For NASA, asking boffins to develop one laser-based tractor beam must not be difficult enough, because they've thrown down $100,000 for three entirely different flavors of the technology, which apparently does in fact exist.
It may seem counterintuitive that you can shoot something with a laser and get it to move towards you. Heck, it is counterintuitive. But you can do some seriously weird things with lasers, and NASA has asked a research team at Goddard Space Flight Center to investigate three different methods of using lasers on spacecraft and planetary exploration rovers to capture and retrieve itty bitty sample particles of dust and gas.
Meet The Tractor Beams
Laser tractor beam type I is a variation on a fairly well known light-based manipulation technique using what's called "optical tweezers." Two lasers coming from different directions overlap with each other, creating a ring of light (or "optical vortex") that can heat the air, trapping particles at the center of the ring. By changing exactly where this heating is taking place, the particles can be moved in either direction along the beam. The upside of this technique is that we know that it works, but since it depends on air for the tractor beam to work, it's not particularly useful in space.
Laser tractor beam type II is an "optical solenoid beam," which is a type of laser beam where the angle of the light that makes up the beam gets tilted a little bit. This creates a spiraling beam that can exert forces on objects contained in it, either towards or away from the source of the beam. These forces are adjustable by slightly tweaking the angle of the light, and you can even get creative and steer objects through three-dimensional curves and loops.
Laser tractor beam type III uses a "Bessel beam," and unlike the first two types, it's never been tried before. A Bessel beam is what you get when you pass a laser beam through a cone-shaped lens, and the resulting beam comes out ring-shaped, with several unique properties: it doesn't spread out much over long distances (making it handy for long-range tractoring), and you can stick something in the middle of it and the beam will reform itself slightly farther on. Hypothetically, a Bessel beam could induce electric and magnetic fields in the path of an object that would pull that object back towards the source of the beam, but again, this is still just a theory, albeit a theory that now has $100k behind it.
A Few Hurdles Remain (For Instance: Incineration)
While all of these techniques are so far limited (by power) to moving around something about the size of a bacterium, on principle there's no reason why they couldn't be scaled up to person-grabbing size. All you'd need is a terawatt laser, although it would have the unfortunate side-effect of instantly incinerating you. Okay, I guess maybe there's one reason, then.
We're looking at about a decade before a laser tractor beam is probably going to be ready for interplanetary deployment, but when it is, it could find a valuable home on (say) orbiters for taking atmospheric samples, and on rovers for long-distance material analysis.