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SciShow Space News explains two of the Hubble Space Telescope’s most important discoveries, and why the MESSENGER probe is about to crash into Mercury.

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Sources:
http://hubblesite.org/reference_desk/faq/all.php.cat=cosmology
http://www.nytimes.com/1999/05/26/us/hubble-telescope-yields-data-for-recalculating-age-of-universe.html
http://www.gizmag.com/feature-hubble-25th-anniversary/37051/
http://www.nasa.gov/press/2015/april/nasa-spacecraft-achieves-unprecedented-success-studying-mercury
http://phys.org/news/2015-04-mercury-messenger-nears-epic-mission.html
http://messenger.jhuapl.edu/news_room/presscon14_multi.html
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Caitlin: 560km above you right now, there's a hunk of metal more than 13 meters long orbiting our planet once every 97 minutes.  You probably know it as the Hubble Space Telescope, and as of this week, it's been up there for exactly 25 years.  Now, that's a long time to be in space, and thanks to Hubble, we've learned some important things about the universe, that, back in 1990, were just big celestial question marks.  Like what, you might ask?  Well, how about the fact that there are other planets.

These days, we've discovered more than 1,000 exoplanets and we keep finding more, they're everywhere.  But astronomers weren't always sure that planets were so common, until 1994, when they pointed Hubble at the Orion Nebula.  Nebulae like Orion's are basically star factories, and Hubble allowed astronomers to see new baby stars forming in there, in clearer detail than ever.  Its observations showed that many stars had what looked like large, swirling discs of dust around them, kind of like the one we think formed the planets in our solar system.  That was some of the best early evidence that, with so many stars out there with planet-making material around them, Earth probably isn't that special.  And since then, Hubble's fellow space telescope, Kepler, has confirmed that suspicion, observing new exoplanets that we tell you about here all the time.  

But we can thank Hubble for more than just making us feel less alone in the universe.  It also has allowed us to finally figure out the age of the universe.  In the 1990s, most scientists agreed that the age of, well, everything, was in the billions of years, but their estimates ran anywhere from 10 to 20 billion years old, a huge range.  Physicists arrived at this 10 billion year window based on the rate at which the universe is expanding, but they couldn't accurately measure that rate until they called on Hubble.  They used the space telescope to study stars called Cepheid Variables, which fluctuate in brightness in a predictable way.  By giving us the clearest observations yet of these stars, Hubble was able to precisely determine how fast they were moving away from us, and this in turn allowed us astronomers to hone in on the rate of the universe's expansion.  Using this new, more precise figure, physicists then calculated that the age of the universe was actually toward the younger end of the range, around 12 billion years old, though, today, we think it's closer to 13.8 billion.  

Hubble's made so many important discoveries in its very long life that it would be impossible for us to go through them all, but it won't be around forever.  Originally the plan was to use the space shuttle to safely transport the telescope back to Earth and maybe display it in a museum, but since Hubble has outlived its ride home, the new plan is to let it break up into large pieces within Earth's atmosphere, probably sometime around 2024, though it'll likely stop working long before that, which is sad to think about.  

But there's another mission that's coming to a planned violent end and soon!  On April 30th, the Messenger spacecraft, which is in orbit around Mercury right now, will crash into the surface at more than 14,000 km/hr.  The probe was launched in 2004, and when it arrived at Mercury in 2011, the mission was only expected to last for a year, but then it lasted four years.  Along the way, Messenger has taught us a lot about our smallest planet, for example, we now know that Mercury's thin atmosphere is mostly made of Sodium and Oxygen, and even though Mercury's surface can get as hot as 427 degrees, it turns out there's water ice hiding deep within its craters.  The probe itself still works just fine, but on April 6th, the Messenger ran out of Hydrazine, its fuel of choice.  To get as much life out of the probe as possible, scientists decided to use some of the helium gas that was onboard, originally used for pressurizing fuel tanks as a propellant.  But now even that's nearly gone, so on April 24th, Messenger will perform its last orbital maneuver, and six days later, the little probe that could will collide with Mercury's surface.  It won't burn up since Mercury doesn't have much of an atmosphere.  Instead, the craft will create a 16 meter wide crater on the opposite side of Mercury from us, where we won't be able to see it.  But even though the mission will be over, our interest in Mercury still lives on.  In 2017, the space agencies of Europe and Japan will launch their joint BepiColombo mission, which will send two orbiters to Mercury.  They'll arrive in 2024, just around the same time the Hubble will be going out in a blaze of glory.

Thanks for joining me for this week's news from around the universe, and if you wanna help support SciShow Space, just go to Patreon.com/SciShow.

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