That is the number of people in the entire history of human civilization who have left Low Earth Orbit (LEO). You heard me right, only 24 people (all Apollo astronauts) have left the protection of Earth’s magnetic field. The prospects of journeying past LEO is a daunting one. There is dangerous radiation in deep space that the magnetic field protects us from.
There are two types of radiation that pose a risk to astronauts: those that originate outside the solar system, the Galactic Cosmic Rays (GCR), and those that come from the sun, called Solar Proton Events (SPE). The GCR consist mainly of heavy atomic nuclei, while the SPE, as the name suggests, consists mostly of protons. Both of these types of radiation are high energy, so if they hit an unshielded astronaut they could cause damage to DNA, cell replication, and even lead to cell death.
The SPE, released during solar flares and coronal mass ejections, are especially dangerous as they emit so much radiation that it could be fatal to an unprotected astronaut. Luckily SPEs are rare and none occurred during the Apollo missions. Most of the damage from radiation is from prolonged exposure to it, which increases an astronaut’s risk of developing problems such as cancer and cataracts.
Radiation is not the only danger to astronauts on a deep space mission, though. On a long mission, such as our proposed 308 day DFD powered mission to Mars, the extended period in weightlessness can cause issues as well. Bones and muscles that normally have to deal with gravity suddenly do not have any load on them. For this reason astronaut’s bones and muscles (including the heart!) begin to atrophy and lose mass. The ones most affected are those that fight with gravity: the bones and muscles in the lower back and legs.
Image Source: NASA
http://spinoff.nasa.gov/Spinoff2009/hm_5.html
Astronauts will need to exercise daily to minimize these losses, but even that will not be 100% effective. Similarly there will be radiation shielding on the spacecraft and a storm shelter for the SPE, but nothing is perfect. These are just some of the risks associated with a voyage to Mars. Despite the risks, I do not think we will have any problems finding volunteers to be number 25!