Up, Up, And Away (Bit By Bit) -- Satellites On A String

So proclaimed comic strip character Diet Smith in the 1960s when he would fly Dick Tracy from Earth to Moon in a magnetically driven Space Coupe. Smith's proclamation was a bit over the top, but it carried a kernel of truth: you can cut space travel costs by using an extension cord to tap into a planet's magnetic field.

Scientists and engineers at NASA's Marshall Space Flight Center are developing a test model of such a device that will use Earth's magnetic field to make a rocket stage re-enter the atmosphere in a few days instead of months. If it works, then America will have a powerful new tool to keep satellites up - even to explore the solar system - without using rockets.

It could even trim $2 billion a year off the cost of operating the International Space Station.

It won't quite work the same as a Space Coupe with steerable magnets. Instead, ProSEDS will use a 20 km (12-mile) extension cord that plugs into the magnetosphere and turns the cord into an electric motor that slowly raises or lowers a satellite's orbit.

The concepts behind ProSEDS - the Propulsive Small Expendable Deployer System - are derived from the Tethered Satellite System flown on the Space Shuttle in 1995 and 1996. Although the tether broke as it reached its 19.6 km (12-mile) length on its 1996 flight, scientists gathered a great deal of data about tether behavior during five hours of operation.

"There's a new model out there on how you collect electrical current in space," said Dr. Nobie Stone, project scientist for the Tethered Satellite System. Stone and Dr. Dennis Gallagher, both in NASA Marshall's Space Sciences Laboratory, are advising Marshall engineers on the electrodynamic aspects of the ProSEDS experiment.

The Tethered Satellite System employed a large deployment mechanism, resembling a deck winch, in the Space Shuttle payload bay. The winch unreeled 20 km of insulated, conducting tether with a spherical satellite at the end. As the Shuttle orbited the Earth, the electrical wire cut through the Earth's magnetic field , and the motion produced an electrical current. Electrons - which make up a current - were collected by the satellite, through the tether, and flowed out the Shuttle by way of an electron gun that dumped the charge as it built up.

What Stone and other scientists found was that the tethered system produced more current that expected.

"The theoretical models were not accurate on tether," Stone said, "and the currents were higher than we expected." Specifically, the models require that the voltage be 10 times greater to collect a current than what was observed. Before the flight, the models predicted that the tether would produce 0.5 amp (0.5 A) under ideal conditions. Instead, it produced more than 1 amp under less than ideal conditions.

"The models were a factor of two or three off because they don't include the effects of orbital motion through the plasma (electrified gas) of the ionosphere," Stone said. While motion of a conductor through the magnetic field is crucial (it's also how a generator in a power plant works), motion through the electrons in space was thought to be a miniscule effect. The Shuttle moves at 7.7 km/s (17,500 mph) while the electrons move at 200 km/s (115,000 mph).

It turns out that the current carried by those electrons connected nicely with the tethered system and "contributed significantly" to the power generated.

Juan San Martine of Spain predicted that a tethered system did not need a large sphere at the end of the line to work. The motion of a satellite through space generate a plasma shield that stands off about 1 cm (0.4 in) away from the spacecraft surface. On the 1.8 m (6 ft) diameter TSS, that 1 cm standoff adds only about 2 percent to the collecting area. On a wire, it increases the collecting area 400-fold or more, so that an 82-meter wire now has as much effective collecting area as the 1.8-meter sphere.

"If this new bare wire tether works as advertised," Stone said. "it would allow us to collect considerably more current for a given length of tether." As a result, shorter tethers could be used for propulsion or to generate electrical power.

"The applications of this are, potentially, to produce power or thrust on the International Space Station," Gallagher explained. The tether could provide extra electricity to the station, or help maintain its altitude so it does not re-enter Earth's atmosphere.

The tether will produce just a little force. The force on the Shuttle was 0.4 newton (0.1 lb). But applied steadily, for hours or days, it makes a difference. Les Johnson of Marshall's Advanced Systems and Technologies Office predicts that a 10 km (6 mi), 10 kilowatt tether system could boost a 1,000 kg (2,200 lb) satellite as much as 400 to 540 km in one day, depending on the orbit and other conditions.

Johnson and others in the Advanced Systems and Technologies Office are developing the ProSEDS concept to test this innovative idea.

"The big thing we're trying to do is demonstrate the propulsive utility of an electrodynamic tether," Johnson said. "We view this as a precursor to a lot of different approaches that we've been studying."

ProSEDS is much smaller than the Shuttle's Tethered Satellite System. In operation on future satellites, the tether, which will look more like dental floss than TSS's high-tech rope, will unreel from a bobbin in a can. The can is released from the satellite and the difference in Earth's gravity - even across a difference of a few feet in altitude - pulls the can down. Eventually, the tether is unwound to a distance of 25 km.About 5 km (3 miles) of tether near the spacecraft would be bare; the rest is non-conducting and provided to put enough distance on the tether so it stays taut.

In the ProSEDS demonstration flight, the satellite will be the second stage of a Delta rocket. ProSEDS will ride as a piggyback payload for the launch of a larger satellite (an assignment is being sought). The tether bobbin will stay on the rocket and a weight will be unreeled upward on the tether. After the satellite is injected into orbit, the second stage normally would be slowly pulled back from 400 km (240 mi) to Earth by atmospheric drag. After 120 days, it re-enters and burns up.

Johnson, Stone, and Gallagher want to do that in about 15 days. Their plan is for the tether to increase drag by acting as an electrical generator to power batteries on the ProSEDS telemetry package. This package, equipped with sensors that Stone will develop from designs proven on the Space Shuttle and satellites, will measure precisely how well the bare-wire tether concept works). Among other things, the ProSEDS team wants to study the tether's behavior to see whether the differences between night and day would make the tether swing like a pendulum out of control.

Operated the other way around, a tether powered by solar cells - such as those on International Space Station - would boost a satellite's orbit and keep it from re-entering.

"We're not as fast as chemical rockets, but we have the efficiency of electrical propulsion," Johnson said. Electrical engines provide more thrust per pround of propellant than chemical enbgines. Since a propulsive tether expends no propellant, its efficiency will be measured in its cost compared to rockets. And that can be as little as 8 percent the cost of chemical rockets.

A propulsive tether would weigh about 90 kg (200 lbs.). In turn, it would eliminate the need to haul up to 4,000 kg (8,800 lbs.) of chemical propellants to the station. Atmospheric drag on the station will be about 0.3 to 1.1 newton (depending on the time of year), and the tether could produce 0.5 to 0.8 newton of thrust.

A reusable space tug - called an electrodynamic tether upper stage - could be built using the propulsive tether to haul satellites from a launch vehicle in low orbit to higher orbits. The sky is not quite the limit on propulsive tethers. The technique requires an ionosphere, a region of electrified gas which acts as part of the electrical circuit. Around the Earth, it tapers off around 1,500 km (900 miles).

The solar plasma and magnetic field of the sun are too weak for interplanetary voyages. Most of the planets do not have the right conditions for propulsive tether operations. Even Jupiter, with its intense magnetic field, does not have the right gravity gradient to keep a tether strung out to help move a satellite in exploring its moons.

However, conditions are right around the moons themselves. This would eliminate the problem of storing chemical propellants at the right temperatures so they work in deep space, or carrying complex electrical thrusters. At the request of the Jet Propulsion Laboratory, the MSFC Program Development Office is studying a propulsive tethert o help explore icy Europa, and Gallagher has studied science data to determine which Jovian moons would be the best candidates for such a mission.

"Io is most interesting because it is electrically connected" through its volcanoes which spew sulfur dioxide into space, Gallagher said.

All that is a few years off. The ProSEDS demonstration has been approved and is being developed for flight, possibly in 1999 or 2000.