Answer:
1.3734 years pass on the astronauts' clock.
The distance measured by astronauts is [tex]1.22*10^{16} \ m[/tex]
Explanation:
The fomula for time dilation is:
[tex]t'=t\sqrt{1-\frac{v^2}{c^2} }[/tex]
where t' is the time observed by the astronauts, t is the time observed form Earth, v is the velocity of the astronaut and c is the velocity of light in a vacuum.
Replacing our values: t=4.2 year, v=0.945c
[tex]t'=4.2\sqrt{1-\frac{{(0.945c)}^2}{c^2} } =\\\\=4.2\sqrt{1-0.8930 }=4.2*0.3270=1.3734\ years[/tex]
The distance measured by astronauts will be their speed multolied by the time is takes them to get to alpha centauri:
[tex]D=v*t'=0.945*c*1.3734=0.945*3.00*10^8 m/s*1.3734*3.154*10^7[/tex]
To acomodate fot units we write the speed of light in m/s and the amount of seconds in a year, so the resulting distance will be in meters.[tex]D=1.22*10^{16} \ m[/tex]
We should compare this to the distance measured from Earth which is [tex]D=c*t=3.00*10^8 m/s*4.2*3.154*10^7=3.974*10^{16}\ m[/tex]