what are the problems that face scientists about modern space travel?

THE MONEY AND THE DANGER

money
any danger can be remedied with the proper equipment, which costs money.

Some of the problems that scientists face are the locations of life preserving planets. Most case scenarios, the planet is light years away or in another galaxy. That being that case, our spacecraft does not travel at the speed of light, not even close to half the speed of light. That is just about the spacecraft.

We also know that when astronauts travel to outer space, their bone density decreases, also impacting the muscle, it slowly deteriorates, even with exercise. For a small period of time, the astronauts cannot maintain a body with muscle that does not deteriorate. Space missions will take a long time to complete again due to the fact that the destinations are far away.

For example, if we decided to choose Mars as our planet for sustaining life, due to the presence of ice, maintaining life on that planet is a big responsibility. Not to mention that if people , say billionaires decided to live on Mars, it would be tough to gather the correct amount of chemicals for the atmosphere. I know this sounds ridiculous, but think of how much energy we would waste to create that atmosphere for people to live, for the plants to grow and release the oxygen. Not just plants we need various bacterial cultures to create the Nitrogen Cycle and get the similar ratio of chemicals, Nitrogen to Oxygen and other small chemicals.

Space travel is not just about having the greatest shuttle design, its also about the purpose behind it. A very interesting question, indeed.

Someone mentioned loss of bone density in microgravity -- of course, long-duration spaceflights can make artificial gravity by rotation, so that's kind of not so intractable a barrier to long duration missions as might be supposed.

Another is radiation, particularly from large solar flares. Astronauts beyond Earth's Van Allen radiation belts will need to be prepared for that contingency. They may need a foot or two of water, for example. I'm not sure how much, really.

Then th biggest IMHO is just the cost and complexity, currently, of getting a kilo of cargo into orbit. We need to bring that down before anything big can happen in space.