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Bruce BettsOctober 13, 2014

Video Introduction to Comet Siding Spring’s Near Miss at Mars

On October 19, 2014, Comet Siding Spring will fly very close to Mars. Here’s a 5 minute video introduction to get you up to speed on this planetary near miss, and some suggestions on how to find out more now, during, and after the encounter.

How to Learn More

Below are the blogs from covering various aspects of this rare planetary encounter. The list will automatically update with new ones so you can check back here or the home page or blog page of

Science observations related to the comet and its encounter are discussed and updated on the Coordinated Investigations of Comets (CIOC) website. This is a run by a group of scientists who first organized this group to track and put out data on Comet ISON. You can also check NASA’s graphics heavy Comet Siding Spring at Mars page.  Update: Check in particular Emily's new blog on ways to Watch Siding Spring's Encounter with Mars for links to webcasts and websites that will likely have updates during the encounter including some telescopes doing live observing from Earth.

Twitter is always a great way to get quick updates. On the day of encounter, both Emily Lakdawalla @elakdawalla and I @RandomSpaceFact will be covering the encounter on Twitter for The Planetary Society. Karl Battams is an astrophysicist with the Naval Research Lab and part of CIOC and provides Twitter updates based on CIOC information @SungrazerComets (you’ll note in our list of blogs below we also republish some of his blogs here).

Comet Siding Spring Blogs

Comet Siding Spring FAQ

What’s the deal with a comet flying close to Mars?
On October 19, 2014, Comet Siding Spring (more formally, C/2013 A1 Siding Spring) will fly very close to Mars.

Why is it called Siding Spring?
It is named after the observatory in Australia where the discovery was made by Robert McNaught.

Will it hit Mars?
No. Early on, when its orbit was not well known, we weren’t sure, but now we are.

How far from Mars will it be?
At closest approach, it will be about 140,000 km (88,000 mi) from Mars. That is 10 times closer to Mars than any recorded comet flyby of Earth. The distance is equivalent to only about 1/3 the distance from the Earth to the Moon. Mars moons are much closer to the planet, so the comet will still be much farther from Mars than they are.

How fast will the comet be traveling relative to Mars?
About 56 km per second, or about 126,000 miles per hour.

How big is the comet?
The exact size of the nucleus (the solid part of the comet) is not well known, but current estimates put it at 700 meters diameter, which is much smaller than some of the initial estimates.

Will the seven working spacecraft at Mars be safe?
They should be. As more information has become known about the comet, including its low dust production and the distance it will be from Mars, the risk to spacecraft is thought to be very low. Still, some of the orbiters have been phased so they will be on the other side of Mars when the maximum dust and gas from the comet is expected to pass by them. The atmosphere of Mars will protect the rovers.

Will there be science obtained?
The fleet of Mars spacecraft, though not designed for comet observations, will be taking lots of data ranging from attempted imaging, to studying the dust and gas given off by the comet and the interaction of that dust and gas with Mars. Read articles in the blogs at to learn more including

What is new about these comet observations?
All previous comets studied by relatively nearby spacecraft have been short period comets, comets that orbit the Sun in a few years or a few tens of years. This comet in contrast is coming in from the distant Oort cloud and may be taking its first voyage near the Sun. We don’t know how that will affect what we see.

Should we expect great pictures like from other spacecraft that have visited comets?
There will be attempts at imaging, but the comet is likely to only be a few pixels in size. There may or may not be images with Mars in them as well. It is wise to temper expectations about imaging.

Will it be visible from Earth?
Possibly with binoculars, but probably requiring a descent telescope, and to have a good view, you’ll need to be in the Southern Hemisphere.

Read more: comets, comet Siding Spring, Planetary Society Video, explaining science, Mars

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Bruce Betts

Chief Scientist / LightSail Program Manager for The Planetary Society
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