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.
Today, Dr. Dennis Gallagher of NASA's Marshall Space Flight
Center will discuss electrodynamic tethers at the Ninth Annual Advanced
Propulsion Research Workshop and Conference being held at the Jet Propulsion
Laboratory in Pasadena, Calif.
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."
Images sent back by the Galileo spacecraft orbiting Jupiter show that Europa
is covered with sheets of ice that move, break open, and expose slush and
possibly liquid water. NASA/Marshall is also studying ice from Earth's
Antarctic which contains microorganism preserved in conditions like those
on Europa.
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."
