The Japanese spacecraft Icarus has apparently spread its solar sail, and we will soon see whether the novel propulsion system, familiar in the realm of science fiction, will also work in practice. There’s no question about the fact that photons from the sun will exert pressure on the sail. The goal of the mission is to determine whether the resulting acceleration can be controlled for accurate navigation. The efficiency of thin film solar panels embedded in the sail will also be measured to determine their potential for powering an auxiliary ion engine that might be used on future flights for more precise navigational control. Such an engine powers the NASA Dawn asteroid exploration probe, which recently established a new speed change record of 9600 miles per hour and counting, using only 363 pounds of xenon propellant in the process.
Sail technology is not necessarily limited to the vicinity of the sun. The photons required for propulsion might also be supplied by ground based lasers or masers to enable sail-based interstellar travel. (Related links are here, here and here.) Unfortunately, human travel using such systems is out of the question for the time being because of the inordinate amount of energy that would be needed to drive a manned spacecraft to the speeds required to cross interstellar distances within a human lifetime. However, it may be feasible to accelerate very small payloads of a few tens of grams to the speeds necessary to cross interstellar distances in times on the order of decades and decelerate them at their targets. Given continued advances in nanotechnology, useful scientific instruments of sufficiently small size might be developed to fit in such tiny craft. More importantly, life in the form of spores or bacteria might be sent to seed promising planets. Given the very real possiblity that we will exterminate ourselves here on earth, and that survival trumps any other purpose or goal that we might reasonably set for ourselves, it seems to me that this should be one of our highest priorities. We are related to every other life form on the planet, but are the only species capable of preserving that life indefinitely. We should do so.
Given the energy limitations noted above, it will probably be impractical to send spacecraft large enough to support a human crew over interstellar distances any time in the foreseeable future, barring some unforeseen advance in an enabling technology. However, All the information necessary to assemble a human being is contained in the nucleus of every cell in our bodies. It may prove more practical to send self-replicating nano-robots, programmed to eventually build the larger machines necessary to create dwellings, begin agriculture, etc., and finally build artificial wombs and “nannies” of human size. At that point, eggs and sperm, which it would be much more practical to send over interstellar distances in a reasonable time, could be combined to form a human population. Fanciful? Certainly, but it sounds better, to me at least, than waiting around for our extinction, which is inveitable and will probably occur sooner rather than later if we are foolhardy enough to remain on one planet.