Quote:
Originally Posted by NightBazaar
An interesting related article from The Daily Galaxy. Estimates are that " Andromeda and the Milky Way are approaching one another at a speed of 100 to 140 kilometers per second (62â€“87 miles/sec)", which amounts to 223,200 to 313,200 miles per hour (roughly 1/4 to 1/3 of a million mph). That's like watching paint dry in comparison to the speed of light, but it's still blisteringly fast and gives it an interesting point of view. Hang on to yer hat!
The Coming Titanic Milky WayAndromedaTriangulum Galaxy Mashup!

Or 300,000 km/sec divided by 120 km/sec (taking the average) equals one 2,500th of the speed of light (or 0.04% the speed of light). Voyager I, the fastest craft made by man, is currently traveling about one 25,000th the speed of light (or 0.004% the speed of light), at around 13 km/sec.
The ion engines being used by today's spacecraft have the potential of reaching 99.9% the speed of light, given enough fuel and time. A spacecraft under a constant one gravity thrust could reach Alpha Proxima (4.2 light years away) in 3.124 years and reach 90% the speed of light.
The outside observer would note that the time to reach some speed v is given by:
T = (v/a)/SQRT[1  (v/c)^2]
While the person on the ship would measure:
Tship = (c/a)LN[SQRT(1 + g^2t^2/c^2) + g*t/c]
To move at 0.9c these would not be that much different.
T = 6.314x10^7 seconds = 731 days = 2 years
Tship = 4.502x10^07 seconds = 521 days = 1.43 years
Therefore, even under 1G of constant acceleration (assuming you brought all your own fuel), it would take just over 22,500 years for those on the ship to reach the Andromeda galaxy. To an outside observer on Earth it would take two million years.