Speedy impacts send microwave distress calls. (Physics).

Earth-orbiting space junk zips along so fast that even small pieces striking the International Space Station and other satellites could cause serious damage. Based on laboratory experiments, a Japanese team now reports that these hypervelocity impacts emit microwaves. The finding suggests a new way to remotely detect space-junk hits, say engineers who study such impacts.

Researchers have long known that hypervelocity impacts give off bursts of heat and visible light. Those occur because the kinetic energies of the colliding objects convert rapidly to heat, setting aglow the materials involved. However, earlier attempts to measure other electromagnetic emissions from such impacts produced ambiguous results, says Tadashi Takano of the Institute of Space and Astronautical Science in Sagamihara.

In the new search for a microwave signature of collisions, he and his colleagues equipped a vacuum chamber with an antenna tuned to a midrange microwave frequency, 22 gigahertz. Then they placed aluminum plates of different thicknesses in the chamber and fired bulletlike projectiles of nylon and metal at the plates at speeds of more than 14,000 kilometers per hour.

Within microseconds after each collision, the antenna picked up multiple bursts of microwaves, the scientists say. They present their observations in the Nov. 1 Journal of Applied Physics. The pattern of microwave pulses suggests that the emissions may arise from the collapse of metallic-crystal lattices rather than from rapid heating, the researchers speculate.

If other experiments confirm the microwave releases, the phenomenon might provide the basis for simple debris-impact detectors using modified cell-phone equipment, comments Eric L. Christiansen of NASA's Johnson Space enter in Houston.--P.W.