Gravity Probe B is an elaborate package that is designed to directly demonstrate two effects. The first, the geodetic effect, measures the precession of a gyroscope orbiting the earth. If a perfect gyroscope is placed in a free falling orbit around a massive object the angular momentum of the gyroscope will be connected with the orbit through parallel transport. Since the space-time is curved the geometry of the orbit is not Euclidean. As Everitt put it at the press conference before the launch: ``If the orbit were perfectly circular its circumference would be just a couple of centimeters less than times the orbital diameter." Thus the gyro angular momentum exhibits a precession in the plane of the orbit. The second effect, the Lense-Thirring precession, results from the dragging of the orbital inertial frame by the rotating earth. This precession can be made to be perpendicular to the geodetic precession if the orbit is polar.
The predicted precessions are very small. The geodetic precession is calculated to be 6614.4 milliarcseconds per year; the frame dragging precession is supposed to be 40.9 milliarcseconds per year. These incredibly small precession angles required the development of many new technologies. The experiment's orbit has to be continuously corrected for drag, i.e. it must be a zero g orbit. The gyroscopes need to be perfect spheres to eliminate the torques due to gravitational gradients and techniques had to be developed to make them and to measure their orientation without inducing torques. A technique needed to be developed to maintain a fixed direction in inertial space and a whole new type of telescope was developed to sense this direction and couple it to the gyroscopes. All of this means that literally hundreds of people have been involved in, or have contributed to, various aspects of the GPB spacecraft.
Not everyone who came or who tried to come was able to see the launch because, in the best tradition of Gravity Probe B, there were delays. The original launch of 6 December 2003 had to be postponed to rework some grounding problems in the experimental control unit. It was then scheduled for Saturday, 17 April 2004. Most of us who had tickets already were able to change them to the later date. NASA had been very explicit that we had to have official letters of invitation to attend and they were reissued for the later date. On 7 April we were informed that the launch was postponed for two days, until Monday, 19 April because of a short circuit in the launch tower. Everyone I talked to said that they just decided that it was ``this time for sure" and that they weren't going to change their tickets.
The launch activities were a virtual Woodstock of old gravitational physicists. NASA's official visitor's headquarters was in the small town of Buellton. It was there that the press conferences were staged and it was from there that buses were scheduled to take people out to the launch site. A number of people, many of those from Stanford, stayed nearer to the Vandenberg gates at Lompoc. I saw people that I hadn't seen in 15 or 20 years. There were several NASA briefings at the Marriott in Buellton. The powers-that-be had been thoughtful enough to invite the children of Leonard Schiff and Bill Fairbank to see the launch and to participate in the briefings.
On Monday morning, 19 April, a vast fleet of buses appeared in front of the Marriott and hauled us all out to the official viewing site. We discovered that we couldn't see the rocket on the launch site at all: it was hidden behind a small hill. We did have television monitors underneath a big tent and loud speakers connected to launch control but there was no view of the launch tower. A number of us with binoculars and absolutely no knowledge speculated about just what things visible on the horizon might be the launch tower. We all took pictures of the horizon and each other and eagerly listened to the countdown until t= -3 when it was announced that the winds aloft were too high and the launch would be postponed until tomorrow: Tuesday 20 March.
The delay was unfortunate for a number of the prospective viewers because of class commitments or plane reservations. Several had to leave. Some of us with rental cars decided to scout the area to see if there was a better viewing spot. We found a place behind the weather station where the launch tower was in full view at a distance of about 4 km and we were told that the weather station personnel would hook up a loud speaker to listen to mission control. Talking to old time Vandenberg personnel at a party for the invitees hosted by Brad Parkinson reinforced the desirability of this spot.
The next morning we drove to our newfound viewing site and, by the time of the launch, were sharing it with about 250 other people including many past and present members of the Stanford team. Rumors were passed about the reason for the previous day's delay, the prevalent one being that there had been a checksum error in loading the wind profile into the guidance system. Knowledgeable people spoke about the reasons for various gas ventings from the rocket. Finally when the count got to t=0 there was a tremendous cloud of smoke from the solid rocket boosters and the vehicle began to rise above the tower. It took fewer than 3 minutes until the rocket was completely out of sight, leaving only a trail of smoke in the sky. My main impression from the launch was that the brightness of the rocket's flame was much greater than anything I could have imagined from what I had seen of launches on television. The other impression, of course, was of how many people's work had gone into this project to get it this far and how much more there still was to do.
As an experimentalist I am sometimes accused of being over-awed by technology. GPB is, to me, much more than a technological demonstration. It is an attempt to measure, directly, things that we now believe we know. We didn't know these things when the experiment started. We still don't have direct measurements of them. This experiment is a direct measurement in an environment we believe we understand. I can only be amazed at the persistence of the Stanford team and Francis Everitt. It took more than 40 years to develop the technology and methodology to make these measurements. In a year we should have a firmer foundation for our theoretical speculations.
At this writing the experiment is working well. To check on its
progress look at