A pleasant autumn evening is a good time for a little jaunt — on foot, in your car, or in your personal spaceship. You might be tempted, for example, to cruise to a pair of bright objects that are in the southwest this evening: the Moon and the planet Saturn.
So you power up your ship and head toward the Moon, which is about a quarter of a million miles away. At escape velocity — about 25,000 miles an hour — it would take you 10 hours to get there. But that’s only if you could ignore the effects of gravity. As you move away from Earth, the old home planet is reluctant to let you go — it tugs at your spaceship, trying to pull it back. So that slows you down — until you get close to the Moon. Then its gravity takes over and starts making you go faster. So the total travel time is close to three days.
If you’ve packed enough supplies, you might want to go on to Saturn, which looks like a bright star below the Moon this evening. At that steady speed of 25,000 miles per hour, it would take you more than four years to get to where Saturn is tonight.
By the time you arrive, though, Saturn would be nowhere around. That’s because its orbit around the Sun would have carried it hundreds of millions of miles from its current position.
So when you take off for astronomical adventures, it helps to know about gravity and orbits and other essential details for exploring the cosmos.
Script by Damond Benningfield