The Concept of Solar Sails - Continued
A solar sail-powered spacecraft wouldn't need traditional propellant
for power, because its propellant would be sunlight and the sun would be its engine. Light is composed of electromagnetic
radiation that exerts force on objects it comes in contact with. NASA researchers have found that at 1 astronomical unit (AU),
which is the distance from the sun to Earth, equal to 93 million miles (150 million km), sunlight can produce about 1.4 kilowatts
(kw) of power. If you take 1.4 kw and divide it by the speed of light, you would find that the force exerted by the sun is
about 9 newtons (N)/square mile (i.e., 2 lb/km2 or .78 lb/mi2). In comparison, a space shuttle main engine can produce 1.67 million N of force during liftoff
and 2.1 million N of thrust in a vacuum. Eventually, however, the continuous force of the sunlight on a solar sail could propel
a spacecraft to speeds five times faster than traditional rockets.
With just sunlight as power, a solar sail would never be launched
from the ground. More likely, a second spacecraft would launch the solar sail, which would then be deployed in space. However,
another possible way to launch a solar sail would be with microwave or laser beams provided by a satellite or other spacecraft.
These energy beams could be directed at the sail to launch it into space and provide a secondary power source during its journey.
In a recent experiment at NASA's Jet Propulsion Laboratory, sails were driven to liftoff using microwave beams, while laser
beams were used to push the sail forward.
As we found out in the last section, solar sails would not initially
be driven by the amount of force that is used to launch the space shuttle. NASA believes that the exploration of space is
similar to the tale of the "Tortoise and the Hare," with rocket-propelled spacecraft being the hare. In this race, the rocket-propelled
spacecraft will quickly jump out, moving quickly toward its destination. On the other hand, a rocketless spacecraft powered
by a solar sail would begin its journey at a slow but steady pace, gradually picking up speed as the sun continues to exert
force upon it. Sooner or later, no matter how fast it goes, the rocket ship will run out of power. In contrast, the solar
sail craft has an endless supply of power from the sun. Additionally, the solar sail could potentially return to Earth, whereas
the rocket powered vehicle would not have any propellant to bring it back.
As it continues to be pushed by sunlight, the solar sail-propelled
vehicle will build up speeds that rocket powered vehicles would never be able to achieve. Such a vehicle would eventually
travel at about 56 mi/sec (90 km/sec), which would be more than 200,000 mph (324,000 kph). That speed is about 10 times faster
than the space shuttle's orbital speed of 5 mi/sec (8 km/sec). To give you an idea how fast that is, you could travel from
New York to Los Angeles in less than a minute with a solar sail vehicle traveling at top speed.
If NASA were to launch an interstellar probe powered by
solar sails, it would take only eight years for it to catch the Voyager 1 spacecraft (the most distant spacecraft from Earth),
which has been traveling for more than 20 years. By adding a laser or magnetic beam transmitter, NASA said it could push speeds
to 18,600 mi/sec (30,000 km/sec), which is one-tenth the speed of light. At those speeds, interstellar travel would be an