ONE HUNDRED years from now, a space probe the size of an ice hockey puck will coast silently past an insignificant star system in a remote corner of the Milky Way galaxy. A collection of microscopic cameras and sensors will record the star system鈥檚 intimate behaviour and the data will be beamed home using an infrared laser focused with an inflatable mirror. The information will take four years to reach the group of ageing scientists who run the mission 鈥� a mere fraction of the forty years they have waited to receive it.
Science fiction? Perhaps not. 杏吧原创s are now seriously evaluating the problems of interstellar travel. According to a group of scientists, engineers and space enthusiasts who met recently at New York University to discuss interstellar travel, a probe designed to visit our nearest neighbour, Alpha Centauri, could one day be built. It would travel at a tenth of the speed of light, protect itself against impacts with dust particles and be piloted by a computer capable of controlling the craft without help from Earth.
But before visiting the stars, the technologies needed to get there should be tested on a shorter mission within the Solar neighbourhood, says Claudio Maccone, a space scientist at Alenia Spazio, a space technology company based in Italy. The most promising target is the solar focus, a mere 80 billion kilometres away, a distance light travels in a little over three days. This is where radiation from distant stars is brought to a focus by the Sun鈥檚 gravitational field, which would allow a visiting probe to resolve objects at the centre of the Galaxy that are a few hundred thousand kilometres apart. Maccone has named the mission FOCAL and believes it could be launched within the next few decades.
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One of the drawbacks of interstellar missions is the time required to travel such vast distances. The Apollo spacecraft took three days to travel the 400 000 kilometres to the Moon. A probe travelling at 200 000 kilometres per hour would take 45 years to reach the solar focus and the 4.3 light year journey to Alpha Centauri at 100 million kilometres per hour, a tenth of the speed of light, would take almost as long. The total length of each mission would be even longer since the probe would have to be approved before the launch.
The average working lifespan of a scientist, however, puts a practical limit on the length of a space mission, argues Robert Forward, an independent space consultant based in California. He believes that such a mission should aim to produce results within the working life of the scientists that run it and the people who fund it. There would be little incentive for scientists to design and build experiments when they would never see the results. 鈥淚t doesn鈥檛 make sense to have a mission that lasts more than fifty years,鈥� he says.
To keep missions to a reasonable length, the vehicle would have to travel at enormous speed. And because a heavy craft is more difficult to push, weight becomes a crucial factor in the design. 鈥淭he sensible minimum size for a package of scientific instruments is a few kilograms. Add the power supply and the radio and it鈥檚 a few tens of kilograms,鈥� says Jordin Kare, a sensor specialist at the Lawrence Livermore National Laboratory in California. Eventually, he expects this to drop by a factor of 10 as equipment such as cameras and sensors become smaller and lighter. However, Kare says that reductions in size beyond this are unlikely. A camera aperture, for example, cannot be made arbitrarily small without the image suffering. One way to produce major weight benefits is by replacing actuators, mechanisms that position cameras and aerials, with 鈥渕uscle鈥� wire. This changes length according to the current passing through it and can be used to push and pull objects into place.
Another way to make the craft lighter is to leave the propulsion units and the fuel at home. According to Ed Belbruno, a mathematician and expert on spacecraft trajectories at the University of Minnesota, one option is to accelerate the craft by bombarding it with a beam of high-energy particles or laser light produced from a base within the Solar System.
This approach is fraught with problems that have yet to be solved. For example, any beam naturally spreads out as it travels through space dramatically reducing the amount of energy it can transfer to the probe. Even if a way can be found to stop the beam from spreading, the amount of energy required is enormous. Curt Miliekowsky, a nuclear power engineer based in Sweden, has calculated that the energy needed to create the beams would be equivalent to the world鈥檚 monthly consumption of electricity. One way to provide the energy would be to build solar-powered facilities orbiting the Sun or to mine the asteroid belt for resources that could power a laser or particle accelerator.
Another problem is that interstellar space is filled with small particles that represent a significant hazard to a craft travelling at such speeds, says Geoffrey Landis, a physicist at NASA鈥檚 Lewis Research Center in Cleveland. The most common are hydrogen atoms, on average about 100 000 of them in every cubic metre of space, and the metal skin of the vehicle would have to able to absorb the energy generated by their impact. A collision with a grain of dust weighing only 0.1 grams, on the other hand, would create enough energy to destroy the craft. In interstellar space, the density of small particles of silica and ice is about 100 000 per cubic kilometre and so protection is essential.
Plasma shield
Landis suggests using a plasma shield 鈥� a volume of ionised gas stretching 10 metres in front of the craft 鈥� to absorb the energy of the dust grains in the same way that the Earth鈥檚 atmosphere protects the surface from impacts with micrometeoroids. The plasma would be contained in a powerful magnetic field generated by two superconducting hoops attached to the craft. Although the hoops would be 30 metres in diameter, Landis calculates that they could be powered by as little as a tenth of a watt. This could be supplied by the sort of radioisotope thermoelectric generators that have been used successfully on spacecraft such as Pioneer, Voyager and Galileo. They work by converting heat produced by radioactive materials into electricity and could easily last the trip if powered with a material that remains radioactive for a long time.
Little can be done to protect against collisions with larger particles. An impact with a 10-gram particle would produce a 1-kiloton explosion, says Landis. Fortunately, such gravel is rare in interstellar space. In the 1980s, James Wolfe, an astrophysicist at the NASA Ames Research Center in California, used the density of gravel in the Solar System to calculate that a probe would be likely to encounter only one such particle on a 20-light-year trip. In interstellar space, Landis expects the probability to be even lower.
Assuming the probe reaches its destination, sending data to Earth presents yet another challenge. Since any signal will be very weak by the time it reaches the Solar System, data must be sent at a precise frequency so that they can be filtered from background noise and radiation produced by the star. Robert Cesarone, an engineer at the Jet Propulsion Laboratory in Pasadena, says that the most promising method is to send the data using lasers. Solid-state lasers are several times lighter than radio transmitters and can send up to fifty times more information. But given the power restrictions, even that amounts to no more than 10 bits per second, he explains.
This method has a big drawback: to focus the beam towards Earth requires a mirror with a diameter of 3 metres. And to receive the signal requires an Earth-orbiting telescope as big as the largest ground-based instruments available today. With current technology, the mirror alone could weigh tonnes, making the craft unfeasibly large.
While the design for an interstellar spacecraft verges on science fiction, the FOCAL mission could stand a real chance of reaching the solar focus within fifty years from a launch, says Maccone, if it was built using the technology that is available now. For example, instead of accelerating the craft with impossibly powerful beams, he suggests using the gravity of the Sun or a large planet like a catapult to hurl the craft out of the Solar System. The same method was used to catapult Galileo towards Jupiter. In addition, the craft will use a solar sail, little more than a large sheet of plastic, to harness the continuous flow of photons away from the Sun in the same way that a sheet of canvas catches the wind to propel a yacht on Earth.
Coated with aluminium, the sail also doubles as a radio telescope dish to scan the heavens. And to increase the area of coverage, FOCAL will carry a second inflatable dish connected to the craft by a 20 kilometre tether. By rotating the craft, the antenna will sweep a circle with a radius of 20 kilometres.
Target practice
Aiming the spacecraft could be tricky. It is possible to view any distant star at its focus by positioning the spacecraft within 200 metres of a straight line through the Sun towards the object being studied. Even with FOCAL鈥檚 20 kilometre coverage, finding a focus would be like hitting an envelope on Mars with a dart thrown from Earth, explains Maccone.
The best object to study is the centre of our Galaxy because it contains a high density of stars that appear to overlap. This provides a much larger target to aim at and should contain many objects of interest. Maccone has computed a trajectory that will take FOCAL out of the plane of the Solar System towards Elnath, a star in the constellation of Auriga in the northern sky.
Much of the technology needed for FOCAL is available now. Tethers have been used in space since the 1960s and a solar sail has been built and tested in laboratory conditions by the World Space Foundation, a space technology institute in California. In addition, an inflatable antenna designed by the Jet Propulsion Laboratory and L鈥橤arde, an aerospace company also in California, is due to be tested on a shuttle flight in 1996. The entire instrument is made of plastic film and comprises a 14-metre dish coated with aluminium to reflect radio waves, supported by 28-metre struts. During the flight astronauts will fill the device with nitrogen to see if the structure can maintain the precise shape needed to broadcast and receive radio signals.
Perhaps the biggest obstacle is finding the money to fund FOCAL, estimated at more than $500 million. In 1993, the European Space Agency rejected a proposal from Alenia Spazio to build and launch the vehicle early next century. Maccone says the mission is clearly too ambitious for one space agency alone. An international collaboration, on the other hand, could have FOCAL heading out of the Solar System by 2020. He believes the vehicle could be built in Europe, launched by the Russians and tracked through deep space by the Americans.
One way of justifying the expense is to carry out other experiments at the solar focus and throughout the journey. For example, Maccone suggests that the 1-metre mirror used for focusing the laser beam towards Earth could also be used as an optical telescope. Such an instrument could help carry out parallax measurements 鈥� the only reliable way to determine distances to stars. The technique involves comparing images of the same star taken from two different places. But the maximum distance that can be measured is limited by the distance between these sites (the baseline). The most distant measurements now possible are made by comparing photographs of stars taken on opposite sides of the Earth鈥檚 orbit round the Sun. This allows measurements on stars up to 300 light years away.
FOCAL could create a baseline several hundred times longer allowing measurements to be made on stars up to 30 000 light years distant. This data could then be used to calibrate less reliable methods of measurement used for larger distances. In theory, the results could be used to work out the size of the Universe and the rate at which it is expanding, and help determine the intrinsic brightness of stars, their size and mass.
For the moment, however, there is little incentive to visit the stars or carry out a mission like FOCAL. All that could change if we found an Earth-like planet orbiting a nearby star or convincing evidence of extraterrestrial intelligence, argues Roger Gilbertson of Mondotronics, a Californian technology firm that makes muscle wire. 鈥淭hat would provide a real incentive to get images back by whatever means we can,鈥� he says. 鈥淓ven if it means sending an Instamatic with 12 snapshots.鈥�