JOIN USA recent report of a proposal supported by Mark Zuckerberg, Yuri Milner and Stephen Hawking among others, envisages the launch of several miniature spaceships pushed by the light pressure exerted by powerful laser beams, which would accelerate them to a fifth of the speed of light.
This would enable these spacecrafts to reach the Alpha Centauri star system (nearest to the Solar System), 40 trillion km away, in about 20 years. It is believed that an earthlike planet could be orbiting one of the stars in the system.
Stephen supports this by saying that if we are to survive as a species, we must ultimately spread out to the stars. Conventional spacecraft (including the ones to the Moon or Mars) have to carry considerable amount of fuel. As an example, the Apollo spacecraft which put men on the moon, weighed 3,000 tonnes at start, while the final payload which came back to Earth was barely 6 tonnes! More than 90% was fuel. The idea of using solar sails for propulsion has been around for quite some time. Sunlight also exerts pressure; the thrust is about 10 Newton per kilometre square area of sail. The sail is made of lightweight Teflon, coated with a highly reflective material like aluminium or gold foil. Its material must be no denser than 1 g/m².
Better alternatives
Carbon fibres, graphene and related materials are better than this limit. A sail 5 km² can deliver 30 tonnes from Earth to Mars within a year. The fuel is just sunlight which exerts pressure on sail. Already, many sails have been launched, like the Russian Znamya (300 m²) and the planetary Society’s 600 m² sail. Spacecraft to Pluto by such sails are planned.
Lasers operating from the ground can push up interstellar spacecraft with highly directional powerful beams. As sunlight is perennial, a solar sail would be continuously accelerated and can cross the solar system in months. A previous plan had a 3 TW (trillion watts) laser continuously operating which would get a 500 tonne payload to Barnard’s Star (next nearest star) in 50 years. Again recently, there was a proposal to have a laser-powered spacecraft to reach Mars in a few days. It can be calculated that a 50 GW (billion watts) laser (quite common now) illuminating a 50 m diameter gold plated sail (gold is very malleable) can send 40 kg to Mars in a week!
As far as Alpha Centauri is concerned, the current plan is to launch several spacecrafts, each weighing a few gram only, containing a camera, communication devices, navigation equipment (all miniaturised). They would be attached to light spacecrafts of few hundred atoms thick. These chip-sized crafts could be accelerated by directed gigawatt beams to several thousand times Earth’s gravity (g) and reach 1/5 light velocity in about 10 minutes. This push would enable them to reach the nearest star in 20 years. They need not carry any fuel. Laser beams would be ubiquitously used in future rocket propulsion enabling us to reach the stars within a human life time.
Our conventional spacecraft like Voyager or even the new Horizons spacecraft to Pluto would take 1,00,000 years or more to reach the nearest stars.