Since Sputnik 1 launched the Space Race in 1957,
we've sent thousands of things into space.
Lots have spent their entire lives orbiting Earth
from communications satellites to the International Space Station
and the Hubble Space Telescope.
But others have gone a lot farther away, and today,
there are way too many active probes out there to fit in a single video.
And they're all returning amazing data, so it's not even fair
to say that some are better than others -- but some are especially neat.
Here are seven active space probes that we think are just the coolest.
The Opportunity rover arrived at Mars in 2004.
It was supposed to spend about 92 Earth days there,
searching mostly for signs of ancient water and helping us
understand why Mars and Earth look so different today,
even though they were both warm and wet billions of years ago.
That was its primary mission,
or the set of goals it was designed to accomplish in a certain amount of time.
After that, scientists figured that Martian dust would build up
on Opportunity's solar panels, and the rover's batteries would go dead.
But that didn't happen.
Opportunity kept working.
And if a probe still works well after its primary mission ends,
its space agency can give the mission an extension
so that scientists can keep using it.
Engineers working on probes tend to be pretty good at their jobs,
so missions get extended kind of a lot:
About three-quarters of NASA's current projects are in extended phases.
But even among other NASA missions, Opportunity stands out,
because it wasn't just extended once or twice: It's had ten mission extensions.
So more than fourteen years after landing and more than a marathon of driving,
Opportunity is still roving the Red Planet.
All that success started early.
Opportunity landed inside a crater, and, when it analyzed nearby rocks,
it proved that the plains around the crater were once an ancient ocean floor.
And by examining the composition of other Martian rocks,
Opportunity and its partner Spirit -- which stopped working in 2010 --
discovered that Mars's ancient water
went through some pretty dynamic changes while it was still around.
It was super-salty in some places and completely fresh in others,
and some locations on Mars went from deep to shallow water and back again.
Starting in 2005, Opportunity also found meteorites hanging out on the surface
and measured them to show scientists that when the meteorites arrived,
Mars's surface was still pretty wet and its atmosphere was pretty thick.
And the discoveries won't stop soon.
A couple of Opportunity's instruments and computers might not work any more,
and the rover could definitely use a tune-up.
But it's still doing incredible science.
There was a lot of fanfare when Juno got to Jupiter in the middle of 2016.
Because everyone just loves Jupiter.
Juno's sensitive instruments are designed to do a whole bunch of things,
including measuring Jupiter's magnetic field, taking its temperature,
revealing what kinds of molecules make up its colorful clouds,
and figuring out how much solid rock is beneath all that swirling gas.
And hopefully all of that will teach us how Jupiter formed in the first place.
So far, those instruments haven't disappointed.
Juno has already found big differences between Jupiter's
northern and southern hemispheres
everything from their gravitational pulls to the auroras at the poles.
And thanks to Juno's measurements, scientists found that
something toward the planet's center is making heat and gas
flow in some directions more than others.
Juno also regularly sends back stunning pictures of Earth-sized cyclones
raging all over Jupiter's outer cloud layers.
Aside from just being beautiful,
all those pictures let scientists track how Jupiter changes over time,
which gives them hints about what's driving those changes from deeper down.
But NASA scientists aren't always the ones choosing
what Juno's camera captures next:
They let regular internet citizens like you vote on it.
And so far, you've collectively made some fantastic decisions.
So thanks for that.
Juno's primary mission is set to end this July 2018.
Then, in 2021, the probe will intentionally crash into Jupiter.
That way, there's no chance of it hitting a moon like Europa
and contaminating it with life from Earth.
But if history is any guide, as long as everything's working well,
there might be a good chance we have a few more years left of Juno.
Some of these missions are pretty well-known,
but a lot of space nuts haven't even heard of the
Advanced Composition Explorer, or ACE.
Although, if you have, you've probably already left us a comment about it.
ACE's five-year primary mission started in 1997.
It was designed to study high-energy charged particles coming from the Sun
and even higher-energy particles from interstellar space.
And of course, fifteen years after its primary mission ended,
ACE is still out there sending back data,
and it will have enough power to maintain its orbit until around 2024.
One of ACE's major targets has been the solar wind
the constant stream of charged particles flying off the Sun's surface.
It studied how the solar wind is affected by changes on the Sun's surface
and in its magnetic field,
giving scientists a better idea of what's going on inside our star.
And it's also part of a system that warns scientists about solar storms
before particles reach Earth.
Those particles can damage satellites and people,
so it gives us time to protect our equipment
and astronauts on the International Space Station.
That way, we don't end up with the Fantastic Four.
ACE also helped researchers figure out that those super-energetic particles
from interstellar space probably came from
black holes or supernova explosions.
And it's helped scientists research why the Sun's corona
is so much hotter than its surface.
And although they still don't have an answer for that one,
ACE's measurements have given us a better understanding of the problem.
Later this year, NASA will build on ACE's data by launching the
Parker Space Probe, which will eventually skim through the solar corona
and hopefully solve the mystery for good.
Dawn has done a lot.
Among other things, it was the first probe to visit and
orbit a dwarf planet, and the first to orbit two different extraterrestrial bodies.
In 2011, It arrived at the giant asteroid Vesta, which is in the asteroid belt
between Mars and Jupiter, and spent a year there.
Then, it flew to the dwarf
planet Ceres, which is also in the asteroid belt.
In 2016, after a year at Ceres, Dawn was supposed to power down,
but two years and two mission extensions later, it's still there
returning new data all the time.
Vesta and Ceres have both been around since the solar system began,
and they have a lot to teach us about those early days
and about what's happened since.
But they're also very different places, and scientists wanted to know why.
Vesta, where Dawn started, is an oddball in the asteroid belt:
It's the third-largest rock out there, but it's denser and more reflective
than most other asteroids.
Even before we got there, scientists knew that about one in sixteen meteorites
found here on Earth looks a lot like Vesta,
and they'd wondered if the resemblance was more than coincidental.
And shortly after it arrived, Dawn proved their hunch right.
It measured the composition of Vesta's surface, and then completely mapped it.
And that map shows that Vesta's been pummeled by other asteroids over the
eons, with some of those impacts sending off pieces that ended up here.
Then, at the much rockier dwarf planet Ceres, the probe proved that,
like other bodies in the solar system, Ceres had oceans billions of years ago.
By mapping Ceres and measuring things like the density of its rocks,
Dawn has found evidence that lots of that ancient water is still trapped in
rocks in and beneath the surface.
On top of all that, it also found ice volcanoes and organic,
carbon-containing molecules sitting on Ceres, making it the first
main-belt asteroid where we've ever spotted organic molecules.
And until its engines run dry sometime later this year, it'll orbit closer to
Ceres than ever before so that it can keep returning new revelations.
When it opened its eyes in 2009, the Kepler Space Telescope had
pretty much one job: Stare at a little more than a hundred thousand stars.
If there were planets orbiting any of those stars, they might pass between
the star and Kepler at some point.
And when they did, they would block
some starlight, and the star would periodically look dimmer.
If that happened a few times, scientists would take a closer look to confirm
the planet and its measurements.
In the last nine years, Kepler has found around three thousand exoplanets,
and there are another two thousand candidates just waiting to be confirmed.
The telescope discovered the vast majority of exoplanets we know about
everything from broiling gas giants to rocky, Earth-sized planets.
And it's still discovering new ones,
but it isn't staring at those same stars any more.
It received a mission extension back in 2012, but then a second of its four
reaction wheels, devices that stabilized the telescope, failed a few months later.
It had lost a reaction wheel earlier in the mission, but without two of the reaction
wheels, the telescope couldn't consistently stay pointed in any one direction.
After months of work, the mission's scientists figured out a way to use
the pressure of sunlight itself to keep Kepler stable.
As particles of light hit the telescope, they apply just enough pressure
to hold the telescope steady and make up for the broken reaction wheels.
So now, Kepler spends a few months at a time pointed at whatever patch
of the sky is roughly opposite the Sun, and it's now called the K2 mission.
But those reaction wheels are pretty much the only things
that don't work anymore, so the telescope's images and data
are still just as good as when it first started staring.
Carl Sagan wrote that the Voyager probes are
"triumphs of human engineering".
And he wasn't wrong.
Voyager I and II took off a few weeks apart in 1977,
each carrying the best scientific instruments of their day
and a gold-plated copper record with messages from Earth.
Just in case they encountered any aliens.
Their combined primary mission was to explore
the Jupiter and Saturn systems, and they did a great job.
At Jupiter, they discovered new moons and faint rings,
and even accidentally found active volcanoes on Jupiter's moon Io.
And years later, those measurements influenced crucial decisions
about the Galileo probe in the 1990s and the Juno probe today.
The Voyagers were no less successful at Saturn in 1980 and 1981
discovering moons, flying near the rings, and gathering information
for what became the famous Cassini-Huygens mission that ended last year.
Then, of course, their missions were extended.
Voyager 1 used gravity from Saturn and its moon Titan
to slingshot toward the top of the solar system.
Nine years later, it took the famous Pale Blue Dot picture of the Earth
from six billion kilometers away,
which inspired future astronomers and space fans.
Meanwhile, Voyager 2 became the only ship to ever
explore Uranus and Neptune, and it made groundbreaking discoveries
at each planet -- like finding boiling water beneath Uranus' clouds
and a gigantic storm called the Great Dark Spot on Neptune.
Since then, the Voyagers have been speeding out of the solar system
at about 16 kilometers per second,
and they're still sending back radiation and magnetic field measurements.
In the last couple years, the they've made the first direct measurements
of the heliosheath, the region where the Sun's radiation
gives way to the radiation of interstellar space.
They won't last forever, though.
After their power sources finally die -- which might happen around 2025
the two Voyagers will silently drift through the galaxy,
carrying messages from Earth on golden records
that should still work in a billion years.
And maybe -- just maybe -- someone else will find one.
After decades of messages to Earth,
it might become a friendly message from Earth.
Thanks for watching this episode of SciShow!
If you'd like to learn even more about these space probes
and other missions like them,
you can find plenty of videos at youtube.com/scishowspace.
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