Solar sail spacecraft. Computer illustration of a solar sail spacecraft flying past comet Halley. The solar sail is a hypothetical design for a spacecraft which is powered by the pressure of photons (light) emitted from the Sun. Such a craft would require little fuel and could reach high speeds due to the lack of friction slowing it down. Halley is a comet which orbits the Sun on a 76 year cycle. It has two tails visible here, a dust tail (yellow) formed of dust cast from the nucleus, and an ion tail (blue/grey) formed of ionised gas particles. Both tails always face away from the Sun.