Did you know NASA is exploring the asteroid belt with an ion-propulsion spacecraft? Neither did we.
This year, we’ve had the thrill-packed landing of Curiosity on Mars, and then Voyager 1 blew our collective mind by officially exiting our solar system. Yet, another equally amazing interplanetary space mission has gone strangely overlooked.
In early September, the Dawn spacecraft left its orbit around the asteroid Vesta after a yearlong exploratory mission, and in two and a half years it will reach Ceres, a “proto-planet” located between Mars and Jupiter and the largest asteroid in our planetary system.
Dawn is making this remarkable journey using ion propulsion: the electrical excitation of atoms, in this case xenon atoms, creating a plume of illuminated thrust powered only by the electricity generated by its solar array. It’s a technology capable of driving Dawn at speeds ten times that of conventional chemical rockets, and it’s also the stuff of science fiction — the technology that drives the TIE fighters of Star Wars and Star Trek’s shuttlecraft Galileo.
Marc Rayman, a mission director at the Jet Propulsion Laboratory (JPL) in Pasadena, California, is Dawn’s chief engineer, and it was his idea to promote the ion propulsion aspect of the mission.
Something very much like ion propulsion was proposed as early as 1906 by rocketry pioneer Robert Goddard and tested in laboratories on Earth in the second half of the 20th century, but the technology wasn’t put to use in space until 1998 on a probe called Deep Space 1. “There’d been a program within NASA to develop this technology for a long time,” Rayman says, “but it was usually not called ‘ion propulsion’ but SEP — Solar Electric Propulsion. The technologists that had been working on the system were going to provide it to DS1 didn’t want to call it the ‘ion propulsion’ system — they didn’t see that calling it ion propulsion was not only accurate but would excite people about the project. ‘Electric propulsion’ sounds like it might be a Toyota Prius — it doesn’t sound that cool — but if you call it ‘ion propulsion,’ people connect that to Star Wars and Star Trek.” Which is cool.
Deep Space 1 performed so well that after it finished its mission in 2001, Rayman and his team proposed to reroute the craft and send it in pursuit of the comet Borrelly, where it eventually obtained the first close-up photographs of a comet nucleus. “It was an absolutely spectacular experience — in fact, it was truly one of the high points of my entire life when that picture came down to mission control. But we did that in September of 2001, when the world news was focused on another topic. It was 11 days after 9/11, and scientifically and technologically it was no less successful because of that but it wasn’t a story people were interested in hearing about at that time.”
Strangely, Dawn seems to be suffering a bit of the same fate in the media, as its success has been somewhat overshadowed by other news, but its asteroid study mission is every bit as daring as Deep Space 1’s comet encounter. After being launched from the Earth by a traditional chemical rocket, Dawn whipped around Mars, using the planet’s gravity well to help it accelerate and finally reached Vesta, a huge object that is in some ways a mighty, 300-mile wide impact crater floating in space. Almost half of its mass was blasted away by a collision with another object that occurred around a billion years ago — and one sixth of the meteorites that have fallen on Earth can be traced back to this fantastic impact on Vesta — far more than objects that originated from Mars and the Earth’s moon.
Dawn’s next destination, Ceres, is the largest object in the solar system that isn’t a planet or a star. In fact, it’s classified as a dwarf planet, one of the reasons the similarly sized Pluto was downgraded from planet to dwarf planet status several years ago. Ceres is a spherical mixture of rock and ice and may contain more fresh water than Earth. And after Dawn wraps it up at Ceres? “Ceres is such a big place that I don’t think we’ll ever run out of things to observe there,” Rayman says, “but Dawn is funded for a certain duration in order to accomplish a certain set of objectives, and at the end of that if the spacecraft is still healthy, NASA might propose an extended mission. That’s still so far off that we’re not putting a lot of energy into that right now. We probably will have plenty of xenon even at the end of the primary mission.”
Rayman has become one of NASA’s most entertaining lecturers — he even appeared on the TV special How William Shatner Changed the World, talking about the connections between Star Trek technology and the ion propulsion system he works on. But his gateway into science wasn’t Star Trek — it was The Wizard of Oz. As he remembers, “When I was 4 years old I used to be very afraid of witches, and I was out one evening with my parents and saw a meteor, and I was afraid because I thought it was a witch flying through the sky, and my parents assured me it was not and probably said it was a shooting star, a rock from space burning up in the earth’s atmosphere. And I just thought that was so cool and I’ve been interested in space ever since. And although I was a huge Star Trek fan, my interest in science came first. I’ve been fascinated by the universe and have had a passionate hunger for an understanding of the cosmos my whole life.”
Dropping Old-School Ion Science
An engineer and a technician check out a prototype ion engine in the Electronic Propulsion Research Building at the Lewis Research Center (now NASA’s Glenn Research Center) in 1961. Ion engines use electrostatic charge, something like pulling hot socks out of a clothes dryer. The electrostatic charge pushes the socks away from each other. The fuel used by this device is xenon, a gas that is four times heavier than air. Although ion engines have been around for decades, they weren’t used by NASA to propel spacecraft until the late 1990s.
If you’re planning an intergalactic war, is ion propulsion the best way to power your marauding spacecraft? The Star Wars universe’s Galactic Empire made their decision to go with TIE (Twin Ion Engine) fighters, but NASA engineer Marc Rayman isn’t so sure they chose wisely. “Ion propulsion is effective because it’s so efficient, but if someone’s trying to kill you, you care more about maneuverability than your gas mileage,” he points out. Okay, but what about those big TIE fighter solar arrays? They got those right, didn’t they? Nope. “They’d be useless,” Rayman attests. “When you’re far away from a star in interstellar space, what good are those going to do? Those solar arrays point to the left and right and if the sun’s in front of you, you’re not getting much energy. It would be difficult to make the case that that’s an optimal design from any standpoint except for having a cool spaceship.”
2. High-gain Antenna
This parabolic dish is for primary communication, but it must be pointed at the Earth to be used.
3. Solar Array
With the panels fully extended, they stretch 65′ across — the length of a tractor-trailor rig.
4. Ion Thruster
This is one of the three electric engines that keep Dawn moving, but only one is in use at any given time.
5. Low-Gain Antenna
There are three of these on the Dawn, allowing the craft to comunicate with Earth from any position.