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Headshot of Emily Lakdawalla

Comet to whiz past Mars in October 2014

Posted by Emily Lakdawalla

27-02-2013 17:36 CST

Topics: comets, astronomy by planetary missions, comet Siding Spring

A recently discovered comet, C/2013 A1 (Siding Spring), is going to be passing very close to Mars on October 19, 2014. The latest observations from the ISON-NM observatory, reported by Leonid Elenin, suggest that the comet will pass just 41,000 kilometers from Mars. Here's a diagram I made using the JPL Small-body Database Browser:

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

JPL Small-body Database Browser/Emily Lakdawalla

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.

But wait a minute. This is not a close-approaching asteroid we're talking about here. It is a comet. And the thing that makes comets comets is that as they approach the Sun they start evaporating, spouting jets of formerly frozen gases that entrain dust and ice particles into a gigantic coma easily spanning tens of thousands of kilometers. The coma can be observed telescopically at distances up to 100,000 kilometers from the nucleus, and I imagine that there is more material beyond that, though it's too sparse to be observed. So when Siding Spring visits Mars it will be bringing with it a lot of dust particles to which Mars and every spacecraft orbiting or on it will be exposed.

So, are spacecraft at risk from comet C/2013 A1 (Siding Spring)? I am sure that there are "tiger teams" being convened by space agencies to investigate this question. I don't know enough to know the answer, though I can get some clues by querying the scientific literature. Of major importance is the size of the particles in the comet's coma. According to this review article in the Comets II tome, "the dust emitted from comets spans a broad size range, from submicrometer to millimeter or centimeter and larger." In fact, "most of the particulate mass shed from the nucleus is concentrated in large particles." Of course, these large particles are few and far between, much sparser than tiny (sub-micrometer) dust particles, which pose no risk to spacecraft. Still, I am nervous about what the larger-than-dust particles could do if they were in the wrong place at the wrong time.

One thing that adds to the risk is that the particles will be moving at a very high relative speed. The comet has a hyperbolic, retrograde orbit, so its velocity with respect to Mars will be 56 kilometers per second. Wow.

Still, space is very empty; the density of material in a sphere of radius 100,000 kilometers centered on a cometary nucleus only a couple of kilometers in diameter is near zero. So I suspect that the risk to orbiters is either "small" or "vanishingly small". I'm looking forward to reading what experts have to say.

As for the rovers, my initial instinct is that spacecraft on the surface should be safe from dust; most particles would burn up in the atmosphere, briefly flaring into meteors. Still, the smallest impact craters yet seen on Mars are only 10 centimeters wide. According to one study, the smallest object that could survive to the surface would be about 5 millimeters in diameter. Which is pretty small. Something that small would be greatly slowed by its passage through the atmosphere. How many of these particles are there, and how dense a "rain" would they present? Is it only one particle per square kilometer? Then we're probably fine. Is it one per square meter? Then I'm more scared.

Leaving aside the question of safety, what will the rovers be able to see from the surface? Again, I'm not sure. Comets can be bright but this one will be spread out over so much of the sky that I'm not sure what we'll see. It's way too soon to plan rover observations though. As we learn more about the comet and can better predict how bright it will be, the imaging teams will do their calculations to figure out whether the rovers will see anything if they gaze skyward. If they will -- and if the rovers are both still functioning then -- I'm sure they'll shoot photos!

More observations of the comet are needed to pin down its orbit. It's getting close to the Sun now in our sky now, so it's getting harder to see it, but will be in a better position for observation by late summer. I'll sure be keeping my eye on this one!

See other posts from February 2013


Or read more blog entries about: comets, astronomy by planetary missions, comet Siding Spring


Ethan Walker: 02/28/2013 01:25 CST

Has anyone seen a detailed analysis of the size of this comet? I have seen "up to 50 km." If this were the case, a direct impact would likely result in the loss of all resources on and around mars, among other things.

Alexander: 02/28/2013 08:23 CST

Hazard risks notwithstanding, it is still amazing to think we'd have an actual comet passing right through the range of our assets in orbit and on the surface. Even if they don't end up being able to see much of it, isn't it the first time ever we'd be so close to a comet?

Ethan Walker: 02/28/2013 10:34 CST

Alexander: I think the Vega missions made it closer to Halley. Any word on whether it is expected to pass Mars on its day or night side? That seems relavent both for observations and the debris risk.

Douglas Rau: 02/28/2013 12:18 CST

the threat to ground rovers wouldn't be distinctly different than the threat to orbital craft, would it? if we see one particle impact per square meter on the ground, an orbiter should see one particle per square meter pass by as well.

joedeath: 02/28/2013 05:03 CST

I say time to get terraforming, and releasing some of that CO2 into the air, and heat up all those hungry underground microbes. Have at it, Mother Nature. It's our ticket! Let's nudge it down to the surface. Duh. I know we'd lose Oppy and Curiosity, but it would be an awfully good headstart for poor old Mars, blue mars.

Eccentric & Anomalous: 02/28/2013 07:25 CST

An 'areostationary' spacecraft would orbit at a distance of approximately 20,430 km from the center of Mars (a small refinement on Emily's essentially correct estimate), while Deimos orbits at about 23,460 km. It'd be interesting to see how close this comet actually passes!

Michael Hutchinson: 02/28/2013 08:53 CST

Amazing, I have learned of the news.

Alex Forster: 03/01/2013 08:15 CST

I can't imagine that, with our multiple electronic eyes on and around Mars, that there won't be some sort of recorded passage of Siding Spring. Even it weren't directly visible through conventional photographs I think it'd be obersvable in some manner. I'm excited to see if we can gain any kind of compositional or other useful knowledge from it.

Leo Metcalfe: 03/01/2013 01:02 CST

Alexander / Ethan, on 13 March 1986, the European Space Agency (ESA) probe giotto approached Halley's Comet nucleus at a distance of 596 kilometers.

Ralph Lorenz: 03/04/2013 12:04 CST

The severe damage encountered by the VEGAs at ~6000km (a jet?), and that by Giotto (at 500km, but perhaps less fluence) is summarized in

Jeff Rabb: 03/05/2013 02:27 CST

Don't you know that the MAVEN folks are just chomping at the bit. With the present launch window, MAVEN is scheduled to arrive at Mars just before the comet makes it's flyby. What an incredible, once-in-a-lifetime, bonus science opportunity they could never have dreamed of when they were preparing for this mission

Stanton: 03/05/2013 03:20 CST

Haven't been this excited since Roberta Bondar was the coin for the superbowl! So many questions! If it misses, will it escape the sun's gravity? Is it too late for any kind of dedicated mission? If it hits, could any ejected material reach earth? What if it was headed towards earth with this kind of uncertainty? How would people be reacting? Could it be kept secret? Would Bruce Willis save us? :)

Adolfo Reig: 03/06/2013 02:18 CST

Should this comet hits Mars (i know that chances are very very low) wonder if it could raise an ocean and change the global planet environment to a more earth like (increase of atmosphere, increase in temps,...)

Robert Walker: 03/10/2013 07:33 CDT

Yes according to some theories part of the global ocean is still there beneath the northern plains Vastitas Borealis. Quite a large target and if it hit there and melted part of it we might find out if that hypothesis is true :) Wouldn't last for long sadly with current Mars conditions. Air so thin water boils at just a few degrees above its freezing point. So would either boil away rapidly if warm enough, or freeze quickly in the amazingly cold Martian nights (far colder than the coldest temperatures recorded on Earth). Terraforming Mars if ever possible is likely to be a tricky process with many gotchas and possibilities for it to go disastously wrong maybe irrecoverably so, so it needs great care and foresight. Earth can keep going because of Gaia (weak gaia hypothesis supported by most scientists just existence of many feedback loops keeping it at the right temperature and pressure etc etc). To have a hope of terraforming Mars we need feedback loops also - but not the same ones. Mars has no continental drift and that is a major part of the long term Earth cycles. So need to invent and recreate a new form of Gaia unlike Earth. That could easily go horribly wrong. Not sure we are yet too close to knowing how to do that, maybe in a century or two, or if knowledge progresses amazingly quickly.

Robert Walker: 03/10/2013 07:57 CDT

There's another idea here though. A large impact could melt enough water, so the surface freezes but it remains liquid underneath the surface maybe even for a few thousand years. Not sure how big that is maybe pretty huge but maybe even this size impact the ocean could survive surprisingly long within an insulating ice surface. Just thoughts. Would be really interesting if it happened, interesting thought, thanks :)

Jeff Rabb: 03/19/2013 05:35 CDT

I've another thought on this subject. A number of us, including myself have pointed out that MAVEN should have made orbit insertion by the time the comet makes it's pass of Mars. However, I've just remembered, when looking at Olaf's "What's Up in the Solar System" for March, that there should be a 2nd spacecraft also arriving at that time. The new Indian Mars probe that's slated for launch this October and scheduled to arrive at Mars around Sept 22nd. Granted this spacecraft is more of a technology trial spacecraft with science taking a backseat, but still I would imagine the Indian scientist are just beside themselves that if they can make this year's launch window. They'll now have a ring side seat to one of the greatest astronomical event in recent history. Kuddos to them!

Fall of a Thousand Suns: 05/09/2013 01:52 CDT

JPL's only giving it a 1 in 120,000 chance of Comet Siding-Spring hitting Mars now. It will, however, make for some stunning images from orbiting spacecraft. Looking forward to ISON and Siding Spring. Hopefully, they wake everyone around the world up. Money is an invention. The shooting gallery in the universe is real. How quickly we've all forgotten SL-9. For info and facts on individual comets take a look

Steven: 06/08/2013 03:14 CDT indicates there will be interaction. I wonder what the post-Mars encounter orbit will be. I know the error margin for encounter is huge to the breadth of changes to the orbit of the comet are huge….

Steven: 06/08/2013 04:19 CDT

Sorry about the dupe. In other news - Mars will be fairly far from us but visible….

Steven: 06/08/2013 04:24 CDT

I wonder where the moons of Mars will be. Since this will get exciting I suppose even the old spacecraft orbiting Mars could be watched….

SpaceCommando: 07/14/2013 06:53 CDT

I know I'm a bit late posting to this thread but I wanted to point out that there is a very good chance that we will see some extremely interesting electrical phenomena as "Sliding Spring" passes Mars at a near miss distance of about 23,500 miles. Sorry, I didn't mention I am a proud follower of the anti-mainstream science, paradigm busting, free thinkers who have recently advanced theories related to the "electric universe" hypothesis. The EU adherents provide a convincing argument of the highly charged electrical nature of inbound objects into our solar system such as comets. Sadly, this line of thinking has faced outright rejection by the science PTB (Powers That Be), because acceptance of even part of their promulgations would mean essentially throwing out the current "accepted knowledge" governing much of cosmology/astronomy/solar science. If the E.U. theories hold true (particularly those about the electric nature of comets) there should be a tremendous exchange of electrical energy between the comet and Mars as it makes it's closest approach. Many scientists that have accepted the E.U. based version of cosmology/astronomy as a plausible (even a more attractive) alternative to the main-stream "Gravity Based" nature of the universe believe that some inexplicable surface features of Mars, such as the Valles Marineris, were formed either partially or in total by past electrical interactions between Mars and other bodies in, or passing through, the solar system. For those free-thinkers out there that would like to learn more about the Electric Universe hypothesis a good place to start would be There is also a video on the "" site that covers the bulk of the E.U. nature of comets - go to (video posted from 6-20-13 to ~9-20-13 for peer review/comments/etc.).

SpaceCommando: 07/14/2013 06:57 CDT

Sorry - I didn't realize the E.U. comet video had been taken down on the "" site. It can be found on YouTube here:

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