Space is a very big place. The distances involved are literally astronomical. Even our closest neighbour, the Moon, is about 250,000 miles from us. However, back in 1969 two American astronauts, Neil Armstrong and Edwin “Buzz” Aldrin became the first humans to step out onto another world. Armstrong’s immortal first words, “It’s one small step for Man…” was absolutely correct. It was the first and very small step in our exploration of space because from this starting point, the distances to other planets and star systems become much harder to contemplate.
It took approximately three days for the Apollo 11 astronauts to reach the Moon. Their spacecraft travelled at about 40,000 kilometres per hour. But a mission to Mars, our favourite planetary neighbour to explore, would take a lot longer at this speed – about eight months. To fly to the outermost planet in our solar system, Neptune at 2.8 billion miles distance would take approximately 13 years. Incidentally, an unmanned space probe called Voyager-2 has already travelled so far from the Earth that it has officially left our solar system and is continuing to fly into deep space. So it can be done, but it is important to realise that to travel this far the probe was launched back in August 1977.
As we begin to look at the distances to other stars and galaxies we have to stop talking in terms of miles or kilometres. Instead we use one of the standard astronomical units – the light year. One light year is the distance travelled by light in one year. This figure has to be very large because in just one second light can travel over 186,000 miles. That means that if we could travel at the speed of light, the astronauts would have reached the Moon in less than two seconds. So in one year, light will travel 5,878 billion miles.
Okay, with this unit of distance in mind, we can begin to work out how big the observable universe is. I use the term observable because that is how far we can see out into the cosmos. The universe may be much bigger, we just don’t know for sure.
Let’s start with the closest star to our own star, the Sun. This is Proxima Centuri and is about 4.35 light years away. The closest galaxy to our own Milky Way is in the constellation of Andromeda and is known as Messier 31 or M 31. This is 2.5 million light years away or 15 million trillion miles. In astronomical terms we are still in the local neighbourhood. Millions and millions of other galaxies exist and as we look out further and further, we begin to get an idea of the true scale of the universe we live in. At the present time, the furthest object we have observed is an amazing 13.7 billion light years from us or 77 billion trillion miles.
So space is definitely a very, very, very big place. And because we know that other galaxies are moving away from us, it is getting bigger all the time. In the next article we will ask one of the most fundamental questions that humans have asked; how did it all begin?