After getting back into astronomy recently, one new development I’m really enjoying is the great abundance of astronomical events, exploration and research on social media. Following progress, discoveries and images from the New Horizons mission in near real-time on Twitter, for example, has been a thrill and quite addicting!

But there has always been a fringe element around this subject, and that comes up in SM as well.Last month I saw some fear mongering about a major comet or asteroid due to hit the Earth in September, and subsequent rebuttal and denial by professionals. I don’t even want to post a link to the sites purporting this nonsense – but you could look it up! But it’s got me thinking about whether a really big celestial object could catch us by surprise..

I’ve also re-joined a local astronomy group and am looking forward to getting out with them again. Many of the members have changed, but they still have a core group of very dedicated observers. One of the former members I knew was a dedicated comet hunter, and would go out on many clear nights with a wide-field telescope or binoculars and scan the sky for fuzzy objects.

To hunt for comets by eye, you have to become acquainted with existing nebula, galaxies and other fuzzy objects. In fact, Messier developed his catalog for this very reason! He had identified over 100 objects (and these have been added to), and today there are a number of more comprehensive catalogs for all of the “fixed” deep sky objects visible in the sky.

The club also co-hosted a talk by Thomas Bopp a while back, who, of course, is one of the co-discoverers of the spectacular Hale-Bopp comet. He described stargazing with friends and noticing a faint fuzzy object he did not recognize. After checking start charts for known objects he suspected he had found a comet. After watching it for a while, he noticed movement, confirming that it was not a nebula or galaxy. He did not have a way to take a picture, so he sketched the comet relative to nearby stars and was able to work out a location for the new comet. Then he sent in the discovery the old-fashioned way – by telegram!

Alan Hale is an avid observer and hunter of comets and had also noticed the new visitor and sent in his observations as well. The comet was confirmed to be new and was named for the two co-discoverers.

When Hale and Bopp first viewed the comet, it was determined to be about 9 AU out from the sun or well over 1 million km from Earth. The comet was one of the largest and brightest seen in recent times and would surely cause major damage if it happened to hit the Earth, but how likely is that?

After a newly discovered comet or minor planet is observed for a few days, it’s quite straight forward to calculate it’s orbital path. The motion of any body in orbit around the Sun was determined by Kepler and others in the 16-17th centuries and explained by Newton. Any body captured by the Sun will travel in an ellipse – or a perfectly round circle which is a specific type of ellipse. The orbit of the planets is roughly circular for the most part, but long period comets like Hale Bopp move in a very elongated or eccentric track. Hale-Bopp makes a trip around the Sun every 2500 years or so and when it comes to visit it approaches 0.93 AU before traveling way out to the fringes of the Solar System at 370 AU. It’s thought that it used to go further out before it’s orbit was brought in a bit closer by Jupiter.

Given the orbital parameters for a comet, one can work out the probability for impact developed by Opik over 50 years ago and refined by scientists in the field since. The probability is determined by whether the orbits cross and how often, and the chance that both objects will be in the same place at the same time. This gets fairly complicated as the orbit of a comet or other smaller body is subject to precession as well as perturbation by other bodies. So the orbits can align from time to time increasing the chance of a crossing. But since there is lots of room out there and the visits infrequent, the collision probability from a long-period comet is estimated to be in the tens or hundreds of millions per year per potential impact.

Perhaps people imagine the probability of these collisions to be more likely than this very small number because of the pictures we’ve all seen of the Solar System. These are fine conceptually but impossible to draw at actual scale. As this page from the NOAO shows, if the Earth were represented as a small peppercorn, or about 1/10 of an inch across, it would be located about 25m from the Sun. And Hale Bopp would travel about 10,000 m away on every orbit, coming back every 2500 year for another shot. Another awesome, live scale model was made recently by Wylie Overstreet and Alex Gorosh in the Nevada desert and posted here.

So I don’t see how a big comet could take us by surprise, but we’ll soon see! Anyhow I’m still waiting for Planet-X to end us, which was another doomsday hoax propagated on Usenet a while back. Perhaps things have not changed so much after all..