2) Solar energy for propulsion (solar sail)
A solar sail is a spacecraft propelled by sunlight. Whereas a conventional rocket is propelled by the thrust produced by its internal engine burn, a solar sail is pushed forward simply by light from the Sun. This is possible because light is made up of packets of energy known as “photons,” that
act like atomic particles, but with more energy. When a beam of light is pointed at a bright mirror-like surface, its photons reflect right back, just like a ball bouncing off a wall. In the process the photons transmit their momentum to the surface twice – once by the initial impact, and again by reflecting back from it. Thus propelled by a steady stream of reflecting photons, the bright surface is pushed forward.
There are three components to a solar sail-powered spacecraft:
When the bright sails face the Sun directly, they are subjected to a steady barrage of photons that reflect off the shiny surfaces and impel the spacecraft forward, away from the Sun. By changing the angle of the sail relative to the Sun it is possible to change the direction in which the sail is propelled. It is even possible to direct the spacecraft towards the Sun, rather than away from it, by using the photon’s pressure on the sails to slow down the spacecraft’s speed and bring its orbit closer to the Sun.
A solar sail spacecraft will accelerate very slowly when compared to a conventional rocket. Under optimal conditions, a solar sail on an interplanetary mission would gain only 1 millimeter per second in speed every second it is pushed along by solar radiation.
But the incomparable advantage of a solar sail is that it accelerates CONSTANTLY. A rocket only burns for a few minutes, before releasing its payload and letting it cruise at a constant speed the rest of the way. A solar sail, in contrast, keeps on accelerating, and can ultimately reach speeds much greater than those of a rocket-launched craft.
Given time, however, with small but constant acceleration, a solar sail spacecraft can reach any desired speed. If the acceleration diminishes due to an increasing distance from the Sun, some scientists have proposed pointing powerful laser beams at the spacecraft to propel it forward.
Solar sails don't work well in low Earth orbit below 800 km altitude due to erosion or air drag .Above that altitude they give very small accelerations that take months to build up to useful speeds. Solar sails have to be physically large, and payload size is often small. Deploying solar sails is also highly challenging to date.