Today on Crash Course Astronomy, Phil explains comets. Comets are chunks of ice and rock that orbit the Sun. When they get near the Sun the ice turns into gas, forming the long tail, and also releases dust that forms a different tail. We’ve visited comets up close and found them to be lumpy, with vents on the surface that release the gas as ice sublimates. Eons ago, comets (and asteroids) may have brought a lot of water to Earth -- as well as the ingredients for life.

Check out the Crash Course Astronomy solar system poster here: http://store.dftba.com/products/crash...

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Chapters:
Introduction: Comets 00:00
Comets = Dirty Snowballs 1:19
Comets Have Two Tails 2:52
Short-Term vs Long-Term Comets 4:35
Where do comets come from? 5:06
Comets Up-Close 6:17
What Happens When Comets Hit Earth? 9:15
Review 10:59
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PHOTOS/VIDEOS
Halley's Comet, 1910 http://en.wikipedia.org/wiki/File:Hal... [credit: New York Times, Wikimedia Commons]
Bayeux Tapestry http://upload.wikimedia.org/wikipedia... [credit: Wikimedia Commons]
Comet McNaught [credit: Phil Plait]
Comet McNaught + tail http://en.wikipedia.org/wiki/C/2006_P... [credit: Chris North, Wikimedia Commons]
Comet surface http://rosetta.jpl.nasa.gov/sites/def... [credit: ESA/Rosetta/NAVCAM]
Fine structure in the comet’s jets http://blogs.esa.int/rosetta/2015/01/... [credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA]
Comet Halley http://apod.nasa.gov/apod/image/1001/... [credit: Halley Multicolor Camera Team, Giotto Project, ESA]
Comet McNaught https://solarsystem.nasa.gov/multimed... [credit: ESO]
Hubble's Last Look at Comet ISON Before Perihelion http://en.wikipedia.org/wiki/File:Hub...
[credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)]
Hale-Bopp: The Great Comet of 1997 http://apod.nasa.gov/apod/ap131013.html [credit: Jerry Lodriguss, used with permission]
Sungrazer video http://svs.gsfc.nasa.gov/cgi-bin/deta... [credit: NASA's Goddard Space Flight Center]
“Large Web” graphic http://www.nasa.gov/sites/default/fil... [credit: JHUAPL/SwRI/Dan Durda]
Comet Daniel http://commons.wikimedia.org/wiki/Fil... [credit: Max Wolf, Wikimedia Commons]
Vega 1 Low Res http://nssdc.gsfc.nasa.gov/planetary/... [credit: Russian Academy of Sciences]
The Nucleus of Comet Halley http://sci.esa.int/giotto/51183-the-n... [credit: ESA/MPS]
Comet Hartley 2 http://en.wikipedia.org/wiki/103P/Har... [credit: NASA/JPL-Caltech/UMD]
Comet 67P/Churyumov-Gerasimenko http://sci.esa.int/rosetta/54472-come... [credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA]
OSIRIS Catches Activity in the Act http://blogs.esa.int/rosetta/2015/04/... [credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA]
Active Pit http://www.esa.int/spaceinimages/Imag... [credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA]
Depiction of Philae‍‍ '​‍s planned touchdown on the comet http://en.wikipedia.org/wiki/Philae_(... [credit: DLR, CC-BY 3.0, Wikimedia Commons]
Colour Image of a Comet http://www.esa.int/spaceinimages/Imag... [credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA]
Comet Lovejoy http://en.wikipedia.org/wiki/C/2011_W... [credit: NASA/Dan Burbank]
NASA’s Stardust http://www.nasa.gov/images/content/36... [credit: NASA/JPL]
Comet dust https://www.nasa.gov/images/content/1... [credit: NASA]
Comet Lovejoy http://www.eso.org/public/usa/images/... [credit: ESO/G. Brammer]