Posted on 01/05/2006 5:12:38 PM PST by blam
Longest laser link bridges the gulf of space
19:00 05 January 2006
NewScientist.com news service
Jeff Hecht
Messenger blasted off in 2004, and will begin a yearlong orbital study of Mercury in March 2011 (Artist's impression: NASA)
A laser communication link has been made across a record 24 million kilometres (15 million miles), between the Messenger spacecraft and instruments on Earth.
The craft and the ground station transmitted pulses back and forth to each other, and although no actual information was transmitted, the experiment shows the potential for interplanetary laser links, says David Smith of NASA's Goddard Space Flight Center in Maryland, US.
Interplanetary space probes currently communicate via microwaves, but those transmitters are not as tightly focused as laser beams. This spreading reduces microwave power received, and thus the maximum data rate. For example, NASA's Mars Odyssey probe can send only 128,000 bits per second to Earth.
Because laser beams spread much more slowly, they can deliver more power to ground-based optical receivers, allowing higher data rates. This advantage was clear as far back as 1960 when Theodore Maiman – who had just made the first laser – listed space communications as an important potential application.
However, laser beams proved so narrow that space communications only worked if moving spacecraft were precisely tracked. This difficulty meant the development of practical laser links has taken decades.
Pinpoint accuracy
It was only in late 2005 that the first laser link was demonstrated between two satellites in different Earth orbits: the low-orbit Japanese Kirari satellite and the European Artemis satellite in geosynchronous orbit. NASA had planned laser communications for its Mars Telecommunications Orbiter, but budget cuts forced cancellation of the mission that was to launch in 2009.
The laser carried by Messenger is not in fact designed as a communications tool, but as an altimeter that will map the surface of Mercury starting in 2011. It will do this by timing the return of laser pulses fired toward the planet.
The immediate purpose of the interplanetary tests, performed in May 2005 but reported in Science on Thursday, was to check the altimeter was working and to precisely measure its orientation. At 24 million kilometres distance, the laser beam pulses sent by Messenger had expanded to about 1900 kilometres wide, so they were only received at Goddard when the laser was pointed almost directly at it.
At the same time, a ground-based laser fired pulses toward Messenger, timed so the satellite receiver would spot the pulses during the intervals when it was looking for the light reflected from its own laser. The tests also yielded precise measurements of the spacecraft's range, clock timing, and movement.
Journal reference: Science (vol 311, p 53)
Really? Which? I'd not heard about that.
Record Set for Space Laser Communication
space.com | 01/05/06 | Ker Than
Posted on 01/05/2006 5:06:18 PM PST by KevinDavis
http://www.freerepublic.com/focus/f-news/1552749/posts
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