There are many challenges to space travel but propulsion is the biggest challenge.
Let's see:
- Human lifetime = 85 years.
- Max. distance = 85 lightyears / 2 (signal round-trip) = 42.5
Number of stars within 40 light-years = approx. 2.000 (according to: Stars within 50 light years). Not a lot, but still .. bound to be something interesting around. However, propulsion isn't the biggest issue? What about hitting anything at a fraction of light-speed.
The lower limit is about 4.3 light years away in 43 years = 0.1c. 30.000 km/s. Say your spacecraft weighs 100 ton. Energy = 1/2 x m x v^2
1/2 x 10^5 kg x 9x10^14 (m/s)^2 = 4.5x10^19 joules. Hiroshima was about 8 x 10^13 joules. *Boom* Even hitting a tiny rock weighing 200 grams is one hiroshima.
(unless I missed my napkin calculations?)