Astronomy and Space articles
by Martin George of the Launceston Planetarium
25 October 2014
Comet Siding Spring and Mars
One of the most talked-about astronomical events in recent times has been the close
approach of a comet called C/2013 A1 Siding Spring to Mars early on Monday morning
our time. Just after 5am Tasmanian time, the comet passed to within 140,000 kilometres
of Mars, making this the closest-ever predicted approach between a comet and a planet
without involving a collision.
Comet Siding Spring is on its first visit to the inner Solar System from the distant region
known as the Oort Cloud, which is home to a large number of these icy objects. It was
discovered early in 2013 by Robert McNaught at the Siding Spring Observatory in New
South Wales. Not surprisingly, he also discovered the Comet McNaught that was a
spectacular sight in our southern-hemisphere skies in January 2007.
Of course, there has indeed been a collision that was predicted, and that occurred 20
years ago. That was when a comet called Shoemaker-Levy 9 ended up in orbit around
Jupiter, the largest of the planets, and was broken up into many pieces by tidal forces.
Subsequently, each of the pieces slammed into Jupiter's atmosphere, resulting in several
black spots that were clearly visible even through fairly small telescopes.
It is interesting to compare the close approach distance earlier this week with the size of
Mars itself. Mars is about 6,800 kilometres in diameter, so if we were to represent Mars
by a cricket ball, the comet's closest approach would have been about one and a half
metres away.
At the planetarium, I received a few enquiries as to whether this event was going to be
visible with the unaided eye or binoculars. This followed at least one media report that
people with binoculars would get a good view. Unfortunately, this was never the case,
with visibility of the comet being limited to those with sizeable amateur telescopes at the
very least. Not only that, the glare of Mars itself was of course a significant factor in
being able to witness and photograph the event.
A major issue with the comet passing so close to Mars was the health of the many
spacecraft up there. Even though the nucleus - the relatively solid part - of the comet
itself is relatively small (well under a kilometre in diameter), the debris being released
from the comet could have posed a hazard.
Indeed, I remember that in 1986, many pieces of Comet Halley impacted the Giotto
spacecraft. I had been at the University of Kent in the United Kingdom during the
assembly stage of Giotto's protective shield, and studied how the shield was able to
collect information about the mass and location of impacting objects. At least one of the
Astronomy and Space articles
by Martin George of the Launceston Planetarium
25 October 2014
pieces that hit Giotto was the size of a golf ball, and the encounter with the comet
certainly affected the spacecraft's orientation as it was repeatedly hit.
The good news is that all spacecraft are doing well, and seem to be unaffected.
However, to be sure, NASA had deliberately adjusted the paths of its orbiting Mars craft
so that they would be on the opposite side of Mars at the critical time, protected by the
body of the planet itself.
I would have loved to have been on Mars this week, watching the flyby of the object,
which would have had a total brightness greater than that of brilliant Venus!
An image of the head of Comet Siding Spring,
taken by the Mars Reconnaissance Orbiter
spacecraft. The actual icy nucleus covered only
a few pixels, showing its actual diameter to be
only a few hundred metres.
Article by Martin George, Launceston Planetarium, QVMAG.
Reproduced with permission of the Mercury newspaper.