Record Set for Space Laser Communication

Record Set for Space Laser Communication
By Ker Than, 05 January 2006

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.

The record-setting effort is described in the Jan. 6 issue of the journal
Science.

Does anyone know what frequency light they used for this? Also how big is
the beam spot at earth? I am guessing much bigger than the whole earth?

In article ,
rk wrote:
Does anyone know what frequency light they used for this? Also how big is
the beam spot at earth? I am guessing much bigger than the whole earth?

The best information I can find on the www is he
http://optics.org/articles/ole/9/9/2/1

Supplementary information for the published paper can be found at
http://www.sciencemag.org/cgi/content/full/311/5757/53/DC1.

Both lasers were at 1064nm, probably diode-pumped Nd:YAG lasers like the
ones in the Clementine and NEAR lidars. If I've done the arithmetic
correctly, the spot size at the test distance (about 24_Mkm) was around
2000_km. They scanned the spacecraft slowly over a raster pattern, partly
because the beam was not big enough to cover Earth and partly because
spacecraft pointing was not that accurate. (Not least, because the
attitude sensors and the laser altimeter are on opposite ends of the
spacecraft and their relative alignment was only known to about 0.2_deg --
checking that alignment was part of the purpose of the exercise.)

No data was sent, but the timing was precise enough to reveal that the
spacecraft distance as estimated by radio tracking was 52.6_m off.
--
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rk wrote:
The best information I can find on the www is he

http://optics.org/articles/ole/9/9/2/1

"The scientific instruments will make measurements for one Earth year,
the equivalent of four Mercury years. "The amount of fuel the spacecraft
carries dictates this length of time," explained Sun. "Each Mercury year
we have to do a burn to correct the orbit, and we carry enough fuel to
do at least three burns.""

I'm curious, why isn't the orbit around Mercury stable?

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"Always look on the bright side of life."
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In article ,
Russell Wallace wrote:
..."Each Mercury year we have to do a burn to correct the orbit, and we
carry enough fuel to do at least three burns.""
I'm curious, why isn't the orbit around Mercury stable?

For a guess, precession from the Sun's gravity.

For an orbit around Mercury, any sort of perturbation matters more than
you might think, because choice of orbit is an important part of
spacecraft temperature control. In particular, you typically don't want
an orbit that passes low over the center of the daylight hemisphere,
because there, half the spacecraft's sky is full of furnace-hot planet.
Perigee(*) preferably should be near the day-night line, where surface
temperatures are more moderate. Putting perigee near one of the poles
achieves that over a full Mercury year, but perturbations will make the
perigee location gradually move, requiring occasional corrections.

(I don't know, offhand, exactly what orbit is planned for Messenger, but
the above is a good guess.)

(* Pedantically it should be something like perihermion, but I'm inclined
to just use "perigee" as generic for planets. )
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(Henry Spencer) wrote in :


(* Pedantically it should be something like perihermion, but I'm inclined
to just use "perigee" as generic for planets. )

Periapsis and apoapsis? See also "apsis".

http://www.answers.com/topic/planetary-orbit


--Damon

In article ,
Damon Hill wrote:
(* Pedantically it should be something like perihermion, but I'm inclined
to just use "perigee" as generic for planets. )

Periapsis and apoapsis? See also "apsis".

*Too* generic. Sometimes it is useful to have different (unambiguous)
words for perigee and perihelion, for example,
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