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

[Thomas Dorman]

Robert
My question would be are green fireball reported or observed more in same months than others?It seems to me that this question could be answered by reviewing the records of the AMS and IMO over many years.My view,opinion,is that January to mid-Febuary has been known for green fireball activity for some time.Maybe I wrong on this view?
Regards
Thomas


--- On Tue, 1/18/11, Robert Lunsford <lunro.imo.usa at cox.net> wrote:

> From: Robert Lunsford <lunro.imo.usa at cox.net>
> Subject: Re: (meteorobs) On the connection between nearby NEAs and the recentincrease in fireballs...
> To: "Global Meteor Observing Forum" <meteorobs at meteorobs.org>
> Date: Tuesday, January 18, 2011, 10:33 AM
> Carl,
> 
> Many thanks for your input on this subject.
> 
> The next logical question is what is causing the apparent
> increase in fireballs seen this month? The key word here is
> apparent. It 
> could very well be that there is no increase at all, but
> rather a marked increase in the number of reported
> fireballs. Mr. Hankey 
> has worked with the AMS in providing an easy way to report
> fireball sightings and the general public has responded with
> a record 
> number of reports so far in January. If you look through
> the last five years that the AMS has available, you will see
> an increase in 
> every year. The increase is certainly not as dramatic as
> January 10 vs. January 11, but that again can be attributed
> to the recent 
> change in format. I am also confident that no matter the
> number of NEO's in January 2012, there were still be more
> fireball reports 
> in January 12 vs. January 11.
> 
> Interesting subject and I appreciate the contributions of
> Dirk, Mike, Wayne, and Carl!
> 
> Bob Lunsford
> 
> -----Original Message----- 
> From: chergen
> Sent: Monday, January 17, 2011 11:46 PM
> To: meteorobs at meteorobs.org
> Subject: (meteorobs) On the connection between nearby NEAs
> and the recentincrease in fireballs...
> 
> 
> 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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