NASA Goes Nuclear
When your batteries are dead and solar power is only a distant memory, you're going to need something else in your power pack.
- By Ben Iannotta
- Air & Space magazine, July 2003
NASA Glenn Research Center
(Page 3 of 5)
It will be just such an engine, powered by a nuclear reactor, that pushes JIMO toward Jupiter and lets it maneuver through the Jovian system with a new kind of nimbleness. The thrust produced by ion propulsion—the slow, steady expulsion of ions to accelerate a spacecraft—ranges from a fraction of an ounce to three pounds, minuscule by the standards of most liquid-fuel rockets. Cassini’s twin thrusters, for example, produce 100 pounds each. But, explains Jankovsky, chief of Glenn’s onboard propulsion branch, the ion engine would be so fuel-efficient that it could run continuously, building up speed and eventually outpacing chemical rockets.
When the Galileo probe approached Jupiter in 1995, it fired its liquid-fuel main engine for 49 minutes to slow down so Jupiter’s gravity could pull it into orbit. After that big burst, Galileo had only enough fuel left for minor tweaks to its trajectory. Most of the subsequent course correction came from carefully timed gravity-assisted swing-bys of the Jovian moons. “If you look at a chart of the Galileo orbits, it looks like a line drawing of a flower where each orbit represents a petal of the flower,” explains Ron Greeley, a planetary geologist at Arizona State University.
Mission designers measure such maneuvers in terms of delta-V, or change in velocity—basically, how much energy is needed to change a spacecraft’s speed and direction. Cassini carries enough fuel to provide a total of 6,500 feet per second of delta-V over the lifetime of the mission. JIMO may have 30 or 40 times that. “With JIMO, we’ll orbit Callisto, then slip over and orbit Ganymede, and finally over to orbit Europa,” says Greeley. Such dramatic, energy-demanding orbit shifts were well beyond the capability of earlier planetary spacecraft, because the trajectories would have required many times more liquid fuel than they could affordably carry.
The efficiency of a rocket is generally given in terms of specific impulse, measured in seconds. A typical planetary spacecraft thruster might have a specific impulse of 300 seconds. “People who build chemical rockets would kill for a couple extra seconds of specific impulse,” says Jankovsky. With JIMO’s ion drive, NASA engineers hope to achieve 4,000 seconds.
As he enters the nearby Electric Propulsion Research Building, Jankovsky points to a huge circular engine in a corner. It measures five feet across and looks like the housing of a large industrial fan. Engineers tested this 200-kilowatt ion engine in a vacuum tank here in 1967, back when it was assumed nuclear reactors would be generating millions of watts of electricity for future missions to Mars.
The engines being developed in this building are far less ambitious, but still an advance over the ion engines that have flown in space so far. At one end is a cathode tube that spits out electrons. They collide with a neutral gas, in this case xenon, knocking off more electrons and creating positively charged xenon ions. Other fuels could be used—krypton gives off a greenish glow, neon glows red. Xenon is popular because its electrons orbit farther from their nuclei, and that makes them easier to bump.
In 1998, a NASA technology demonstration mission called Deep Space 1 used a xenon engine; solar panels instead of a nuclear reactor supplied electricity. Although ion propulsion had already flown on U.S. commercial satellites and dozens of Russian military satellites, its use on DS1 was the first time it was included on a spacecraft dispatched beyond Earth orbit—in this case, to a comet and asteroid. The ion drive worked like a champ. The test proved that the engine could be throttled up or down, that its exhaust would not corrupt scientific readings, and that the ions wouldn’t short out electronics or block radio signals.
But DS1’s engine was not a powerful one, even by ion thruster standards. And although it ran for 678 days and was still going when NASA ended the mission in 2001, that wasn’t long enough to demonstrate the years-long operation required for the JIMO mission.