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The story situation is that a generation ship is headed to another solar system where there are several planets in the star's habitable zone and once the ship arrives, it will approach these planets one by one and examine them to see whether they're suitable for colonization.

I'm assuming such a ship would have sophisticated antennae and sensory equipment to examine the planet's atmosphere and topography in great detail. So if that's the case and the people of the ship find an intriguing planet and want to examine it more closely before they start the colonization process, is there any reason why they might want to send a manned recon craft rather than an automated one?

I ask because my story idea requires four people to man a recon craft much like they do with the Ranger craft in the movie Interstellar, and I know that that movie had people wondering why they didn't just send unmanned recon craft with robots instead of risking people's lives. Fair question.

I think the reason given in the movie is that humans' fear of death enables them to improvise more than any robot could. I'm not sure, though, that that's a satisfactory answer. So again, given the story situation I outline above, are there any reasons why a generation ship might want to send a manned recon craft to a planet instead of an unmanned one?

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    $\begingroup$ Actually I'd expect them to do both: First, send a robotic probe, to rule out the most obvious dangers. Then, when the robotic probe found that the conditions are probably well, the next step would be to send a few actual humans, before betting the whole generation ship on the robots not having overlooked something critical. The most reliable test that a world is habitable for humans is by having humans live there for a while. $\endgroup$ – celtschk Jul 12 '16 at 7:34
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    $\begingroup$ one reason is the delay in the commands. someone on the ship can take actions on events that a computer would not anticipate, and much faster than a team back on the origin planet would be able to do. $\endgroup$ – njzk2 Jul 12 '16 at 13:58
  • $\begingroup$ Are there decisions that a computer cannot make but a "man" can? Assuming that machines cannot completely replace people, then there are some things that people would be required for that a machine could not handle. Once you identify the differences, you have possible answers. $\endgroup$ – Noctis Skytower Jul 13 '16 at 15:09

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The same reasons we want to send manned missions towards the planets in our own solar system:

Automated systems can only do so much.

The Apollo 17 mission brought a geologist with them because he was able to pick interesting moon rocks to return. This turned out very well.

The manned Apollo missions were able to move around quite easy and covered a whole lot of ground. If you compare the speed at which a human can safely walk on the moon and the speed at which robotic rovers are driven, you'll easy see that humans can navigate much, much better then robots.

In the 70s, NASA planned a manned Venus fly-by. Humans are much better at exploration and science. We are better in spotting anomalies and deciding what to investigate further and what not to do. An automated mission is much more limited in what it can achieve.

Add to that the delay in communications, and it gets even worse. Communications with neighbouring stars takes years or even decades. The bandwidth is abysmall. So the probes would likely have to return home to relay the data anyways. Having humans on location which are able to decide on demand what things are worth to investigate further will improve any survey mission.

So the question is how far the technology is in your world. If its similar to ours in terms of computing power and radio communications, then a manned mission has a lot of benefits over a robotic one (or probably a combined manned / robotic mission, where the manned mothership orbits the star and sends robotic probes to the planets to gather intel, and then decides which planet to investigate more deeply).

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  • $\begingroup$ The story is set in the near future, so humanity's technology level would be further advanced than it is currently, but not by orders of magnitude. The robotic probes or manned recon craft would be coming and going from the generation ship, not Earth, so there would be some delay, but nothing like communicating from Earth. From reading through these answers, I think now that a robot probe first and then a manned recon craft if things look promising is the way to go. $\endgroup$ – Arbutus Jul 12 '16 at 19:14
  • $\begingroup$ @Arbutus, cutting even a few hours out of your decision loop makes for a huge gain in efficiency. $\endgroup$ – Mark Jul 12 '16 at 21:28
  • $\begingroup$ This is the best answer from my point of view. A while back stargazing live (in partnership with other organisations) organised an event in which they got loads of humans to look at various images and try to identify pulsars. The explicitly stated many times that currently there are no computer programs capable of identifying pulsars to the same accuracy of an average human. A human's ability to identify things by sight far exceeds any modern algorithms. (This is also why capcha systems now use complex images.) $\endgroup$ – Pharap Jul 12 '16 at 23:45
  • $\begingroup$ It's a nice idea. But the OP has already said that the generation ship is swinging by each planet in turn, meaning your probe has comms times measured in seconds at most. So we're looking at real-time conversations and mission monitoring with the ISS as the best example, not Mars rovers. For my money, there's no reason to send a human down, when a telepresence remote can provide at least as much information as a human (and certainly more for a well-designed remote with well-thought-out sensors). That puts human decision-making in the loop, without risking human life. $\endgroup$ – Graham Jul 13 '16 at 11:45
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    $\begingroup$ Computers are not able to solve Captchas (at least the new Captcahas) efficiently. Even googles most advanced OCR software used to scan books still makes a lot of silly mistakes that are obvious at first glance to humans. Computers simply lack the tools to understand things in their context, and are thus not able to recognize things that human are able to see. $\endgroup$ – Polygnome Jul 14 '16 at 18:16
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I'll give it a shot, but my reasoning might cause other plot problems for your story, so apologies in advance!

A generation ship crossing interstellar distance has, presumably, significant velocity and little fuel to use (either because they have none or because they may need it for re-acceleration and dare not use it), so a trajectory is plotted to use encounters with the star and planets to decelerate the ship. This results in a huge elliptical orbit. The ship doesn't just come in and park. Instead, it must pass through the inner system and loop into the outer system and back again, maybe several times. This takes many more years of travel to accomplish. So close, but the physics do not allow them to simply stop on a dime.

If the target planet requires preparation (e.g. terra-forming or the creation of habitation), perhaps this is the reason for deploying a team to the planet in a smaller vessel capable of the necessary deceleration. the team would bring what they need to prepare for the arrival of the generation ship 10's or 100's of years later. They will likely be dead and gone by the time the mother ship arrives to benefit from their work.

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    $\begingroup$ Parking a generation ship in a long elliptic orbit is a great idea. I'm amazed nobody else has thought of it before now. It makes sense in so many ways. Also, the time involved for successive being years. I take my hat off to you and upvoted it. $\endgroup$ – a4android Jul 12 '16 at 4:52
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Robotic sensors are reasonably good at detecting the thing they were built to detect. They won't do well at the detection of the absence of unspecified dangers.

  • If the planet is habitable, are there forests, trees, lumber to build huts? Could a robot fell a tree and check the grain of the wood?
  • If there are animals, could they find humans tasty, but not robots? (It might well be that humans are indigestible or poisonous to the local wildlife, but that won't help the victim.)
  • Last but not least, if there is something unspecified that will be lethal it would be good to find out early. The explorers will open their helmets, if they drop dead after a month that's a bad sign ...
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  • $\begingroup$ I think that a video feed, some fences & guns resolves many problems. For the last point I believe they would analyze the air/soil/water samples with the robotic probe before even sending humans in there $\endgroup$ – beppe9000 Jul 12 '16 at 12:24
  • $\begingroup$ @beppe9000, those fences would not tell if it is safe to wander in the woods without a gun. And if it isn't, perhaps another planet in the system offers better hospitality. $\endgroup$ – o.m. Jul 12 '16 at 20:15
  • $\begingroup$ the first manned expedition could check for / assume wild animals could occur and be prepared to kill them / take samples $\endgroup$ – beppe9000 Jul 12 '16 at 20:53
  • $\begingroup$ @beppe9000, an animal might decide that a robot "doesn't smell like food" and ignore it, while a human "smells tasty" and gets attacked. That should be tested early. $\endgroup$ – o.m. Jul 13 '16 at 4:59
  • $\begingroup$ The landing team would be aware of them (because they studied the video from the robot) and equipped properly because they can't assume the species behaviour: they are dealing with aliens, after all. They will eventually ascertain it when the team goes in the woods to capture some specimen and one of them tries to attack, gets incapacitated and held captive for further studies. However if the alien does only rely on smell to find enemies and be friendly as long as space suits are on, then the biologists will notice the aggressiveness of the caged specimens the moment they remove their suits. $\endgroup$ – beppe9000 Jul 13 '16 at 12:19
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Ground truth is a term used by geologists in surveying what the structure of a piece is like. Robot probes will be sent, but manned reconnaissance will also be dispatched. Because after several generations in transit to another planetary system it will be necessary to have living humans touch down on a planet to get a sense of what it's like to be on a planet again and how well they adapt to the experience. Readjustment to planetary life will be a major factor.

Plus there will be preparatory work to be done prior to landing more personnel. Things like landing sites, habitats, manufacturing centres, and possibly farms and gardens to feed the settlers.

Also, they may want to do research into any lifeforms on the planets before humans come in and trample the landscape. To survey, record and investigate the pristine environment. If there are any sapient inhabitants somebody has to start talking to them because there will have to a lot of adaptation and accommodation. Possibly, inhabited planets will be forbidden for human settlement. But from the perspective of the native sapients they have to share their planetary system with occupying aliens.

Somebody has to do all this work. We can't let robots have all the fun.

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  • $\begingroup$ Good points. For my story, I need the manned craft to only be for short trips, as I need its low food/water/air resources to become a factor. Given that, I'm thinking that maybe a robot probe would be sent first, followed by the manned recon craft for a brief survey/record/investigate trip, as you suggest. The craft would then return to the generation ship and, if the findings were promising, a separate team would be sent down for a longer stay to set up habitats, etc. So if that sort of survey trip could be limited to 2-3 days, it will work for me. $\endgroup$ – Arbutus Jul 13 '16 at 3:33
  • $\begingroup$ That works fine for me too. Robot probes would be a given. Short manned recon makes sense. The various options weren't meant to apply to one trip. Different teams for different missions is most likely. Have fun with your story. $\endgroup$ – a4android Jul 13 '16 at 10:41
  • $\begingroup$ Let's steal the fun from the robots... Let's start by polluting the orbit of the planet with some relay satellites and deploy a fleet of VR-controlled probes. Hoping that the ping isn't too high they could safely explore and rule out even more dangers. $\endgroup$ – beppe9000 Jul 13 '16 at 12:28
  • $\begingroup$ @beppe9000 You know me. I'm all in favour of stealing fun from robots. But polluting planetary environments is a hard ask. There's so, so much beautiful science to be done on a pristine planet. But, on the other hand, VR-controlled probes would be ideal for researching in comfort and safety back on the generation ship. So we can have the best of both worlds. $\endgroup$ – a4android Jul 14 '16 at 1:32
  • $\begingroup$ @a4android Exactly! $\endgroup$ – beppe9000 Jul 14 '16 at 10:56
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I guess for me the answer is obvious. You've got a ship full of people who are clearly natural explorers, are clearly willing to take risks, and have been waiting for this moment for a very long time.

So what's the first thing they will all want to do when presented with a potentially suitable planet? Of course, they're all going to want to get out and explore.

You might talk about the fact that sending a robot probe is safer, but you've got a ship full of people who will bite your arm off to get a chance to be the first one on the surface. You'll probably send some robot probes anyway to support them, but you'll definitely be sending a manned lander as well. You'll have a mutiny if you don't!

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  • $\begingroup$ Actually, the people on a generation ship are most likely to have a severe fear of leaving it, as it's literally the only place they'd ever lived. Contrary to the popular perception that they'd head straight for the most-habitable planet, I think it's far more likely they'd start building Belter communities (w/ similar, if smaller, living environments) to harvest resources available without descending into a planet's gravity well, and stockpile plenty of fuel and spare landing craft before sending anything but one-way robot missions to a planet's surface. Spreading out also hedges their bet. $\endgroup$ – Monty Harder Jul 13 '16 at 15:17
  • $\begingroup$ @MontyHarder I agree. generation ships are more likely to want build more generation ships or space-based habitats than colonising planets. Although generation ships still might do this, by landing a fraction of their population who want to live on planets. Then resupply their ship and move on the next planetary system to do the same thing again. $\endgroup$ – a4android Jul 14 '16 at 6:22
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When overpopulation is such that it's cheaper and more acceptable to send a person on a one way ticket than to send a robot.

OK, so that's harsh, but you don't need to bring probes back, coming back more than doubles the cost and complexity of the mission.

The only real reason to send humans on a recon mission is to drive the plot or if intelligent life is found. Anything else can be done remotely and better by robots. A bot can sit in one place quite happily for years just pumping back data and images, never moaning about potatoes. There's never going to be a panic when you realise the bot is still alive and you have to go back and get it.

Obligatory xkcd: https://xkcd.com/695/

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  • $\begingroup$ It's not harsh, just doesn't make sense. While this depends on the population on generation ship, fertility control is easy to maintain. In space, it will be essential for starfaring populations. A generation ship will have a population the size of a large town. People will know one another. They won't waste lives. Especially when they're the only humans around for tens of light years. If an insane dictator is in charge, lives might be cheap, but revolution could be brewing too. Hey! There's a story in that. $\endgroup$ – a4android Jul 14 '16 at 6:28
  • $\begingroup$ @a4android, we know what should happen on such a voyage, what would happen is an entirely different game. Consider the fact that we can't seem to run a country as a stable entity for more than a couple of hundred years, imagine the equivalent problems when you're on a generational ship. The insane dictator at some point in the voyage is almost a given. $\endgroup$ – Separatrix Jul 18 '16 at 14:49
  • $\begingroup$ "almost a given". Yes then that's a contingency that can be planned for. Countries as stable entities don't last for more than centuries, but that applies to their institutions social and political. The populations live on. It is an interesting problem to devise ways of preventing self-destruction for generation ships. Certainly there will be difficult times, but it's surprising how often people turn away from the edge of the abyss. $\endgroup$ – a4android Jul 19 '16 at 7:36
  • $\begingroup$ @a4android, you say that but Brexit happened and Trump might yet... $\endgroup$ – Separatrix Jul 19 '16 at 7:37
  • $\begingroup$ Quite! To paraphrase Einstein: "There are two things I know of that are infinite. The universe and human stupidity, but I'm not so sure about the universe." This doesn't mean every generation ship will fail, just some of them. $\endgroup$ – a4android Jul 20 '16 at 8:51
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One does not exclude the other.

Send a probe first. It will check for air composition and pressure, temperature, presence of liquid water, maybe take nice pictures of the landscape, take soil samples, and other things a probe can do. If the planet is survivable for humans, then send a recon crew. There's no sense in sending people if the probe dies two minutes after it lands.

The recon crew will take a look around for things a robot may not look for. Maybe your robot can't make the difference between good farmland and good building site. Maybe there are dangerous predators that don't have a taste to robots. A human eye might be necessary for a lot of reasons.

Presumably, the recon ship will also be bigger, and can therefore carry bigger instruments. With the manpower that goes with it, that means you can survey for things deeper. Comparatively, if you have a small probe like a Mars rover, you are only going to scrap the surface.

Presumably also, the probe and recon crew will arrive at different times of the year. The probe will also have collected a lot of information on weather patterns and such. So you'll have a better idea of what the conditions are.

If your probe lands in summer, you may think it's a warm planet, and then winter happens and you'll wish you brought a coat with you. Another thing you could look for is pollen. Maybe there's that one tree that blooms at a specific time that you might not catch until months later. Lot of things happens over the course of a year. Temperature changes, weather changes, air quality changes. If you are surveying for long time settlement, that's something you want to know.

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If something unplanned happens, robots would be clueless. If you are sending a ship to another star system, it is more likely than not to have something unplanned. If the technology level of your civilization is not far advanced (having instant communication and mapping exoplanets with perfect accuracy), they are probably will have to plot the course after entering to the star system.

Also see o.m.'s answer about what unplanned things could happen on surface.

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The most obvious advantage is they know that if they survive, then they can state without doubt that humans can definitely live on the planet. They can study the progress (medically speaking) of their own bodies and be able to predict what will happen to the colonists.

This information is far more valuable than any amount of analysis of the atmosphere content and radiation, etc. that a robot might detect.

Just as an example, there may be a deadly mosquito-like species which remains in hiding until it spots a warm-blooded organic living being giving off pheromones of life before it attacks and uses the sweet living juices to multiply like crazy. They would ignore the robot and so the robot would ignore them.

Or the robot might have no way of telling if a particular species of bacteria loves the taste of brains...

Also, they need the expertise and knowledge of decision makers at the scene, since the data transfer will be one-way only. A robot could not receive orders from Earth as it would take years or decades to receive the orders and the same time again to report back on the findings.

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  • $\begingroup$ Send a dog first? $\endgroup$ – user17228 Jul 14 '16 at 1:49
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Good answers so far, so I've had to think a little outside the box on this one.

Considering its a generation ship, that suggests it has resource issues that mean sending a probe on a one-way journey that might not result in a habitable planet is a use of a precious resource that they don't want to spend (eg they can't manufacture new microchips, lose your robot probe and you've lost a few that you are not getting back). So you send a crew with the capability to return. That way, you lose only fuel (and perhaps fuel is something that can be replenished during the ship's travel - eg by sending a manned ship to collect water or scooped hydrogen etc)

I assume a robot probe would not have the capability to return, or a probe would not have the ability to land, scout sufficiently and return; or that the risk of losing it is too great compared to crew that will be better able to survive any unexpected factors.

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  • $\begingroup$ This is why I say they wouldn't go straight for a planet but instead explore asteroid belt(s) for resources that are less expensive to obtain. Only after they have a thriving Belter economy producing the raw materials for the nanofactory to make those microchips, etc. could they dare to send anything to a planet's surface, where the gravity makes getting it back expensive at best. $\endgroup$ – Monty Harder Jul 13 '16 at 15:30
  • $\begingroup$ @MontyHarder - I think it's quite plausible that a generation ship arriving at its destination might find its crew no longer able to establish such an economy. It is entirely plausible that the crew have been unable to keep up maintenance on anything but critical systems, and no longer have the educational capacity to learn how to produce the raw materials they would require or what to do with them once they had them (because, perhaps, the library of information that they would need for that was stored in a computer system that no longer works). $\endgroup$ – Jules Jul 14 '16 at 2:36
  • $\begingroup$ @Jules Systems failures like that, both technological and sociological, would be safeguarded against. A generation ship won't set off without systems for long-term survival being standardised and normalised to the point no-one realises they're there. There will be many generations of space-habitat living and operating before the first generation ship is launched. It's only a moving space-habitat itself. They will be totally self-sufficient. No chance of spare parts or repair men from Sol. It's do or die in deep space. Nothing else counts. $\endgroup$ – a4android Jul 14 '16 at 6:38
  • $\begingroup$ Generation ships will have to have the capacity to make every part they are made of, and everything else too. They have no back-ups. Time delays to get information from Sol will be too slow and take too long. Survival means complete self-sufficiency. $\endgroup$ – a4android Jul 14 '16 at 6:41
  • $\begingroup$ @a4android great in theory but in practice things always go wrong. Chip fabs are very big, very dirty and require a whole heap of raw materials that require big and dirty processing facilities. Assuming a generation ship smaller than a small country, its more likely they would set off with more than sufficient parts for the journey and then find that unexpected circumstances would deplete these - or the fab gets disabled in an unexpected accident, etc. In the OPs story this could be the case, maybe they can only manufacture 10 Mhz 8 bit chips but no more 4Ghz i7s. $\endgroup$ – gbjbaanb Jul 14 '16 at 7:40
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I'd like to add a slightly different possibility: politics.

You send a manned mission to generate public interest and create heroes, thus ensuring public support, political support, and - therefore - additional funding. Get enough people emotionally vested in "the first interstellar expedition" and the rest becomes a lot easier.

How much of the success of the U.S. Apollo missions was due to the availability of funds and political capital due to public interest? We were racing the Soviets (with some implied concerns over threats if we didn't win), so people were willing to throw resources (money, scientists, etc.) into the project.

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Electronics used outside Earth's atmosphere need to be heavily shielded against radiation to prevent bits from being flipped in software instructions. Biological life is in this sense actually more tolerant to radiation than unshielded electronics; humans exposed to radiation might have to deal with cancer and tissue damage in the long term, but with sufficiently advanced medical technology such concerns are minimized.

A human crew could have the simple but important responsibility of reprogramming or rebooting an automated system after its active software instructions have been inevitably corrupted by cosmic radiation.

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Even Captain Kirk sends out a probe before the Away Team. First, a generation ship with only near-future technology is probably a planetoid so it has plenty of raw materials and reaction mass at hand. Second, in any resource-limited scenario that discovers several planets in a nearby system's goldilocks' zone will send a fast ship with several robot probes on board: one for each candidate planet. (remember that the planetoid must have significant time to make any decision to decel) Then right behind the probe ship it launches a manned scout/recon/shuttle ship that is atmosphere capable but can't do the 6g continuous boost like the probe ship. The probe ship launches robots to each candidate once in-system and are controlled by the manned ship while en route. By the time the manned ship gets to the system, they have selected the best candidate planet and descend to the surface for detailed exploration. If you think fear of death has any value in exploration, you have never met a test pilot or astronaut. They are chosen because they have absolutely no fear of injury or death - none.

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  • $\begingroup$ He does? Seems to me he just has Spock & Scotty scan it from orbit, and then beams down with his pals and possibly some redshirts, ready to spread Federation goodness. More often than not, Kirk's attention, cunning & fist-fighting skills are needed to solve whatever the problem is. $\endgroup$ – Dronz Jul 14 '16 at 16:50
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THEY NEED TO EXPERIENCE GRAVITY.

Say the generation ship can't produce high gravity, like over .3 gee or whatever. They either don't have or can't repair their high gee centrifuge and everything else simulates a lower gee. Everyone now on the ship has lived their entire lives in low gravity. So they need to send down people to experience this "almost earth like gravity" that they have read about but never been able to experience. Maybe they even fear it. Do they choose a planet with earth like gravity, or one with less (but that has a thinner atmosphere or less water, for example)? Sensor readings from a robot won't help them with this decision.

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  • $\begingroup$ Maybe they have a centrifugal device like in kubrick's 2001(on a larger scale) and mandatory physical activity? $\endgroup$ – beppe9000 Jul 14 '16 at 11:19
  • $\begingroup$ Sure, I doubt a generation ship would be built without one. But what if it broke? It is pretty hard to get up to 1 gee in a 2001 torus unless it is REALLY large (like 100m in diameter with 3 rpm rotation). You gotta figure that by the end a ship like this will be held together with spit and duct tape. If the centrifuge was disabled and the occupants couldn't experience high gravity for decades/generations, I think it is quite plausible that there would be substantial fear about whether or not "current" humans could survive it. $\endgroup$ – Jason K Jul 14 '16 at 13:51
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In addition to the technology-level dependent answers above, a possible solution (though one that has to be drastically incorporated into the story), is to create some sort of information which robotic sensors can't detect. The most obvious is some telepathic alien race native to the planet in question.

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Right now, the question of whether life, and in particular complex multicellular life, is common in the galaxy has not been answered. But what if it is extremely common?

If the expectation is that a large proportion of planets in the habitable zone will be host to a large and diverse ecology, a large team of scientists would be necessary, not to survey the atmosphere and topography (which could probably be done remotely or with drones) but to evaluate the biological resources and threats.

It may be that life is more common in this particular sector of the galaxy that science would normally predict because humanity is following in the footsteps of earlier alien terraformers...

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