Things are changing at Nasa. In October it saw the launch of a spacecraft with an engine that thunders with all the force of a small piece of paper resting on your hand. But, according to researchers working on the mission, the almost imperceptible thrust of this ion drive could be the key to the future of space exploration.
Deep Space 1 (DS1) is only 2,5m, uncrewed and launched with the help of three chemical rockets. It may not sound like much, but the launch marks a new era: it is the first of Nasa’s “New Millennium” missions, a programme created in 1994 amid talk of changing to “faster, cheaper, better” technology. Nasa is now aiming to become a frequent flyer, shooting simple craft into space on a monthly basis.
Eventually, working with academic and industrial partners, Nasa hopes to produce spacecraft that are almost disposable; cheap to build, easy to launch and so frequently produced that failures become an irritation rather than a disaster.
The DS1 project took just more than three years and $150-million to bring to the launch pad. These “quick, cheap launches” will enable new generations of technology to be tried almost as soon as they are ready, says Peter Ulrich, director of Advanced Technologies and Mission Studies in Nasa’s Space Science Division. This method also acts as a safeguard against expensive mistakes.
“Flying frequent, smaller missions with simple payloads means we don’t lose the whole programme with a simple failure,” he says.
“Trying to cobble everything together on a single mission meant you lost six or eight instruments – and years of development time – with a simple accident. That is not the way to enter the new millennium.”
The initial missions are designed to test the performance of new technologies; first up, after a wait of almost four decades, is the ion drive.
“They have been trying to find a mission to fly it for years, but nobody would take the risk,” says Marc Rayman, chief mission engineer for DS1. The ion drive was invented in 1959, and has been featured in science fiction stories like Star Wars and Star Trek. Nasa gave it a few (largely unsuccessful) trials on spacecraft that put their trust in the “traditional” chemical fuels. Now, say Nasa scientists, there is no reason why ion drives should not take them into the future.
The drive works on the same reaction principle as a conventional rocket; but instead of being thrust forward by expanding gases, it is propelled by a stream of ions. Solar power, gathered by DS1’s panel-covered wings, provides the energy to strip electrons from atoms of xenon (the gas used in photographic flashbulbs), turning them into positively charged ions.
These ions are attracted to a negatively charged metal grid at the back of the engine and rush through it at 99 200km per hour, leaving a ghostly blue trail. A second stream of electrons outside the spacecraft neutralises the ion beam, preventing the ions from being pulled back in by the charged grid.
Xenon is the preferred fuel because it is inert; it tends not to react with other chemicals, so it is easy and clean to work with. Previous ion drives, using vaporised mercury, coated the spacecraft’s skin with metal as the vapour condensed. This is what put the ion drive on the shelf at a time when Nasa had plenty of money for expensive alternatives.
The ion drive scores in long-haul space travel; it gives 10 times as many miles per gallon as conventional rocket fuels. The weight of the fuel is what pushes up launch costs for long journeys. DS1 will carry 82 kilos of xenon gas, which is expected to last about 20 months. Over a long distance, like the 4,8-billion- kilometre trip to Pluto, an ion drive can accelerate to much higher speeds than a chemical rocket.
Pluto’s distance from the sun does rule out solar power for the last leg of the journey, so the ion drive spacecraft would have to coast part of the way. Even so, the drive could still knock a couple of years off the 12 years it would take a conventional rocket.
The ion drive is just one of 12 technologies to be tested on this trip. DS1 is also carrying a new instrument to investigate the solar wind, a stream of charged particles emitted by the sun.
However, there is a chance that Pepe (Plasma Experiment for Planetary Exploration), designed for data- gathering in future Nasa missions, may be disturbed by the stream of charged particles that the ion drive pumps out.
The ion drive could also hamper the performance of Micas, a compact, lightweight imaging instrument that works across the ultraviolet, visible and infrared regions of the spectrum. Its 10cm diameter telescope acts like a scanner, sweeping across the object of interest and taking light readings. Some ground tests have indicated that the ion drive gives out an ultraviolet glow, which could disrupt Micas.
Rayman thinks it “pretty unlikely” that the ion drive will cause a problem with any of the instruments, but the object of the exercise is to find out for sure. “The purpose of the mission is to take these risks so that future missions don’t have to,” he says.
Micas is a particularly important venture because it supports one of Nasa’s most cherished goals: spacecraft autonomy. Micas’s imaging power will provide DS1’s eyes. The data it gathers will be fed to AutoNav, a self-navigation system that compares the data with its very own “road map”: AutoNav holds the orbits of 250 asteroids and the position of 250 000 stars; every week a few glances at the landmarks of outer space and a quick check with the computer map should enable DS1 to keep itself on course.
That course, if all goes well, includes a very close encounter with an asteroid, 1992 KD in July 1999. This will be a crucial test for AutoNav. The plan is to come within three miles of the asteroid. But in the last hours of the approach AutoNav will have to decide if it is safe to get in as close as three miles and execute what Nasa terms a “bold manoeuvre” all on its own without intervention by mission control.
As the spacecraft will be speeding past the asteroid at 53 600km per hour, there is precious little room for error.
DS1’s primary mission will be completed by September 1999 when all the new technology will have been tested. But if the craft is still in good shape and Nasa chooses to continue the mission, it will visit an object known as Wilson-Harrington in January 2001. This body is believed to be either a dormant comet or in the process of changing from a comet to an asteroid. It has not been observed to behave like a comet – spewing gas with a coma and tail – since 1949 and it is unusual for a comet to exhibit this type of change in behaviour.
Finally, if all goes well, in September 2001 the spacecraft will hop into the tail of Borrelly, a comet that is very much alive.
“It’s a dangerous place to be, but if we can get in there we’ll get some very useful scientific data,” says Rayman. It may knock out most of DS1’s instruments, but by that time every one of the mission goals will have been achieved so no one at Nasa will worry. New Nasa is almost cavalier: danger and risk are the watchwords of the New Millennium programme.
“This is really an experiment in management and approach, and there’s inherent risk in doing something differently – we won’t deny that,” says Ulrich.
But in the new era, there will not be any stigma attached to a mission that goes wrong. “This is a more robust programme: it can take the shock of a failure much better,” he says.
For details of the Deep Space 1 visit