High wire act may be best way to explore Europa
March 13, 1998: As NASA works to make space missions cheaper, it is looking at the possibility of using a long wire to power spacecraft exploring space around Jupiter where Galileo is gathering more hints that icebound Europa may have the right conditions for life.
In theory, a spacecraft could use a 10 km (6.2-mile) wire to augment rockets for propulsion once it reaches Jupiter.
"These are exciting possibilities that are worth exploring. The physics is wonderful," Gallagher said. "The engineering will be a challenge, though."
Which is to say that some very sophisticated controls will be needed to operate an electrical tether in Jupiter's dynamic environment.
An electrical tether uses the same principles as electric motors and generators. Move a wire through a magnetic field and you get an electrical current for power. Send electricity through a wire and you get a magnetic field that drags or pushes on any outside magnetic field.
This runs motors inside toys, appliances, disk drives, and generators in power plants, automobiles, and so on.
It can also generate electrical power for a satellite orbiting a planet with a magnetic field, or raise or lower the satellite's orbit - if the satellite has an electrically conducting tether.
NASA tested a Tethered Satellite System on the Space Shuttle in 1995 and 1996. Although it broke on the second mission, the tether produced some surprises in how electrical currents are produced and conducted by extended objects in space. Marshall Space Flight Center is now developing a Propulsive Small Expendable Deployer System - ProSEDS - that will speed a rocket stage's return to Earth.
If successful, it may be followed by an Electrodynamic Tether Upper Stage that would use the same principles to boost satellites to higher orbits, or a similar system on the International Space Station to help maintain its orbit.
"Jupiter is another path the program could take," said Gallagher, a plasma physicist at NASA/Marshall. "What we're suggesting is getting together with the Jet Propulsion Laboratory and doing an advanced tether study for a Europa orbiter mission."
The concept is to use a tether to propel the spacecraft and power its electrical system, thus saving the most precious of space resources, money. By reducing the amount of propellant needed once the spacecraft arrives at Jupiter, or the size of the electrical power system, the cost of the spacecraft also can be reduced, and it can be launched with a smaller, cheaper rocket.
An electrical tether will work only where nature provides both a magnetic field and a plasma (electrified gas). The motion of the wire through the magnetic field provides the energy, and the electrons in the plasma provide the return path that completes the electrical circuit.
The Earth's magnetic field and its ionosphere, which extends well into "empty" space, would do well for satellites here.
Jupiter is a bit more of a challenge, Gallagher explained.
Near the planet, where the plasma is densest, a 10 km (6.2 mile) tether would produce a 50,000-volt potential and a 20 amp current. That would be 1 megawatt of power flowing through a line just 1 mm (1/25th of an inch) thick.
"This would become a tremendous fuse and vaporize the tether," Gallagher said. This is also where engineering steps in and has to deal with the numbers developed by physics.
"You could only use the tether to conduct for brief intervals," Gallagher said. Theoretically, it could bring the satellite down from a high, 100-day orbit to a tighter, 5-day orbit. And the megawatt of power would be far more than than the 100 watts that the spacecraft would need during normal operations.
While the planet has a large magnetic field, its strength drops out towards the four large Galilean that are of greatest interest to scientists; the plasma density also drops. Europa is 9 Rj - nine Jovian radii, or 630,000 km (391,000 mi) - out.
"If you get that far out, densities have fallen substantially, and the field is pretty weak," Gallagher said. That means a much longer tether would be needed. The extra weight might offset the gains, and the tether would have a greater risk of being hit by a micrometeorite.
Oddly enough, another difficulty is the gravity gradient. The slight difference in gravitational pull across the length of the tether is what keeps it taut. But while Jupiter is the most massive planet in our solar system, it is also the largest. That means its gravity gradient is shallow more than 4 Rj where the probe would need to work.
The solution might be to spin the spacecraft so centripetal force keeps the tether taut. That, of course, complicates the electrical controls.
As for exploring Europa itself, Gallagher said that more needs to be known.
"Europa has a thin atmosphere and may have an ionosphere," he said. "Perhaps it has its own built-in blanket of current carriers." On the other hand, its magnetic field is very weak, so a longer tether might be required to generate enough current to power the spacecraft.
It might even be possible to extend a tether skyward from a Europa science station and power the the craft that way, Gallagher said.
So, the bottom line for now is a definite "maybe."
"One of the objectives of this study was to figure out whether it was worth looking at seriously," Gallagher said. "This study could just as easily have said, 'Don't bother.'" But it didn't.
"Europa is a potentially exciting place to use electrodynamic tethers."
Author: Dave Dooling
Curator: Bryan Walls
NASA Official: John M. Horack