This question will likely be answered "in retrospect" in about 8 days. So it's a great opportunity to "lay it out there" and then let reality prove you right/wrong.

On the 19th of October, Comet Siding is expected to pass Mars very closely (1/3 the distance of the moon from Earth). My assumption is that this means its nucleus. But comet tails can be very long (100s of millions of miles). For the sake of this questions let's keep it to "obviously observable" tail and coma.

Additionally "going through" may need a definition. So let's say that if a line drawn from the outer (obviously observable) coma closest to Mars and the end of the (obviously observable) tail were to intersect with-in 20 miles of the surface of Mars, then we would say that Mars has "gone through the tail" of the comet.

So is Mars expected (or even more fun, do YOU expect Mars) to go through the tail of Comet Siding on the 19th of October?

If so, what will be observable from the surface of Mars? Large number of shooting stars?

NASA is preparing to hide its satellites (repositioning them to the "far side" of Mars) from comet dust traveling at 35 miles/sec. Is there any idea of how long the encounter will last?

  • $\begingroup$ The Earth passed through the tail of Comet Halley in 1910 with no appreciable effect (except mild public panic). In theory, you could use HORIZONS to compute the positions of Mars, the comet, and the Sun (comet tails always point directly away from the Sun), but, of course, you'd get the same results NASA does, since NASA runs HORIZONS $\endgroup$
    – user21
    Commented Oct 14, 2014 at 3:07

2 Answers 2


Short answer: Sort of

Long answer:

One week before the encounter, Wikipedia says no. The comet's nucleus will pass by Mars at a distance somewhere of about 139,000 km from the center of Mars - way too far for any predicted collision. The main tail of the comet, too, will most likely miss Mars by about "10 Mars diameters" - roughly 64,000 km. However, small quantities of dust particles could (and most likely will) stray away from the main tail and hit Mars, Deimos and Phobos. This graphic gives a good picture of the encounter:

enter image description here

However, Mars may pass through the comet's coma and be hit by any dust particles there.

It will certainly be observable from the Martian surface: The comet will have an apparent magnitude of -6 at its peak. It will be quite a show, but nothing extraordinarily special.

The damage to orbiting spacecraft around Mars will likely be minimal. 90 minutes after the comet passes Mars, the worst of the dust particles will hit the spacecraft. The tiny barrage will only last about 20 minutes, though, and there is a low probability of damage. In fact, the various orbiters will take pictures of the comet and try to analyze it to learn more about its properties.

I hope this helps.

Other sources:




As of about an hour after the encounter, Mars still exists. Read this article for some not-so-juicy details.

  • $\begingroup$ Magnitude -6? That'll be brighter than Phobos, which runs to magnitude -5 on the martian surface: astrobob.areavoices.com/tag/deimos $\endgroup$ Commented Oct 12, 2014 at 17:45
  • $\begingroup$ @wayfaringStranger Glad you found that. I was looking for something to compare it to, but the Moon is a bit brighter than -6 a lot of the time. $\endgroup$
    – HDE 226868
    Commented Oct 12, 2014 at 17:47

Emily Lakdawalla at the Planetary Society as of October 17:

The nucleus of comet Siding Spring passes close by Mars on Sunday, October 19, at 18:32 UTC. Mars passes through the densest part of the comet's tail about 100 minutes later. All five orbiters at Mars will be in constant contact with the Madrid and then Goldstone Deep Space Network facilities, just to make sure they're okay.

Article also includes multiple links to live webcasts of tomorrow's events.

Opportunity rover comet image here as well as orbiter status update.

Mars Reconnaissance Orbiter comet images here.


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