How would it slow down at the other end?
> A very difficult challenge is to slow the spacecraft to typical planetary orbital speeds to enable orbital capture once arriving. This task is extremely difficult as the initial entry speeds are so high (~ c) and the orbital speeds are so low (~ 1e-4 c). Dissipating this much energy is challenging. We have considered using the stars photon pressure, the stellar wind (assuming it is like our own solar system), using the magnetic coupling to the exo solar system plasma. None of these techniques appears to be obviously able to accomplish this task and much more work and simulation is needed. A simple fly-by mission is clearly the first type of mission to explore in any case to assess the environment in a given system to design (if possible) an optimized braking strategy.
Even at that, however, you'll be skimming very close to Mars' surface and incurring very high temperatures.
Perhaps (and this is a big perhaps) we could engineer lifting body surfaces to increase the time-in-atmosphere and dissipate heat better.
This is why I love playing Kerbal Space Program.
Fling a device ahead of the craft, detonate it and catch the energy on the back of the sail.