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Glitch hits Juno as it seeks to solve gravity riddle

Paul Sutherland, Feature writer
Oct 10, 2013, 7:00 UTC

Sen—NASA’s Jupiter probe Juno unexpectedly put itself into safe mode as it swooped past the Earth last night, a flyby that scientists hope might help solve a long-standing mystery.

To mission controllers' surprise, they found that the spacecraft had "switched off" during the close encounter. This is a precaution by the robotic probe if it senses there might be a problem and is not seen as a long-term issue for the $1.1 billion mission.

European Space Agency tracking stations across the world monitored the flyby which, as we reported yesterday, would bring the robotic probe swinging just 560 km (350 miles) over South Africa at 19.21 UT.

The racing visit was a manoeuvre practised several times before by space missions to use Earth’s gravity to give a probe a slingshot-style boost in speed on its journey to a distant target.

Puzzlingly, some probes have experienced a puzzling variation in their speed during the flyby that has never been satisfactorily explained. Equally oddly, a European spacecraft called Rosetta, which has zipped past the Earth three times, showed the anomaly on one occasion but not on the others.

Signals sent from 3,225 kg Juno were recorded by ESA tracking stations in Argentina and Australia and will be closely studied by scientists to see if they show some problem with our understanding of gravity.

They were much weaker than expected, due to the probe going temporarily into safe mode. During its encounter with Earth, Juno had flown round the night side, preventing sunlight from reaching its vast solar panels and there is speculation that this change might have triggered the probe's change in status.

Juno also recorded several images of the Earth and Moon during its approach and the mission team plan to release these as a unique movie later.

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

The ESA tracking station at Malargüe, Argentina

The ESA tracking station at Malargüe, Argentina. Credit: ESA/S. Marti

“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.”

Since 1990, mission controllers at ESA and NASA have found that spacecraft sometimes experience a strange variation in the amount of orbital energy they pick up from flybys of Earth.

The unexplained variation is noticed as a tiny difference in the expected speed gained (or lost) during the passage. They are extremely small variations. NASA’s previous Jupiter probe Galileo ended up just 3.9 mm a second faster than expected when it swung past Earth in December 1990.

The largest variation came with NASA’s NEAR asteroid craft in January 1998 which gained a boost of 13.0 mm/s. On the other hand, the differences during swingbys of NASA’s Cassini in 1999 and Messenger in 2005 were so small that they could not be confirmed.

Engineers and the flight dynamics teams at ESOC were watching closely as figures from the Juno flyby came in from ESA’s new 35 metre-diameter deep-space dish in Malargüe, Argentina, and a smaller 15 metre dish in Perth, Australia. They began tracking more than three hours before Juno’s closest approach.


Juno's track across the night side of the Earth when the Sun was blocked. Credit: NASA/JPL

The stations recorded highly precise radio-signal information that will indicate whether Juno speeds up or slows down more or less than predicted by current theories. Results will be closely studied by ESA and NASA scientists as well as other researchers around the world.

ESA’s Daniel Firre, who is responsible for the tracking support at ESOC, said before the event: “Our Malargüe station is designed to track very distant and relatively slow-moving spacecraft, while Juno will pass by moving very, very fast at just 561 km altitude.

“This makes tracking Juno technically very challenging, but it’s how the scientific process works. Gathering more data that can be analysed by experts is critical if we are ever to solve this perplexing mystery.”