It is so difficult to launch men into orbit that so far only the United States, Russia, and very recently China have done so. All of us have seen the breathtaking videos of astronauts floating 400 kilometers up near the space station as the blue marble of Earth turned below. However, many of us had not yet been born when we sent men a thousand times further to the Moon. The fact that nobody has ever duplicated that feat, even 35 years later, underscores the difficulty of that achievement. Some people feel that going just a little bit further from the Moon bring us to Mars. The Red Planet at its closest though is well more than a hundred times further away from us than the Moon is. Comparing the distance to the Space Station and to Mars is similar to comparing the distance across your fingernail to a kilometer. In class, we mentioned the definition of a lightyear, but we really did not appreciate its meaning. Light moves so fast it would zip around the world seven times in a second, and yet it must travel an entire year at that pace to cover that lightyear. Stellar distances are so vast that light would travel for four years to reach our nearest neighbor Proxima Centauri. This is 750,000 times as far away as Mars, and yet many people think of Mars as the next stepping-stone to the stars. Proxima Centauri is just the nearest star, so other stars in our galaxy are vastly further yet. The stars on the far side of our galaxy are perhaps 20,000 times as far as our nearest stellar neighbor. When ET phones home from there, he has to wait for 160,000 years before finding out if anyone picked up the receiver. As unthinkably vast as the distance across our galaxy is, we are not done quite yet. We have other galaxies. The closest one, Andromeda, sits 2 million light years away or 25 times as far away as the furthest star in our own galaxy. Andromeda is the closest galaxy to us, but others are further. The most distant galaxies that we can see are more than 5,000 times further from us than Andromeda. The light from these objects started their journey to us long before our sun and earth existed. Since we see them from all the way across the observable universe, we also see them as they were more than 10 billion years ago, not as they are now. Expanding space was then rushing away from us at nearly the speed of light. In the intervening years, they have continued to move away from us at an ever-accelerating pace. Although we will never be able to measure their distance from us, we estimate that these most distant objects are now perhaps ten times further away, about 150 billion light years from where we are now. The universe is a very big place.
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