What's the
deal with gravity?
We don't know what causes it, but we do know it's a force that occurs when two objects -- such as the Earth and the moon -- attract to one another. We also know that if each object is treated as a perfect sphere, the math describing gravity is fairly simple.
But reality bites, and the Earth is not a perfect sphere. It has deep oceans and towering mountain peaks, and is thicker around the equator. Each little rock, pebble, grain of sand and molecule of water pulls on the moon, planets, the sun and everything else in the universe. Needless to say, accounting for these disturbances is a difficult problem.
Since before the first satellite launch, humanity has studied gravity anomalies, and those investigations have brought us to the Gravity Recovery and Climatology Experiment, aka "GRACE," which uses special satellites to measure this mysterious force.
See right for a 3-D
map of these anomalies that the GRACE mission has teased out. The
red indicates anomalies stronger than Earth's average pull of gravity (about
32.1740486 ft/s2), while the blue shows weaker-than-average
anomalies.
Looks cool, but what do we do with this kind of information?
From modeling satellite orbits to monitoring the melting of glaciers and the polar ice caps, the benefits are numerous. To elaborate on the glacier example: If one is melting, the water drains away. That change in water means a change in mass -- and that translates to a small change in gravity around the glacier. So we can use gravity to study climate change.
Here's where I fit in: Scientists and engineers have used one type of gravity model for the past 100 years -- it's called spherical harmonics and is a bit dense to describe here (just look at the equation). My research looks at gravity from a totally different perspective, and to understand it, let's talk photography.
A photograph helps us preserve a memory, e.g. you as a kid. Years later, the picture reminds us of what we looked like when we were younger. That retrospective approach is similar to the gravity modeling I'm studying: Let's take our knowledge of the Earth's gravity, and store that information like a photograph so we can use it later. And instead of taking a picture of what the Earth visually looks like, we take a snapshot of what the Earth gravitationally looks like.
But one
photograph doesn't show us the whole picture. If we take a picture of one side
of the Earth, what does the other side look like? How do we know what gravity
is like there? Even two pictures don't lend us a clear picture of the edges. We
actually need six photographs to give us the Earth as a cube (see right).
So, why this new way versus the "old" way?
We've all sat there, waiting for a computer game or some piece of software to run... the more complex it is, the longer it takes for the computer to crunch the numbers. The "cubed sphere" model of gravity actually makes the software scientists use run faster -- so you get the speed of a rough test with the accuracy of an incredibly complex one. For people trying to model how their shiny new satellites will react to the Earth's anomalies while in orbit, that's a valuable improvement.
Modeling gravity is especially important for missions to the moon, since the spacecraft will be carrying people, and the moon's gravity is much more irregular than the Earth's. Better mapping of the moon's gravity not only improves simulations for planning trips there, but also helps astronauts figure out precisely where their spaceship is without asking for help from Earth.
It's also useful because, quite simply, someone can easily zoom in on a region they're interested in. Like looking at pictures of ourselves from when we were younger, we can use this cube-like model to easily see changes over time in things such as melting glaciers, mountains and more.
Brandon Jones is PhD student in aerospace engineering at the University of Colorado, Boulder.
Photos, top to bottom: Brandon Jones; NASA. Wikimedia Commons; Brandon Jones
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