On Connection Between Nearby NEAs and the Recent Increase in Fireballs... 19JAN2011

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

by Carl Hergenrother

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

Carl Hergenrother is a professional astronomer at the University of Arizona’s Lunar and Planetary Lab, Tucson., AZ, USA.