From: Matthew Brown <[EMAIL PROTECTED]> Date: 17 September 2004 12:19:39 PM PDT Subject: Mars laser telecoms
http://www.newscientist.com/news/news.jsp?id=ns99996409
A laser that can beam data from Mars to Earth at 10 times the rate of
current radio links will be sent to the Red Planet in 2009, say NASA
scientists. The laser will be the first test of such technology in deep
space and may usher in a new era of space communication.
"It is the next big thing," says Stephen Townes, deputy manager of the Mars
Laser Communication Demonstration at NASA's Jet Propulsion Laboratory in
Pasadena, California. "There is the promise we will be able to get high data
returns with lower power and lower mass than the typical systems out there."
NASA's Mars Odyssey spacecraft currently boasts the highest data
transmission rate at 128,000 bits per second. The new laser will beam back
between one million and 30 million bits per second, depending on the
distance between Mars and Earth.
That leap in capacity is due to the different wavelengths of light carrying
the data. The laser will use infrared light with a wavelength of 1.06
microns, which is thousands of times shorter than radio waves. Since all
light travels at the same speed through space, shorter wavelengths carry
more information in the same time.
That is crucial for the increasing number of ambitious space missions, says
Joss Bland-Hawthorn, head of instrument science at the Anglo-Australian
Observatory in Sydney.
"Astronomers are losing vast amounts of data from recent satellite missions
to Mars," he told New Scientist. "We collect a hundred times more than we
can transmit back."
Cloudy skies
But so-called optical communication has certain drawbacks compared with
time-tested methods. Unlike radio waves, clouds can block the laser's
photons. And laser beams are narrower than the distended radio wave cones
that wash over the entire Earth, making precise pointing of the laser
important.
The new Mars laser project will use two different sites to detect the laser
on Earth - the 5-metre Hale Telescope in southern California and an array of
four 0.8-metre telescopes whose location has yet to be determined. If the
weather is overcast at one location, astronomers can try the next. Future
projects are likely to have a dozen telescopes spaced around the world.
The laser will be transmitted from a 0.3-metre telescope on a spacecraft
orbiting Mars, but the beam will spread to a width of a few hundred
kilometres by the time it reaches Earth. That makes picking up the laser's
relatively weak signal "very difficult," says Townes, adding that the team
is developing photon detectors that are orders of magnitude more sensitive
than those now used.
Live footage
Despite the signal's weak strength, "optical astronomers are quite alarmed
about the prospect of this being generalised", says Tomas Gergely,
electromagnetic spectrum manager at the National Science Foundation in
Arlington, Virginia, US.
A few satellites orbiting Earth are already testing the technology to talk
to each other or to send data to ground-based telescopes. A glut of such
nearby lasers could bounce off the foil shrouds covering satellites and
create light pollution, explains Gergely.
Bland-Hawthorn agrees that space-based lasers can cause unwanted light but
adds that astronomers on Earth already cause similar problems by using laser
dots for telescope calibration. He says that one day space agencies could
send live video footage - which requires data rates of billions of bits per
second - from Jupiter to Earth.
The new Mars laser, allocated $270 million from NASA, will undergo a design
review in early 2005 and will fly on NASA's Mars Telecommunications Orbiter
in 2009. That spacecraft, which will also have traditional radio links, will
be the first mission designed mainly for communication between other space
missions.
