The Mars Encounter of Comet C/2013 A1 (Siding Spring)
You'll have heard that on October 19, 2014 Martians will be treated to a rare spectacle. A recently discovered comet named C/2013 A1 (Siding Spring) will pass the red planet at a very close distance (at most some hundreds of thousands of kilometers; though the latest orbit determination leads to a distance of around 70,000 km). The comet might also hit Mars, and if it does, it will create quite a bang. The diameter of the comet's nucleus can be up to 50 km, and it will encounter Mars at a relative velocity of 56 km/s.
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The orbit determination results are still somewhat inaccurate. With optical observations of a comet that is still far beyond the orbit of planet Jupiter, the plane-of-sky measurements do not discriminate well even for significant differences in the orbital elements. When propagating ahead numerically over 20 months from an inaccurately known starting point, one will inevitably end up with a large uncertainty ellipsoid that encompasses planet Mars and much of the surrounding space. With time, observations will cover a growing arc of the comet's trajectory and become much more accurate. At some point in time, at the latest by the end of 2013, taking into account that from spring to summer the Sun will block our view of the comet, we will have much more reliable predictions of the relative geometry at flyby, and we will also know whether there will be an impact or (more likely) not.
If you make a cut through the dispersion ellipsoid at the time of closest approach to Mars, you get an ellipse, or rather, you get a series of concentric ellipses of different sizes, depending on the number of standard deviations (i.e., the level of uncertainty) you are regarding. Let's assume we are talking about a 3 sigma confidence level, which means that the ellipse will include 99.7% of all expected flyby trajectories. In close approximation, at an encounter velocity of over 55 km/s, we can assume that the impact radius is equal to the planet radius. This means that if planet Mars is completely encompassed by the uncertainty ellipse, the impact probability can, in good approximation, be computed as the ratio between the area of a circle with the Mars body radius (~3400 km) and the area of the uncertainty ellipse.
This in turn means that as the orbit determination accuracy improves, the error ellipse at Mars will shrink. As long as it still covers Mars, the impact probability might then actually appear to increase, though this need not be so. Then, either of two things can happen: Either Mars will cease to lie within the uncertainty ellipse. Then the impact probability - after seeing a continuous increase - will suddenly collapse to near-zero. In fact, the latest information by astronomers tracking the comet indicates that the impact probability is not increasing. Or Mars will continue to lie within the uncertainty ellipse, which will then mean that an impact might be in the realm of the likely. If comet C/2013 A1 however does not hit, it will pass its perihelion - still considerably beyond the Earth orbital radius - after the Mars encounter and then disappear into the depths of space. In all probability, this is what will happen.
C/2013 A1 is on a parabolic orbit. Its perihelion radius, the point where it is closest to the Sun, is at a solar distance of just over 200 million km. The orbital inclination is almost 130 degrees, making the orbit retrograde. If you view the comet's orbit from the North pole direction of the ecliptic, it will appear to be moving clockwise, in contrast to the planets, which are moving counter-clockwise. For comets that came from the Oort cloud (i.e., from very far out) this is not rare - such objects can approach the Sun at any inclination.
Celestial mechanics is rife with apparent paradoxes. Case in point: Currently nobody can say for sure whether this comet will hit Mars or miss by a considerable margin. But we can predict quite accurately from where it will approach Mars and at what local time it will hit, if it does hit. And of course one can plot and analyze the trajectories through the solar system, which is what I did. I obtained the orbital data from JPL Horizons and created two diagrams, one showing the inner solar system seen from the ecliptic North, the other seen from along the ecliptic plane. (please click on the pictures to see them at full resolution) The indicated locations of planets Mercury through Mars correspond to where they will be on October 19, 2014, 12:00 UTC.
These diagrams clearly show one thing: Comet C/2013 A1 will not get close to the Earth, no matter what happens. There is no way Mars can appreciably deflect the comet from its pre-computed path. The relative velocity is much too high for that.The comet will approach Mars from the South and speed off towards the North.
Now let's look at where on Mars an impact could happen, if it happens. This applies not just to the nucleus proper, but also to large and small debris that might have separated from the nucleus and will then be flying on parallel but distant trajectories. Even if the nucleus does give Mars tens or hundreds of thousands of kilometers of berth, debris might still hit. The probability for this rises with the activity the comet will have delivered by then. That is another thing nobody can tell for sure. Experience with previous large comets, e.g., Hale-Bopp in the late 1990s, does indicate that a significant coma and tail might have developed even at a sun distance of 1.4 AU. Every piece of approaching debris will however be approaching from virtually the same direction and therefore all analysis made hereafter applies to the debris as well.
The following statements can be made already now with a high level of confidence.
- If anything from C/2013 A1 hits Mars at a shallow angle of attack (here, I looked at entry angles of up to -15 degrees), it can hit anywhere on the Surface except for the North polar cap. If objects enter steeply, they will impact near the equator.
- The chances for impact at local day or night are 50/50.
- There is a fair chance that a major impact will be observable from the Earth. If entry is steep, visibility from the Earth is certain. If it is shallow, the probability of visibility from the Earth is around 75%.
- Die entry velocity at 100 km of altitude is around 56 km/s
So much for the analysis. However, there is more to this event than just math and physics.
The part of me that is a scientist, actively engaged in space exploration, is of course jubilant. No matter how much of this comet, if anything at all, will reach the Mars surface, this close encounter constitutes a unique opportunity to learn about comets and their interaction with a tellurian planet. In October 2014 there will be up to four active spacecraft in orbit around Mars, each of them equipped with cameras, spectrometers and other science instruments with which the impact of the nucleus or debris can be observed and studied, and also the aftermath. All of this in real time. This is the first time such a golden opportunity came along.
Even if nothing at all reaches the surface, there still will be the chance to observe a vigorous, young comet at close quarters. The nucleus must still be in possession of almost all of its primordial, volatile material. We're talking about very ancient and fragile material here, material that will have remained largely unchanged from when the solar system accreted, because everything out in the Oort cloud will have remained largely unaffected by the violent changes wrought much closer to the Sun. This should allow some fantastic science.
But every scientist is also an earthling. As an earth dweller, this encounter gives me the creeps. Eons ago, a very insignificant orbital perturbation dislodged this 50 kilometer chunk of ice and assorted dirt from its orbit through the Oort cloud, around one light year distant, and sent it hurtling towards the inner solar system, imperceptibly at first, but slowly gaining speed and turning into an unstoppable juggernaut. had this perturbation been different by just a minute amount, the comet might now be on a course that would intersect the Earth orbit exactly at the time when the Earth happens to be at the same spot. There is no reason why this should not have happened, other than sheer chance. For all we know, someone might discover another comet tomorrow that actually is on a collision course with Earth. The resulting impact would de disastrous. It would put an end to civilization and perhaps trigger mass extinction, eradicating most plant and animal species and making room for others. It can happen at any time. One day, it will happen. I perceive C/2013 A1 as a chilling reminder that our existence in the vastness of space is precarious and perilous.
Comet C/2013 A1 was discovered around 2 years before it reaches its perihelion. That is typical for long-period comets. Two years are not enough to mount any meaningful defenses, which in view of the size and speed of a comet nucleus are at any rate extremely challenging, even with sufficient warning time. There is no reason for panic. The fact that a comet was discovered that might hit a neighboring planet does not suddenly expose the Earth to a higher risk than before. The only thing this discovery does is to drive home the point that we have a problem here. A big problem, but one whose probability of occurring can be significantly reduced if we pull our collective socks up, extend the realm of human activity to as far as our technological capabilities allow and implement an effective system for early warning and countermeasures. We can do it, so if we don't and as a consequence get clobbered, then we didn't deserve better.
List of posts related to C/2013 A1 on spaceobs.org