A low-fuel engine that accelerates a spacecraft slowly over many years could open up new possibilities for solar system exploration.
In May 2015, a small unmanned Nasa spacecraft called Dawn entered orbit around Ceres, the largest object in the rubble-strewn asteroid belt that drifts between the orbits of Mars and Jupiter. Planetary scientists are in their element, gathering clues about the origins of the solar system and the processes that make (and occasionally destroy) entire worlds. Ceres has sufficient gravity to have pulled itself into a spherical shape, but it's not a planet or a moon. Instead it's classified as a 'dwarf planet'.
Remember when we were kids, and we were told that the solar system has nine planets? That classroom narrative is more complicated today than it used to be. For instance, Pluto, once classified as the outermost planet, is now relegated to dwarf planet status.
Many hundreds of celestial objects lie within the conceivable range of our technologies: from satellite moons to asteroids, comets and of course, dwarf planets. Now we have the wonderfully named Xenon Ion Thruster to help us reach some of them. Despite sounding like a device from science fiction, this is not a new idea. Konstantin Tsiolkovsky, the Russian pioneer of astronautics theory, talked about something similar in 1911, and American rocket experimenter Robert Goddard built a prototype in 1916. Over 200 small spacecraft have been augmented with ion thrusters, but it took many decades for mission planners to risk using this delicate technique for voyages into the depths of the solar system.
Nasa's Deep Space 1 (DS1) was launched in October 1998 aboard a Delta II rocket, bound for a flyby rendezvous with asteroid 9969 Braille in July 1999 and comet 19P/Borrelly in 2001. The Delta's upper stage pushed the little craft out of Earth's gravitational influence. After that almighty kick, DS1 was on its own, with a mere 81.5 kilograms of xenon propellant on board for the entire voyage. That's all it needed. Over the course of a 20-month engine firing, DS1 accumulated 11,000km/h of additional velocity, using an ion propulsion system whose thrust was equivalent to the weight of a sheet of A4 paper.
In the 'gridded' ion thruster, as used by both the DS1 and Dawn spacecraft, energetic electrons generated by a hot cathode are fired at passing atoms of xenon fuel, knocking off negatively charged electrons and leaving a stream of positively charged xenon ions and loose electrons. This hot plasma is drawn toward a series of charged metal grids at the rear of the engine. Carefully structured potential differences between the grids cause the plasma to accelerate to 36km/s and shoot out of the engine's open end. The electrons from the beam that bombarded the xenon atoms are then dumped into the exhaust trail, preventing any undesirable build-up of charge in the engine.
The thrust from a typical ion engine is so minuscule you wouldn't notice it pushing against your hand, but this is the most efficient propulsion system yet devised. Dennis Gilliam, formerly of the TRW company in California, has tested rocket sledges in the New Mexico desert, yet he's happy to explain why an ion engine sometimes outranks the noisy monsters he works with. 'We think in terms of specific impulse, a measure of thrust per unit time derived from a given quantity of fuel,' he explains. 'The rockets you see going up from Cape Kennedy are only 35 per cent efficient, and the fuel's gone in a few minutes. Ion engines are 90 per cent efficient, and a tiny fuel tank's good for hundreds of days of operation. You get a tremendous acceleration, albeit very gradual. You won't get an inch off the ground with ions, but they're great for space.'
Most planetary missions depend on momentum generated on launch day by the carrier rocket. There isn't the capacity to lift heavy fuel tanks for use by the probe itself. The best bet for missions further out than Mars is often to fall towards intermediate planets and pick up momentum from gravitational slingshot manoeuvres. Scheduling is dependent on planetary alignments calculated many years in advance. According to Jim Green, director of Nasa's planetary science division, 'planets are on well-established orbits, like roads. With ion engines, you don't have to chase them. You can cut across the roads and reach any locations you want. The fuel load for an ion engine is small and easily accommodated.'
It would take Dawn four days to accelerate in space from a standing start to the cruising speed of a car on a motorway, but maintaining that acceleration for 678 days continuously enabled the spacecraft to increase its velocity by 4.3km/s, or 15,450km/h, during its outward voyage, breaking DS1's previous record. In November 2010 Nasa announced that its Evolutionary Xenon Thruster (NEXT) prototype had completed an astonishing 48,000 hours (five and a half years) of continuous operation during ground tests in a vacuum chamber at Nasa's Glenn Research Center in Cleveland, Ohio. It consumed just 860kg of xenon propellant.
For small spacecraft at least, velocity changes of 160,000km/h are now a realistic proposition, albeit over spans measured in years rather than weeks. A conventional chemical rocket would require 10,000kg of propellant to achieve the same result.
Apart from a small xenon fuel tank, a source of electricity is required to charge up the grids of an ion engine. The DS1 and Dawn missions both depended on solar panels for this task, but physicist and former astronaut Franklin Chang-Diaz, a veteran of seven space shuttle missions, believes that ion propulsion could be much more powerful if only a different source of electricity could be harnessed. In 2005 he founded the Ad Astra Rocket Company in Texas to put his theories into practice.
If Chang-Diaz is right, a spacecraft sufficiently large for a human expedition to Mars could reach its target just two months after leaving Earth orbit. More than $30m in funding raised from private investors and Nasa suggest that his idea, the Variable Specific Impulse Magnetoplasma Rocket (VASIMR), might be worth a shot.
'Efficiency increases as a rocket's exhaust gets hotter,' Chang-Diaz explains. 'There's a huge payoff from heating the xenon into superhot plasma, but flowing this past charged grids essentially would melt them, so you can't go to extremely high power with current ion engines.'
VASIMR has no grids. Instead it uses high-frequency radio waves to heat the xenon fuel, ionising it and turning it into extremely energetic plasma. A further system of radio frequency control accelerates the stream and ejects it from the rear of the engine. At no time does any plasma touch the engine's inner walls. It is constrained by a powerful magnetic containment system, a close cousin of the technologies used in nuclear fusion experiments.
One downside is that magnetic containment is tricky to stabilise and calls for strong electromagnetic fields that could interfere with delicate electronics elsewhere in a spacecraft.
The other snag is even more problematic. Dawn's ion thrusters drew 2.6kW of power from the probe's solar arrays. A large VASIMR engine capable of sufficient thrust for a human expedition to Mars would need an input in the megawatt class. This could only come from a nuclear power source.
Radioisotope thermal generators (RTGs) containing tiny amounts of plutonium have been used in a variety of space probes to generate electricity, because solar panels aren't effective beyond the orbit of Mars, where the sun dwindles to a speck. Nasa's Pioneer, Voyager and Galileo missions to the outer planets all used RTGs.
The most recent Nasa probe, augmented with nuclear materials, was Cassini, launched in 1997 with more plutonium (32kg) than on any previous mission. This generated 900W - enough electricity to power a microwave oven. It also generated massive controversy. Nasa conceded the possible dangers of Cassini making an 'inadvertent re-entry' if its carrier rocket failed, but the agency stressed that the casings around the plutonium were designed to survive any kind of disaster.
The risks from RTGs may have been slight but the consequences of failure would have been disastrous. Plutonium is highly carcinogenic. Any break-up of an RTG in the atmosphere could have generated a significant cancer risk for millions of people.
Incredibly, there was a time when Nasa and other agencies thought that full-on nuclear reactors could propel us through space by pumping hydrogen through a plutonium core and ejecting it in a ferocious blast of superheated exhaust. In July 1959, a nuclear rocket codenamed Kiwi was successfully tested at Jackass Flats, Nevada, a remote desert facility. The Nuclear Energy for Rocket Vehicle Applications project (NERVA) was soon under way, firing radioactive plumes into the sky.
Throughout the 1960s, NERVA engines were fired successfully on 23 occasions, generating up to 34,000kg of thrust, but in 1963 radioactive pollution from atomic bomb tests led to an international ban on all nuclear tests in the atmosphere, in space, or underwater. NERVA was cancelled a few years later.
VASIMR most assuredly is not dependent on a full-tilt nuclear reactor. The nuclear components generate the electricity that runs the engine, but not the thrust that drives the spacecraft. Even so, a substantial plutonium load will have to take on the task.
Chang-Diaz insists, 'If humans are ever going to explore Mars and beyond, we do have to develop high-power electricity sources. Nuclear submarines are common, and have been for half a century. Something similar has to happen in space.'
A plasma trail to Jupiter
In the absence of a nuclear power source, even a solar-powered VASIMR ' rated at 200kW ' could propel a small probe close to the sun, where it could pick up tremendous momentum from a gravitational slingshot. According to Chang-Diaz, 'this would get a spacecraft to Jupiter in under two years. Otherwise, that trip takes about six.'
Nasa is studying propulsion systems that could be suitable for a mission to Jupiter space, where three tantalising icy moons await: Callisto, Ganymede and Europa. Callisto is the most heavily cratered object in the solar system. Ganymede is the largest satellite we've ever found; three-quarters the size of Mars. If it orbited the sun instead of Jupiter, it would be classed as a planet. Its mantle is composed of ice and silicates, and its crust is a thick layer of water ice.
Europa is the top prize, though. Its icy crust almost certainly conceals a deep, warm ocean. Surface ice is stained with traces of simple organic chemicals. There's more chance of finding life on Europa than on Mars, and xenon ions could help get us there.