365 days: Nature's 10

CGI Illustration by Peter Crowther Associates c/o Dbut Art

Andrea Accomazzo: Comet chaser | Suzanne Topalian: Cancer combatant | Radhika Nagpal: Robot-maker | Sheik Humarr Khan: Ebola doctor | David Spergel: Cosmic sceptic | Maryam Mirzakhani: Surface explorer | Pete Frates: Ice-bucket challenger | Koppillil Radhakrishnan: Rocket launcher | Masayo Takahashi: Stem-cell tester | Sjors Scheres: Structure solver | Ones to watch

A former test pilot steered the Rosetta mission to an icy world in deep space. By Elizabeth Gibney

Andreas Reeg/Agentur Focus/Eyevine

Nearly two decades ago, Andrea Accomazzo got into trouble with his girlfriend when she found a scrap of paper on his desk. In his handwriting was scrawled a phone number next to a female name: Rosetta.

She thought it was a girl, says Accomazzo. I had to explain to my jealous Italian girlfriend that Rosetta is an interplanetary mission that is flying to a comet in almost 20 years.

Ever since, Accomazzo has divided his attention. He eventually married his girlfriend and has also spent the past 18 years pursuing the comet 67P/ChuryumovGerasimenko. As flight director for the mission, Accomazzo led the team that steered Rosetta to its August rendezvous with the comet, following a 6.4-billion-kilometre journey from Earth. The pinnacle of the project came in November, when Rosetta successfully set down a lander named Philae, providing scientists with the first data from the surface of a comet and making it one of the most successful missions in the history of the European Space Agency (ESA).

Accomazzo did not act alone: it took a large operations team at ESA to manoeuvre Rosetta with enough precision to drop Philae down just 120 metres from the centre of the landing zone. Given that we'd had a 500-metre error circle, that was not a bad shot, says Fred Jansen, who led the mission. When Philae's anchoring systems failed, the craft bounced into a shady site where it could not charge its solar panels, so the lander lost power after 64 hours. But in that time, it gathered a trove of data that will add to the information collected by Rosetta about the comet's structure and composition. Armed with those insights, scientists hope to better understand the origin and evolution of the Solar System, including whether comets could have brought water and organic molecules to Earth during its infancy.

Accomazzo started off his career focused on a different type of flight. He first trained as a test pilot in the Italian Air Force. But although he loved flying, he found the culture too constraining and after two years he quit to study aerospace engineering. With his quiet, hard-working, sometimes no-nonsense nature, colleagues say that Accomazzo brings a bit of the military with him into mission control.

For Accomazzo, the biggest parallel between flying a fighter jet and Rosetta is the need for split-second judgements. You have to prepare and train a lot to be able to make the right decision, very quickly, he says. Between launch and landing, his team ran 87 full-day simulations.

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365 days: Nature's 10