Many people have a vague understanding of the solar system, thinking it is just a small part of the Milky Way, which in turn is just a grain of sand in the universe. But did you know that it takes a year for light emitted from the surface of the sun to reach the edge of the Oort Cloud, marking the outskirts of the solar system? Voyager 1, launched 46 years ago, has only just left the heliosphere, the outermost layer of the sun's sphere, and it will take at least another 30,000 years to exit the solar system.
When photons of sunlight start their journey at the speed of light, the Earth is waiting for them in space 150 million kilometers away. It takes 8 minutes and 20 seconds for sunlight to reach Earth, meaning that if the sun were to suddenly go out, it would take us the same amount of time to find out. Earth is just the third planet from the sun in the solar system. Beyond it are Mars, Jupiter, Saturn, Uranus, Neptune, and a fun fact: the distance between Earth and the moon, 380,000 kilometers, can fit all the planets of the solar system side by side.
Modern astronomers use astronomical units (AU) to measure distances in the solar system. One AU is equal to 150 million kilometers. From the sun, Mercury is 0.4 AU away, Venus is 0.7 AU away, Earth is 1 AU away, Mars is 1.5 AU away, Jupiter is 5.2 AU away, Saturn is 9.6 AU away, Uranus is 19.2 AU away, and Neptune is 30 AU away. Pluto, no longer classified as a planet, is not included. However, astronomers see all the planets, whether it is Neptune or Pluto, as still being part of the solar system and not yet at its true edge.
After a star system forms, the remaining material after star formation forms a torus around the star, known as the circumstellar disk or Oort Cloud for the solar system. The Oort Cloud has a diameter of two light-years and consists mostly of comet fragments left over from the formation of the sun. They act as a protective shield around the planets in the solar system. However, with current technology, it is impossible for us to reach the vicinity of the Oort Cloud. Chemical rockets and gravitational slingshots are too slow; they can't even reach 1% the speed of light. For the Oort Cloud, which is one light-year away, humanity's current space travel speed is that of a turtle. Are there any other propulsion methods available to us?
The answer is yes – it is called solar sail or photon propulsion. With controlled nuclear fusion technology seemingly always 50 years away, NASA has successfully deployed multiple solar sails in near-Earth space. In the future, humans will be able to manufacture solar sails with an area of several square kilometers or even larger, transport them to low Earth orbit, and accelerate using sunlight. Strong lasers from Earth can also be used to accelerate the sail.
In Liu Cixin's science fiction novel "The Three-Body Problem," the protagonist embarks on an interstellar voyage using a similar solar sail device. With the help of cryonics, the sail accelerates to 10% the speed of light or faster using the acceleration of starlight. In theory, as long as the sail is durable enough and not damaged by unexpected asteroids, it could become the fastest spacecraft ever built by humanity.
