ON 8 December 1990, NASA’s flew past the Earth on its roundabout path to Jupiter. It was the first of two such manoeuvres, designed to boost its speed and give it sufficient oomph to reach its destination. The probe skimmed just 960 kilometres above the cloud tops at its closest point before slingshotting back into deep space. Everything went to plan – almost. As Galileo raced away from Earth, NASA’s space-flight engineers began to scratch their heads. To their bafflement, the probe was travelling 3.9 millimetres per second faster than it had any right to be travelling. That may seem a paltry amount, but in fact such a change is startling.
And Galileo is not alone. Almost every spacecraft that has swung around the Earth to speed it on its journey into space has recorded an inexplicable velocity change. The biggest, in 1998, affected NASA’s spacecraft, whose speed was boosted by an additional 13.5 millimetres per second. “The fly-by anomalies appear to be real,” says John Anderson, who recently retired from (JPL) in Pasadena, California. “Yet we don’t have a clue what’s causing them.”
The findings may call into question our understanding of gravity. Could these anomalies yield the first tangible connection between quantum mechanics and relativity, a link that theorists would dearly love to find? Or is there some mysterious, unseen body of matter tugging at our spacecraft? Whatever the answer, these anomalies may compel us to re-examine the forces at work in space.
Planetary fly-by manoeuvres were first proposed in 1961 by Mike Minovitch, then a summer student at JPL. They…
