For hundreds of years, people have dreamt of flying to distant stars
and planets. Unfortunately, the distances are far too great for humans
to travel through with the current technology. However, some proposed
ideas provide hope of making this dream a reality.
nuclear fusion, the process that powers the stars, the nuclei of two
light atoms fuse into one heavier nucleus. During this process, a
small amount of matter is converted into a huge amount of energy (according
to Einstein's famous equation, E=mc2, an amount as small as 10-11
grams of matter can produce a kilojoule of energy). Current fusion
reactors work by heating light elements to many million Kelvins. At
such temperatures, no known substances could contain the fuels for
the fusion reaction, which are plasmas by this point. Luckily, the
plasmas can be contained by a magnetic field and never touch their
The atoms are moving so quickly at these temperatures that they can overcome the repelling forces between them, allowing the nuclei to collide and fuse together.
The energy released by fusion can be used in a number of ways. The plasma could be directed out of the reactor providing thrust directly. The energy could also be used to create electricity to power other propulsion systems. The reaction could also take place outside the ship in the form of a series of explosions next to some sort of pusher plate or magnetic field which would push the ship forward.
Unfortunately, a self-sustaining fusion reactor is beyond our current capabilities. As it is, more energy is put into the reactor to keep it going than the reactor produces. In order to increase the efficiency of fusion reaction enough for them to be self-sustaining, much greater temperatures are needed. Until scientists discover a way to increase the temperature of the reaction enough, fusion-powered propulsion systems won't be plausible.
Scientists are also looking into the possibility of cold fusion, a way of carrying out a fusion reaction at room temperature (or close to it). The concept, however, remains purely theoretical.
Every particle has a antiparticle. For example, the positively charged
proton's antiparticle is the negatively charged antiproton, and the
negatively charged electron's antiparticle is the positively charged
positron. Antimatter is matter (the name is somewhat missleading since
antimatter is still matter, just a different type) that is made up
of antiparticles. Antimatter has the interesting property that when
it collides with regular matter, the two destroy each other and produce
electromagnetic radiation. Matter-antimatter reactions completely
convert matter into energy. Therefore, they are the most efficient
way to produce energy. This tremendous amount of energy could be converted
into electricity, which can power another propulsion system or be
converted into heat. The thermal energy could heat a gas to very high
temperatures, which could be used as a propellant. The energy could
even be converted to light that, when focused in one direction, could
actually propel a ship forward.
Unfortunately, the use of antimatter has two major drawbacks. First, because it destroys all matter in comes in contact with, there is no known way to contain it. Second, and more importantly, antimatter is extremely rare. In fact, the only place it can be found is in laboratories. It has only been produced in extremely small amounts and requires more energy than it produces. The cost of creating antimatter is astronomical--an estimated 62.5 trillion dollars per gram! As technology improves, though, the price is expected to drop to several billion dollars per gram.
wormholes aren't really a form of propulsion, they could certainly
help us get from one point to another in a very short time. A wormhole
is a theoretical shortcut through space. As Einstein's theory of relativity
indicates, nothing in the universe can travel faster than the speed
of light. Thus, the speed of light, approximately equal to 300,000,000
m/s, is the universal speed limit that nothing can break. Unfortunately,
most stars are many, many lightyears away. The only way a person could
travel to one of these distant stars before dying would be to find
some way of decreasing the distance between here and the star. Wormholes
could provide this shortcut. Wormholes can be hard to imagine due
to the fact that they rely on the curvature of space. The picture
to the left illustrates what a wormhole might look like if space were
only 2-dimesional. The 2D rectangle is flat to anyone who is confined
to its surface. However, the rectangle could be bent in three dimensions
and two points can be linked by a wormhole, providing a shortcut.
Wormholes, as far as we know, only exist in theory. Physics needs
to solve many problems including creating and maintaining wormholes
before they can be studied seriously for use in space travel. If they
are ever created, they will be extremely useful. Trips to different
planets could take minutes or even seconds instead of months or years.
Possibly even the most distant starts will then be within our reach.
It's even conceivable that wormholes could provide us with a means
of time travel, since space and time are actually combined into a
single 4-dimensional space by the theory of relativity. Wormholes,
if humans ever learn to create and control them, would revolutionize