(meteorobs) A real Earth-grazer

[Esko Lyytinen]

Hi,

An Earth grazing meteor(oid) was captured here in Finland by two 
fireball cameras in 2008 Sept. 22 at 01.14 UT.
This was not especially bright, only near the fireball-limit, as seen 
almost below in the Jari's images and video, see below.

It was captured by Jari Tuukkanen with an all sky camera and 
UFOC-program ( V2.21 )and by Timo Kantola with a wide field camera and 
SkyPatrol4 program. We received only one eyewitneess report to Ursa. 
This was observable in either of the cameras for about 15 seconds.

This was already bright at the start of Jari's capture and was prbably 
prevented from earlier capturing because of a too slow angular velocity 
earlier, for the UFO.program definitions. Also the end of capturing was 
probably from this reason. Timo had the start of capturing earlier, but 
in this the still earlier capturing was probably prevented by clouds low 
in the sky. In Timos image it went outside the field after about ten 
seconds of visibility.

The data of these two cameras allowed a very accurate trajectory 
derivation (with also the Earth gravity effect applied during the 
flight. It appears that the trajectory had its most low point when 
visible in Timo's image and close to the start of Jari's video at the 
height of 87.3 km, from sea level. The derived velocity then was 17.89 km/s
The trajectory fitting ws done into about 130 half-frame (automatic) 
UFO-position measurement points, these treated as mutually timed 
observations. The SkyPatrol image was measured for 12 (non timed points) 
along the track.

At that height the atmospheric density is very slow. An attempt was made 
to derive the deceleration, with the assumtion of it being constant 
during this.
A value of (as small as) 1 m/s^2 was achieved, but this is probably too 
small a value to trust as distinct from zero, (or from any other 
smallish value).
In any case it appears quite sure that the meteoroid went back into the 
interplanetary space, with almost unaltered velocity relative to the Earth.
If that deceleration value were correct the meteoroid would have been as 
big as about 200 kilograms ( if chondritic density). This is not 
reliable however. So it is very difficult to get any reliable mass value 
for this. This height may be aroud the typical beginning height of this 
slow meteors, so the brightness probably does not give anything to tell 
this.
This was also well visible in my forward scatter radio data, which may 
not be very typical for this low meteors/fireballs, according to my 
experience.
In this, first an overdense reflection appears, then the specular or 
head echo and it continues as an overdense for some time. The visible 
band is only 100Hz wide and the head echo would pass over this in about 
a second, so this deos not allow the measurement of  the frequncy drift 
value here. Probabaly a more wide visible channel would have given 
something on this.

In the video, a weak trail is more or less visible.
You can see the mp4 form video of Jari at:
http://www.horseheadobservatory.com/grazer/

Timo's image can bee seen at:
http://img91.imageshack.us/img91/7792/200809220114utov3.png
Here it goes from lowere left to upper right.

The solar system orbit before the encounter (as derived with Marco 
Langbroek's Excel sheet) has

a=2.67
q=0.869
e=0.674
peri=48.10
node=359.291
i=8.31
The geocentric radiant is
344.18 , -29.96 (angular values J2000.0)  
Vinf 17.9km/s and Vgeo 14.2 km/s

Esko Lyytinen
Jari Tuukkanen
Timo Kantola