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.