(meteorobs) On the connection between nearby NEAs and the recent increase in fireballs...

Mike Hankey mike.hankey at gmail.com
Tue Jan 18 13:55:39 EST 2011


Excellent information and explanation. Thanks for sending out such a
detailed analysis.

To summarize in laymans terms without putting words in your mouth, I
think what you are saying is:

There is no correlation to witnessed fireball activity on Earth and
NEOs that are reported in close approach to Earth, because, these
reported NEOs are only a small subset of the actual NEOs flying by
everyday anyway.

My understanding of what Chris is saying is -- sometimes fireball
rates are going to go up and sometimes they are going to go down and
that is just the way it is. Rates will ebb and flow, and correlating a
perceived increase in fireball rates for a certain time with
approaching NEOs in that same time frame is a fallacy. It would be
like saying, fireball rates went up right after christmas, that must
be why those dead birds are falling from the sky.

And I think what Bob is saying is -- an increase in reported fireballs
does not correlate to an increase in actual fireball rates. It just
means more people are seeing them and reporting them. This is a trend
he has seen with the AMS reports since they were started. Explanations
for this can include: more people with computers and internet access,
increased interested in meteors and fireballs, better news coverage
and better promotion of the AMS reporting form.

Wayne, to your point, yes the ams fireball reporting form was broken
from approximately Nov 2009 - Jan 28 2010. While reports were still
coming in via email, this would certainly impact the number of total
reports as well as the number of events. Good call.

I think it would be interesting to review the fireball reports from
the IMO for the last 12-16 months to see how they correlate. Did IMO
rates in Jan 2011 seem to go up like they 'seemed' to with US rates? I
checked the IMO site and don't see where I can view a fireball log
like I can on the AMS site.

Lastly, shouldn't there be some metric collected by the camera based
fireball networks, like ASGARD that would quantify the fireball rates
in a consistent manner year over year, so that we could get a real
number vs a perceived number? Using counts from a fixed network would
compensate for fluctuations caused by the human factor. I think this
sort of data would be the only thing that could 'prove' a rate
increase theory, one way or the other.


Mike Hankey

On Tue, Jan 18, 2011 at 2:46 AM, chergen <chergen at yahoo.com> wrote:
> Hello Everybody,
> There has been some talk about the recent flurry of fireballs
> being related to the apparently large number of near-Earth
> asteroids (NEA) approaching Earth over the past few days. As a
> NEA researcher, I'll take a quick stab at investigating whether
> the connection is real.
> Lets start with a look at the 5 NEAs that came within ~0.15 AU
> of Earth on January 11. First off, their orbits are not similar.
> Some are Apollos and some Atens with semi-major axes ranging
> from 0.74 AU to 1.74 AU and inclinations ranging from 1.7 deg to
> 23 deg. Also, some are currently inbound towards the Sun while
> others are currently outbound. There is nothing about the
> orbits of these NEAs that suggest that they are related to each
> other (other than that they are all part of the same NEA
> population and have been spending the past couple of million
> years being perturbed from the Main Belt onto planet crossing
> orbits).
> Having 5 known NEAs pass within 0.15 AU of Earth on the same
> day is more than average, but within the expected variation.
> Over the past year, 806 known NEAs passed within 0.20 AU of
> Earth and 485 of those passed within 0.10 AU. That works out
> to an average of 1-3 known NEAs making a close approach within
> 0.15 AU of Earth every day. Considering that the rate of
> discovery is not constant throughout the year (few objects are
> found within a week of Full Moon or during the rainy summer
> months in the American Southwest), the expected daily rate is
> higher for days like January 11. Over the past year there were
> 15 dates that saw 5 or more known NEAs make close approaches to
> within 0.15 AU. So the 5 close approachers on a single day is
> not that uncommon.
> The date of close approach is not very important. Rather it is
> the date when the Earth crosses the asteroid's orbit that tell
> us when we should expect meteors from any particular object.
> For the 5 known Jan 11 NEAs, the time of orbit crossing ranges
> from Dec 27 to Jan 21 with none occurring on Jan 11 (though one
> crossing takes place on Jan 13).
> In the above paragraphs I purposely referred to the close
> approach NEAs as 'known' NEAs which is an extremely important
> distinction. Currently just over 7600 NEAs have been found
> ranging from the 30-km Ganymed to a handful of very small 1-2
> meter objects. Though we have probably found ~90% of the 1 km
> and larger NEAs, our knowledge of the smaller objects is much
> less complete. For example, based on our latest understanding
> of the size distribution of the NEA population there should be
> over 40,000 NEAs larger than 200-meters in diameter, over
> 200,000 that are larger than 100-meters, over 40 million with
> diameters over 10-meters, and 10,000,000,000 larger than a
> meter!
> To further this point, 22 NEAs were observed to pass within
> 1 Lunar Distance (LD) of Earth in 2010. Sounds impressive until
> we realize that 4-5 10-meter asteroids should pass within 1 LD
> of Earth every day. If we drop down to 1-meter objects the rate
> could be as high as a few thousand per day!
> So it doesn't really matter if there are 1, 3, 5 or 20 known
> NEAs passing within 0.15 AU of Earth on any single day because
> the number of unknown objects (even within a few LD) is much
> much greater.
> A fireball stream may be the cause of the recent increase in
> observed fireballs. Honestly, without orbit and/or radiant info
> we can't be sure. But one should not use the number of known
> NEAs in the vicinity of Earth as a predictor of fireball
> activity because the known NEAs are just a fraction of the
> total NEA population. (The caveat being unless one the
> asteroids is on an actual collision course, e.g. 2008 TC3).
> There are always thousands of NEAs down to a meter in size
> flying by Earth every day. Most pass unseen as the current
> generation of asteroid surveys are very inefficient at
> detecting such faint objects.
> All the orbit and close approach data is from the Minor Planet
> Center and the JPL/NASA NEO Project Office. Size distribution
> is from the National Research Council's "Defending Planet Earth:
> Near-Earth Object Surveys and Hazard Mitigation Strategies:
> Final Report".
> - Carl Hergenrother
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