Comet Sliding Spring & Mars near Miss

No doubt sooner or later your will read sensationalized reports of a comet which possibly may crash into the planet Mars.

Once again, the news business is creating a sensation for profit. As you read your local rags, take note on how they report this non news.

I find the event interesting, but then I find solar flares,  eclipses of the Sun by Mercury interesting. Just not news worthy or particularly exciting..

Anyway, here is some straight dope on the event.

Comet to whiz past Mars

in October 2014

Comet C/2013 A1 (Siding Spring)'s trajectory past Mars

C/2013 A1 (Siding Spring) will pass extremely close to Mars on October 19, 2014. This orbital diagram was based on slightly older observational data that showed the closest approach happening on October 20. The comet will be coming up from south of the ecliptic in a direction opposite to that of Mars'. (The orbits are drawn in dark colors where they dip below the ecliptic, and bright colors when they rise above the ecliptic. Since Earth's orbit is, by definition, in the ecliptic, its orbit is bright everywhere.)

When astronomers report the distances between two objects in space, they are almost always referring to the distances of the centers of the two objects. Distances are usually so great in space that this is a perfectly fine approximation that greatly simplifies calculations. But in cases like this one, you can't make that approximation anymore; you have to account for the size of Mars, which is roughly 7000 kilometers in diameter. So the comet will be passing within 38,000 kilometers of Mars' surface. That is very, very close.  Note though that the uncertainty in the orbit is such that the possibility of a Mars impact can't yet be ruled out, though it's very unlikely.

How close is 38,000 kilometers? One way we talk about that for close passes by Earth is to compare the distance to that of Earth-orbiting satellites. If it were Earth, 38,000 kilometers would be within the orbit of geosynchronous satellites. At Mars there are no geosynchronous satellites, and if there were, they'd be in a closer orbit, at 20,000 kilometers. So it's outside that distance. Probably more relevant is the question of whether there are any satellites at Mars that do get out to 38,000 kilometers; the answer to that is no. Mars Express has the largest orbit, and it gets to an altitude of about 10,000 kilometers. All the other active orbiters are down closer to 300 or 400 kilometers. So there's not likely to be a direct hazard to spacecraft in the form of a potential impact.

The Planetary Society