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If you are a Star Trek fan, you will of course be familiar with “tractor beams,” those cool-looking laser beams that can grab an object in space and it pull backwards toward the source of the beam (including trapping spacecraft as evil aliens would often do). They are another long-running staple of science fiction that is now closer to science reality. NASA is now working on developing just such technology, which would help primarily in obtaining material samples in real-life space missions, such as on Mars or an asteroid or comet.
A $100,000 study to look at three possible methods has been awarded to NASA’s Goddard Space Flight Center by the NASA Office of the Chief Technologist (OCT). According to Principal Investigator Paul Stysley, “Though a mainstay in science fiction, and Star Trek in particular, laser-based trapping isn’t fanciful or beyond current technological know-how.”
The methods being developed can trap and move particles of matter or even single molecules, viruses or cells, using the power of light – maybe not another spacecraft yet, but the principle is the same.
NASA has used various methods of sample-retrieving, all with great success, including aerogel on the Stardust spacecraft to obtain dust samples from the comet Wild 2 and scoops, brushes and rock abrasion tools on various Mars landers and rovers to retrieve rock and soil samples. On the next Mars rover, Curiosity, which is due to be launched later this month, there will be a scoop as well as a drill. It will also feature a laser beam to zap rocks so the resulting particles can be analyzed; not quite the same as a tractor beam but still cool.
The first technique being studied is the optical vortex or “optical tweezers” method which uses two counter-propagating beams of light. Particles are confined to the “dark core” of the overlapping beams. Particles can be moved along the ring’s centre by alternating the strength or weakness of one of the beams. The only catch with this method is that it requires an atmosphere to work. Ideal then maybe for on the surface of Mars or Titan for example, but not for an asteroid or other airless body.
The second technique uses optical solenoid beams, where the intensity peaks spiral around the axis of propagation. Particles can be pulled backwards along the entire length of the beam, and it can operate in a vacuum, no atmosphere necessary.
Both of those techniques have been tested in the laboratory, but the third method, as of yet, has not. It uses what is known as a Bessel beam, which, when projected onto a wall for example, features rings of light surrounding the central dot of light. The effect is similar to looking at ripples surrounding the spot where a pebble has been dropped into a pool of water. Other types of laser beams do not exhibit that however, appearing only as a single point of light. Such a beam could induce electric and magnetic fields in the path of an object, which could then pull the object backwards.
According to team member Barry Coyle, “We want to make sure we thoroughly understand these methods. We have hope that one of these will work for our purposes.” He added, “We’re at the starting gate on this. This is a new application that no one has claimed yet.”
A more technical overview of the practicality of tractor beams is here.