Satellite Experiments

A relatively cheap way of conducting experiments in space is to put them on the upper stage of a rocket used to launch other satellites into orbit. An upper stage needs to achieve orbital velocity to deliver its cargo, so after deploying the payload satellites it is carrying into space, the stage itself becomes an independent and passive "satellite'' (in fact, when people thought they were seeing tiny Sputnik-1 fly across the night sky in 1957, they were actually looking at the much larger upper stage of its launch rocket that went into orbit with it). If the mass of the cargo satellites is less than the maximum capability of the launcher, small experiments can be attached to the upper stage and flown along at often very little cost.

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Figure 4.8: The severed TSS tether and satellite photographed from Earth, with several reference stars nearby. (Courtesy of Kym Thalassoudis.)

Figure 4.8: The severed TSS tether and satellite photographed from Earth, with several reference stars nearby. (Courtesy of Kym Thalassoudis.)

Several tether experiments involved putting equipment on upper stages and deploying tethers after delivery of the rocket's main cargo into orbit (so that they would not pose a risk to the expensive primary cargo). The first of these was SEDS-1, an acronym for Small Expendable Deployer System. It consisted of a 33-cm-long (13-inch-long) deployer cylinder with rolled up tether attached to the rocket stage, and a tiny "end-mass" 26-kg (57-pound) payload satellite. This satellite was a completely autonomous system that carried its own batteries, electronics, telemetry system, and sensors. It was connected to the rocket stage by a 20-km-long (12-mile-long) tether made of an especially strong, Kevlar-like material called Spectra-1000. SEDS-1 was developed and built by NASA's Marshall Space Flight Center, the Harvard-Smithsonian Center for Astrophysics, and Tether Applications of San Diego. The end mass satellite was built by NASA-Langley (Fig. 4.9).

SEDS-1 was launched from Cape Canaveral Air Force Station on March 29, 1993, attached to the second stage of a Delta II rocket with a GPS navigation satellite on board. An hour after launch, a spring ejected the endmass satellite downward with a velocity of 1.6 meters per second (5.2 feet per second). This impulse carried the small probe far enough to allow gravity-

Figure 4.9: The SEDS system. (Courtesy of NASA.)

gradient effects to pull on and therefore deploy the tether faster and faster. When only 1 km (0.5 mile) of tether remained on the spool, active braking was applied. However, the braking was insufficient, so that when the tether was completely paid out the satellite was still flying away from the rocket stage. This resulted in a series of gentle "bungee" bounces at the end of deployment.

Because of the gravity gradient, the tether then swung to the vertical. One orbit after the start of deployment the tether was cut, slinging the satellite and tether onto a trajectory that made them reenter the atmosphere off the coast of Mexico. The reentry was accurate enough that prepositioned NASA observers were able to film the satellite and tether burning up high in the atmosphere. The last data collected from the satellite before reentry showed that the tension in the tether was rising as the aerodynamic drag began to blow the cable back, turning it into a kite tail.

SEDS-2 was launched on March 9, 1994, again onboard the second stage of a Delta II launching a GPS satellite. This time the tether and end-mass satellite would not be cut loose, but left attached to the rocket stage to determine long-term tether stability and the risks of micrometeoroid cutting tethers in orbit. SEDS-2 had an improved braking system that applied braking force as a function of the measured speed of the unrolling tether, and also started braking earlier than on SEDS-1. This was to ensure that the satellite stopped flying out just when the whole tether was deployed and to prevent the bounces experienced during the previous mission. The so-called "feedback control'' worked well and limited the residual swing after deployment to only 4 degrees.

The small satellite with its 19.7-km (12.2-mile) tether returned data for 10 hours until its battery died. After 3.7 days the single-strand tether suffered a cut, probably due to a hit from a micrometeoroid or piece of space junk. The loose end of the tether and the attached satellite reentered within hours, but 7.2 km (4.5 miles) of cable remained connected to the Delta stage and survived with no apparent further cuts until the whole assembly reentered on May 7, 1994. Before that, the thin tether turned out to be rather easy to spot with the naked eye when front-lit by the sun and viewed against a dark sky. Many videos were made, all showing that the tether remained stabilized near a vertical position with respect to Earth's surface, even after the cut. The SEDS-2 mission thus proved that a tether can be accurately deployed to a stable vertical position in orbit by feedback control and a relatively simple friction brake.

On June 26,1993, NASA and Tether Applications Inc. launched the Plasma Motor Generator (PMG) experiment onboard the second stage of an Air Force Delta II rocket. The system consisted of a far-end package satellite connected to the spent Delta stage by 500 meters (1640 feet) of conducting tether, orbiting Earth in a vertical configuration. The mission was designed to demonstrate that such a configuration could be used either to generate an electric current between the ionosphere and a spacecraft, or as an orbit-boosting motor (as described in the section Electrodynamic Tethers in Chapter 1). The current going through the tether was shown to be completely reversible. In the passive mode a current ran down the tether, proving that the system was capable of generating electrical power. As this power generation meant that orbital energy was converted into electrical energy and thus that the orbital velocity was decreasing, it was also demonstrated that the system could be used for braking and thus potentially for de-orbiting a spacecraft. When instead a current was actively driven up the tether, the Lorentz forces were shown to speed up the spacecraft and thus that the system could be used as a boosting motor, raising the orbit's altitude by converting electrical energy into orbital energy. The experiment lasted 7 hours, until the spacecraft's batteries expired.

The SEDS series should have been continued with a new mission similar to PMG named ProSEDS, for Propulsive Small Expendable Deployer System. ProSEDS involved a 4.4-km (2.7-mile) ultra-thin bare-wire conducting tether connected to a 8.7-km-long (5.4-mile-long) nonconducting tether made of Spectra and Kevlar (a strong synthetic fiber also used for making bulletproof vests), to be deployed from a Delta II second stage. Sweeping through Earth's magnetic field, the tether would have generated an electrodynamic braking force that would have lowered the orbit of the rocket stage. It was expected that this would have made it reenter the atmosphere in 15 days instead of the usual 120 days after launch. ProSEDS should have flown in 2003. However, after the tragedy with Space Shuttle Columbia in February 2003, NASA scientists reevaluated the risk that several already-planned space missions posed to astronauts onboard the Space Shuttle and International Space Station. They decided that the danger of the long ProSEDS tether at some time colliding with the Space Station was too high, and canceled the mission (Fig. 4.10).

TiPS, an acronym for Tether Physics and Survivability, was a relatively simple experiment designed to study how a tether would behave in space

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