(Image: Michael Kelley/Getty) The VASIMR ion engine could – if powered by an onboard nuclear reactor – take astronauts to Mars in just 39 days (Illustration: Ad Astra Rocket Company) The Bussard ramjet would ionise interstellar hydrogen and then collect it using an electromagentic field for use as fuel. In this illustration, an onboard laser heats the ionised gas; the laser could be used to trigger fusion pulses for propulsion (Image: NASA-MSFC) Solar sails can pick up a lot of speed, thanks to the constant pressure from sunlight. In the future, they might be able to traverse the solar system in the span of just a few years (Illustration: Rick Sternbach/Planetary Society) Positively charged wires extending from a spacecraft would repel the heavy positive ions in the solar wind (Illustration: Allt om vetenskap/alltomvetenskap.se) Wormholes are distortions in the fabric of space that can link two distant locations, sucking in objects at one end and spitting them out at the other (Illustration: NASA/Les Bossinas)

In 1961, Yuri Gagarin became the first human being to reach outer space. Eight years later, Neil Armstrong and Buzz Aldrin made it to the surface of the moon. And that is as far as any of us has ventured.


Apart from the mundane problems of budgets and political will, the major roadblock is that our dominant space-flight technology – chemically fuelled rockets – just isn’t up to the distances involved. We can send robot probes to the outer planets, but they take years to get there.

And as for visiting other stars, forget it. As an example of why, the Apollo 10 moon probe is currently listed as the fastest manned vehicle in history, having reached a maximum speed of 39,895 kilometres per hour. At this speed, it would take 120,000 years to cover the 4 light years to Alpha Centauri, the nearest star system.

So if we want to explore the depths of deep space and journey to Alpha Centauri and beyond, we’re going to need some new technologies. Here, we look at 10 of the most intriguing.

The technologies range widely in their plausibility. Some, we could more or less build tomorrow if we wanted to, while others may well be fundamentally impossible.

Ion thruster

Conventional rockets work by shooting gases out of their rear exhausts at high speeds, thus generating thrust. Ion thrusters use the same principle, but instead of blasting out hot gases, they shoot out a beam of electrically charged particles, or ions.

They provide quite a weak thrust, but crucially they use far less fuel than a rocket to get the same amount of thrust. Providing they can be made to keep working steadily for a long time, they could eventually accelerate a craft to high speeds.

They have already been used on several spacecraft, such as Japan’s Hayabusa probe and Europe’s SMART-1 lunar mission, and the technology has been improving steadily.

A particularly promising variant is the variable specific impulse magnetoplasma rocket (VASIMR). This works on a slightly different principle to other ion thrusters, which accelerate the ions using a strong electric field. Instead, VASIMR uses a radio-frequency generator, rather like the transmitters used to broadcast radio shows, to heat ions to 1 million °C.

It does this by taking advantage of the fact that in a strong magnetic field, like those produced by the superconducting magnets in the engine, ions spin at a fixed frequency. The radio-frequency generator is then tuned to that frequency, injecting extra energy into the ions and massively increasing the thrust.

Initial tests have been promising, and if all goes well, VASIMR could be used to take humans to Mars in 39 days (see illustration).

Plausibility: just a few years away

Nuclear pulse propulsion

If some of the ideas here strike you as a little unlikely, this one will seem downright reckless. The notion here is to power your spacecraft by periodically throwing a nuclear bomb out of the back and setting it off.

Nuclear pulse propulsion was studied seriously by the US government’s military technology agency DARPA, under the code name Project Orion. The aim was to come up with a design for rapid interplanetary travel.

The design DARPA came up with was huge even by today’s standards, and was built to be a giant shock absorber, with heavy radiation shielding to protect the passengers.

It seemed workable, but there were concerns about fallout if it was launched in the atmosphere as planned. The project was eventually dropped in the 1960s when the first nuclear test bans came into force.

Despite these worries, the Orion design remains one that could be built using existing technology, and some researchers are still coming up with new approaches to nuclear pulse propulsion. Theoretically, a nuclear-bomb-powered ship could reach up to 10 per cent of the speed of light, allowing a journey to the nearest star in about 40 years.

Plausibility: perfectly possible, if a tad hazardous

Fusion rocket

Nuclear pulse propulsion is far from the only space-flight technology that depends on nuclear power.

For instance, nuclear rockets could use the heat from an onboard fission reactor to expel gases, providing thrust. But in terms of power, these pale in comparison to fusion rockets.

Nuclear fusion, in which the nuclei of atoms are forced to join together, could produce vast amounts of energy. Most designs for fusion reactors drive the reaction by confining the fuel in a magnetic field, using a device called a tokamak.

Unfortunately, tokamaks are prohibitively heavy, so designs for fusion rockets tend to focus on another method of triggering fusion, called inertial confinement fusion.

This design replaces the tokamak’s magnetic fields with high-powered energy beams, usually lasers. These blast a small pellet of fuel so intensely that the outer layers explode. This in turn crushes the inner layers, triggering fusion. Magnetic fields could then direct the resulting hot plasma out of the back of a spacecraft. Hey presto: a fusion rocket.

Fusion rockets of this type were famously explored in the 1970s by the British Interplanetary Society in its Project Daedalus. They were the basis for a craft that could travel to another star within 50 years – a journey time that a human could reasonably be expected to survive.

There is just one fly in the ointment: despite decades of work, we still do not have a working fusion reactor.

Plausibility: possible, but a few decades away at best

Bussard ramjet

All rockets, including fusion rockets, have the same fundamental problem. To get more acceleration, you need to carry more fuel, which makes your craft heavier, reducing your acceleration. If you are serious about interstellar travel, you need to avoid carrying any fuel at all.

The Bussard ramjet, suggested by physicist Robert Bussard in 1960, neatly solves this problem. It is a fusion rocket as described above but, rather than carrying its supply of nuclear fuel, it ionises hydrogen from the surrounding space, then sucks it in using a huge “electromagnetic field” scoop (see illustration).

One problem for the ramjet, other than the aforementioned lack of a working fusion reactor, is the sheer size of the electromagnetic field required. Because there is so little hydrogen (or anything else, for that matter) in interstellar space, the field might have to be hundreds or even thousands of kilometres across.

One possible “cheat” is to pre-launch the ramjet’s fuel from Earth on a carefully calculated trajectory, so that the craft could pick it up without the need for a huge electromagnetic field. However, this would mean that the ramjet could not deviate from its planned course, and would also make round trips to other stars rather difficult.

Plausibility: huge technical challenge

Solar sail

This is another technology that dispenses with the problem of carrying fuel and could therefore reach extremely high speeds – though it would take its time to do so.

Just as conventional sails pick up energy from the winds of Earth’s atmosphere, solar sails pick up energy from light streaming from the sun (see illustration). They have been successfully tested in vacuum chambers on Earth, but attempts to test them in orbit have been plagued by misfortune.

For instance, in 2005, the independent Planetary Society, based in Pasadena, California, sent up a craft called Cosmos 1, but the rocket carrying it into space failed and crashed. Another mission called NanoSail-D was also lost because of rocket failure.

Teething problems notwithstanding, solar sails remain a very promising technology – at least for travel within the solar system, where the sun’s light provides the strongest push. Humans may weigh too much to use them for interstellar travel anytime soon.

Plausibility: completely possible but limited

Magnetic sail

A variant on the solar sail, the magnetic sail is pushed by the solar wind rather than by light.

The solar wind is a stream of charged particles that have their own magnetic field. One idea is to simply surround a spacecraft with a magnetic field that repels that of the solar wind, thereby propelling the craft out of the solar system. This could be accomplished by “inflating” a small initial field with plasma, like a balloon.

Another variant is the “space spiderweb“, which uses positively charged wires extending from the spacecraft to repel the heavy positive ions in the solar wind (see illustration).

Magnetic sails, or similar technologies, can also be used to surf the magnetic fields of planets, enabling the spacecraft to change orbit and even escape to interplanetary space.

However, on their own, solar and magnetic sails are no good for interstellar travel. As they get farther from the sun, the intensity of sunlight and of the solar wind drop sharply. As a result, they cannot achieve the speeds necessary to travel to other stars.

Plausibility: local trips only

Beam-powered propulsion

Or can they? If the sun doesn’t supply enough energy to push an interstellar craft to really high speeds, maybe we can do it ourselves, by sending a powerful beam of energy into space.

One such technology is laser ablation, in which a metal plate on the craft is gradually vaporised by an intense laser that is beamed from the ground. The metal vapour provides thrust.

Similarly, physicist and science-fiction author Gregory Benford and his brother James have proposed equipping a spaceship with a solar sail covered with a specially formulated paint. A microwave beam from Earth would boil off molecules from the paint, again generating thrust. This could make interplanetary trips go faster.

Another version that could be used to get around the solar system is magnetised beamed plasma propulsion, in which a craft with a magnetic sail is boosted by a beam of ions.

When it comes to interstellar travel, the best approach may be a laser-pushed light sail. First proposed by Robert Forward in a 1984 paper, this is just like a solar sail that is pushed along by light – but it is driven along by an intense laser.

Beam-powered propulsion poses several key challenges. The beam must be focused accurately over huge distances, the craft must be able to use almost all of the supplied energy without losses, and the beam-generating equipment needs to be enormously powerful – in some cases, the amount of energy needed is greater than human civilisation’s total current energy output.

Plausibility: extremely challenging

Alcubierre drive

This is essentially the warp drive from Star Trek. It was first proposed in 1994 by Miguel Alcubierre, a physicist working at the University of Wales in Cardiff.

The drive would use as-yet-undiscovered stuff called “exotic matter”: particles that have a negative mass and exert a negative pressure. This could distort space-time, causing the space ahead of the spaceship to contract and that behind it to expand. The ship, cocooned in a “warp bubble”, could effectively travel faster than light without breaking the laws of relativity.

Unfortunately, the Alcubierre drive has a host of problems. For one thing, the amount of energy needed to sustain the warp is greater than the total energy of the universe, although modifications to the shape of the bubble might help. The drive would also kick up a lot of radiation, which would threaten the astronauts’ lives. And there is no evidence that exotic matter even exists.

Perhaps crucially, calculations published in 2002 showed that it would be impossible for the ship to send signals to the front of the bubble, meaning that crew members could not control, steer or stop the ship. In fact, it seems that no matter how much energy is available, it would be physically impossible to generate the warp bubble.

Plausibility: apparently impossible

Wormholes

Ever since Einstein’s theory of general relativity became widely accepted, it has been clear that it seems to allow for the existence of wormholes: tunnel-like shortcuts across space and time (see illustration). The term was coined by the quantum physicist John Wheeler, who also came up with the term “black hole”.

The question is, do they really exist? And if they do, could we ever travel through them? Sadly, the answers to both questions may well be “no”.

For a wormhole to exist, it would have to be stabilised by the same sort of exotic matter as found in an Alcubierre drive (see above) – and it may be that no such stuff exists.

Furthermore, any matter or energy that entered a wormhole would immediately cause it to close up – though it may be possible to hold the wormhole open with a strange negative energy field called ghost radiation.

Nevertheless, a different type of wormhole, put forward by physicist Serguei Krasnikov in the 1990s, could be traversible. Krasnikov’s version is self-sustaining, as it produces its own exotic matter to hold itself open.

There is another significant objection to the idea of wormholes. If they could be used to transport matter across space, they could also be used to create a kind of time machine. This would violate the laws of cause and effect.

Plausibility: almost certainly impossible

Bonus technology: Hyperspace

If the universe has more spatial dimensions than the three we observe, it may be possible to drive a spacecraft through them, perhaps at extreme speeds. However, this idea depends on the work of an obscure physicist called Burkhard Heim, whose ideas have never passed peer review and are dismissed by most modern physicists as largely incomprehensible.

And if that isn’t enough, we reported recently on two more theoretical technologies: dark matter rockets and black hole starships.