Project planned to pin down moon's distance from Earth
UNIVERSITY OF WASHINGTON NEWS RELEASE
Posted: January 15, 2002

Tom Murphy plans to spend much of the next five years using the Apache Point telescope in New Mexico as a tape measure 239,000 miles long - give or take a millimeter.

He'll employ the telescope, a laser beam and reflectors left by several lunar missions in a technique known as laser ranging to provide the most exacting measure yet of the Earth's distance from the moon.

Scientists have long known the center of the moon is about 238,700 miles from the center of Earth. In the early 1970s, the distance was known to within about 25 centimeters (10 inches) but technological advances since the mid-1980s have sharply reduced that margin to about 2 centimeters (less than an inch).

Now Murphy, a University of Washington postdoctoral researcher in physics and astronomy, hopes to reduce that uncertainty to a millimeter - about the thickness of a paper clip. He will lead a team that includes Christopher Stubbs, a UW astronomy professor; Eric Adelberger, a UW physics professor; and Jana Strasburg, a physics graduate student.

In making such a precise measurement, the team will perform the most sensitive tests ever done on several features of gravity. One involves Einstein's equivalence principal, which essentially states that bodies of different compositions accelerate at the same rate in a gravitational field. Another deals with variability of the strength of the gravitational interaction - testing to determine whether there are signs that the force of gravity is diluted as the universe expands.

"We don't know enough about gravity, so we have to probe gravity with every tool we have available," Murphy said.

He will use the 3.5-meter telescope at Apache Point, near Sunspot, N.M., owned and operated by the Astrophysical Research Consortium of which the UW is a member. He will attach a laser that generates an average power of 2 watts, but that will jump to a peak power of a gigawatt (1 billion watts) long enough to generate a 1-inch "bullet" of light aimed through the telescope at the lunar surface. The distance is calculated by measuring the light pulse's round-trip travel time and multiplying that figure by the speed of light.

Each laser bullet will be aimed at one of five retroreflectors, banks of 100 to 300 special prisms that reflect a beam of light back to its point of origin. The retroreflectors, each about the size of a suitcase, were left behind by three Apollo missions (including Apollo 11, the first manned mission to land on the moon) and two unmanned Soviet missions.

"You pick which retroreflector you want to aim at, then you focus the beam as tightly as you can. But even then, the atmosphere distorts the beam so that when it hits the moon it's 2 kilometers in diameter," Murphy said.

"Only one in 30 million of the photons that you launch to the moon will actually find the retroreflector. It's like winning the lottery - very tall odds," he said. "And then for a photon to make it back to the telescope, the odds again are about one in 30 million." That's because once the light makes it back to Earth, it has expanded to about 15 kilometers - or 9.3 miles - in diameter.

The number of photons detected depends a lot on the technology and the size of the telescope. Current laser-ranging experiments detect just a single photon from every 100 laser pulses sent. But this will be the first time advanced measuring technology has been used in conjunction with a telescope as large as that at Apache Point, so Murphy hopes to detect five to 10 photons for each laser pulse.

"We're going to shoot 20 pulses per second, so at any given time we'll have 50 pulses in the air coming and going from the lunar surface," he said.

For each 30-minute session, Murphy plans to use all five retroreflectors, which will remove any ambiguous measurements related to the moon's orientation to the Earth. He expects to complete preparations and begin taking measurements in about a year, and says the work is likely to last five years. The project, paid for by the National Aeronautics and Space Administration, is actually a feasibility study for performing laser-ranging experiments from space.

When he's done, Murphy also expects to add to the understanding of how the sun's gravitational field exerts a pull on the Earth's gravitational field.

"This is essentially measuring the weight of gravity, and this is the only type of project that can currently do that," he said.

Apollo 16
NEW! The latest in Apogee Book's acclaimed NASA Mission Reports series features the Apollo 16 expedition to the lunar highland area of Descartes. Includes CD-ROM.
 U.S. STORE
 U.K. & WORLDWIDE STORE

Gemini 7
Gemini 7: The NASA Mission Reports covers this 14-day mission by Borman and Lovell as they demonstrated some of the more essential facts of space flight. Includes CD-ROM.
 Choose your store:
U.S. - U.K. - E.U. - Worldwide

Apollo patches
The Apollo Patch Collection: Includes all 12 Apollo mission patches plus the Apollo Program Patch. Save over 20% off the Individual price.
 Choose your store:
U.S. - U.K. - E.U. - Worldwide

Mars Rover mission patch
A mission patch featuring NASA's Mars Exploration Rover is available from our online.
 Choose your store:
U.S. - U.K. - E.U. - Worldwide

Apollo 9 DVD
On the road to the moon, the mission of Apollo 9 stands as an important gateway in experience and procedures. This 2-DVD collection presents the crucial mission on the voyage to the moon.
 Choose your store:
U.S. - U.K. - E.U. - Worldwide

Gemini 12
Gemini 12: The NASA Mission Reports covers the voyage of James Lovell and Buzz Aldrin that capped the Gemini program's efforts to prove the technologies and techniques that would be needed for the Apollo Moon landings. Includes CD-ROM.
 Choose your store:
U.S. - U.K. - E.U. - Worldwide

STS-134 Patch

Free shipping to U.S. addresses!

The final planned flight of space shuttle Endeavour is symbolized in the official embroidered crew patch for STS-134. Available in our store!
 U.S. STORE
 WORLDWIDE STORE

Final Shuttle Mission Patch

Free shipping to U.S. addresses!

The crew emblem for the final space shuttle mission is now available in our store. Get this piece of history!
 U.S. STORE
 WORLDWIDE STORE

Apollo Collage
This beautiful one piece set features the Apollo program emblem surrounded by the individual mission logos.
 U.S. STORE

STS-133 Patch

Free shipping to U.S. addresses!

The final planned flight of space shuttle Discovery is symbolized in the official embroidered crew patch for STS-133. Available in our store!
 U.S. STORE
 WORLDWIDE STORE

Anniversary Shuttle Patch

Free shipping to U.S. addresses!

This embroidered patch commemorates the 30th anniversary of the Space Shuttle Program. The design features the space shuttle Columbia's historic maiden flight of April 12, 1981.
 U.S. STORE
 WORLDWIDE STORE

Mercury anniversary

Free shipping to U.S. addresses!


Celebrate the 50th anniversary of Alan Shephard's historic Mercury mission with this collectors' item, the official commemorative embroidered patch.
 U.S. STORE
 WORLDWIDE STORE

Hubble Posters
Stunning posters featuring images from the Hubble Space Telescope and world-renowned astrophotographer David Malin are now available from the Astronomy Now Store.
 U.S. STORE
 U.K. & WORLDWIDE STORE

Get e-mail updates
Sign up for our NewsAlert service and have the latest news in astronomy and space e-mailed direct to your desktop (privacy note: your e-mail address will not be used for any other purpose).
Enter your e-mail address:

INDEX | PLUS | NEWS ARCHIVE | LAUNCH SCHEDULE
ASTRONOMY NOW | STORE

ADVERTISE

© 2014 Spaceflight Now Inc.