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."
Laser
ruler
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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