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Here on Earth we all
know about gravity.

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But what you might
not realize

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is that, depending where
you are on the planet,

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the strength of gravity
is different.

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For example, up here
on a mountain

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with all this mass underneath,

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gravity is stronger.

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Water also has mass.

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And the Earth has a
whole lot of water.

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It's moving around.
It's changing phases.

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If you can track the
change in gravity,

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you can track the
change in mass.

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And that means you're
understanding

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the movement of water.

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NASA's about to send the
GRACE Follow-On mission

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which will continue
to do just that.

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Let's learn about it on this
episode of Crazy Engineering.

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[chomp]

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[ ♪ ]

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[whoosh]

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OK, everybody. We're here
with Neil, he's one of the

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key engineers on the
GRACE Follow-On mission.

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Neil, thank you so
much for joiining us

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and answering our questions.

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Why do we call it
"GRACE Follow-On"?

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So, GRACE stands for Gravity
Recovery and Climate Experiment.

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And we're using gravity to track
water motion around the planet.

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And the "Follow-On" because
we've done this before--

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and we're doing it again
with two new satellites.

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The original GRACE mission
was launched in 2002.

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They lasted for 15 years and
provided amazing science

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for the scientists.

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Both of these missions
have two satellites.

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Can you explain to us why
we need two satellites

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instead of just one?

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So, we need two satellites
because we're trying to

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measure very precise, small
amounts of gravity changes

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in the Earth.

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The Earth is actually lumpy,
when it comes to gravity.

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Far away from Earth, gravity's
just a single number.

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But as you get closer to
the Earth the gravity changes

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because the Himalayas have a
little bit more mass...

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certain areas have less mass.

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So as the satellite is
orbiting the planet,

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as it comes towards a
large mass like a mountain

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it actually will speed up.

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And then as it leaves
it will slow down.

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And so with two satellites,
we're able to measure

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the distance between the two.

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How far apart are
these satellites?

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And just how precise? Do they
have to have knowledge

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of each other's position?

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We have the satellites at
200 kilometers apart,

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but we're measuring down to an
accuracy of a micron level,

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which is about a tenth
of a human hair.

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That is extremely precise!

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It sounds like a very hard
engineering problem.

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What's the technology
that lets us do this?

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We're using microwave
technologies at about 30 GHz.

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Here you see one
of the satellites.

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And 200 kilometers away
is the other satellite.

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And what we're doing is
we're firing our signal

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to the other satellite,

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and the other satellite's
measuring that.

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And as you move
backwards and forwards

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relative to this sine wave,

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you can measure the distance
changes to the levels we need.

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Neil, it's been well
more than a decade

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since the original
GRACE mission.

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I assume we're upgrading some
of these technologies.

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Can you describe some of those?

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Sure. Like technology
advances in general,

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we have upgraded
computer systems.

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We have more efficient
solar cells.

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We have better star cameras.

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And, like everything else
in the future,

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we now have lasers!

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Lasers! Yes!

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What do lasers
actually get for us?

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So, we're actually using the
lasers to do the measurement

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between the two satellites.

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We're using RF systems from
the previous missions.

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On top of that, we have this new
technology demonstrator

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where we're going to fire
laser beams between each other

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to make a more accurate
measurement

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between the two satellites.

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Neil, this is without a doubt

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one of the coolest
missions we've seen!

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Thank you so much for
answering our questions.

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When can we hope to
see this launch?

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So we're hoping to hitch a ride
with a SpaceX rocket

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later this year and we should
be collecting science data

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shortly after that.

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All right. Well, we'll
certainly check that out.

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And, everyone out there, check
back soon for some more

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Crazy Engineering!

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[ ♪ ]

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NASA Jet Propulsion Laboratory