Too speedy for gravity?

Ya got trouble, my friend. Trouble with a capital T and that rhymes with G and that stands for gravity.

MYSTERY KICK. When the Near-Earth Asteroid Rendezvous mission (pictured) flew past Earth on Jan. 23, 1998, it received a kick the standard theory of gravity may not explain. NASA, JHU APL

A new analysis suggests that when five different spacecraft flew past Earth several years ago, they gained more speed than can be accounted for by Einstein’s theory of gravitation.

The unexplained gain in speed is tiny, between 1.8 and 13.5 millimeters per second—only about one-millionth the total velocity of the spacecraft examined. But with radar tracking able to clock spacecraft speeds as small as 0.1 mm per second, that excess is enough to warrant further scrutiny, assert John D. Anderson of NASA’s Jet Propulsion Laboratory in Pasadena, Calif., and his collaborators in the March 7 Physical Review Letters.

The largest speed deviations occurred during the flyby of NASA’s Near Earth Asteroid Rendezvous craft. Anderson’s team proposes that Earth’s rotation is somehow imparting an extra kick to the craft. The kick might be similar to, but much larger than, an effect predicted by Einstein’s theory in which rapidly spinning bodies warp surrounding space and drag orbiting objects with them.

A decade ago, Anderson and other researchers reported that the two Pioneer spacecraft seemed to be heading out of the solar system at speeds too high to be explained by the standard theory of gravity, but that claim remains controversial. Anderson says that because the new findings include a multitude of craft in the easier-to-measure near-Earth environment, the results appear to be on firmer footing.

With other, more mundane explanations, such as errors in software tracking, still to be explored, “it’s way too early to get excited about this,” says Ron Hellings of Montana State University in Bozeman.

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