A 1960’s Dream Comes True
By Davide Castelvecchi
Testing Albert Einstein’s ideas was never going to be easy, but Gravity
Probe B (GP-B) turned out to be possibly the hardest—and certainly the
longest-running—NASA experiment in history.
Pief Panofsky’s great-grandson,
Julian Pardeilhan, watched the launch from San Luis Obispo. (Photo
courtesy of W.K.H. Panofsky)
After more than 40 years of research and
development, a Delta II rocket finally took the three ton, $700 million
probe into orbit on April 20. Its 18-month mission is to measure some of
the most subtle predictions of Einstein’s theory of relativity.
The successful launch was a remarkable personal achievement for Francis
Everitt, the project’s leader and driving force. The British-born
physicist joined Stanford in 1962 during what was supposed to be a short
visit to the U. S., and he has devoted almost his entire career to GP-B.
Several generations of physicists and rocket scientists, family and fans
were on hand at Vandenberg Air Force Base on April 19 to see the launch.
NASA could not verify whether some last-minute data had been loaded
aboard the rocket, so the launch was put on hold until the next day. But
at 9:57 a.m. on April 20, the wait was over.
“When it finally lifted off, when it wasn’t going to be one more day, it
was a tremendous feeling,” says Robert Cannon, a Stanford Aeronautics
and Astronautics emeritus who helped initiate the project in 1959.
An expert in gyroscopes and other navigation systems, Cannon joined the
late William Fairbank and Leonard Schiff in a series of brainstorming
sessions to figure out how to do what most experts thought impossible.
Another scientist had the same idea just weeks before. “[It] was
independently suggested by George Pugh of the Department of Defense in a
little-known document,” Everitt says.
The three Stanford scientists realized that to achieve their goal they
would have to invent much of the technology from scratch.
While orbiting 400 miles above earth GP-B will test two predictions of
General Relativity, Einstein’s theory that mass and energy deform space
and time. The geodetic effect says that apples falling to earth and
satellites going around earth are following the shortest path in the
deformed space. Although previously confirmed by less precise
measurements, GP-B will improve precision by a factor between 2.5 and
The frame dragging effect says a rotating mass (earth) causes nearby
space itself to rotate with the mass. This will be directly tested for
the first time ever to a precision of one percent by measuring
deviations in the rotations of four gyroscopes. The tricky part is,
those deviations are so tiny that it would take more than a million
years for the axis of rotation to go around in a full circle. GP-B will
need to see deviations in the order of a millionth of a degree,
comparable to seeing the width of a human hair from 10 miles’ distance.
An on-board telescope will keep the probe aligned by pointing at the
center of a star with an unprecedented precision of one ten-thousandth
of the star’s diameter.
In 1964, NASA began supporting the project. Its long history has brought
advances in several fields, including cryogenics and superconductor and
telescope technology, churning out 79 PhDs at Stanford and 13 at other
universities in the process.
In 1984, Stanford hired Lockheed Martin to build the spacecraft. Jack
Goodman (now at SLAC working on GLAST) was on the original Lockheed GP-B
team for 11 years. “I was very elated that the launch was successful.
Gravity Probe B was a large part of my career at Lockheed,” he says.
Over the years, GP-B has generated much controversy because of its
technical boldness, its delays and its enormous budget, and NASA had
threatened to cancel it as many as seven times. Just last year, after a
technical problem during some tests, the agency appointed two
independent review panels, and space science administrator Ed Weiler
asked “if the time has come to put an end to GP-B,” Science magazine
Now one month into the mission, the spacecraft is performing well, NASA
says, and is ready to enter into the science phase of the mission this
For more information, see: einstein.stanford.edu/