It is believed that at any point in time, a handful of small asteroids (TCO's) are temporarily orbiting the Sun-Earth Lagrange points 1 & 2. They are random samples from the Asteroid Belt and therefor represent that size distribution, mostly up to a couple of meters in diameter. They have irregular orbits which last about 9 months on average until they escape. (Btw, what are such orbits called, Lissajous although they are so short lived?)
I understand that it is really cheap to travel from LEO to Lagrange points. When I add up what I easily find online, going from LEO via an Earth-Moon L-point to a Sun-Earth L-point could require only 1 km/s. Is that reasonable? How long time would such a trip take? How much of an extra challenge would it be to rendezvous with an asteroid in a particular orbit around an L-point?
What would a long-term mission look like to redirect a handful such TCO's per year to LEO for studies by astronauts? I imagine one would like to have a small observatory in Lagrange orbit to detect the small candidates, and a mobile Solar electric tow vehicle to push them to LEO one by one.
In this very recent Keck talk the leading asteroid astronomer Dr. William Bottke mentions a mission to TCO's as an alternative to the controversial ARM. Here's a short news article and a paper about such asteroids/moons.
2006 RH120 is the only TCO yet observed, thanks to its large size of about 5 meters. That is about the size that the NASA Asteroid Redirect Mission aims to deal with. One of the advantages of going to a SEL point instead of to a NEO, is that one can go to SEL any day while a NEO has specific launch windows.