At two-tenths the speed of light, dust and atoms might not do significant damage even in a voyage of 40 years, but the faster you go, the worse it is - space begins to become abrasive. When you begin to approach the speed of light, hydrogen atoms become cosmic-ray particles, and they will fry the crew. . . . So 60,000 kilometers per second may be the practical speed limit for space travel.
Source: ‘Sail On! Sail On!’ in The Relativity of Wrong, Kensington Books, NY, 1996, p 220.
Contributed by: Zaady