5. Accidents
On Mars they may well be fatal if they result in damage of your spacesuit or habitat.
Also, in a vacuum, you can die just because you have forgotten one step in your
checklist while you put on your spacesuit - or because you get interested in what you
are doing and forget to allow enough time to get back within your oxygen reserves.
In the future we may use robots for exploring most of the time rather than humans
for safety reasons even when there are humans close by who could go.