Have you ever thought about what mankind has discovered happens to the human body after lengthy trips in space travel? Watch this interesting video from Tech Insider.
Some have proposed creating artificial gravity as a solution for the lack of gravity health issues but this is not an easy accomplishment which the Institute of Physics points out in its article, Long-duration space travel.
Health Issues Learned from the International Space Station
Besides all the health issues discovered in space travel on the international space station due to lack of gravity discussed in the above video, there will be much more learned when and if humans travel to the moon and beyond since the earth's magnetic field helps protect humans on the international space station. There is no protection from cosmic radiation once you travel beyond the orbit of the international space station. The Van Allen Belts protect the earth and the international space station. "Spacecraft travelling beyond low Earth orbit enter the zone of radiation of the Van Allen belts. Beyond the belts, they face additional hazards from cosmic rays and solar particle events. A region between the inner and outer Van Allen belts lies at two to four Earth radii and is sometimes referred to as the 'safe zone'. " Wikipedia
Psychological Stress and Health Issues
In the initial video it was mentioned that the psychological stress of being in a confined space for long duration presents some challenges, but the physical health issues seem to be of more concern. As Wired points out, "Outside the safe cocoon of Earth’s atmosphere and magnetic field, subatomic particles zip around at close to the speed of light. This is space radiation, and it’s deadly. Aside from cancer, it can also cause cataracts and possibly Alzheimer’s."
One of the most challenging problems of space travel is the cosmic radiation (gamma rays, solar energetic particles and galactic cosmic rays) penetration through the hull of the space craft. The space craft would have to have very thick walls of some kind of non penetrable material, i.e., concrete cement, water, etc., to protect humans safely when traveling through space. On a long voyage, for example to Mars (a round-trip to Mars will last at least 2 years), the humans would be cooked before they arrive without some counter measures to prevent the health risks associated with cosmic radiation which includes "damage to all parts of the body including the central nervous system, skin, gastrointestinal tract, skeletal system, and the blood forming organs." Actually, there is no real data on such long term trips since humans have only gone to the moon and back rather quickly and is a comparatively short distance. NASA has dubbed this 'The Radiation Challenge.'
"Currently, the main operational countermeasure against the adverse affects of radiation is simply limiting astronaut exposure, which means limiting the amount of time astronauts are allowed to be in space." The Radiation Challenge
Since the above countermeasure can't be implemented on a trip to Mars, what has NASA proposed as countermeasures for such a voyage? NASA proposes "better shielding and mitigation strategies."
So far all the proposals for better shielding for a trip to Mars are problematic since the material that is able to protect humans has to be so thick that it is too expensive and heavy to produce and impracticable.
NASA states that mitigation includes 'dietary countermeasures.'
"Dietary countermeasures are drugs, that when ingested by an astronaut, may have the potential to reduce effects of ionizing radiation." So far such dietary and drug countermeasures are wanting.
Traveling Through the Van Allen Belts
It can be quite dangerous traveling through the Van Allen Belts. The Apollo missions all traveled through them so if you wonder how this is possible, Jillian Scudder, PhD, explains in a Forbes article, Why Aren't The Van Allen Belts A Barrier To Spaceflight? So while traveling through the Van Allen Belts can be accomplished since the Apollo missions proved that, after leaving these belts the cosmic galactic radiation still has to be dealt with.
Safely Traveling Through Space
While NASA or any space agency hasn't resolved the problem of safely transporting humans from the earth to Mars and back, nevertheless, NASA is optimistic. Note this statement, “Some people think that radiation will keep NASA from sending people to Mars, but that’s not the current situation,” said, Pat Troutman, NASA Human Exploration Strategic Analysis Lead. “When we add the various mitigation techniques up, we are optimistic it will lead to a successful Mars mission with a healthy crew that will live a very long and productive life after they return to Earth.” Space Radiation Won’t Stop NASA’s Human Exploration, Oct. 12, 2017, NASA
"At the 2017 International Astronautical Congress in Adelaide, Australia, Musk announced his plans to build large spaceships to reach Mars." SpaceX, Wikipedia
If humans ever do take the challenge of traveling to Mars and back, these first travelers into space beyond the Van Allen Belts will be the first humans to experience what is considered one of the biggest challenges, exposure to cosmic galactic radiation. Without a doubt, there would be volunteers to take such a voyage, but so far, the problems associated with space travel will probably be highly detrimental to human health and safety resulting in serious health risks and/or death.
Physics of Space Travel to Another Star Other Than The Sun
On the subject of traveling to another star other than the Sun, Michel Mayor, an astrophysicist who was a co-recipient of the Nobel Prize in physics in 2019 for discovering the first planet orbiting a sun-like star outside of our solar system. is reported to say on this subject, "in this case, the required physics to reach the stars, if it exists, is not known to us and it would require a fundamental change in our understanding of the relationship between mass, acceleration and energy." 
 Humans Will Never Live on Another Planet, Nobel Laureate Says. Here's Why., Tasemin Saplakoglu, Live Science