The idea for travelling to the nearest stars has been discussed in the science fiction literature for many years. However, the first technical paper addressing the problem in a mathematical way was written in 1952 by Les Shepherd, a long serving Fellow of The British Interplanetary Society. The theme of interstellar research has then be maintained by members of the BIS throughout its history, culminating in the 1970s Project Daedalus and the recent 2009 successor Project Icarus.
Throughout the 1970s and 1980s the Journal of the British Interplanetary Society (JBIS) published the now infamous ‘red cover’ issues (edited by Tony Martin & Alan Bond) dealing with Interstellar Studies. These contained paper by notable contributors such as Robert Forward and Greg Matloff – pioneers in the field. JBIS has always been the main publication prepared to allow discussions on such a speculative topic.
When addressing the interstellar problem, the immediate issue that comes up is distance. Because transit time is proportional to distance, the only way one can get to the distance stars in a short duration time is to go fast. This then leads to the most important problem associated with interstellar travel – propulsion. What any propulsion system aims to achieve is high exhaust velocity (high specific impulse) energy release so as to lead to a high velocity of the spacecraft. This will then allow for moderate to high acceleration, a reasonable cruise duration of order decades and perhaps reaching the nearby stars within a human life.
Current chemical propulsion systems such as used on the Saturn V rocket or the Space Shuttle simply do not release enough energy to address the problem. Hence designers are driven to more exotic fuels. Electric, Nuclear Electric and Nuclear Thermal propulsion systems do offer performances capable of sending probes to the entire system and beyond, perhaps as far as the Oort cloud. But such spacecraft would have to carry low mass science payloads and would not be capable of interstellar travel. Fusion propulsion does offer this potential, due to the energetic fuel performance, and may enable missions to the nearest stars within decades.
If a Bussard Ramjet system could be built, then the fuel could be collected en route and relativistic speeds may even be possible – but there are several technical problems with making the interstellar ramjet work, including the increased drag as the spacecraft travels through the interstellar medium.
Antimatter based fuels also offer such potential, but the technology readiness of antimatter based manufacturing facilities is very low and there are many containment issues that must be addressed when handling anti-particles.
One way around the interstellar propulsion issue (e.g. the need to carry a reaction mass) is to leave the fuel source at home. This is the method proposed for schemes like the solar sail which use the photons of starlight to propel a spacecraft on its interstellar journey. However, because starlight falls off to the inverse squared with distance from the source, eventually the intensity of this starlight will drop off and no further acceleration of the sail will be possible. A laser-drive sail system gets around this problem by deploying a giant laser in orbit around the Sun, sending the intense narrow monochromatic beam towards the sail which can (in theory) be sustained indefinitely using giant Fresnel lenses. Another alternative is to use a microwave beaming system.
More exotic ideas for getting to the nearest stars throw away the need to carry a reaction mass altogether. So called ‘Space Drive’ concepts will engineer the quantum vacuum energy of space itself.
Another alternative is known as the ‘warp Drive’ and so beloved of science fiction fans, manipulates the gravitational fields of space to collapse and expand the distances between the spacecraft and any distant targets. The existence of collapsed gravitational fields could also be utilised in the form of ‘Black Holes’, ‘Worm Holes’ and ‘White Holes’ , which derive from dying stars. If it were possible for a spacecraft to traverse these intense gravity wells and survive the trip then any location in the Universe could be reached within minutes to hours. Anyone entering the subject of interstellar travel soon learns that there is no shortage of ideas – what distinguishes all the ideas is credible engineering assessments using the known laws of physics.
As well as propulsion, interstellar travel presents many other technical challenges for spacecraft designers. This includes long distance communications using high gain antenna systems or optical lasers. Another problem is the bombardment of interstellar dust onto the frontal area of the spacecraft, the impact kinetic energy for which increases the faster the spacecraft is travelling. Long distance navigation is also an issue, with instruments having to make high precision parallax measurements on distance star systems in order to carefully navigate its way safely to its target. The larger the mass of the vehicle, the more propellant will be required so there is a design driver to maintain a low mass science payload.
Interplanetary space probes deployed into our own solar system will typically mass between 0.5 – 3 tons, and this includes probes such as Pioneer 10 and 11 probes or Voyager 1 & 2 destined for the outer solar system heliosphere and beyond, or spacecraft such as Galileo or Cassini-Huygens, destined for Jupiter and Saturn.
The Project Daedalus study had a science payload mass of 450 tons and this required 50,000 tons of fusion fuel to push it to over 12 percent of light speed reaching its target star in half a century. The determination of the payload mass will be a trade-off between science requirements, engine performance and distance to the target. Ultimately, if any planets exist in the target system, a likely prospect, then atmospheric re-entry probes and landers may also be required which drive up the payload mass. And this raises another issue, the prospects for discovering life.
As of 2010, the evidence for life originating outside of the Earth’s biosphere is none. Despite this, many astronomers look to the vast number of stars in the night sky and find it bewildering that this could be the only life in the galaxy or even the Universe. This is then one of the primary motivations for interstellar travel, to seek out new life.
If evidence could be found for extraterrestrial life, whether microbial or intelligent, it would be one of the greatest discoveries ever made and would change our view of the Cosmos in a very profound way. On the other hand, if after searching the galaxy we found no such evidence, this would also be very profound and place a greater importance on humans as the caretaker of life to spread out seed and propagate. Whatever argument turns out to be true – the case for interstellar travel is a very good one.