While many fictional spacecraft can achieve the speed of light and beyond, the real spacecraft that exist today still rely on conventional propulsion. These types of engines are capable of limited speeds, even in the low-resistance vacuum of space.
Today's rocket engines are reaction engines. That means the engines propel the rockets forward using an equal and opposite reaction -- the rocket shoots something with mass out the rear of the ship, and the rocket is propelled forward by a force equal to the force produced by the discarded matter. For example, a chemical rocket burns fuel to accelerate. The burning process causes expanding gas to rush out of a small opening in the back of the rocket. The rocket is then pushed forward by the equal and opposite reaction. Therefore, conventional rockets have to carry large masses of fuel to keep them going. The important thing is that they have enough fuel mass and can shoot it at a high enough velocity to produce a force that will propel the heavy spaceship to its destination.
Within our atmosphere, friction places natural limits on the maximum speed that vehicles can achieve. Speed is a rather different animal in space. Just consider a space station, which we would tend to think of as stationary (as its name would imply). In reality, space stations and other Earth-orbiting satellites typically zip along at more than 17,000 miles per hour (about 27,400 kilometers per hour) [source: Axline]. Thus far, the Voyager 1 probe has attained the greatest solar escape velocity at about 38,600 miles per hour (62,120 kilometers per hour). Meanwhile, the Helios probes take the cake for absolute fastest spacecraft speeds -- and indeed, the fastest speeds ever attained by any human-made object. With the aid of the Sun's immense gravity, the Helios 2 probe reached a staggering speed exceeding 150,000 miles per hour (about 241,400 kilometers per hour) as it orbited the Sun in April 1976 [source: Scott].
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