For example, if any robot probe returns evidence of biological activity, either current or fossil, then I believe the expense and risk of sending a crew of human biologists would definitely be justified! A probe might photograph or even return a specimen, but it could hardly monitor an ecosystem or study exobiology in situ.

In some cases, it may even turn out cheaper to send people: for example, exploring the alleged seas under the ice in Europa may prove much easier if there are human crews operating the drills, submarines, etc that may be required. Even the slightest error, one easily anticipated or corrected by a human hand, might doom this type of mission to an expensive and preventable failure. Humans would give it some flexibility. Humans need not necessarily be directly present themselves, that might be too dangerous, but they could be in a spacecraft nearby, operating robots by remote control. This might be a type of hybrid man/machine mission where radio time delay would make eal-time rover operations from earth impractical.

Any potential space resource which might resupply or provide direct support for future manned missions would require direct human evaluation. A robot probe could tell us if there's water ice on an asteroid, but you'd need mining engineers and surveyors to help design your extraction facility.

If you really want to stretch it, if an automated probe located an alien artifact in the asteroid belt do you think a more elaborate probe would satisfy anyone? Not hardly, we'd throw everything we had at it. Even archaeologists!

In general, automated probes would dominate the discovery and exploration phases, humans would be involved in the survey and exploitation phases. The more valuable or inaccessible the treasure revealed by the probe, the greater the justification for sending people. The high value targets will be those with a high possibility of yielding unanticipated discoveries which will require human decision-making and improvisation.

What I do question is sending a man just to take a picture, or pick up a rock. A machine can do that.