An Unusually Close Comet

On Sunday, October 19 2014, humanity will experience its closest brush ever with a comet. Unfortunately, the comet will not be passing by the Earth on that date, but the planet Mars. While there will be no people on Mars to enjoy the sight, several human controlled spacecraft are there to give scientists – and us amateur science nerds – a ringside seat to observe this spectacle.


 Image credit  NASA/JPL-Caltech (click to enlarge)

Comet C/2013 A1, known also by its informal name Siding Spring (after the Australian observatory where it was discovered last year) will barnstorm Mars in a breathtakingly close passage. While its visibility from here on Earth can’t be estimated yet, the Mars science community who control spacecraft such as Mars Express, and the Curiosity and Opportunity rovers are already deep into preparations to observe its passage.

For perspective, the closest comet pass by the Earth in modern times (and for the purposes of this blog, I’ll define “modern” as post-telescope, e.g., >1609 CE), was Comet Lexell, which passed within 1.4 million miles of Earth on July 1, 1770. It had been discovered just two weeks earlier by none other than Charles Messier and was observed at its closest passage by many astronomers, when the apparent size of its coma reached over 2 degrees (4 times the width of the full moon). On the day of its closest approach to Earth, it was observed to cross 42 degrees of sky in just 24 hours! (Lexell, by the way, was named for Swedish astronomer Anders Johan Lexell, who computed its orbit).

In more modern times (e.g., our lifetimes), a notable close approach was Comet Hyakutake, which in March 1996 passed just under 9.5 million miles from Earth. It was recorded at a visible magnitude of about 0 and about a degree in angular diameter. For a short period of time, it was a big, bright comet.

By comparison, Siding Spring will pass Mars at a distance of just 85,000 miles. In astronomical terms, that’s astoundingly close! So close in fact, that the comet’s coma (its visible cloud of debris) will likely encompass the planet. Cometary comas are believed to be made up of particles that range from dust size up to a significant fraction of an inch. For the rovers on the surface, this means a high likelihood of a really good meteor shower to accompany the visual spectacle of the comet itself. For any spacecraft in Mars orbit however, this could spell significant danger.

Orbiting spacecraft, as we know from our experience with Earth-orbiting satellites, are vulnerable to hits from both natural and artificial orbital debris. At orbital speeds, an impact with an object the size of a speck of dust can be serious. When Mars and Siding Spring find themselves so close this coming October, their speed relative to one-another will be well over 100,000 miles per hour. At those speeds, an impact with even a speck of dust would likely be catastrophic.

Currently, there are three active spacecraft orbiting Mars, NASA’s Mars Reconnaissance Orbiter (MRO) and Mars Odyssey, and ESA’s Mars Express. Just before the encounter with the comet, two additional spacecraft, launched recently, will arrive in orbit (NASA’s Mars Atmosphere and Volatile EvolutioN (MAVEN) and India’s Mars Orbiter Mission). Since missions like these cost anywhere from hundreds of millions to billions of dollars, you can imagine that the engineers are already busy brainstorming the best ways to protect their investments.

The comet encounter at Mars will be fun to watch. Interest from the science community will likely be high and news of the event will lead to lots of web coverage and perhaps some stories in the mainstream news outlets. It will also bring to the surface an ever present tension between the science and engineering teams that run spacecraft operations. The engineers, who are responsible for the safety and operation of missions like the Mars orbiters, will want to batten down the hatches and place the instruments in safe mode, put the covers on all the cameras, and angle the spacecraft so that sensitive instruments are protected as much as possible from the assault of debris. The scientists, naturally, will want to take advantage of this unique opportunity to gather as much data as possible. But you can’t have it both ways and in the end, the safety of these valuable assets will have to be protected for the future, and even the scientists will grudgingly admit that safe is better than sorry.

And, as we learned this past Thanksgiving with comet ISON, you never know what a comet will do. It could be a wondrous and unforgettable experience, or it could be a fizzle. Stay tuned to see.


About John Rummel

Amateur astronomer and photographer. Observer of the universe.
This entry was posted in comets, Uncategorized and tagged , , , . Bookmark the permalink.

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