So Close and Yet So Far: Why isn't Siding Spring going to sandblast Mars?

The following originally appeared at the Coordinated Investigations of Comets website and is reposted here with permission.

Comet Siding Spring (C/2013 A1) is going to make a very close approach to Mars on October 19, 2014, passing only 131,000 kilometers from the planet (from the current orbit solution). With a relative speed of 56 kilometers per second, any comet dust that reaches Mars has the potential to inflict significant damage on the spacecraft orbiting the planet. As it turns out, however, Mars and its orbiters are likely to see very few, if any, impacts. Why?

I was the leader of a group at the University of Maryland (with Mike Kelley and Dennis Bodewits) who performed analyses to predict how the dust from comet Siding Spring would affect Mars and its immediate vicinity. (Groups from the Planetary Science Institute and the Jet Propulsion Laboratory performed similar analyses, to see if results from different approaches produced the same result. The papers that came out of this work are cited below if you are interested in more details.) Our goal was to provide credible information that the spacecraft teams could use to evaluate the potential risks and develop any mitigation procedures that might be necessary to protect the spacecraft. Karl Battams presented an overview of this analysis in his article on the hazard assessment. I wanted to expand on that account, to discuss some of the issues that I find fascinating and somewhat counterintuitive about the encounter.

Our study of the comet and models of the dust environment led to some very interesting conclusions:

As I noted, these results are fascinating and counterintuitive. How can the comet get so close to Mars, with so few impacts? Why is it that only grains of a particular size can reach Mars, and any impacts will happen, not around close approach, but over an hour later when the comet is more than twice as far away? And how is it that any impacting dust had to have left the nucleus so long ago, while the comet's activity throughout 2014 is irrelevant to the encounter?

The answer to all of these questions comes from understanding the physics of the comet, the forces on the dust grains and the dynamics of the grains after they have left the nucleus. So I'll deviate here with a short description of these topics to lead into the explanations about Mars.

Comet Physics and Dust Dynamics

The fundamental physics of comets dictate that as the nucleus approaches the Sun, ices start to vaporize, producing outflowing gas that lifts dust grains off the surface. The outflowing gas transfers some of its momentum to the dust grains, accelerating them away from the nucleus. This process continues until the density of the expanding gas drops too low to have any significant effect on the dust (usually at a distance of a few comet radii). Because the acceleration is driven by momentum transfer, small, low-mass grains reach higher emission velocities than the larger, more massive grains.

The amount of gas the comet produces controls the dust emission velocity, as well as the amount of dust that is produced and the size of the grains that can be lifted from the surface (for a given amount of gas, there is an upper limit to what size grain can be accelerated to the nucleus' escape velocity). So as the comet gets closer to the Sun and generates more gas, it also tends to emit more dust with higher emission velocities and more large grains.

After the particles have left the sphere of influence of the nucleus (when they are beyond the pull of its weak gravity and have decoupled from the gas), their motions are controlled by two opposing forces: the inward pull of the Sun's gravity and the outward push of solar radiation pressure. Radiation pressure is similar to gas drag, in that photons are transferring their momentum to the dust grains. And as with the gas drag, radiation pressure acts most efficiently on small grains, accelerating them away from the sun more rapidly than large grains, which effectively "sorts" the particles into different parts of the tail. Surprisingly, the smaller cometary dust grains (in the micrometer size range) start to feel the effects of radiation pressure very quickly, and can be swept away from the nucleus in as little as a few hours. The larger grains are also influenced by radiation pressure, though they react more slowly. Millimeter and centimeter sized grains can remain in the vicinity of the nucleus for months or even years, slowly accelerating away from the Sun under the gradual accumulation of momentum transferred from the photons.

So the motions of the grains are governed by these two dominant forces. Gravity causes them to move according to the laws of orbital mechanics, while radiation pressure perturbs that motion. Our reference point, the comet's nucleus, follows a Keplerian orbit (at least for our purposes) and the dust grains start out with essentially the same motion. Over time, radiation pressure pushes a grain away from the Sun, increasing the size of its orbit. Because larger orbits have longer orbital periods, the outward moving grain will also lag behind the nucleus (see the left panel in the picture below). For small grains, which tend to make up most of a comet's dust environment, the radial motions dominate, producing the classic curved, anti-sunward dust tail. For larger grains, however, the radial perturbations are slow, and the orbital lag dominates their motions, causing them to trail behind the nucleus. (This is the mechanism that can form a dust trail of large particles along the comet's orbit). Several fragments of comet SW3 are shown in the right panel, illustrating how the small grains form a broad, curved tail, while the large grains lie concentrated along the comet's path.

For the most part, the dust tends to remain near the comet's orbital plane, because both gravity and radiation pressure are radial forces, providing no acceleration perpendicular to the comet-Sun line. The tail does have some width, however, because the initial emission velocity imparts some perpendicular motion. Conceptually, we can think of the dust as a cloud, expanding away from the nucleus in all directions. This cloud is then pushed by radiation pressure, continuing to grow, but remaining centered on the comet's orbital plane as it moves away from the Sun. Thus, the tail starts out fairly narrow at the nucleus and broadens with distance, with the dimensions determined by the dust velocities.

Siding Spring Dust at Mars

With this basic understanding of comet dust behavior, we can now look at the events that will occur when Siding Spring encounters Mars, and see how they lead to the conclusions outlined earlier?

We used observations of the comet, from 2013 and 2014, to constrain its dust properties, and then produced a model to predict what the comet would be doing at other times. (Actually we used several different models in case any particular one was not representative of the real situation.) We then produced a simulation with a billion dust particles, consisting of sizes from microns to centimeters, velocities spanning the range naturally produced in comets, and emission times starting at 13 AU and continuing all the way up to perihelion. We tracked these grains under the forces outlined above to see where they would be located when the comet passed Mars. These simulations revealed that the dust impacts would be rare, mainly due to the basic geometry of Siding Spring and Mars.

Since the geometry is critical to understanding the outcome, see a movie depicting the encounter below. The red plane is the orbital plane of Mars and the green plane is the orbital plane of comet Siding Spring. The viewpoint is somewhat oblique, showing the intersection of the planes and Mars' orbital path lying just outside the comet's. The dust tail is shown, in a somewhat stylized form, as the brown lines in the comet's orbital plane. As described above, the smallest grains move primarily away from the Sun, while the larger grains trail behind the nucleus and stay closer to the comet's orbit.

At the start of the movie (16:30 UT at the comet), Siding Spring is approaching Mars from below. At around 17:21, the comet crosses Mars' orbital plane, but this event has little significance because it occurs long before Mars reaches that point.

At 18:33, Siding Spring reaches its closest point to Mars, but there will be little danger of impacts at this point. Because Mars is nearly perpendicular to the comet's orbit, the dust cloud would need to expand quickly enough to cover the 131,000-kilometer gap before the particles are swept away from the Sun by radiation pressure. As noted above, there is a limit to the speed that a grain can attain under the effects of gas drag (or any other natural cometary process), and it turns out that the effects of radiation pressure will always sweep the dust into the tail before it can cross the gap to Mars. This same situation holds throughout much of the next hour and a half, as the comet gets even farther from Mars.

It is not until about 100 minutes after closest approach, around 20:12, that conditions begin to allow impacts (even though the nucleus is over twice as far away than it was at closest approach). The key, as seen in the movie, is that this is the time when Mars crosses the comet's orbit, and it does so just outside of the comet's path. Recall that the largest dust grains tend to lag behind the nucleus, just outside it's orbit, and Mars could encounter some of these particles. From the dust dynamics and our simulations, we found that Mars will pass through a region of the tail where particles around 1-3 mm in size reside. Due to the sorting effects of radiation pressure, smaller particles will be pushed downstream of Mars, and bigger ones will lie upstream.

As for when the dust left the nucleus, only millimeter-sized grains that were emitted at least a year and a half before the encounter would have drifted (via orbital lag) far enough from the comet to reach this point in the dust trail. Any particles emitted since then would either be blown down the tail (small grains) or not had time to drift into the path of Mars (large grains). In either case, this means that the comet's activity for the past year or more is irrelevant to the dust environment that Mars will encounter.

From observations of Siding Spring, we also determined how much dust it produces, and then computed an upper limit on the number of grains in the 1-3 mm range. The result suggests that the column density at the encounter (the number of particles along Mars' path) will be at most, one grain for every ~10 square kilometer cross section. So from the lower and upper limits in our analysis, we find that Mars could experience anywhere from no impacts to as many as a million impacts of millimeter-sized grains, spread over the planet's leading hemisphere. Depending on the emission velocities of the dust (recall that this defines the "width" of the tail), the duration of the impact events is likely to span 20-40 minutes, centered ~100 minutes after close approach. Even in the case of the upper limit scenario, this would produce a rate of only a few meteors per hour, as seen from Mars' surface, so it will not be a particularly impressive meteor shower.

So, even though Siding Spring will pass Mars at a record close approach distance (for known encounters that miss the planet, anyway), the dynamics of the dust and the particular geometric circumstances make it a truly glancing blow.

The "Up" Side

We presented our results to the spacecraft teams so they could evaluate the hazards to the orbiters. The consensus was that the risks were minimal (below the 5-year background levels), and so no major changes were needed to protect their assets. However, the teams did generally opt to execute some minimal hazard mitigation procedures that could be easily implemented. The simplest task for most orbiters is to phase the orbit so that the spacecraft is "downstream" from Mars when it passes through the comet's orbital plane, and thus protected from any incoming dust grains. Another protective measure, being implemented by MAVEN, is to turn off their high voltage instruments during the hours around closest approach, to minimize potential damage (a dust impact vaporizes material into a conductive plasma that can facilitate short circuits). As for the planet's surface, it is protected by the atmosphere, so (to my knowledge) no measures are currently planned for the rovers.

The really good news is that, because the risks are fairly minimal, the spacecraft teams are planning to study the comet and its effects on Mars.

The Mars Reconnaissance Orbiter (MRO), Mars Odyssey and Mars Express (MEX) are already planning their observations, and although their instruments are not designed for studying diffuse, faint objects, they will attempt to characterize the nucleus, coma and tail of Siding Spring. Some of the observations will span several days around the encounter to look for both temporal and spatial variations. The HiRISE camera on MRO has the potential to resolve the comet's nucleus (which would be a first for a dynamically new comet) with a pixel scale up to 140 meters at closest approach. MAVEN may try to do some measurements of the composition of the comet during its approach and departure, but since they will have just arrived at Mars, their first priority is to transition to their science orbit and to check out the spacecraft for its primary mission. India's Mars Orbiter Mission (MOM) will still be in their arrival phase at the time of close approach, so at this time, it has not been decided if they will perform any observations. Opportunity and Curiosity will attempt to image the comet from the surface, but as with the orbiters, their cameras were not designed for observing comets. Furthermore, they are both in daylight at closest approach, so any detections could be a long shot and may come before sunrise or after sunset, when the comet is farther away.

Another science contribution may come from studying what effect the comet's gas and dust have on Mars' atmosphere. As discussed above, some dust is likely to reach the planet. However, because gas is faster than dust, a significant amount of the comet's gas could reach Mars, and although it is not a hazard to the spacecraft, it will deposit a lot of energy into the upper levels of the planet's atmosphere. The Mars spacecraft have instruments specifically designed for this type of measurement and so they may be able to detect any heating or ionization that is produced by the comet's passage.

Look for results from these observations in a few months time.

Publications detailing the dust hazard analyses:

• Tricarico et al. Ap. J. Let. 787, eid L35, 2014.
• Farnocchia et al. Ap. J. 790, eid 114, 2014.
• Kelley et al. Ap. J. Let. 792, eid L16, 2014.