There have been a number of interstellar propulsion systems proposed, and many are on the drawing board today. They range from the practical using today’s technology to the fantastical requiring great leaps in technological advancement to achieve. Here are a few of the more well known, in chronological order starting with the earliest.
Project Orion, 1958
In 1958, physicist Freeman Dyson, Theodore Taylor, and a team of scientists started a top secret space vehicle project trying to utilize nuclear explosives behind a massive pusher plate cushioned by duel shock absorber columns. They intended to use it for both interplanetary and interstellar flight, so called it Project Orion after the constellation. The goal was to create a less expensive means of exploring deep space than chemical rockets, as nuclear fission provides millions of times more thrust than chemical combustion. Smaller, lighter, easier to carry, and more effective equals larger payloads and greater speeds.
No actual spacecraft emerged from Project Orion then, or even today as scientists take another look at an old idea, but there were several flight test vehicles several that were damaged or destroyed. The first successful test flight was in 1959 using six explosions to gain 10 meters of altitude using chemical explosives for the proof of concept.
Bussard Interstellar Ramjet, 1960
The whole idea behind this Bussard Interstellar Ramjet concept, from the 1960’s, is that it uses free-floating protons in interstellar space to convert into nuclear fusion.
Using enormous electromagnetic fields hundreds of kilometers in diameter to “scoop” and compress hydrogen *hopefully* available in the vast distances between stars, high speeds would theoretically force the reactive mass into progressively more constricting magnetic fields much the way a jet engine’s fans force compressed air into smaller and smaller circumferences, compressing it until thermonuclear fusion occurs (or is caused to occur).
The same magnetic field would then focus the force of the continual nuclear reactions out of the back side of the craft, propelling it in the desired direction. The faster the vehicle travels, the more material it scoops, the greater the nuclear reactions, and faster the vehicle travels…
Practical problems include not knowing the dispersion or availability of atoms in deep space, and how exactly to get fusion to occur.
Project Daedalus, 1973
The challenge: use current or near-future technology, be able to reach its destination within a human lifetime, and be flexible enough in its design that it could be sent to any of a number of target stars. British Interplanetary Society‘s answer?
Daedalus was the first serious and thorough design for an interstellar vehicle, a robotic probe that would target Barnard’s Star which is 5.9 light years from the sun, reaching it in fifty years and moving at 12% the speed of light.
The British Interplanetary Society conducted their research on Daedalus between 1973 and 1977. It was essentially a re-engineered “nuclear-pulse” rocket of Project Orion origin. Instead of the rocket using nuclear fission for propulsion however, it utilized nuclear fusion. The rocket was propelled by a process coined as “internal confinement fusion.” Small pellets of helium-3 and deuterium were each to be hit by electron beams in a combustion chamber and exploded like mini thermonuclear bombs.
Like the Ramjet, magnetic fields would then channel the hot plasma of the continual explosions out of the back of the craft, accelerating the ship with increasing speed.
Solar Sailing, 1984
Solar sails work because photons (light particles) have momentum and force even though they have no rest mass. This resulting “solar wind” hits a craft’s reflective surfaced-sail, giving it a push just like the wind on an ocean sail. The effect builds up over time accelerating a solar sailing ship to tremendous speeds without the need for propellant.
The technology has already been tested in space, with Japan’s Ikaros probe deploying a 46-foot-wide (14 meters) sail in June 2010 and NASA launching an even smaller craft called NanoSail-D five months later.
Some scientist believe that a sail as large as Texas would be needed for an effective interstellar craft. However, recent advances in nanotechnology and the production of nano materials makes the daunting task of manufacturing and deploying such a large scale component more within reach than was even thought 2 years ago.
Alcubierres Warp Drive, 1994
The idea came to White while he was considering a rather remarkable equation formulated by physicist Miguel Alcubierre. In his 1994 paper titled, Alcubierre suggested a mechanism by which space-time could be “warped” both in front of and behind a spacecraft.
In 1994 Miguel Alcubierre authored a paper entitled “The Warp Drive: Hyper-Fast Travel Within General Relativity,” in which he proved the mathematical possibility of “warping” space-time via some as yet undiscovered exotic particle or power. But the idea was to dramatically expand space behind an interstellar craft and to condense the space in front of it; the middle (holding the space craft) would then be perpetually moved forward. Perhaps reaching stars in weeks, not thousands of years. Theoretically. The problem was that there would be no way to create enough power to generate such a warp field.
Enter physicist and NASA scientist Harold White. He announced in 2012 that he had made some revisions to Alcubierre’s equations based on the thickness of the negative vacuum energy ring; he realized mathematically, that if you make that thicker and oscillate the warp bubble it’s plausible to dramatically decrease the energy needed. The revised equations were compelling enough to get him funding for continued warp drive research at NASA.
Deep Space 1, 1998
Ion propulsion is a technology that involves ionizing a gas to propel a craft. Instead of a spacecraft being propelled with standard chemicals, the gas xenon (which is like neon or helium, but heavier) is given an electrical charge, or ionized. It is then electrically accelerated to a speed of about 30 km/second. When xenon ions are emitted at such high speed as exhaust from a spacecraft, they push the spacecraft in the opposite direction.
SOLAR ELECTRIC ION PROPULSION – Unlike chemical rocket engines, ion engines accelerate nearly continuously, giving each ion a tremendous burst of speed. The DS1 engine provided about 10 times the specific impulse (ratio of thrust to propellant used) of chemical propulsion.
SOLAR CONCENTRATOR ARRAY – The advanced solar concentrator arrays that provide power to the ion engine are more efficient than conventional arrays, and cost and weigh less.
Anti-Matter Drive, 2000
In October 2000, NASA scientists announced early designs for an antimatter engine that could generate enormous thrust with only small amounts of antimatter fueling it. Matter-antimatter propulsion will be the most efficient propulsion ever developed, because 100 percent of the mass of the matter and antimatter is converted into energy.
When matter and antimatter collide, the energy released by their annihilation releases about 10 billion times the energy that chemical energy such as hydrogen and oxygen combustion, the kind used by the space shuttle, releases.
Matter-antimatter reactions are 1,000 times more powerful than the nuclear fission produced in nuclear power plants and 300 times more powerful than nuclear fusion energy.
First Successful Solar Sail
IKAROS (Interplanetary Kite-craft Accelerated by Radiation Of the Sun) was an experimental spaceship built and launched by the Japan Aerospace Exploration Agency (JAXA). It launched May 21st, 2010, alongside the Venus Climate probe Akatsuki Orbier and four other small spacecraft. IKAROS was the first space vehicle to successfully demonstrate interplanetary solar-sail technology as its main propulsion.
IKAROS flew by Venus at about 80,800 km (50,200 mi)On 8 December 2010,
Technologies proven during the mission included:
- Deployment and control of a 20 m (66 ft) square, micro-thin (0.00030 in) polyimide solar sail
- Solar cells integrated into the sail to power its payload
- Accurate acceleration measurement provided by the solar wind
- Variable reflectance liquid crystal panels for attitude control.
Unmanned Spacecraft Already Near Interstellar Space
The twin Voyager 1 and 2 spacecraft continue exploring where nothing from Earth has flown before. In the 36th year after their 1977 launches, they each are much farther away from Earth and the Sun than Pluto. Voyager 1 and 2 are now in the “Heliosheath” – the outermost layer of the heliosphere where the solar wind is slowed by the pressure of interstellar gas. Both spacecraft are still sending scientific information about their surroundings through the Deep Space Network (DSN).
The mission objective of the Voyager Interstellar Mission (VIM) is to extend the NASA exploration of the solar system beyond the neighborhood of the outer planets to the outer limits of the Sun’s sphere of influence, and possibly beyond. This extended mission is continuing to characterize the outer solar system environment and search for the heliopause boundary, the outer limits of the Sun’s magnetic field and outward flow of the solar wind. Penetration of the heliopause boundary between the solar wind and the interstellar medium will allow measurements to be made of the interstellar fields, particles and waves unaffected by the solar wind.
The Voyagers should cross the heliopause 10 to 20 years after reaching the termination shock. The Voyagers have enough electrical power and thruster fuel to operate at least until 2020. By that time, Voyager 1 will be 12.4 billion miles (19.9 billion KM) from the Sun and Voyager 2 will be 10.5 billion miles (16.9 billion KM) away. Eventually, the Voyagers will pass other stars. In about 40,000 years, Voyager 1 will drift within 1.6 light-years (9.3 trillion miles) of AC+79 3888, a star in the constellation of Camelopardalis which is heading toward the constellation Ophiuchus.
In about 40,000 years, Voyager 2 will pass 1.7 light-years (9.7 trillion miles) from the star Ross 248 and in about 296,000 years, it will pass 4.3 light-years (25 trillion miles) from Sirius, the brightest star in the sky . The Voyagers are destined—perhaps eternally—to wander the Milky Way.
Project Icarus is a volunteer theoretical engineering study to design an interstellar spacecraft. The project was launched on September 30th 2009 at the British Interplanetary Society HQ in London, and is a five year study. The purpose of Project Icarus is four-fold:
- To motivate a new generation of scientists in designing space missions that can explore beyond our solar system.
- To generate greater interest in the real term prospects for interstellar precursor missions that are based on credible science.
- To design a credible interstellar probe that is a concept design for a potential mission in the coming centuries so as to allow a direct technology comparison with Daedalus and to provide an assessment of the maturity of fusion based space propulsion for future precursor missions.
- To allow a direct technology comparison with Daedalus and provide an assessment of the maturity of fusion based space propulsion for future precursor missions.
Research into Deep Space Exploration
Alpha Centauri and other nearby stars seem impossible destinations not just for manned missions but even for robotic probes like Cassini or Galileo. Nonetheless, serious work on propulsion, communications, long-life electronics and spacecraft autonomy continues at NASA, ESA and many other venues, some in academia, some in private industry. The goal of reaching the stars is a distant one and the work remains low-key, but fascinating ideas continue to emerge. This site will track current research. I’ll also throw in the occasional musing about the literary and cultural implications of interstellar flight. Ultimately, the challenge may be as much philosophical as technological: to reassert the value of the long haul in a time of jittery short-term thinking.