Given constant accel, it would be a lot less time than that for a mere 100 LY.


"So lets be very generous and say that they are coming 100 light years instead of four and the trip takes 200 years real time and say 30 years subjective (dilated) time for the crew. For a round trip it would take 400 years."


http://www.projectrho.com/rocket/rocket3aj.html

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Another advantage of a constant 1g acceleration is that it would allow the pilot to make very long journeys. To an observer on Earth, such a ship would take hundreds of thousands of years to reach the centre of the galaxy. Thanks to relativistic time dilation, however, the pilot would be only 20 years older on arrival. So, for the pilot, the centre of our galaxy is only 20 years away!

A Science Fiction Dream

Leaving aside the fact that we are not yet able to build fusion engines or sufficiently powerful superconducting coils, the Bussard ramjet sounds at first like an excellent prospect for interstellar propulsion. Unfortunately, there are strong theoretical objections to the principle of the Bussard ramjet.

Fusion as generated on Earth requires deuterium3, which accounts for only about 0.01% of interstellar hydrogen. Fusion in the Sun uses normal hydrogen, but achieving the conditions necessary for that would be very difficult. An optimistic estimate would be that only 1% of the hydrogen would be actually usable as fuel. So in fact much of the propulsive power would be used up slogging through a soup of useless hydrogen.

Also, one of the byproducts of the fusion reaction is neutrons4. Any crew compartment would need extremely heavy shielding against this radiation, adding to the mass of the ship.

Unless these and other serious problems can be addressed, the Bussard ramjet will remain a science fiction concept. Of course, we literally cannot imagine the capabilities of future technology, so the stated objections may eventually seem trivial.

Bussard Ramjets in Science Fiction

Tau Zero by Poul Anderson is the quintessential ramjet story. It also deals extensively with the concept of relativistic time dilation. This is not to say it is a dry, technical book, however. Like all classics of literature, it succeeds because it is, at heart, about people, and because it is a cracking story. It has been called 'the ultimate in hard science fiction' and is strongly recommended to anyone with an interest in the concept of the Bussard ramjet.

Rammer is a short story by Larry Niven. It was later reworked into the opening chapter of the novel A World Out Of Time. The short story is more of a cautionary tale of the unforeseeable consequences of cryonic preservation, and the novel is a fantasy of the far future, but both rely on the concept of the Bussard ramjet in passing. Niven's Known Space stories, particularly Protector, feature extensive use of ramships.

In Star Trek: The Next Generation, the starship Enterprise has 'Bussard collectors' on its warp engine nacelles. They are mentioned explicitly in two episodes, 'Samaritan Snare' and 'Night Terrors'. The technical manual states that they are an emergency fuel collection system only.

http://www.bbc.co.uk/dna/h2g2/alabaster/A600436