This week, space agencies around the world will have a historic opportunity to watch a comet make its first visit to the inner solar system.

(0:07)  It will also be the closest look we've ever gotten at one of these brand new comets.

(0:11) On October 19th, comet C/2013 A1, also known by the classy sounding nick-name of Siding Spring, will pass within about 140,000 km of Mars.

(0:20) That's just 1/3 of the distance between us and the Moon, and less than 1/10 as far as the closest known comet flyby of Earth.

(0:26) Conveniently enough, we have a ton of spacecraft on and around Mars that are going to be right there when Siding Spring passes - ready to make the most of it.

(00:33) But first we have to make sure they all survive the encounter.

(Intro)

(0:40) Astronomer Robert McNaught discovered comet C/2013 A1 at Siding Spring Observatory in Australia on January 3rd, 2013. Hence, the name.

(0:48) Judging by its trajectory the comet hails from the distant reaches of the Oort cloud; a remote region surrounding our solar system that's about 100,000 times farther from us than the sun.

(0:56) Hurdling toward our neighborhood at 56 km/sec, Siding Spring has taken at least a million years to make it this far,

(1:02) and for a while it looked like its journey might be coming to an abrupt and messy end.

(1:06) At first, astronomers that Siding Spring was going to make a direct impact with Mars, a planet that we are heavily invested in,

(1:12) but in recent months they've determined that the comet will miss the planet, but just barely.

(1:16) Specifically the comet's nucleus, a small chunk of rock and ice between about one and eight kilometers wide, will likely pass Mars right by.

(1:22) But that may not be the case for the coma - the tail of gas and dust that the comet gives off as it approaches the sun.

(1:27) But it's just dust, right?

(1:30) Well, even a single particle a few millimeters wide can damage a spacecraft if it's traveling at 56 km/sec.

(1:36) To give you a sense of how fat that is, the speed of sound at Earth's sea level is about 1/3 of a kilometer per second.

(1:43) Plus Siding Spring's coma is enormous - 160,000 km wide and 480,000 km long.

(1:49) So even though we have a pretty good sense of where the comet's nucleus is going to go, the coma is much less predictable.

(1:54) As Canadian astronomer David Levy once said,

(1:56) "Comets are like cats: they have tails, and they do precisely what they want."

(2:00) Which is adorable, but still we have a lot of really important stuff laying around Mars right now that we'd very much not like to be destroyed by some sassy comet swinging its tail.

(2:09) Like ESA's Mars Express for example, and NASA's Mars Reconnaissance Orbiter, and the Mars Odyssey.
All in orbit right around the red planet.

(2:16) Then there's India's Mars Orbiter Mission and NASA's Maven, both of which just arrived in orbit a few weeks ago.

(2:21) Not to mention NASA's two remaining robots that are still active on the ground - Curiosity and Opportunity.

(2:26) Fortunately, astronomers think Mars's atmosphere, though much thinner than Earth's, is still thick enough to slow and burn up any dust particles, which will help protect the rovers.

(2:34) But to protect the orbiters, space agencies are sending them behind Mars while the coma is expected to be closest about 90 minutes after the nucleus passes.

(2:42) So once everyone has put their hardware out of harms way, they can think about how best to use that hardware.

(2:46) and make the most of this historic encounter.

(2:49) After all according to NASA, we may never again be this close to a comet fresh from the Oort Cloud.

(2:53) Comets carve enormous elliptical orbits through the solar system, and essentially start to melt as they approach the sun.

(2:59) After a hundred orbits or so, many comets eventually disintegrate.

(3:02) But new comets keep showing up, and astronomers think they originate in the Oort cloud. A sort of fluffy, icy nimbus of material left over from when the solar system first formed.

(3:10) And since this seems to be Siding Spring's first time out of the cloud, that means it's never before been exposed to the Sun's heat and radiation.

(3:16) So the chance to observe something that's never been close to the Sun is a chance to study an object that has stayed pretty much unchanged since the birth of the solar system some 4.6 billion years ago.

(3:25) If we happen to find important life-giving materials on or around Siding Spring like water or carbon compounds, that would mean that these things were present early in our solar system's history.

(3:34) So dozens of instruments in the metro-mars area will be trained on the comet looking for these materials and compiling data about the overall makeup of this amazing celestial relic.

(3:42) What could be more in your face than that?

(3:44) Well don't forget the Rosetta spacecraft is going to send a lander to the surface of its target comet in just a couple of weeks.

(3:49) So if you want to make sure you get the latest from these events and all kinds of goings-ons around the universe, check out subable.com/scishow to learn how you can help support us.

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