Conventional Propulsion to Support an Interstellar Probe

Guest contributor Andreas Tziolas discusses the propulsion techniques available today that could assist an interstellar mission.

Project Icarus is an ambitious five-year study into launching an unmanned spacecraft to an interstellar destination. Headed by the Tau Zero Foundation and British Interplanetary Society, a non-profit group of scientists dedicated to interstellar spaceflight, Icarus is working to develop a spacecraft that can travel to a nearby star.

Andreas Tziolas, secondary propulsion lead for Project Icarus, discusses how the propulsion techniques we use today will still have a part to play during the construction of the Icarus vehicle and the science probes Icarus will deploy when exploring the target star system.

One of the central issues critical to an interstellar mission, which could be completed on timescales of a human lifetime, will be the method of propulsion.

For the Project Icarus space vehicle, the decision was made early on to use a fusion-based main engine, which can theoretically have almost 1 million times better performance than currently used chemical rockets. This would allow an Icarus top speed of around 10 percent the speed of light, assuming its size and mass are similar to the Daedalus starship.

Apart from tackling the engineering challenges of the Icarus main propulsion, the Icarus Project's mission includes any number of scientific objectives surrounding the exploration of interstellar space en-route and the detailed study of the target star solar system. To accomplish these objectives, a wide range of secondary propulsion systems would be necessary, many of which may evolve from current and near future systems.

For instance, when the Icarus vehicle arrives at its destination, it would release probes designed to explore the target system. These probes would need to move into their respective orbits around objects of interest, for which they require some method of propulsion.

Some may have solar sails that would allow them to slowly move around, perhaps taking pictures and making measurements of the chemical elements in the atmospheres of remarkable exoplanets and exomoons. Others might need to be moving faster, perhaps using nuclear powered rockets to steer their way towards a promising landing site eventually using a parachute to break its descent and release a glider, rover or even a submarine.

The Rich Landscape of Propulsion Options

It is quite remarkable how secondary propulsion options for interstellar missions, are in fact primary propulsion systems for all contemporary space missions. There is still a significant gap in current propulsion technology and what may be needed for an interstellar mission.

As space exploration is still very expensive, it is difficult for organizations such as NASA and ESA to take risks on testing new and innovative systems. Progress has been slow, but the gap is becoming more narrow, with some very promising technologies being realized and others peeking over the horizon.

The successful deployment and operation of the Japanese IKAROS and NASA's NanoSail-D solar sails have demonstrated a key method for getting around in space, without having to use any propellant. These advances are of particular interest to Icarus, which would otherwise have to carry the fuel for its many probes all the way to the target system.

In general, solar sails may not be a good option for accelerating, what would be, a very massive Icarus to another star, but offers unique advantages for exploring the target solar system with probes.

The advances in ion (NASA's NEXT and NSTAR) and electromagnetic thrusters (VASIMR) in recent years seem to indicate a path towards some new and exciting propulsion technologies being developed. Some of these systems might be used on the Icarus interstellar spacecraft itself, and others may play supporting roles. For example, the technologies necessary for the spacecraft construction and fuel gathering stages of the mission.

The Daedalus spacecraft weighed around 50,000 tonnes, almost all of which would have been a very rare Helium isotope called Helium-3. Icarus has not made a final decision on the type of fuel to be used for its fusion engine, but it is almost certain it would require a substantial amount of propellant.

Even if the fuel could be refined on the Earth, it would still need to be ferried to orbit. A cheap and reliable delivery vehicle to low-Earth orbit (LEO) would be needed, most likely capable of frequent trips to space, a single-stage-to-orbit craft such as SKYLON being an ideal solution (pictured top).

Chemical rockets have the high thrust necessary to lift rockets into space, overcoming the Earth's gravitational pull. The significant investment made in developing these rockets over the last 50 years, will most likely still hold our best options for getting off the Earth and into space, for the next several decades. There is certainly room for improvement however, through high-performance fuels, turbo pumps, etc., with many groups working on re-imagined and re-engineered rocket systems.

Relatively new space programs from countries like India and China, as well as companies, such as SpaceX and Virgin Galactic, will hopefully galvanize spaceflight into a sustainable industry.

The proud announcement by SpaceX for their plans to test the new Falcon-Heavy lift vehicle, was met very positively by the public; the vehicle being capable of delivering more mass to orbit than any other rocket, other than the Saturn V's that took the Apollo landers to the Moon.

Innovations and the Future of Interstellar Propulsion

If we were to exclude speculative methods of propulsion such as warp drives, we find a rich landscape of promising ideas that may play a crucial role in interstellar travel.

Exciting proposals for advanced systems using magnetic sails is one example. The idea is similar to that of solar sails, only instead of solar radiation one uses the magnetic particles found in the solar wind. A large metallic mesh acts as a net to catch these particles and have them push a spacecraft along.

Beamed propulsion is yet another interesting concept. In this scheme, a large orbiting solar collector or nuclear reactor generates the power needed to produce a beam of concentrated light or charged particles, which is then directed towards the spacecraft you want to push.

Combining these two ideas may lead to an advanced space tug, responsible for pushing spacecraft equipped with magsails into their orbits. Perhaps the Icarus could serve as the beam generator which pushes its planetary explorer probes into place. Alternatively Icarus could use an extremely large magnetic sail to help decelerate, once within reach of the target star's solar wind or magnetosphere.

The Icarus team is on the hunt for the right combination of systems and ideas that will make a plausible case for interstellar flight using current and near future technologies.