Superconducting magnets could help a fleet of spacecraft fly in precise formation without using up limited fuel reserves, two groups of researchers say. But others foresee problems with the technology.

Many proposals for groundbreaking space missions require multiple spacecraft to fly in formation, including NASA's Terrestrial Planet Finder, which would hunt for Earth-like planets around other stars, and the Laser Interferometer Space Antenna (LISA), which would search for ripples in the fabric of space called gravitational waves.

One way to keep spacecraft in the right arrangement is to use thrusters, which fire jets of gas to push a craft in the opposite direction. Unfortunately, thrusters can limit the lifetime of a mission because they rely on limited supplies of fuel.

But two groups of researchers are developing a technology that replaces thrusters with electromagnets.

In this scenario, each spacecraft in the fleet would be outfitted with coils made of superconducting wire. Running electric currents through the coils turns each spacecraft into a magnet with a north and south pole.

By adjusting the current, the orientation of the poles can be changed to either attract multiple spacecraft towards each other or push them farther apart, keeping them at the desired distance.

Solar arrays

A group headed by David Miller of MIT's Space Systems Laboratory in Cambridge, US, has been testing out the concept in the laboratory using mock spacecraft.

The test vehicles squirt jets of air down from their bases in order to hover nearly frictionless on a glass table to simulate floating in space. Using superconducting coils, the vehicles attract and repel each other, and even move sideways relative to one another, (see video at right).

"The biggest advantage is that you have no fuel that can run out," Miller told New Scientist. "The magnetic coils work purely on electrical energy, which you can generate through solar arrays that point at the Sun."

Cooling system

The superconducting coils would need to be kept at low temperatures in order for them to work properly – a temperature of 77 K (-196 °C) is required for one commercially available superconducting material, for example. This temperature could be achieved with a combination of insulation and an electrically powered cooling system, Miller says.

But there are some potential drawbacks to the technology, which is called electromagnetic formation flight (EMFF).

One worry is that the electromagnetic fields generated by the coils could interfere with electronics on board the spacecraft, says Fred Hadaegh, who heads NASA's formation flying efforts at the agency's Jet Propulsion Laboratory in Pasadena, California, US.

Miller counters that the magnetic fields do not need to be very strong, and are weaker than Earth's naturally occurring magnetic field. Some sensitive equipment could be wrapped in a thin layer of nickel-iron alloy called mu-metal, which shields against magnetic fields, he says.

Earth interference

For some equipment, though, shielding is not an option. Wrapping a camera in mu-metal would block light from entering it, making it useless. And shielding a radio antenna would prevent it from sending or receiving signals. But Miller says such devices could be protected by putting small secondary electromagnet coils next to them, tuned in a way that locally cancels the field from the main coils.

Earth's naturally occurring magnetic field could also be a problem for the technique. It would pull on the magnets, which could set the spacecraft spinning.

But a team led by Shin-ichiro Sakai of the Japan Aerospace Exploration Agency (JAXA) believes it has a way to overcome this problem. They recently presented their work at a conference on formation flying in Noordwijk, The Netherlands.

To prevent Earth's magnetic field from disturbing the spacecraft, they propose switching the polarity of the magnets several times each minute. This would prevent the spacecraft from building up any unwanted spin, they say, without interfering with forces between the magnets needed to keep the spacecraft in formation.

Military funding

Hadaegh's team is responsible for developing formation flying technology for NASA's Terrestrial Planet Finder, a mission that the agency postponed indefinitely in 2006, in part because of concerns that formation flying techniques were unproven.

He says his team studied EMFF for possible use on TPF. "As a technologist, I get extremely excited about seeing new technology advances, so I welcome any type of breakthrough ideas," he told New Scientist. But he said the team decided that traditional thrusters were a better choice for the mission.

TPF would reside far from any gravitational disturbances, at a spot in space where Earth's gravity balances that of the Sun. In this quiet zone, very little fuel would actually be needed for the thrusters to maintain the spacecraft formation, he says. And he says he is not satisfied that problems with EMFF like the need for shielding have been solved.

Still, the concept is promising enough that the US Defense Advanced Research Projects Agency (DARPA) is funding the MIT group to develop it for possible use in a project called F6. That project aims to create clusters of little spacecraft that can collectively do everything a single, large satellite can.

Sputnik’s Legacy - Learn more about humanity’s first 50 years in space in our special report.

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