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The GP-B spacecraft
Posted: April 17, 2004

All of the Gravity Probe B technologies are integrated into one of the most elegant and sophisticated satellites ever to be launched into space. Over four decades in development, the GP-B space vehicle is a marvel of engineering and truly a beautiful sight to behold.

From it largest to smallest parts, it is filled with the cutting edge technologies and materials described in the previous section, many of which were invented specifically for use in the Gravity Probe B mission.

Inside the Dewar

A Dewar is a sophisticated Thermos bottle for holding cryogenic liquids. The Gravity Probe B Dewar will be one of the largest and most sophisticated ever put into space. It is nine-feet tall and forms the main structure of the space vehicle. The vacuum area just inside the Dewar's shell contains multiple reflective surfaces that cut down on heat radiation. The Dewar also contains vapor-cooled metal shields that help maintain its internal cryogenic temperature and slosh baffles that help suppress tidal motions in the superfluid helium inside. When cooled to almost absolute zero temperature, liquid helium transforms into a state called "superfluid," in which it becomes a completely uniform thermal conductor. Only helium exhibits this, and other special properties of superfluidity.

Inside the neck area of the Dewar is a doughnut-shaped tank called the "guard tank." Once the Dewar has been conditioned for launch - an iterative process of successive evacuations and fills that transforms the liquid helium inside the Dewar to the superfluid state - the guard tank keeps the Dewar supercooled for weeks, without the need for additional conditioning, while the space vehicle is being readied for launch.

Science Instrument Assembly - Quartz Block and Telescope

The Science Instrument Assembly (SIA) contains the quartz block and the telescope. The SIA is located at the center of mass of the Dewar, along its main axis. It is mounted inside a vacuum canister called the probe (described in the next section).

The quartz block houses the four gyroscopes and SQUID readout instruments (Super Conducting Quantum Interference Devices - the magnetometers that read the gyroscopes' spin axis orientation). Each gyroscope is enclosed in a quartz clamshell housing, mounted in the quartz block and surrounded by antimagnetic shielding. The gyroscopes are electrically suspended with only 0.001 inch clearance from the housing walls, and they spin at up to 10,000 rpm during the science phase of the mission.

Optically bonded to the top of the quartz block is the quartz reflecting Cassegran astronomical telescope, which focuses on the guide star, IM Pegasi. Optical bonding is a patented method of fusing together quartz parts, without the use of any "glue" or fasteners to ensure that the SIA does not distort or break when cooled to cryogenic temperatures. The line of sight of the telescope is rigidly aligned to the SQUID readout loop of each gyroscope. As such, the quartz telescope provides the frame of reference for measuring any drift in the spin axis of the gyroscopes.

The Probe

The SIA is mounted in a cigar-shaped canister, called the "probe," which is inserted into the Dewar. The probe is an amazing feat of cryogenic engineering, designed by Lockheed Martin Space Systems in Palo Alto, California. It provides both mechanical and structural stability for the SIA. The probe is designed to provide a free optical path for the telescope to view distant space through a series of four windows, mounted in its upper section. These windows also serve to reduce thermal conductivity into the Dewar.

The inside of the probe is maintained at an extremely high vacuum - much greater than the vacuum of space. The probe is surrounded by a superconducting lead bag between it and the Dewar. The superconducting lead bag provides an impenetrable shield from electromagnetic signals that could disturb the gyroscopes.

Taken together, all of these measures create an ultra pristine, cryogenic environment, free of any external forces or disturbances, in which the gyroscopes spin.

At the upper end of the probe, capping off the Dewar, is the "top hat." The top hat serves as a thermal interface for connecting over 450 plumbing and electrical lines, that run from various electronics and control systems, mounted on the space vehicle's truss system outside the Dewar, to the cryogenic vacuum chamber inside the probe and Dewar.

Before Gravity Probe B was completely integrated, each component went through years of testing and construction. Some parts even had to be de-constructed and rebuilt. The entire probe was assembled in a Class-10 clean room, as any particles larger than a single micron would disrupt the precise structure.

Outside the Dewar

Outside the Dewar are all of the systems that provide power, navigation, communication, and control of the space vehicle.

Sun Shield

The sun shield is a long, conical tube that keeps stray light from entering the telescope. Inside the sun shield are a series of black, metal baffles that absorb incoming stray light before it can reach the telescope. In addition to blocking out stray light from the Sun, the sun shield also blocks stray light from the Earth, Moon, and major planets.

Proportional Micro Thrusters

The proportional micro thrusters on the space vehicle provide a very means of controlling its attitude or orientation in space. In the case of Gravity Probe B, an unprecedented amount of on-orbit control is required for the vehicle to maintain its drag-free orbit. This is accomplished by harnessing the helium gas that continually evaporates from the Dewar's porous plug and venting it as a propellant through eight pairs of opposing or balanced proportional micro thrusters.

These micro thrusters continually meter out a flow of helium gas - at the rate of about 1/100th the amount of a human "puff" exhalation that one might use to clean eyeglasses. This metered flow of helium keeps the space vehicle's center of mass balanced around one of the gyroscopes, called a "proof mass" - a predetermined test mass that serves as a reference for measurement. The thrusters are set up in pairs, so that they counterbalance each other. As long as the same amount of helium is flowing from two opposing thrusters, the space vehicle will not change its position along that axis. However, if the telescope or the SQUID readout for the proof mass gyroscope requires the space vehicle's position to change, it is simply a matter of unbalancing the flow, ever so slightly, in the appropriate thruster pair to move the vehicle in the desired direction. These proportional micro thrusters also control the roll rate of the GP-B space vehicle.

Solar Arrays

Solar arrays convert energy from the Sun into electrical power that is stored in the space vehicle's two batteries and then used to run the various electrical systems on board. The position of each solar array can be controlled to maximize its power output.

GPS Sensors & Antennae

GPS (Global Positioning System) sensors calculate and transmit information about the space vehicle's position. In the case of the Gravity Probe B space vehicle, the number and placement of the GPS sensors provide positioning information that is over 100 times more accurate than traditional ground-based GPS navigation systems. For example, a high quality handheld GPS sensor on Earth can locate your position to within about a meter, whereas the GPS sensors onboard the GP-B space vehicle can locate its position to within a centimeter.

Telemetry & Communications Antennae

These antennae enable both inbound and outbound communications with the space vehicle - this includes communications with ground stations and with orbiting communications satellites in the Tracking Data Relay Satellite System (TDRSS). Telemetry data from the space vehicle and science data from the experiment are transmitted to ground stations using these communications systems. These systems also enable the GP-B Mission Operations Center (MOC) to send daily batches of commands to the space vehicle. Communications between the space vehicle and orbiting satellites is limited to a 2K data format, whereas communications with ground stations uses a 32K data format, enabling far more data to be transmitted per unit of time.

Star Trackers

A star tracker is basically a camera and pattern matching system that uses constellations and stars to determine the direction in which a satellite is pointing. The Gravity Probe B satellite contains two star trackers - one wide field and one narrow field (called the star sensor). The wide field star tracker is used to locate the general region of the heavens containing the guide star, and then the narrow field star tracker helps align the space vehicle with the guide star.

The Gravity Probe B on-board telescope basically performs the same function, but it uses a different technique, and it is orders of magnitude more precise and more accurate. The narrow field star tracker has a field of view on the order of one degree (60 arcminutes), and it can focus to a position within perhaps one arcminute - about the same as the whole field of view of GP-B on-board telescope, which can pinpoint the guide star's position to within a milliarcsecond.

With such a small field of view, it would be nearly impossible to locate the guide star using only the onboard telescope, so the star trackers function like "spotting scopes" for initially pointing the space vehicle towards the guide star. Once the narrow field star tracker has focused on the guide star, the onboard telescope takes over the job of maintaining the precise alignment required for measuring gyroscope drift.

Navigational Gyroscope

GP-B uses a standard, flight-qualified gyroscope, equivalent to those found on other spacecraft (and also airplanes, ships, and other vehicles). This gyroscope is not part of the relativity experiment, but rather it is part of the general navigation system used for monitoring the general direction and position of the space vehicle.

Electro-mechanical Control Systems

Surrounding the Dewar, is a lattice of trusses that forms the structure of the space vehicle. Attached to these trusses are a number of electrical and mechanical systems that control the operation of space vehicle and enable the relativistic measurements to be carried out. These control systems include the following:

  • Attitude Control System (ATC) - Controls all of the proportional micro thrusters that determine and maintain the space vehicle's precise positioning.

  • Mass Trim Mechanism (MTM) - A system of movable weights that can be adjusted during flight to restore rotational balance of the space vehicle (similar to spin balancing the tires on an automobile)

  • Gyro Suspension System (GSS) - The electronics that levitate and precisely control the suspension of the four gyroscopes at the heart of the Gravity Probe B experiment. The GSS control boxes are mounted in the truss work, outside the Dewar. The wiring goes through the top hat section of the probe and down to each gyroscope.

  • Gas Management Assembly (GMA) - A very complex set of valves, pipes, and tubing, that runs from a triangular assembly on the truss work, through the top hat and down the probe to each of the gyroscopes. The critical job of the GMA is to spin up each of the four gyroscopes by blowing a stream of 99.99999% pure helium gas over them, through a channel built into one half of each gyroscope's quartz housing.

  • Experiment Control Unit (ECU) - The ECU controls many of the systems onboard the space vehicle, including the GMA, the UV system, and various thermal devices.