A spacecraft has sent a laser signal to Earth from 24 million km (15 million miles) away in interplanetary space.
Messenger is en route to Mercury
The Messenger probe exchanged laser signals with a US ground station partway through its journey to the planet Mercury.
Messenger blasted off in 2004 on a mission to map the surface of the nearest planet to the Sun.
After arriving in 2011, the probe will orbit Mercury for a year to explore its atmosphere, composition and structure.
The US space agency (Nasa) craft is equipped with a laser altimeter that will map the topography of Mercury by timing the return of laser pulses fired at the planet.
The instrument and the ground station in Maryland transmitted laser pulses back and forth in a test of the payload carried out in May.
David Smith of the Nasa Goddard SpaceFlight Centre in Greenbelt, Maryland, US, and colleagues, reported the experiment in the latest edition of the journal Science.
"This experiment has demonstrated subnanosecond laser pulse timing and accomplished a two-way laser link at interplanetary distance," they said.
"In addition, it established a distance record for laser transmission and detection."
The scientists say the experiment shows the potential of laser in deep space communications.
Most current missions use microwave transmitters but laser beams allow higher data rates.
The only other deep-space ranging demonstration took place in 1992, when two ground-based lasers were detected by cameras on the Galileo spacecraft from a distance of 6 million km (3.7 million miles).