This article originally appeared in Orbital Debris Quarterly News, Volume 18, Issue 4, October 2014.

A. MOORHEAD, NASA METEOROID ENVIRONMENT OFFICE

On 19 October 2014, Comet C/2013 A1 (Siding Spring) will pass within 140,000 km of Mars, making a closer approach to the red planet than any comet has to Earth in recorded history (the closest known approach to Earth by a comet was made by comet Lexell at 2.3 million kilometers – more than 15 times the distance). Comet Siding Spring was discovered in early 2013 by Robert McNaught and named after its discovery site, Siding Spring Observatory [1]. These discovery images, combined with prediscovery images from the Catalina Sky Survey and subsequent observations, enabled Jet Propulsion Laboratory scientists to rule out a “nightmare scenario” in which the comet nucleus hits Mars within a few months of the comet’s discovery.

Missing the comet nucleus, however, does not alone remove the danger posed to the Martian system. The nucleus will be embedded in a coma of gas and dust particles, all of which will be moving 56 km/s (125,000 mph) relative to Mars. Depending on if and how deeply Mars penetrates the coma, its orbiting manmade satellites could be impacted and possibly damaged by high velocity dust grains, or micrometeoroids. Thus, determining whether Mars passes through the coma is critical for assessing the level of meteoroid impact risk posed to Mars Atmosphere and Volatile Evolution (MAVEN), Mars Reconnaissance Orbiter (MRO), and other orbiters.

Early observations of the comet revealed little information about it other than its brightness, but the comet’s similarities to fellow Oort-cloud comet Hale-Bopp gave some cause for concern. Both Siding Spring and Hale-Bopp are comets on near-parabolic orbits making their first passage through the inner Solar System. Both comets showed signs of early activity: Hale-Bopp was active out at 7.2 AU (between the orbits of Jupiter and Saturn) while Siding Spring was active out at 7.8 AU. Hale-Bopp produced a coma that grew to millions of kilometers in size – were Siding Spring to follow Hale-Bopp’s example, Mars would certainly pass within its coma.

For this reason, several groups made early attempts at modeling the particle flux near Mars during the close encounter. A group led by NASA’s Meteoroid Environment Office produced the earliest model: a spherically symmetric coma that borrowed dust properties from Giotto measurements of Halley’s coma and matched brightness measurements of Siding Spring from early 2013 [2]. The largest uncertainty in this model was the assumed comet radius; the group adopted a value of 200,000 km, which is comparable to Halley’s coma radius and significantly less than Hale-Bopp’s radius.

This coma radius was comparable to that resulting from early computer simulations of the comet produced by Paul Wiegert and Jeremie Vaubaillon [2], [3]. These early measurements predicted a significant risk to spacecraft; roughly every 10 square meters of vehicle area would be hit by a potentially hazardous meteoroid (approximately 0.1 mm or larger). In the 90 minutes it would take for Martian spacecraft to pass through the comet’s coma, they would be exposed to 300 times the equivalent risk from spending a year in low Earth orbit. The event at Mars would also have been significant; the resulting meteor shower on Mars would be orders of magnitude stronger than any seen from Earth and was dubbed a “meteor hurricane” [3].

In response to this prospective threat, the Mars program office selected several comet and small body astronomers, including Tony Farnham, Pasquale Tricarico, Davide Farnocchia, and Jian-Yang Li, to undertake a second observation and modeling effort. Images of Siding Spring taken with the Hubble Space Telescope in late 2013 and early 2014 showed that the comet was much more compact than initial estimates, indicating that dust was being ejected at velocities about a factor of 50 lower than assumed in earlier models [4]. Newer models, taking this low ejection velocity into account, predicted that the coma would miss Mars entirely, driving the risk down by five to six orders of magnitude [5], [6], [7]. A similar but independent study led by Quanzhi Ye reported similar findings [8]. Current expectations are that the comet poses less meteoroid impact risk to spacecraft than the sporadic meteoroid background flux. Furthermore, this risk is confined to a region in the comet’s tail containing only a few, millimeter-sized particles, which will pass by Mars within half an hour [9]. The expected effect on the Martian atmosphere is also reduced; the meteor hurricane has become a shower.

Although the risk to spacecraft is now projected to be quite low, the Mars program office has opted to further reduce the risk to Odyssey, MRO, and MAVEN by phasing their orbits to use Mars as a shield. This strategy takes advantage of the risk window’s short duration (30 minutes) compared to the spacecraft’s orbital periods. High voltage components may also be shut down during the encounter in order to reduce the possibility of electric anomalies from high velocity dust particles, which have been hypothesized to cause problems for the Earth-orbiting satellites OLYMPUS and Landsat 5 [10], [11].

Now that the potential impact risk posed to spacecraft has been substantially reduced and mitigation strategies selected, the community is turning its attention to the science opportunity this comet encounter offers. At the Comet Ison Observing Campaign (CIOC) workshop on 11 August, Mars Program scientists reported that all four of its operational spacecraft, including MAVEN, will attempt to observe Comet Siding Spring. The combination of these Mars-based observations with Earth-based and space-based observations will produce a wealth of information on the comet. The result will be a valuable, detailed characterization of an Oort-cloud comet and its meteoroid tail and the validation or refutation of model predictions.

Thus, what first appeared to be a possible hazard to Martian spacecraft has instead turned out to be an exciting opportunity for Mars scientists and comet and meteor astronomers.

References

1. McNaught, R. H., Sato, H., and Williams, G. V. Comet C/2013 A1 (Siding Spring). Central Bureau Electronic Telegrams, 3368, 1, (2013).
2. Moorhead, A. V., Wiegert, P. A., and Cooke, W. J. The meteoroid fluence at Mars due to Comet C/2013 A1 (Siding Spring). Icarus, (2014).
3. Vaubaillon, J. J., Maquet, L., and Soja, R. Meteor hurricane at Mars on 2014 October 19 from comet C/2013 A1. Monthly Notices of the Royal Astronomical Society, (2014).
4. Li, J.-Y., Kelley, M. S. P., Farnham, T. L., Samarasinha, N. H., Lisse, C. M., and A’Hearn, M. F., et al. Imaging C/2013 A1 (Siding Spring) with the Hubble Space Telescope. Asteroids, Comets, Meteors, (2014).
5. Farnham, T. L., Kelley, M. S. P., Bodewits, D., Kleyna, J., Li, J.-Y., Stevenson, R., and Bauer, J. M. Comet Siding Spring (C/2013 A1) and Its Close Approach to Mars. Asteroids, Comets, Meteors, (2014).
6. Farnocchia, D., Chesley, S. R., Chodas, P. W., Tricarico, P., Kelley, M. S. P., and Farnham, T. L. Trajectory Analysis for the Nucleus and Dust of Comet C/2013 A1 (Siding Spring). The Astrophysical Journal, (2014).
7. Tricarico, P., Samarasinha, N. H., Sykes, M. V., Li, J.-Y., Farnham, T. L., and Kelley, M. S. P., et al. Delivery of Dust Grains from Comet C/2013 A1 (Siding Spring) to Mars. The Astrophysical Journal, (2014).
8. Ye, Q., and Hui, M.-T. An Early Look of Comet C/2013 A1 (Siding Spring): Breathtaker or Nightmare? The Astrophysical Journal, (2014).
9. Kelley, M. S. P., Farnham, T. L., Bodewits, D., Tricarico, P., and Farnocchia, D. A. Study of Dust and Gas at Mars from Comet C/2013 A1 (Siding Spring). The Astrophysical Journal Letters, (2014).
10. Caswell, R. D., McBride, N., and Taylor, A. D. Olympus end of life anomaly - A Perseid meteoroid impact event? International Journal of Impact Engineering, (1995).
11. Cooke, W. J. The 2009 Perseid Meteoroid Environment and Landsat 5. NASA MEO Internal Report, (2009).