Every once in a while, we have a celestial body that passes through our solar system. What would be needed to reel one of these in? Not crashing down to Earth, that would be bad, but say into a high earth orbit (above the satellite belt)?

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    Can we assume that it's a small asteroid, something that can be handled with today's propulsion technology? Also, this might be of interest: What ever happened to the Asteroid Redirect Mission? and some of the links within Have there been any documented mini-moons since 2006 RH120? as well. – uhoh Nov 7 at 2:20
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    So basically no political will to do it at the moment then. Probably take a small fleet of rockets to send enough booster rockets out to one to really influence a path. Wonder how long until we get a "gravity beam" that can snag one? Probably a world-building/SciFi question – CrossRoads Nov 7 at 12:23
  • A political will, or a political mandate is definitely required. But if the mass is quite low, then you don't need a "gravity beam". There have already been small sample retrieval missions from asteroids, and there's more going on now. It might make more sense to bring kilogram-sized "chunks" of many asteroids and comets back to orbit rather than one big one. – uhoh Nov 7 at 14:35
  • If you have more questions about space exploration missions, you might consider asking them in Space Exploration SE, or just browsing through questions and answers there. – uhoh Nov 7 at 14:38
  • I was thinking more along the lines of the recently reported 'alien probe' "Oumuamua" that passed thru the solar system google.com/… – CrossRoads Nov 7 at 14:41

You could look it up :-) . The basic problem is that it takes a [bleep]-load of energy to force a distant object to stop orbiting way out there and fall towards the sun, or to be exact, towards Earth. It then takes another [bleep]-load of energy to kick it back up to the needed velocity relative to Earth to be able to orbit Earth.

To some extent, you can reduce the energy required by sending it on a slow spiral, as is more or less done when sending probes to Mars, but that leads to tens of years (at best) waiting for it to arrive.

If you're thinking of comets in their extreme elliptical orbits, well, same problem, since their path is nowhere near tangent to Earth's orbit.

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