A spacecraft has sent a laser signal to Earth from 24 million km (15 million miles) away in interplanetary space. The Messenger probe exchanged laser signals with a US ground station partway through its journey to the planet Mercury.

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.

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.”

Don’t look up! Don’t look!