander Philae, Rosetta got a first peek at the destination comet. After a long wait, the spacecraft will encounter Churyumov-Gerasimenko in May of this year.

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Images credit: ESA. Read more about the complicated orbital dance necessary to get Rosetta to Churyumov-Gerasimenko.