Apollo 11 Experiment Still Going Strong after 35 Years

http://www.jpl.nasa.gov/news/features.cfm?feature=605

Apollo 11 Experiment Still Going Strong after 35 Years
Jet Propulsion Laboratory
Media contact: Charli Schuler (818) 393-5467
July 20, 2004

It was the summer of '69. Director John Schlesinger's "Midnight Cowboy"
had won the Oscar for Best Picture; the Rolling Stones' newly released
"Honky Tonk Women" was climbing the charts; 400,000 people were gearing
up to attend Woodstock?and America landed on the Moon, making "one giant
leap for mankind."

On the afternoon of July 20, 1969, Apollo 11 astronauts Neil Armstrong
and Edwin "Buzz" Aldrin explored the surface of the Moon for two and a
half hours, collecting samples and taking photographs while Michael
Collins orbited in the command module Columbia. On July 21, about an
hour before the end of their final moonwalk, they left an experiment on
the lunar surface which, after 35 years, continues to work as well as it
did the day it got there. Called the lunar laser ranging experiment, it
studies the Earth-Moon system and returns data to scientific centers
around the world, including NASA's Jet Propulsion Laboratory.

"An accurate knowledge of the Moon's orbit and orientation is needed for
future robotic and manned missions to our satellite," said Dr. James G.
Williams, one of four JPL scientists who analyze the data from the Lunar
Laser Ranging Experiment. "Scientists have been able to use the data
they received through lunar laser ranging to study the Earth, the Moon
and the character of gravity."

Scientists from various institutions who analyze the data from the lunar
laser ranging experiment have observed, among other things, that the
Moon is moving away from the Earth and has a fluid core, and that
Einstein's Theory of Relativity is accurate.

The experiment consists of an instrument called the lunar laser ranging
reflector, designed to reflect pulses of laser light fired from the
Earth. The idea was to determine the round-trip travel time of a laser
pulse from the Earth to the Moon and back again, thereby calculating the
distance between the two bodies to unprecedented accuracy. Unlike the
other scientific experiments left on the Moon, this reflector requires
no power and is still functioning perfectly after 35 years.

The Apollo 11 laser reflector consists of 100 fused silica half cubes,
called corner cubes, mounted in a 46-centimeter (18-inch) square
aluminum panel. Each corner cube is 3.8 centimeters (1.5 inches) in
diameter. Corner cubes reflect a beam of light directly back toward its
point of origin; it is this fact that also makes them so useful in Earth
surveying.

Three more reflectors have since been left on the Moon, including two by
later Apollo 14 and 15 missions and one (built by the French) on the
unmanned Soviet Lunokhod 2 rover. Each of the reflectors rests on the
lunar surface in such a way that its flat face points toward the Earth.

The McDonald Observatory in Western Texas and a second observatory near
the city of Grasse in southern France regularly send a laser beam
through an optical telescope to hit one of the reflectors. The
reflectors are too small to be seen from Earth, so even when the beam is
correctly aligned in the telescope, actually hitting a lunar reflector
is quite challenging. At the Moon's surface, the beam is a few
kilometers or miles wide and scientists liken the task of properly
aiming the beam to using a rifle to hit a moving dime 3.2 kilometers
(two miles) away.

Once the laser beam hits a reflector, scientists at the observatories
use sensitive filtering and amplification equipment to detect any return
signal. The reflected light is too weak to be seen with the human eye,
but under good conditions, one photon -- the fundamental particle of
light -- will be received every few seconds.

The lunar laser ranging experiment is the only lunar investigation
continuously operating since the Apollo project. Improvements in lasers
and electronics over the years have led to measurements currently
accurate to about 2 centimeters (less than one inch).

Scientists know the average distance between the centers of the Earth
and the Moon is 385,000 kilometers (239,000 miles), implying that the
modern lunar ranges have relative accuracies of better than one part in
10 billion. This level of accuracy represents one of the most precise
distance measurements ever made and is equivalent to determining the
distance between Los Angeles and New York to one hundredth of an inch.

"Technical improvements at the observatories rejuvenate the lunar laser
ranging effort," Williams said. "When the range accuracy improves, it is
like getting a new experiment on the Moon."

To this end, a new lunar ranging instrument with significantly improved
accuracy is being constructed at Apache Point Observatory in New Mexico
by the University of California at San Diego and the University of
Washington.

"The usefulness of continued improvements in range determinations for
further advancing our understanding of the Earth-Moon system will keep
the lunar reflectors in service for years to come," Williams said.

I have a couple of small question about the laser reflectors.

We know abouyt the corner prisms, and the reflectors are postioned to point
at the earth.

Was there any need for design tolerances so that the actual angle of the
whole reflector assembly was optimised to point at the eart, or was this
simply unneccessary?

Did the astronauts simply position them in a "that looks close enough" way,
or was there some method to the procedure?

I have a couple of small question about the laser reflectors.

We know abouyt the corner prisms, and the reflectors are postioned to point
at the earth.

Was there any need for design tolerances so that the actual angle of the
whole reflector assembly was optimised to point at the eart, or was this
simply unneccessary?

Did the astronauts simply position them in a "that looks close enough" way,
or was there some method to the procedure?

Wally Anglesea wrote:

I have a couple of small question about the laser reflectors.

We know abouyt the corner prisms, and the reflectors are postioned to
point at the earth.

Was there any need for design tolerances so that the actual angle of the
whole reflector assembly was optimised to point at the eart, or was this
simply unneccessary?

Did the astronauts simply position them in a "that looks close enough"
way, or was there some method to the procedure?

The whole reason they used "Corner Cubes" is that "Looks close enough" works
with them, any light falling onto them from a reasonable angle (margin of
20-30 degrees if I remember corrrectly).
Besides, if they had to point extremely accurately, the experiment couldn't
have kept on working, as the moon is continually wobbling, so much so that
in a telescope you can easily see the perspective changing (see
http://www.stargazing.net/david/moon/moonlibration.html ) which would turn
the reflector a considerable ways away from where it was originally
pointing.

Bart

Wally Anglesea wrote:

I have a couple of small question about the laser reflectors.

We know abouyt the corner prisms, and the reflectors are postioned to
point at the earth.

Was there any need for design tolerances so that the actual angle of the
whole reflector assembly was optimised to point at the eart, or was this
simply unneccessary?

Did the astronauts simply position them in a "that looks close enough"
way, or was there some method to the procedure?

The whole reason they used "Corner Cubes" is that "Looks close enough" works
with them, any light falling onto them from a reasonable angle (margin of
20-30 degrees if I remember corrrectly).
Besides, if they had to point extremely accurately, the experiment couldn't
have kept on working, as the moon is continually wobbling, so much so that
in a telescope you can easily see the perspective changing (see
http://www.stargazing.net/david/moon/moonlibration.html ) which would turn
the reflector a considerable ways away from where it was originally
pointing.

Bart

In article ,
Wally Anglesea wrote:
Was there any need for design tolerances so that the actual angle of the
whole reflector assembly was optimised to point at the eart, or was this
simply unneccessary?
Did the astronauts simply position them in a "that looks close enough" way,
or was there some method to the procedure?

There was a systematic procedure for getting the thing pointed roughly
correctly (and likewise for the ALSEP radio antenna, which was also
moderately directional), but no requirement that it be exact. Corner-cube
reflectors reflect light back in the direction it came from, independent
of which direction that is (within limits).
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |

In article ,
Wally Anglesea wrote:
Was there any need for design tolerances so that the actual angle of the
whole reflector assembly was optimised to point at the eart, or was this
simply unneccessary?
Did the astronauts simply position them in a "that looks close enough" way,
or was there some method to the procedure?

There was a systematic procedure for getting the thing pointed roughly
correctly (and likewise for the ALSEP radio antenna, which was also
moderately directional), but no requirement that it be exact. Corner-cube
reflectors reflect light back in the direction it came from, independent
of which direction that is (within limits).
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |

Henry Spencer wrote:



There was a systematic procedure for getting the thing pointed roughly
correctly (and likewise for the ALSEP radio antenna, which was also
moderately directional), but no requirement that it be exact. Corner-cube
reflectors reflect light back in the direction it came from, independent
of which direction that is (within limits).


You'd already know the correct elevation (assuming you were on fairly
flat ground) based on the latitude of the chosen landing site.

Pat

Henry Spencer wrote:



There was a systematic procedure for getting the thing pointed roughly
correctly (and likewise for the ALSEP radio antenna, which was also
moderately directional), but no requirement that it be exact. Corner-cube
reflectors reflect light back in the direction it came from, independent
of which direction that is (within limits).


You'd already know the correct elevation (assuming you were on fairly
flat ground) based on the latitude of the chosen landing site.

Pat

Henry Spencer wrote:
Corner-cube
reflectors reflect light back in the direction it came from, independent
of which direction that is (within limits).

How accurate do the corner cube angles have to be? Of course you're
not building them on the Moon, but back at some fabrication lab it
must have been hard to get them just right. Getting the surfaces
flat plus minus say 1/4 wavelength of the laser light to be used
would be not hard for people experienced in making telescope optics,
but how did they get perfect 90 degree angles? Parallel flat
surfaces shouldn't be that hard to get right if you can make the
1/4 wavelength flat surface above.

Once you have a good cube, then I suppose the diagonal cut isn't
that critical as long as you get it 1/4 wave flat.