Astronomy Cast Ep. 354: Mars vs Comet Siding Spring

Article written: 23 Oct , 2014
Updated: 28 Dec , 2015
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We were witness to a once in a million year event. A close approach of Comet Siding Spring to the Planet Mars. And fortunately, humanity had a fleet of spacecraft orbiting the Red Planet, ready to capture this monumental event in real time. What did we see? What will we learn?

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1 Response

  1. Smokey says

    Thanks for the “once-in-a-million-years” clarification!

    Siding Spring did not actually pass closer to Mars than Earth’s own geosynchronous satellites do to the Earth. (I don’t know if that’s what Ms. Pamela was trying to say, but that’s how it sounded.) Geosynchronous / geostationary chickens – if any! – orbit the Earth’s equator at ~22,300 miles up, which is only a little more than 1/4 of the distance the comet was to Mars. That altitude is where the orbital velocity matches the rotational speed of observers on the surface & thus appear to hold their positions in the sky relative to the local landscape rather than the stellar background, as you guys touched on briefly.

    Higher flying birds would travel across the sky much as the Moon does, rising in the east & setting in the west but moving progressively eastward against the background stars despite having higher orbital velocities. This contrasts with mid- to low-orbit fliers (e.g., GPS, Hubble, the ISS, etc.) which even at slower velocities still manage to out-run the Earth’s rotation to rise in the ~west while setting in the ~east (depending on the original launch angles, inclination, etc.).

    Orbital dynamics. Woof.

    In Ms. Pamela’s defense, Siding Spring WAS much closer to Mars than Luna is to Terra, less than half that distance actually. Also, the visual size of the coma in the image she referenced would have neatly filled that distance between our geosynchronous satellites and the bulk of Earth’s atmosphere. THAT’s pretty awesome in its own right!

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