How UT's experimental mission uncovered secrets of our planet

AUSTIN (KXAN) — A ‘gravity hole’ on Earth? Earlier this summer, researchers with the Indian Institute of Science published a paper exploring one of these ‘gravity holes’. Found all over the planet, these are spots on the planet’s surface where there is less gravity than in other spots. An interesting curio that we wouldn’t know about if it wasn’t for GRACE.

“It’s an acronym for Gravity Recovery and Climate Experiment,” said Professor Byron Tapley with the University of Texas’ Center for Space Research in North Austin.

GRACE was Tapley’s brain child. A new type of satellite, designed to study changes in mass on the planet. Launched in 2002, the mission was run by Tapley and a team of civilians, engineered by NASA and launched by Germans.

The pair of satellites orbited the entire planet every month and mapping its mass. A tiny laser connected the two and was used to measure the distance between them down to 1/10 of a human hair.

The mission not only showed changes in gravity but also proved the melting of the ice caps.

What exactly does GRACE do?

To understand GRACE, you need a little physics lesson. First, the greater something’s mass, the greater its gravity.

Jupiter has more mass than Earth and thus has greater gravity. This is true of all things. You, dear reader, have a slight gravitational pull. As does your friends and family.

Now the Earth isn’t smooth. Oceans, mountains, rivers and valleys all have varying degrees of mass and thus have different gravitational pulls. This is why there are spots on our planet with “gravity holes”. These are simply spots with less mass.

The GRACE Satellite helped researchers study gravity changes on Earth, including a gravity hole in the Indian Ocean. (Credit: NASA)

GRACE utilizes some physics to understand these mass differences.

“There’s a fundamental law that says if you’ve got two mass points that the gravitation attraction between those will be proportional to their mass inversely proportional to the distance between them,” Tapley said.

GRACE uses this law to its advantage. As the the pair of satellites fly over the surface of the Earth, different masses pull on the two ships.

Let’s say they’re approaching a mountain. The ship in front would speed up as it is pulled towards the mountain, then the back ship would get pulled. As the front ship moves past the mountain, it will slow down. Then it will speed up as it escapes the mountain’s gravity. The ship in back will slow down, creating more distance between it and the ship in front, before it speeds up again.

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“If the satellites are flying over smooth Earth, they would maintain the same distance they wouldn’t separate apart,” Tapley said.

So as the ships orbit the planet, they grow further apart then closer as they are pulled by gravity from objects on Earth. The laser connecting them is used to measure the distance between them, which can then be used to show changes in mass over the planet.

Measuring a changing Earth

Over 15 years, GRACE mapped the entire planet each month. Over time it was able to track changes, including changes in water flowing around Earth, including where it was melting.

“The melting of the polar ice caps was kind of at a peak at that point,” Tapley said.

Using the data GRACE collected, scientists were able to show that ice was melting, that aquifers around the world were draining and that the seas were rising.

A model of GRACE. This pair of satellites helped researchers discover differences in gravity on Earth and melting in the polar ice caps. (Credit: Eric Henrikson/KXAN)

“It turns out that observing the polar ice cap melt turned out to be critically important for science,” Tapley said.

The mission concluded in 2017. However, the data it provided was so important that NASA launched a follow up mission, GRACE-FO, in 2018. They plan to launch an additional follow up mission this decade.

“It’s been an extremely rewarding mission to go into, it’s one that I’ve enjoyed a great deal,” Tapley said about the project. While he retired several years ago, he continues to help with the GRACE-FO mission today.

“It’s one of those things that when you go through life, you think you’re lucky to have been been involved with it.”

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