In a cosmic version of laser tag, NASA's MESSENGER spacecraft and an Earth-based observatory successfully exchanged laser pulses with each other while millions of miles apart.
The feat sets a new record for laser transmission in space, a process which may one day be used to communicate across interplanetary distances and provide scientists with a powerful tool to measure the movement of planets and test fundamental principles in physics.
MESSENGER was launched in 2004 on a six-year voyage to Mercury. In late May of 2005, scientists used the spacecraft's Mercury Laser Altimeter (MLA), an instrument designed to map Mercury's surface, to exchange laser pulses with NASA's Goddard Geophysical and Astronomical Observatory in Maryland. MESSENGER was approximately 15 million miles (25 million km) away at the time.
The experiment, reported today, marks the first successful back-and-forth exchange of laser signals between Earth and space. In 1992, laser pulses were successfully transmitted from Earth and detected by a receiver aboard the Galileo probe while it was about 4 million miles (6 million km), but the transmission was only one way since Galileo did not have a transmitter of its own.
Broadband in space
Two-way laser communication in space has long been a goal for NASA because it would enable data transmission rates that are 10 to 1,000 times higher than traditional radio waves. While lasers and radio transmissions both travel at light-speed, lasers can pack more data. It's similar to moving from a dial-up Internet connection to broadband.
"We've been trying to do this kind of thing for about a decade," said David Smith, a researcher from Goddard Space Flight Center who was involved in the experiment. "We attempted to do it on one of our Mars probe but either we got weathered out or the spacecraft misread some stars and everything closed down."
The Mars Telecommunications Orbiter spacecraft, set to launch in 2010, but cancelled last summer due to budget problems, would have used lasers to transmit data between Earth and Mars at a rate of between 1 to 30 million bits per second, depending on how close the two planets are to each other.
Currently, the maximum data rate between Earth and Mars is about 128,000 bits per second.
Laser tag in space
A major challenge with laser communications in space is keeping transmitter and receiver locked onto each other. This is like trying to aim the beam of a very strong laser pointer, akin to the type used in a conference room, at a target millions of miles away.
Radio waves radiate outwards from a transmitter in spherical ripples rather than pencil-thin beams like lasers. So the receiver and transmitter in a laser-based communication system have to be pointed very precisely.
"You don't need to do that with [radiowaves]," Smith said. "The beam divergence is sufficiently large that if you point the antenna at about the right place, and if you're within half a degree, you're usually in great shape."
If the technical hurdles can be overcome, lasers would benefit not only communications, but basic science as well. Astronomers could use lasers like very accurate rulers to measure the movement of planets with unprecedented precision.
"With microwaves, we're limited to numbers like a meter or two in distance, whereas [lasers have] a potential for getting down into well beyond the centimeter range," Smith told SPACE.com.
Lasers could also be used refine basic principles of fundamental physics.
"If you could make planetary scale measurements at the centimeter or millimeter level--which we can't at the moment--then we could understand some principles of relativistic physics which can only be tested at very extreme accuracies at very large distances," Smith said.
The record-setting effort is described in the Jan. 6 issue of the journal Science.
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