[CraigS]

An interesting article by Phil Plait on recent study which poses the question:
"Does the impact rate for asteroids and comets vary periodically with time?"

Plait says:

And now it looks like my thoughts are being supported: a new study finds no such pattern in the ages of craters, and concludes all the periods found previously are probably due to errors in the analyses.

The difference is that the author, {of the study above}, Coryn Bailer-Jones, used Bayesian statistical methods. This is different than standard statistics, and is less prone to bias due to uncertainties in age and size of craters. In using standard statistics, clusters in crater ages can always be found, but it’s hard to know if that’s just a random clump or has an actual physical cause — like flipping a coin 10 times and having it come up heads 5 times in a row. It’s unlikely, but how do you know if it’s coincidence or not? Bayesian methods circumvent that issue.

Interesting also is Bailer-Jones words about Bayesian inference (similar to what was outlined in our thread about Speigel's paper about the probability of abiogenesis).

Bailer Jones says ..

Bayesian inference is not a cure-all for statistical problems. It has some subtleties of its own, notably concerning the proper choice of prior probabilities. For this particular task, that is, the analysis of time series data with different uncertainties (and, in some cases, upper age limits for a crater only), it is a highly suitable tool that allows well-founded statements about the different hypotheses under discussion.

Periodic variation in the cratering rate is strongly disfavoured in all data sets, and there is no evidence for a periodicity added to an underlying general trend. This result is found to be quite robust to the specific assumptions about the priors.

Oh … and yes .. the conclusions:

i) We’re not bombarded every 60 million years (or whatever)... it was simply an artifact of the way the math was being done.

ii) huge craters, ones bigger than 35 km (21 miles) across — in other words, more likely to cause extinction-level events — there is no trend in the rate of impacts over the past 400 million years.

iii) for craters of all sizes (not just big ones), there is a trend of increasing rate over the past 250 million years. Are we getting hit more by smaller objects now than we were in the past? … Not necessarily .. there are other possibilities ....

Interesting !

Cheers