* Preliminary experiments have been performed on two
particularly interesting concepts in spaceflight propulsion.
"Lightsails" involve traveling between planets using a large
lightweight reflective "sail" that captures light from the Sun
or a large laser. "Lightcraft" involve blasting payloads into
orbit on a column of laser or microwave energy. This chapter
describes the lightsail and lightcraft concepts, and outlines
progress in these fields. * Light is emitted in the form of
particles called photons. Photons have momentum, and when they
are reflected off a surface, they transfer momentum to that
surface. In principle, if a spacecraft is tethered to a large
reflecting surface, or "lightsail", sunlight falling on that
surface would provide a gentle but continuous pressure that
could propel the spacecraft between planets without any need to
carry propellants for primary propulsion. The concept of
lightsail spacecraft has been around for decades. The first
formal description of the idea was published in 1924 b