Correction: Mars Global Surveyor, while still in orbit about Mars, is not operational.

Correction: Spitzer images and captions have been updated with credits.

A Date with Mars

Comet Siding Spring was discovered in January 2013 by Robert McNaught at the Siding Spring Observatory [1]. Its orbit was loosely determined at first, but after a few months of observations, it was clear the comet was on a celestial bee line to Mars. The comet will pass by Mars on October 19, 2014, and miss the planet by a mere 135,000 km (84,000 mi). To put that distance in perspective, if the comet were to miss Earth by the same distance, it would be almost three times closer to us than the moon! Thanks to our satellites orbiting Mars, and our rovers on the surface, the people of Earth will get a front row seat to this unique event.

Comet Siding Spring is an Oort cloud comet. The Oort cloud is, in a way, the edge of our solar system. Astronomers commonly use the Astronomical Unit (AU), the Earth-Sun distance, to describe large distances in the solar system. In these terms, the moon is very close: 0.003 AU. At its closest, Jupiter is about 4 AU away, and tiny Pluto, at the inner-edge of the Kuiper Belt, is 30 to 50 AU away. But the Oort cloud is immense, tens to hundreds of thousands of astronomical units across, and filled with a trillion frozen objects [4]. Occasionally, gravitational perturbations from the galaxy push one of these objects in toward the sun. This is likely what happened to Siding Spring millions of years ago, and as it nears the sun, its warming ices cause the surface of the nucleus to erode, which in turn creates a nice coma and tail.

For a comet to pass so closely to Mars must be a rare event. In a paper published in the Astrophysical Journal, astronomers Quan-Zhi Ye and Man-To Hui estimate that such flybys occur once every 100,000 years for the terrestrial planets [2]. Of all the known comets in history, the closest well-determined flyby of Earth was by comet Lexell, which passed in July 1770, but that flyby was a whole 16 times farther [3]. Although we've seen several objects crash into the massive planet Jupiter, including comet Shoemaker-Levy 9 in 1994, we may never see another Oort cloud comet breeze past Mars so closely again.

A Hazardous Tail?

Extremely close encounters are potentially dangerous situations for orbiting spacecraft. NASA and the European Space Agency have four spacecraft three operational spacecraft orbiting the planet (Mars Odyssey, Mars Global Surveyor, Mars Express, and Mars Reconaissance Orbiter), with one due to arrive this year (MAVEN), and the Indian Space Research Organization also has thier first, the Mars Orbiter Mission, on the way. If the orientation and timing of the comet's orbit is just right, the planet could move right through the comet's tail, causing a meteor storm. With dust particles coming in at 125,000 miles per hour, that would be bad news for our spacecraft.

For the particular circumstances of comet Siding Spring and Mars, a meteor storm doesn't seem likely. I wrote to Emily Lakdawalla at The Planetary Society about the problem back in March 2013 (Will comet Siding Spring make a meteor shower at Mars?). A few early papers predicted some dire circumstances [5, 6], but more recent papers [2, 7], including one of my own in preparation, describe a much more favorable encounter. These latest investigations are based on observations of the comet taken between October 2013 and March 2014, and are essentially predicting a safe flyby for the spacecraft. One of those data sets was taken with the Spitzer Space Telescope.

Spitzer's View

Comet Siding Spring, IRAC, 3.6 μm Comet Siding Spring, IRAC, 4.5 μm

Spitzer Space Telescope images of comet C/2013 A1 (Siding Spring), taken with the 3.6 μm (top) and 4.5 μm (bottom) cameras of the IRAC instrument. The images have been colored and enhanced to show both the comet's coma and tail. Some artifacts due to background stars are apparent in the images. Image credit: NASA/JPL-Caltech/M. Kelley (Univ. Maryland)

Spitzer's Infrared Array Camera (IRAC), observed comet Siding Spring in March 2014. At the time, the comet was 3.1 AU (about 300,000,000 miles) from the sun, just at the edge of what we call the solar system's "snow line." Interior to the snow line, the energy from the sun easily converts water from the solid phase into the gas phase, a transition called sublimation. So a comet nucleus, which is primarily a mix of water ice and dust, coming from the great distance of the Oort cloud, should be a near-lifeless, frozen object at 3 AU, right? Not necessarily. Instead, less abundant, but more volatile, ices could be sublimating and causing the erosion needed to generate the comet's tail.

Observations of comet Siding Spring have shown it was active as far out as 10 AU from the sun. If water is not driving that activity, then what is? Spitzer's IRAC instrument is sensitive to carbon dioxide and carbon monoxide gases in comets. Being the second and third most abundant molecules in comets, one of theses gases must be the culprit.

Examine the IRAC images on the right. Do you see a difference between the 3.6 μm (blue) and 4.5 μm (red) images? The 4.5-μm image has a faint halo surrounding the nucleus, that is missing from the 3.6-μm image. The halo is more apparent in the color combined image; the dust coma and tail is blue-white, but the halo is red-orange. Considering the brightness and shape of the halo, we have concluded that it is mostly carbon dioxide gas being produced at a rate of almost 30 kilograms per second. By confirming the rate of erosion beyond the snow line, this data is just one piece of the puzzle that has helped us understand the dust threat — rather, the non-threat — the comet poses to our Mars spacecraft.

Comet Siding Spring, IRAC, 3.6 and 4.5 μm color       combination

A color combination of the IRAC images: blue-white corresponds to dust, red-orange to gas.

Image credit: NASA/JPL-Caltech/M. Kelley (Univ. Maryland)

The Future: An Observing Campaign

Now that we think the Mars-orbiting spacecraft will be safe in October, NASA can concentrate on this exciting opportunity to observe the comet up close. Astronomers, amateur and professional alike, will also be looking at this comet from Earth. Similar to the comet ISON observing campaign of 2013, NASA has started a comet Siding Spring observing campaign, to focus comet observers, and help get the best science out of their data, and the data from the Mars spacecraft. Find more details at


  1. R. H. McNaught, H. Sato, and G. V. Williams 2013, Comet C/2013 A1 (Siding Spring). Central Bureau Electronic Telegrams, 3368.
  2. Q.-Z. Ye and M.-T. Hui 2014, An Early Look of Comet C/2013 A1 (Siding Spring): Breathtaker or Nightmare? Astrophysical Journal 787, 115.
  3. Closest Approaches to Earth by Comets, The Minor Planet Center.
  4. P. R. Weissman 1983, The mass of the Oort cloud. Astronomy and Astrophysics 118, 90.
  5. A. V. Moorhead, P. A. Wiegert, and W. J. Cooke 2014, The meteoroid fluence at Mars due to Comet C/2013 A1 (Siding Spring). Icarus 231, 13.
  6. J. Vaubaillon, L. Maquet, R. Soja 2014, Meteor hurricane at Mars on 2014 October 19 from comet C/2013 A1. Monthly Notices of the Royal Astronomical Society 439, 3294.
  7. P. Tricarico et al. 2014, Delivery of Dust Grains from Comet C/2013 A1 (Siding Spring) to Mars. Astrophysical Journal Letters 787, L35.


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