In the age of sail, ships traveled around the world on voyages of discovery and trade. This era ended in the 19th century when coal-fired steam engines began to replace wind power. We may now be entering a recent era of sail – but this time in space. In a reversal of history, engines and fuel could be replaced by sails on some spacecraft, pushed not by wind but by sunlight.
The idea is still in the development phase, but we know it works. Just a few weeks ago, NASA raised the sail on its most ambitious test vessel ever, a satellite called the Advanced Composite Solar Sail System (ACS3). It has a 9-meter-wide square sail that allows it to adjust its orbital path.
To really go somewhere you would need a much larger sail and NASA effort building one with an area of 1,650 m2 was abandoned in 2022 as unfeasible due to the budget. However, this is an implementation problem and I’m sure sharp people can solve it.
To be clear, this is not like installing solar panels on your roof to generate electricity. Many spacecraft and planetary rovers already employ them. They are actually shiny, ultra-light sails pushed by solar radiation. Well, you may ask: How the hell is that possible lightweight move a physical object?
Comet tails
Good question! After all, when someone says they were “blown away by the beauty of the sunrise,” we don’t imagine that they were actually blown away. But lightweight bouncing off a surface does exert a physical force, however miniature it may be.
One example is the tail of a comet. You might think it’s like a contrail ejected as a comet hurtles through space. NO. You see, comets are basically massive, grubby snowballs. As we get closer to the Sun, some of this ice turns into gas, releasing clouds of dust. Sunlight then pushes this dust into a stream that can extend for millions of miles – sideways to the comet’s path!
Speaking of which, a comet is approaching and may put on a spectacular show in October. It’s called Tsuchinshan – ATLASand its tail can even be evident to the naked eye.
Electromagnetic waves
Now lightweight travels in waves, a kind of “moving displacement.” Look at an ocean wave: water only moves up and down, but this vertical displacement moves horizontally across the surface. It can definitely knock you over if you go into the water.
But lightweight waves are different from ocean waves or sound waves. Take away the water from the sea and you will have no waves to surf. The same applies to sound: there is no wave if there is no atmosphere in which to “wave”. That’s why the space is so strangely tranquil.
On the other hand, lightweight can travel through empty space. This is because, in a sense, the lightweight wave is its own medium. The reason is that it actually consists of two waves – there is an electric field wave and a magnetic field wave. That’s why we call it electromagnetic radiation.