From Wai Wong, Melbourne, Australia
There is a major problem for a spacecraft travelling at relativistic speeds, as suggested in the proposal for a mission to a black hole: damage by particles in space(16 August, p 13).
At such speeds, any particle that a craft collided with would be like an energetic cosmic ray particle carrying kinetic energy more than a million times bigger than the dissociation energy of the strongest chemical bond. Even though the density of matter in intergalactic space is as low as an atom per cubic metre, an object moving at a third the speed of light can encounter 100,000,000 atoms per square metre per second, or about 10,000 times the rate of cosmic ray particles hitting astronauts and satellites in space. Could the thin sail and delicate electronics of such a fast probe survive 75 years in space?
