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

[chergen]

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