October was kind of a bittersweet month for space scientists.

NASA successfully fixed problems with two satellites, the Hubble Space Telescope and the Chandra X-Ray Observatory,

but they also had to say goodbye to two more.

Last week, both the Kepler Space Telescope and Dawn mission

ran out of fuel and came to an end.

But over their 16 cumulative years of operation,

they showed us that our universe is even more amazing than we'd imagined.

Here's some of what we learned.

NASA's Kepler Space Telescope was originally going to be called FRESIP,

which stood for FRequency of Earth-sized Inner Planets.

But thankfully, somebody changed their mind and named it after the famous astronomer instead.

The mission launched in 2009, and it used the transit method to find planets beyond our solar system,

monitoring over half a million stars

to try and detect tiny dips in light that could indicate a planet was passing by.

And it totally worked!

Less than a year after launch, Kepler found its first planets,

a bunch of what scientists call hot Jupiters.

They're large gas giants that orbit super close to their stars.

We don't have any of those planets around here,

so discoveries like this offer huge insights into how other star systems evolve.

Over its two missions, the primary Kepler mission and the later K2,

the telescope went on to find more than 2700 confirmed exoplanets.

That's more than two-thirds of all the confirmed exoplanets we've discovered.

And along the way, it painted a picture of just how diverse our galaxy is.

Even accounting for those hot Jupiters,

most of the planets Kepler observed aren't anything like what we have in our solar system.

Instead, they're somewhere in size between Earth and Neptune.

And while it has been historically easier to find larger planets,

the fact that there are so many suggests our solar system could be at least a little special.

But not all of Kepler's discoveries were super weird and exotic.

It also found planets a little more similar to Earth,

and that could help us understand how our home got to be the way it is.

For example, in 2011, Kepler found the first solid evidence for a rocky exoplanet.

Then, in 2014, it found the first roughly Earth-sized planet in its star's habitable zone.

That's the distance from a star where, if a planet has a thick enough atmosphere,

water could exist on the surface.

And in 2018, the telescope may have even found the first moon around an exoplanet!

Unfortunately, for some time now,

Kepler had been running low on the fuel it needed to perform steering maneuvers.

And on October 30th, NASA announced that it had finally run out.

That means we can't point it toward anything we want to study,

or point it back toward Earth to send data home.

So now, it's just stuck in orbit around the Sun.

But it was a really good telescope while it lasted!

While Kepler was studying distant solar systems, the Dawn mission was investigating the asteroid belt:

the rocky leftovers from when our solar system formed.

Studying those leftovers gives us insight into what exactly happened those four and a half billion years ago.

Dawn launched back in 2007, and became both the first spacecraft to orbit something in the asteroid belt

and the first one to orbit two foreign worlds.

In 2011, it made its first stop at the asteroid Vesta, the second largest body in the belt.

We've talked about some of Dawn's accomplishments at Vesta before,

like how it studied a giant mountain on the south pole.

But this spacecraft did a lot more, too.

For example, Dawn revealed the presence of certain minerals we hadn't expected to find on the asteroid.

Specifically, ones with water molecules stored in them.

They likely came from small collisions,

and their existence contradicts the old idea that Vesta is just a big, dry rock.

Dawn also confirmed that Vesta has a layered interior with a separate crust, mantle, and core,

kind of like the inner planets.

That makes it an important piece of evidence in understanding planetary formation.

Because if a little asteroid has these layers,

that suggests planetesimals, the building blocks of planets, might have had them, too.

In 2012, Dawn left Vesta for Ceres, the only dwarf planet closer than Pluto.

It arrived in 2015, and started discovering a ton about the object's composition,

like that it had clays full of ammonia.

That's actually pretty strange because, given its distance from the Sun,

Ceres is too hot for ammonia to condense into a solid.

So scientists now think that Ceres either formed in the outer solar system and migrated inward,

or is partially made up of smaller bodies that formed out there.

Dawn also spotted an abundance of organic molecules in one of Ceres's craters,

which may have been formed in the dwarf planet's interior.

Organic compounds are commonly made by life, but don't get your alien-hunting gear out yet.

So far, the only molecule that's been identified is some kind of aliphatic,

which is a pretty simple molecule made of hydrogen and carbon chains.

So this might just mean that organic molecules aren't that rare.

During its mission, Dawn traveled nearly 7 billion kilometers and taught us a lot.

But it couldn't last forever.

On October 31st, Dawn ran out of its propellant.

That meant it was unable to rotate its antenna to check in with Earth,

so the team ultimately had to declare the spacecraft defunct.

It will continue to orbit Ceres for up to 50 years, but eventually, it will come crashing down.

The good news is, Dawn will be teaching us about the asteroid belt for years to come,

since the mission collected tons of data, including nearly 100,000 images.

The end of Kepler and Dawn is a sad moment in astronomy, but at least there's a silver lining.

Kepler's replacement, called TESS, launched earlier this year

and has already started capturing data.

And while it will likely be a while before another craft visits Vesta or Ceres,

we have plenty of spacecraft working on other asteroids.

That means more exciting space news is always on the horizon.

Thanks for watching this episode of SciShow Space News!

If you'd like to learn even more about what Dawn did during its mission,

you can watch our episode from 2017 about some of its other accomplishments.

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