January 17, 2002
Researcher to measure moon, Earth distance
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 be within about 25 centimeters, or 10 inches. Technological advances since the mid-1980s have sharply reduced that margin to about 2 centimeters, or less than an inch.
Now Murphy, a UW 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.
