Published online 4 March 2008 | Nature | doi:10.1038/news.2008.640


Anomalies spotted in spacecraft flight

Could a mystery force be altering their course?

Rosetta is amongst the craft that were given a strange bump on their way past Earth.ESA

The speed and direction of some spacecraft are being mysteriously altered as they pass near the Earth. This unexplained velocity shift hints that there is either something missing from current models used for space navigation, or, possibly, something missing from our understanding of physics.

A team from NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California, analysed six flybys by five different spacecraft between 1990 and 2005. Mission planners staged the flybys in order to use Earth's gravity as a slingshot, catapulting the spacecraft to other destinations in the Solar System.

Transponders on board the craft and instruments on the ground are used to track one component of a craft's velocity as it whizzes past Earth. In five out of the six cases studied, this speed was inexplicably shifted faster or slower than expected as the craft passed the point of closest approach to the Earth. The results appear in the 7 March edition of Physical Review Letters1.

In every case the change in speed was of the order of millimetres per second: a tiny fraction of the crafts' typical speeds of kilometres per second, but large enough to be well beyond the margins of error for the measurements, says John Anderson, a retired astronomer now at NASA's Jet Propulsion Laboratory, who led the study. "It's small, but you can see it clearly in the data," he says.

Weird pull

This is not the first time that Anderson and his collaborators have spotted a strange anomaly in spacecraft velocity. An earlier analysis of the Pioneer 10 and 11 spacecraft, which are among the furthest-flung man-made instruments, showed that the two seemed to be inexplicably slowing down as they sped out of the Solar System. Some believe that this so-called 'Pioneer anomaly' might be due to a previously unrecognized facet of gravity, while others suspect the crafts' power supplies might be behind the problem.

Anderson says that he believes there may be a relationship between the new flyby data and his previous work on Pioneer. "I think that they might be connected," he says. "Anomalies like this just don't happen that often."

Most are sceptical about the problem being due to anything more than human error, or something perfectly ordinary not yet included in the model.

Room for error

Trevor Morley, an engineer at the European Space Agency's (ESA) operations centre in Darmstadt, Germany, notes that the trajectories of spacecraft flybys are calculated using detailed models that must incorporate a lot of phenomena, including the pull of other planets, relativistic effects (changes in time and length for objects travelling near the speed of light), and even the radiation pressure of sunlight striking the craft. One of these may be incorrectly accounted for, or there may be an effect missing from the equations entirely.

The data used to measure craft velocity also requires complex processing to interpret. So the changes could well be down to an error "buried deep in the dynamics or signal modelling", Morley says.


Morley, who heads navigation for one of the affected spacecraft — ESA's mission to the comet Rosetta — says that his team has so far been unable to find the source of the velocity shift seen in their craft. "We scratched our heads for quite a long time", but in the end, he says, it is not a high priority: the velocity shift was too small to require a course correction. "It's absolute peanuts," he says.

Morley adds that some European physicists are interested in studying the phenomenon further, and he applauds their efforts. "The chances that this brings new physics is extremely small," he says. "But you never know." 

  • References

    1. Anderson, J. D., Campbell, J. K., Ekelund, J. E., Ellis, J. & Jordan. J. F. Phys. Rev. Lett. 100, 091102 2008). | Article |
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