The Space Tricycle

To understand the Gravity Well, just for a moment stop thinking of it as a well. Imagine instead a bowl-shaped valley, and a little kid riding a tricycle on the sidewalk at the lowest point. The valley happens to be steepest at the bottom. To climb the hill, the kid has to peddle furiously. Fortunately, there are a few flat places where she can rest without her tricycle going backwards and plummeting back down. She takes her feet off the pedals and breathes.

The kid could stay there forever, but she’s hungry and wants to get home for a snack. So she peddles again, gaining speed. The hill eases off, becoming less steep, and she keeps pumping. Finally she reaches the top—whew! Down below, in the next valley, she can see her house and her mother, arms akimbo. The kid scoots the trike forward and then lifts her feet as she picks up speed. This kid is flying!

While this valley isn’t quite as steep as the one she went up, it gets steeper the farther down she travels, and she realizes one big problem with her tricycle: it has no brakes. Gaining some serious velocity, heading straight for her mom, she closes her eyes and…feels a pair of hands grasping her shoulders. She looks up to see her mother right behind her, running while gently pulling back. Mother and daughter slow to a stop. The mother leads her into the house for a snack and a lecture, and the kid looks up and says, “I want to do that again!”

And so you have a rather liberal analogy of gravity wells. The first valley represents the Gravity Well from Earth.. The Gravity Well, like the first valley, rises the most steeply at the bottom and eases off as we approach the top. If you look at some maps of space between the Sun and Mars, with Earth in between, the blackness of space is intersected with curved lines that look a lot like a topographic map of a wilderness on Earth. Instead of a rising and falling landscape, space has centrifugal and centripetal forces, the pushing and pulling of masses. The “hill” of the Gravity Well is steepest near the bottom because Earth’s gravity pulls strongest against the closest objects. The farther up you climb, the less Earth pulls.

The little girl’s tricycle begins picking up speed after she reaches the top of the valley (or the Gravity Well), because she’s now entering a second valley. That second valley represents Mars’s gravity well. (It turns out that mother and daughter are Martians.) One big challenge of going to Mars is finding ways to slow the craft down to zero miles per hour. In this case, Mom did the job. Crafts going to Mars can deploy parachutes or other drags, but they only provide so much deceleration in the thin Martian atmosphere. A landing module must burn fuel to slow down. And, like our little girl who wants to go back up and down into the next valley, a spacecraft must have enough fuel to leave Mars’s gravity well and then to slow down within the Gravity Well of Earth. 

In short, space is nothing like a void. It’s a terrain of hills and valleys (not to mention actual rocks, dust and planets), with the steep climbs and descents and flat spots made by gravitational forces.