Right above you is the sky – or, as scientists call it, the atmosphere. It extends for about 32 kilometers above the Earth.
The atmosphere is made up of a mixture of molecules – tiny bits of air so small you take in billions of them every time you breathe.
Above the atmosphere is space. It is so called because it contains far fewer molecules and a lot of empty space between them.
Have you ever wondered what it would be like to travel to space – and keep going? What would you find? Scientists like me are able to explain much of what you would see. But there are things we don’t know yet, like whether space is infinite.
Planets, stars and galaxies
At the beginning of your space journey, you may recognize some curiosities. Earth is one of a group of planets that all revolve around the Sun – with a few orbiting asteroids and comets, too.
You may know that the Sun is actually just an ordinary star, and it seems bigger and brighter than other stars just because it’s closer. To reach the nearest star would require traversing trillions of miles of space. Even if you could ride the fastest space probe NASA has ever made, it would still take you thousands of years to get there.
If stars are like houses, then galaxies are like cities full of houses. Scientists estimate that there are 100 billion stars in Earth’s galaxy.
If you could zoom out, far beyond the galaxy of Earth, those 100 billion stars would blend together, like the lights of city buildings seen from an airplane.
Recently, astronomers have learned that many, if not most, stars have their own orbiting planets. Some even look like Earth, so it’s possible they harbor other beings who also wonder what’s out there.
You would have to cross millions of trillions of miles of space to reach another galaxy. Most of this space is almost entirely empty, with only a few wandering molecules and mysterious invisible particles that scientists call “dark matter”.
Using large telescopes, astronomers see millions of galaxies, and they keep expanding, in all directions.
If you could look long enough, over millions of years, it would seem that new space is gradually being added between all the galaxies. You can visualize this phenomenon by imagining tiny dots on a deflated balloon and then thinking about inflating it. The dots would continue to move away from each other, just like the galaxies.
Is there an end?
If you could go as far as you want, would you walk past galaxies forever? Are there an infinite number of galaxies in all directions? Or does it all come to an end? And if so, how does it end?
Scientists do not yet have definitive answers to these questions. Many believe it is likely that we will continue to cross galaxies in all directions, forever. In this case, the universe would be infinite, without end.
Some scientists think it’s possible that the universe will end up wrapping around itself. So, if you keep going out, you will one day come back to where you started, but in the other direction.
One way to think about it is to imagine a globe, and imagine that you are a creature that can only move on the surface.
If you start walking in any direction, such as east, and keep walking, you’ll eventually come back to where you started. If that were the case for the universe, that would mean it’s not infinitely big – although it’s still bigger than you can imagine.
Either way, you could never get to the end of the universe or space. Scientists today consider the universe unlikely to have an end – a region where galaxies come to a halt or where there is some sort of barrier marking the end of space.
But no one can be sure. The answer to this question will have to be found by a future scientist.