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ScienceWise - Summer 2011

Eye on the sky

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Rob McNaught with the Uppsala Schmidt telescope used for the Southern Hemisphere observations of the Catalina Sky Survey
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The brightness of asteroids varies enormously depending how far from the Earth they are in their orbits. The very largest ones are just barely visible to the naked eye but the vast majority of smaller potentially hazardous asteroids are all but invisible in even the largest telescopes except when they pass fairly close by us.
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Needle in a haystack! Asteroids look exactly like the billions of background stars. The only effective way to detect them is to use multiple images and look for movement

The Catalina Sky Survey’s search for hazardous asteroids

The idea of a large asteroid colliding with the Earth has formed the basis of a number of movies such as Deep Impact and Armageddon, but is such an event purely a Hollywood fantasy? Any scientist would tell you no, such events do and will continue to happen. But they’d also say that the chances are small.  But in the world of risk management how small does a chance have to be for us to simply ignore it?

For example, the chance of your house being destroyed by fire is also very small, especially if you live in a major city, but how would you feel about simply not bothering with insurance? In situations of low risk but catastrophic consequences the only prudent cause of action is to take what precautions you can. In the case of protecting the Earth from a massive impact, a large part of that precaution is the Catalina Sky Survey.

The mission of the Catalina Sky Survey is to contribute to the inventory of Near-Earth Objects (NEOs), or more specifically, the Potentially Hazardous Asteroids (PHAs) that pose an impact risk to Earth and its inhabitants.  Because the Northern and Southern Hemispheres each see a different region of the sky, this is necessarily an international effort, two instruments being located in Arizona and one in Siding Spring, NSW.  Although the optical specification of the telescopes varies slightly, they all share common detectors and common software so that their observations can be combined into one consistent data set.

But how exactly does the search for potentially dangerous asteroids work? In the world of Hollywood, you might imagine a scientist sitting at the eyepiece of huge refracting telescope peering into space looking for asteroids, but anyone familiar with modern astronomy would know how futile such an approach would be. 

Asteroids are comparatively small and made of dark material, a combination of which makes them incredibly dim. To make matters worse the sky is filled with billions of stars of similar brightens and it’s impossible to tell one from another by simply looking through a telescope, even at high magnifications. In fact the name asteroid literally means “star like” chosen to describe such objects when they were first discovered.

The only practical way to detect asteroids is to take a long exposure image of a section of sky, then wait a while and take another image. Because bodies like asteroids and comets are much closer to the Earth than the background stars, they appear to move over time. If you can identify objects that move and map their position against the background stars, you can compute their orbits and predict the likelihood of them colliding with the Earth. However aside from the complex mathematics involved, there are two major practical difficulties.

Firstly the typical field of view of a professional telescope, even a wide field ones like Schmidt telescopes, is not that big – typically less than one thousandth of the whole sky. Now if the asteroids are very dim and require a reasonably long exposure to detect, you can see the problem – it takes a long time to completely cover the sky. Added to that, asteroids have variable brightness depending on how far from the Earth they are at the time. This means that all but the largest ones may be completely undetectable for the major part of their orbits, which can have periods of several years.

The second difficulty is the size distribution of asteroids. There are relatively few planet-sized ones but thousands that are a few kilometers across, millions in the tens of meters range and countless trillions of smaller ones.

Tackling all these difficulties are Siding Spring astronomers Rob Mc naught and Gordon Garradd forming the Catalina Sky Survey’s Southern arm. Rob spends many nights collecting data and analysing it to identify bodies in orbit close to the Earth.

“We can’t hope to catalogue every asteroid out there, so we have to focus’s on those that have the potential do most damage, that is the ones that are 1km across and larger and we currently know of 815 of these that come within relatively close proximity to the Earth.” He says. “However, Significant local damage can caused by asteroids right down to around 30m in size. Whilst the smallest asteroids we’ve discovered are around 3m across these represent only a tiny proportion of those out there. Realistically, current surveys are inadequate to seriously identify any significant proportion of asteroids in the tens of metres range.”

However you have to start somewhere, and fortunately NASA take the threat seriously enough to have funded the Catalina Sky Survey until at least 2012.

Of course having identified a potentially dangerous asteroid, the next step is what to do about it? Even ten years ago the answer might have been, evacuate the projected impact site. Fine if it’s the Sahara Desert but what if it were Sydney or London? And in addition to the direct impact damage, the widespread environmental disruption could be disastrous, after all, it was a series of impacts that wiped out the dinosaurs!

However, the recent success of space missions such as Japan’s Hayabusa space probe that successfully met and touched down on an asteroid suggest that if we knew in time, we’d have a good chance. It’s probably impractical to try to destroy the asteroid. However soft landing a space probe on the surface and using something like an ion thruster to apply a small but very prolonged thrust, may provide an eminently practical method of deflecting one and is theoretically possible using out existing technology. The key to success would be having sufficient warning to prepare such a mission and of course that’s where Catalina comes in.

Of course not everything identified by the survey poses a hazard, there have been a number of valuable scientific discoveries from the data too. Rob discovered the spectacular 2007 comet that bears his name whilst looking for asteroids.

“I used to observe asteroids as an amateur hobby when in the UK. When I got a job at Siding Spring in the early 1980s,” Rob says, “I continued my hobby in my spare time taking photographs with the Uppsala Schmidt telescope and measuring asteroids and comets discovered at the UK Schmidt Telescope. Later, Duncan Steel started an NEO survey based at Siding Spring and employed me to scan the photos.”

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