As your eyes roll over these words, Dawn, a robotic NASA probe, is coursing away from us, heading into the hinterlands of an asteroid belt, a region of roving rocks that lies between Mars and Jupiter.
It’s en route to the rocky ice ball of Ceres, one of the two dwarf planets in the solar system—the other is Pluto—and is expected to arrive there, its second destination, sometime in April, 2015.
For a little over a year, from July, 2011 to September, 2012, it aerially perambulated over Vesta—the other massive celestial body in that neck of the cosmic woods—taking in its dry vista; gliding over a crater, a mound; watching the Sun come up over a wide equatorial trough called Divalia Fossa.
Launched in September, 2007, it’s a trailblazer in space exploration, being the first mission to explore not one, but two deep-space destinations.
This so-called stopover flight wouldn’t have been possible had it not been for its innovative propulsion technology. What has so long fired the space-faring battlecruisers and clippers of science-fiction is now, being realized in reality as NASA engineers move full steam ahead in developing electric spacecraft.
Perhaps that’s why Dawn looks like more the T.I.E. Fighters (short for Twin Ion Engines) from the “Star Wars” cosmos, and less like the Apollo module.
A tractor trailer-size vessel, with a pair of spacious, dirigible solar wings, it strikes one as a behemoth, angular, butterfly. Each of these panels—27 feet long and 7 feet wide—is coated with 5,740 photovoltaic cells that harvest the energy from the Sun.
Together, they whip up about 10,000 watts, which powers, at a time, one of a set of its three ion engines. Each is a small—a mere 12 inches across and 9 inches deep—nondescript, easy-to-miss device that resembles a “coffee can,” said Mike Patterson, senior technologist at NASA Glenn, in a phone interview.
A traditional, chemical rocket moves forward by burning fuel in the presence of an oxidizer (an oxygen source), creating a mass of scorching gas, which is then pushed out of a bell-shaped nozzle at the stern of the vessel at a very high speed.
An electrical system employs electricity to expel the propellant by a process that’s far less raucous. The kernel of such a mechanism is plasma—the very ingredient that causes neon signs to glow, and the aurora borealis to be shimmering sashes of light.
Unlike, say, the Soyuz, which uses a liquid fuel mix of kerosene and oxygen or the Mars rover, Curiosity, which runs on a solid, plutonium-238, Dawn works with xenon, an inert, colorless gas.
A carefully calibrated quantity of it is pumped into a magnetically sealed chamber, and is converted into plasma by firing into it an electron mist. On hitting a neutral atom, a negatively charged electron strips it off its electron, giving it a net positive charge. It’s from this “ionized” mix that the positive ions are then extracted.
The exit end of the chamber is covered by a row of two electrodes, each of which has scores of tiny perforations on them. The inner plate, the “screen,” is charged positive and the outer, the “accelerator,” is charged negative.
Obeying the law of electrical affinity, the ions emerging out of chamber make a beeline toward the accelerator. Sandwiched in the vanishingly narrow zone between the two electrodes, they are then sped up by an electrical field. The higher the voltage applied, the more powerful is the thrust generated.
Electrical propulsion, being a lot more fuel-efficient than its chemical cousin—as much as by 10 times—allows a spacecraft to be smaller, lighter, and economical. With the pedal to the metal, the Dawn engines consume only about half a teaspoon of fuel. Its tank holds 937 pounds.
Also, it’s free of the risk of explosion. Xenon is a non-combustible element. It poses no radiation hazard either.
Capable, in theory, of traveling at speeds over 200,000 m.p.h., an ion engine-powered spaceship would beat the Space Shuttle, with could go as fast as 18,000 m.p.h., hands down.
But this swiftness comes at a price: slow acceleration. That’s to say that it’d acquire this phenomenal speed at a far more leisurely pace.
A single Dawn engine, translates about 2,300 watts of electrical power—what two-and-a-half hair dryers would drink up, going non-stop—into 0.02 pounds of thrust as compared to anywhere between 100 and 500 pounds, as provided by a chemical rocket. At that rate, it’d take four days to move from standstill to 60 m.p.h.
Granted, an ion engine won’t zoom off like a Delta II rocket (which carried it into space). For that reason, it couldn’t be used to launch from the Earth’s surface. It simply, wouldn’t be able to escape its gravitational grip.
But once in vacuum, it’ll far outshine its chemical counterpart, which does offer more puissance, but only in short bursts. Where the latter will have to coast (fly, without power) till the next boost, an ion engine will run continually, at a gentle pace, over a prolonged period, and like the tortoise, will leave the bunny in dust. Such spaceships make excellent candidates for long-haul odysseys.
Over the course of its journey, Dawn is expected to ratchet up its speed to about 25,000 m.p.h., which surpasses what any previous vessel has been able to achieve independently, after cutting cord with its launch vehicle.
NASA Glenn is presently working on the next generation of this technology: NEXT (short for NASA Evolutionary Xenon Thruster). At 12 inches deep and 20 inches across, it’ll be bigger, and have a correspondingly greater ballast. It should be ready to roll out at the end of the decade or early, the next, said Patterson.
NASA Glenn has been working on electric propulsion since the 1950s. NASA physicist, Harold Kaufman, designed and built the very first ion engine, in 1959. In 1964, the research center sent it on a suborbital spin on the Space Electric Rocket Test I. It returned home after a successful 30-minute run.
The earlier models used mercury or cesium. But neither was easy to work with. Both had to be heated to turn them into gases over and above being messy and environmentally unsafe.
In the early 1990s, Glenn partnered with the Jet Propulsion Laboratory in the NSTAR (acronym for NASA Solar Electric Propulsion Technology Application Readiness) project to develop a xenon-based electric propulsion system.
In 1997, HRL Laboratories—an aerospace think-tank, now owned by General Motors and Boeing—was the first to equip a geostationary commercial satellite with such an engine with some help from NASA.
Deep Space 1, which lifted off in October, 1998, became the first NASA spaceflight to be powered by it. Dawn inherited the same know-how.
In the future, if such spaceships were to embark on interstellar trips, they’d need to strap on a nuclear reactor—for once they stepped beyond the toasty sphere of the Sun, they’d be out in the cold, literally.
Those waiting to hear whether that will happen soon, are asked not to hold their breaths. For now, the technology will be deployed in keeping commercial satellites in their designated orbits around Earth, and for robotic science expeditions within the solar system.