A mystery that has stumped scientists for decades might be one step closer to solution after ESA tracking stations carefully record signals from NASA’s Juno spacecraft as it swings by Earth today.

NASA’s deep-space probe will zip past to within 561 km at 19:21 GMT as it picks up a gravitational speed boost to help it reach Jupiter in 2016.

During the high-speed event, radio signals from the 3225 kg Juno will be carefully recorded by ESA tracking stations in Argentina and Australia.

Engineers hope that the new measurements will unravel the decades-old ‘flyby anomaly’ – an unexplained variation in spacecraft speeds detected during some swingbys.

“We detected the flyby anomaly during Rosetta’s first Earth visit in March 2005,” says Trevor Morley, flight dynamics expert at ESA’s ESOC operations centre in Darmstadt, Germany.

“Frustratingly, no anomaly was seen during Rosetta's subsequent Earth flybys in 2007 and 2011. This is a real cosmic mystery that no one has yet figured out.”

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