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[♪ INTRO]

It's been 50 years since humans last walked on the Moon,

but that doesn't mean we've stopped exploring our closest neighbor.

Countries from all over the world have sent robotic missions to study it,

but none has been as important, or as successful, as NASA's Lunar Reconnaissance Orbiter.

NASA launched the mission, often called LRO, in 2009 as part of its Vision for Space Exploration program.

This was a long-term plan designed to guide the U.S. space program

after the loss of the space shuttle Columbia.

The goal with LRO wasn't just to do science, but to pave the way for a new era of human exploration.

And while we have yet to send people back to the Moon, we can definitely say the LRO has succeeded.

The things it's teaching us about our closest neighbor are transforming the way we think about the Moon,

and the information we're learning from it will make it much easier for future astronauts.

So, in honor of all it's done so far, and to celebrate its 10th birthday,

let's look back at three of LRO's biggest accomplishments.

For one, this mission showed us that the Moon is not as dry as we used to think.

When researchers examined lunar rocks brought back by the Apollo astronauts,

they turned out to be almost completely waterless; way drier than rocks found on Earth.

That painted a picture of a pretty arid Moon.

But some scientists suspected that frozen water

might be hiding out in the Moon's deepest craters, which the Apollo program didn't visit.

Those spots are so deep that they've never seen sunlight,

and can have surface temperatures colder than Pluto, just 35°C above absolute zero.

Temperatures like that are too cold for water to sublimate away,

and they make it possible for ice to survive in the vacuum of space.

But just because water could exist on the Moon didn't mean it had to be there.

To find out more, LRO's companion mission, called LCROSS, took one for the team.

It sent part of its rocket to smash into the Moon's Cabeus crater,

sending a plume of surface debris into space.

Then, the LCROSS spacecraft and LRO studied the light from this material

as it passed in front of the Sun, and researchers later found that

the plume was full of grains of mostly-pure water ice.

They pronounced this crater “wetter than the Sahara.”

That might not sound very impressive, but it showed us that ice can accumulate on the Moon,

and that this supposedly-arid place has pockets full of frozen water.

Someday, that water could be used to help astronauts on the Moon's surface.

But even if not, it's still pretty amazing that it's up there.

That's not the only thing LRO has uncovered in hidden ice, either.

In 2016, scientists published another big discovery made by the orbiter: the Moon's ancient poles.

Besides finding ice at today's poles,

LRO found ice deposits offset by about 200 kilometers in opposite directions.

If you draw a line between them, it passes directly through the Moon's center.

Researchers interpreted this arrangement as evidence of the Moon's old poles,

meaning the Moon used to spin on a different axis.

Which isn't unheard of in the solar system, but still feels downright weird to think about.

Models show that that axis shifted around 3 billion years ago,

most likely as mass moved around deep below the surface.

This process was slow.

The poles drifted only around 2 centimeters every century,

but that was enough to knock the Moon off kilter by about 5°,

like if Earth's axis shifted from the South Pole to Australia.

As sunlight leaked into the once-shady areas, some of that old polar ice probably evaporated,

but the rest traced out the path of the moving axis for us to uncover billions of years later.

Besides being cool, this finding is also significant because

it helps us understand when the inside of the Moon was molten,

which is important for studying how the Moon changed and evolved.

I mean, it doesn't give us a very specific estimate, but you have to start somewhere!

Finally, although LRO is meant to study the Moon,

it's actually taught us a lot about the entire solar system.

For instance, it took enough images for scientists to create a detailed,

billion-year-long timeline of large asteroid impacts on the Moon.

And this January, scientists reported that that timeline revealed something strange:

Asteroid collisions seem to have more than doubled around 290 million years ago.

While that's interesting by itself, that also tells us something surprising about Earth.

Scientists have known for a while that Earth has surprisingly few craters older than 300 million years,

but they'd always assumed that the craters had just eroded away.

Now, LRO is telling us that these aren't just gaps in the record,

there were simply fewer impacts back then than the period that followed.

Scientists still don't know exactly what happened 290 million years ago,

but one idea is the sudden bombardment might point to

large collisions in the asteroid belt between Jupiter and Mars.

Whatever it was, though, the scars on the Moon point to some major event in the distant past.

And figuring out what it was would help us continue to understand

how the solar system has changed over its lifetime.

Also, on a much broader note, cratering rates on the Moon are also used

to determine the ages of surfaces all around the solar system.

Like, when scientists first saw Pluto's smooth surface,

they assumed it was young because it doesn't have as many craters as the old lunar surface.

So the more we understand about the Moon's craters,

the more we can infer about all the other objects out there.

Of course, even though LRO has taught us so many specific things over the last decade,

it isn't valuable just because of those discoveries.

LRO is especially important because it lives around the Moon.

That means that, if scientists find something interesting,

they can ask questions and get answers without having to wait for a new mission,

which is pretty unusual in planetary science.

Also, as much as space exploration is always pushing new horizons,

LRO has shown us that there are still endless things to explore right here in our neighborhood.

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[♪ OUTRO]