Now orbiting Earth, Gravity Probe B is a technological tour de force.
It is on a mission to test an unproven aspect of Einstein's theory
by Patrick L Barry
Engineers don't often
indulge in poetic flourish when discussing the things they build.
So when words like "beautiful" and "elegant" and "artful" frequently
cross the lips of scientists and engineers as they talk about the
design of Gravity Probe B (GP-B), one might suspect that this spacecraft
is truly something special.
The probe, which launched
April 20th on a mission to test an unproven aspect of Einstein's
theory of relativity, is by all accounts a marvel of human ingenuity
and know-how. Only recently has it even become technologically possible
to build Gravity Probe B, despite the fact that the idea for the
experiment has been around since the 1950s.
"If experimental science
is an art, then I would look at GP-B as a Renaissance masterpiece,"
says Jeff Kolodziejczak, NASA's Project Scientist for GP-B at the
Marshall Space Flight Center.
The beauty of GP-B's
design lies in part in its ability to create, in the messy real
world, a pocket of near-perfection. The goal of the experiment demands
it. Researchers hope to detect a bending of spacetime around Earth
so subtle that even a minute interference from some outside force
or a tiny internal imperfection in the spacecraft itself would mask
the effect they're hunting for.
Einstein's theory of
General Relativity predicts that Earth, by rotating, twists space
and time around with it, forming a mild vortex in the fabric of
spacetime around our planet. Researchers call this "frame dragging."
Most physicists believe the spacetime vortex is real, but no experiment
to date has been sensitive enough to detect it unequivocally.
Using the most perfect spheres humans have ever created,
Gravity Probe B just might turn Einstein's theories upside-down.
Enter Gravity Probe
The idea behind the
experiment is simple: Put a spinning gyroscope into orbit around
the Earth, with the spin axis pointed toward some distant star as
a fixed reference point. Free from external forces, the gyroscope's
axis should continue pointing at the star - forever. But if the
region of space through which the gyroscope orbits is slightly twisted,
as Einstein's theory predicts, the direction of the gyroscope's
axis would drift ever-so-slightly over time. By noting this change
in direction relative to the star, the subtle frame-dragging effect
can be measured.
It sounds like a straightforward
experiment; the trick is in actually building it. The gyroscope's
axis won't drift much, only 0.042 arc seconds over a year, according
to calculations. (An arc second is only 1/3600th of a degree.) To
measure this angle reasonably well, GP-B must have a precision of
0.0005 arc seconds.
"Every aspect of the experiment has to be nearly perfect," Kolodziejczak
says. Meeting this challenge has taken almost 40 years of effort
from many bright scientists and engineers, primarily at Stanford
University, NASA's Marshall Space Flight Center, and Lockheed-Martin.
The Gravity Probe B
team had to create the roundest gyroscopes ever made, and set them
orbiting Earth inside a force-free pocket. No form of atmospheric
drag or magnetic forces could be allowed to penetrate the gyro-chambers.
That's tricky because Earth's far-flung magnetic field envelops
GP-B and, even at an altitude of 400 miles, Earth's outermost atmosphere
exerts drag on the spacecraft. Furthermore, it would be necessary
to measure the tilt of the gyroscope's spin axis ... without ever
touching the gyroscope itself.
One of the spherical gyroscopes used in Gravity Probe B.
The gyroscopes in GP-B
are the most perfect spheres ever made by humans. (The experiment
actually carries four gyroscopes for redundancy.) These ping pong-sized
balls of fused quartz and silicon are 1.5 inches across and never
vary from a perfect sphere by more than 40 atomic layers. That means
that if these gyroscopes were the size of the Earth, the elevation
of the entire surface would vary by no more than 12 feet! If these
gyroscopes weren't so spherical, their spin axes would wobble even
without the effects of frame-dragging, thus ruining the experiment.
Being in orbit allows
the spheres to float within their housings as if weightless, but
without other controls, the spinning spheres would still tend to
drift and bump into the walls of their containers. The reason is
that the spacecraft is being slowed slightly by aerodynamic drag,
while the free-floating spheres within the spacecraft's belly are
The GP-B team solved
this problem by developing a drag-free satellite.
Inside the spacecraft
instruments monitor the distance between one of the gyroscopes and
its chamber walls with extraordinary precision - to within less
than a nanometer (a millionth of a millimetre). The spacecraft's
thrusters respond to any changes in that separation. In effect,
the spacecraft chases the gyroscope and flies along the same "drag
free" orbital path that it does.
The spheres must also
be protected from Earth's magnetic field. Why? Because a faint magnetic
signal from the gyroscopes themselves will ultimately be used to
detect the all-important change in angle of their spin axes. The
intrusion of Earth's magnetic field would swamp that signal.
But how do you block
a planet's magnetic field?
"We used superconducting
bags," says Kolodziejczak. The gyroscope assembly is placed inside
lead bags, which in turn are placed inside a large cryogenic container
called a "dewar" holding 400 gallons of liquid helium. The helium
cools the lead bags to 1.7 degrees above absolute zero (1.7 K, or
about -271 °C). At this temperature the lead becomes a superconductor,
thus blocking out Earth's magnetic field. The ambient magnetic field
within these bags is reduced to less than 3 micro-gauss, which is
about the same as in deep interstellar space.
The extreme cold also
helps create an ultra-low pressure vacuum in the gyroscope chamber;
after pumping out most of the gas, the molecules of gas that remain
are very cold and thus hardly moving, which means they exert almost
zero pressure. In this pristine, high-vacuum environment, the spherical
gyroscope could spin at its operating speed of 10,000 rpm for 1,000
years without slowing by more than 1 percent.
Finally, it's necessary
to measure the gyroscopes' spin without nudging the gyroscopes in
Gravity Probe B's big dewar holds hundreds of gallons of
Once again, superconductivity
comes to the rescue. A superconducting sphere, when spun, will produce
a weak magnetic field that is precisely aligned with the axis of rotation.
The gyroscopes are therefore coated with a metallic layer of niobium
of near-perfect uniformity. At the cryogenic temperature in the
core of GP-B, niobium becomes a superconductor and it produces a
magnetic field when the spheres are spun. By monitoring the magnetic
field, engineers can monitor the spin of the gyroscopes - no touching
To do this, the GP-B
scientists use a remarkable device called a SQUID - short for "Superconducting
QUantum Interference Device." Attached to a loop of superconducting
wire closely encircling each gyroscope, a SQUID functions as an
ultra-sensitive magnetic field detector. SQUIDs can detect a change
in this field of only 50 billionths of a micro-gauss (5 x 10-14
gauss), which equates to a change of the gyroscope's angle of 0.0001
A telescope onboard
the spacecraft constantly watches a distant star named IM Pegasus.
This serves as an external reference point for measuring the tilt
of the gyroscopes. IM Pegasus isn't truly a fixed point, though.
It will drift ever-so-slightly during the 2 year lifetime of the
GP-B mission. Fortunately, astronomers know very precisely how far
it will drift, so that motion can be accounted for.
Superconducting lead bags and SQUIDs. These are odd materials for
art. Among engineers and physicists, though, there's no doubt: Gravity
Probe B is a masterpiece.