Current space propulsion methods, such as solid and liquid rockets, are expensive and limited. Alternative methods include ion thrusters, nuclear pulse propulsion, solar sails, and antimatter. These technologies have the potential to accelerate spacecraft to near-light speeds, making them the future of space travel.
Typical forms of space propulsion today are solid rockets, liquid rockets and hybrid rockets. All bring their own fuel aboard and use chemical energy to produce thrust. Unfortunately, they can be very expensive: 25-200 kilograms of rocket may be required to carry a 1 kg payload into low Earth orbit. Getting one kg into low Earth orbit costs a minimum of $4,000 US Dollars (USD), as of 2008. $10,000 USD might be more typical.
The chemical rocket’s approach to launch and space travel is fundamentally limited. Since a rocket has to push its fuel up through the densest part of the atmosphere, it’s not very convenient. A more recent invention is the private spacecraft SpaceShipOne, which used a carrier vehicle (White Knight) to carry it to an altitude of 14 km (8.7 mi) before launch. At this height, greater in altitude than Mt. Everest, SpaceShipOne is already above 90% of the atmosphere and is able to use its small hybrid engine to travel to the edge of space (100km altitude). The first inexpensive, reusable tourist spacecraft are likely to be based on this model.
In addition to the chemical rocket paradigm, several other forms of space propulsion have been investigated. Ion thrusters, in particular, have already been used successfully by several spacecraft, including Deep Space 1, which visited comet Borrelly and asteroid Braille in 2001. Ion thrusters work like a particle accelerator, emitting ions from the rear of the engine using a field. For longer journeys, such as from Earth to Mars, ion thrusters outperform conventional forms of space propulsion, but only by a small margin.
More advanced forms of space propulsion include nuclear pulse propulsion and other nuclear propulsion approaches. The power density of a nuclear power plant or nuclear bomb is many times greater than that of any chemical source, and nuclear rockets would be more effective as a result. Nuclear Pulse Propulsion that a 1960s reference design, called the Orion — not to be confused with the 2000s Orion Crew Exploration Vehicle — that could carry a crew of 200 to Mars and back in just four weeks, compared to 12 months for NASA’s current chemical-powered reference mission, or the moons of Saturn in seven months.
Another project called Project Daedalus would have taken only about 50 years to arrive at Bernard’s Star, 6 light-years away, but would require some technological advances in the area of inertial confinement fusion (ICF). Most research into nuclear pulse propulsion was canceled due to the Partial Test Ban Treaty in 1965, although the idea has received renewed attention in recent times.
Another form of space propulsion, solar sails, was examined in detail in the 1980s and 1990s. Solar sails would use a reflective sail to accelerate the payload using radiation pressure from the sun. Carrying no reaction mass, solar sails could be ideal for fast travel away from the Sun. Although solar sails could take weeks or months to accelerate to appreciable speed, this process could be overcome by using ground-based or space-based lasers to direct the radiation on the sail. Unfortunately, the technology to fold and unfold an extremely thin solar sail is not yet available, so construction may have to take place in space, greatly complicating matters.
Another more futuristic form of space propulsion would be to use antimatter as fuel for propulsion, like some spaceships in science fiction. Today, antimatter is the most expensive substance on Earth, costing about US$300 billion per milligram. Only several nanograms of antimatter have been produced so far, enough to light up a light bulb for several minutes.
The key distinction between many of the technologies mentioned and chemical rockets is that these technologies may be able to accelerate spacecraft to near-light speeds, whereas chemical rockets cannot. Therefore, the long-term future of space travel lies with one of these technologies.
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