From: Bill Taylor (bt4_1284@yahoo.com)
Date: Wed Jul 21 2004 - 20:34:51 PDT
Message-ID: <20040722033451.16814.qmail@web50202.mail.yahoo.com> Date: Wed, 21 Jul 2004 20:34:51 -0700 (PDT) From: Bill Taylor <bt4_1284@yahoo.com> Subject: Pinging the Moon
Kudos to Rick Rees for sending me this article:
http://www.nasa.gov/vision/space/features/21jul_llr.html
Apollo 11 Science Experiment Still Running
A cutting-edge Apollo 11 science experiment left behind in the Sea of
Tranquility is still running today.
The most famous thing Neil Armstrong left on the moon 35 years ago is a
footprint, a boot-shaped depression in the gray moondust. Millions of
people have seen pictures of it, and one day, years from now, lunar
tourists will flock to the Sea of Tranquility to see it in person. Peering
over the rails ... ''hey, Mom, is that the first one?''
Will anyone notice, 100 feet away, something else Armstrong left behind?
Ringed by footprints, sitting in the moondust, lies a 2-foot wide panel
studded with 100 mirrors pointing at Earth: the ''lunar laser ranging
retroreflector array.'' Apollo 11 astronauts Neil Armstrong and Buzz
Aldrin
put it there on July 21, 1969, about an hour before the end of their final
moonwalk. Thirty-five years later, it's the only Apollo science experiment
still running.
University of Maryland physics professor Carroll Alley was the project's
principal investigator during the Apollo years, and he follows its
progress
today. ''Using these mirrors,'' explains Alley, ''we can 'ping' the moon
with laser pulses and measure the Earth-moon distance very precisely. This
is a wonderful way to learn about the moon's orbit and to test theories of
gravity.''
Here's how it works: A laser pulse shoots out of a telescope on Earth,
crosses the Earth-moon divide, and hits the array. Because the mirrors are
''corner-cube reflectors,'' they send the pulse straight back where it
came
from. ''It's like hitting a ball into the corner of a squash court,''
explains Alley. Back on Earth, telescopes intercept the returning
pulse--''usually just a single photon,'' he marvels.
The round-trip travel time pinpoints the moon's distance with staggering
precision: better than a few centimeters out of 385,000 km (about 240,000
miles), typically.
Targeting the mirrors and catching their faint reflections is a challenge,
but astronomers have been doing it for 35 years. A key observing site is
the McDonald Observatory in Texas where a 0.7 meter (2.3 foot) telescope
regularly pings reflectors in the Sea of Tranquility (Apollo 11), at Fra
Mauro (Apollo 14) and Hadley Rille (Apollo 15), and, sometimes, in the Sea
of Serenity. There's a set of mirrors there onboard the parked Soviet
Lunokhud 2 moon rover--maybe the coolest-looking robot ever built.
In this way, for decades, researchers have carefully traced the moon's
orbit, and they've learned some remarkable things, among them:
(1) The moon is spiraling away from Earth at a rate of 3.8 cm (1.5 inches)
per year. Why? Earth's ocean tides are responsible.
(2) The moon probably has a liquid core.
(3) The universal force of gravity is very stable. Newton's gravitational
constant G has changed less than 1 part in 100-billion since the laser
experiments began.
Lunar laser ranging at the McDonald Observatory.
Physicists have also used the laser results to check Einstein's theory of
gravity, the general theory of relativity. So far, so good: Einstein's
equations predict the shape of the moon's orbit as well as laser ranging
can measure it. But Einstein, constantly tested, isn't out of the woods
yet. Some physicists (Alley is one of them) believe his general theory of
relativity is flawed. If there is a flaw, lunar laser ranging might yet
find it.
NASA and the National Science Foundation are funding a new facility in New
Mexico, the Apache Point Observatory Lunar Laser-ranging Operation or,
appropriately, ''APOLLO'' for short. Using a 3.5-meter telescope with good
atmospheric ''seeing,'' researchers there will examine the moon's orbit
with millimeter precision, 10 times better than before.
''Who knows what they'll discover?'' wonders Alley.
More and better data could reveal strange fluctuations in gravity,
amendments to Einstein, the ''sloshing'' of the moon's core. Time will
tell
... and there's plenty of time. Lunar mirrors require no power source.
They
haven't been covered with moondust or pelted by meteoroids, as early
Apollo
planners feared. Lunar ranging should continue for decades, perhaps for
centuries.
Picture this: Tourists in the Sea of Tranquility, looking up at Earth.
Half
of the planet is dark, including New Mexico where a pinprick of light
appears. A laser.
''Hey, Mom,'' stepping over a footprint, ''what's that star?''
Editor's Note: Among many early contributors to the the lunar laser
ranging
retroreflector array project, Prof. Alley credits Robert Henry Dicke,
James
Faller, Peter Bender, Douglas Currie and Bendix Corporation. A complete
list may be found in Alley's account of the project, ''Laser ranging to
retro-reflectors on the Moon as a test of theories of gravity,'' published
in Quantum Optics, Experimental Gravitation, and Measurement Theory, Eds.
P
Meystre and M.O. Scully, Plenum Publishing (1982).
=====
I am in Santa Cruz during the week most of July.
Bill Taylor
Drew School
San Francisco
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