The Bussard ramjet is an interstellar drive, proposed by physicist Robert W. Bussard in the 1950s, that would harvest interstellar hydrogen to feed a fusion rocket. It's been a staple of science fiction ever since (see stories by Larry Niven and Vernor Vinge in particular).
Unfortunately, a recent study shows that it isn't practical.
Their results were something of a “good news, bad news” situation. Consistent with what Fishback proposed, a “static ‘slowly-varying’ magnetic field is capable of funneling interstellar matter and guiding it into a fusion reactor. In this way, a consistent acceleration of one Earth gravity (1 g) can be sustained until relativistic speeds are achieved. However, when they calculated the size of the magnetic funnel, that’s where the bad news began.
To achieve a thrust of 10 million newtons (N) – equivalent to twice the main propulsion of the Space Shuttle – the magnetic field would need to be 4000 km (2485 mi) in diameter. Even worse, the field would need to be 150 million km (93 million mi) long to adequately capture and funnel ISM material into the ship’s fusion reaction. This is equivalent to the distance between the Sun and Earth, also known as one Astronomical Unit (1 AU).
I guess it's time to start looking at warp drives again.
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