[bluGill]

Eventually you cannot run high risks in unmanned. If a rocket fails getting a satellite to orbit just build a new one. However missions to the outer planets are often only possible once every several hundred years (when orbits line up) and so if you fail you can't retry. Mars you get a retry every year and a half (though you get about a month). If you want to hit 5 planets that is a several hundred year event. And the trip time means if you fail once you reach the outer planet all the engineers who knew how the system works have retired and so you start from scratch on the retry (assuming you even get the orbits to line up)

[eru]

> However missions to the outer planets are often only possible once every several hundred years (when orbits line up) and so if you fail you can't retry.

Just send ten missions at the same time. No need to wait until you fail.

[bluGill]

Ten that fail in the exact same way is a real possibility.

[eru]

Have them build by ten different teams in different countries by different companies.