Some scientists think that man may approach the speed of light — 300,000 kilometers a second in a distant future. This forecast is based on Einstein’s theory of Relativity which holds, in part, that an object may approach the speed of light but never reach or surpass it.
Long-range forecasts envision incredible rocket speeds. The main reason such speeds will be possible is that outer space is almost a perfect Vacuum filled with widely spaced hydrogen particles. In the Earth’s atmosphere, on the other hand, aircraft have to plough through a blanket of heavy air which produces drag and acts as a brake on their speed. The near-perfect vacuum beyond Earth’s atmosphere will offer practically no resistance to space ships. Even so, to approach the speed of light will call for propulsion devices of almost infinite energy.
The highest speed recorded so far has been 252,800 kilometers per hour, achieved by NASA’s Helios A and B probes (photo, above) each time they reached the perihelion (nearest point to the Sun) in their solar orbits.