[UPDATED] The 21 September fireball: a small Aten asteroid?
In my previous post I presented clear evidence that the splendid fireball seen over NW Europe on September 21st, 2012, was a meteoric fireball. I also presented a first, very preliminary idea of its trajectory.
Based on that trajectory, I can now present some very first, very cautious conclusions about the heliocentric orbit of this meteoroid.The solutions strongly favour an identification as an Aten asteroid.
The entry azimuth of the fireball from the reconstructed preliminary trajectory is around
I used these values and an 18-20 km speed estimate to compute a nominal heliocentric orbit: and then played around by widely varying the values for speed, entry angle, entry azimuth around these nominal values.
The interesting point is, that for all of these, I get an Aten orbit as a result. Aten asteroids are asteroids whose perihelion lies within the orbit of the earth and who's aphelion lies only just outside the orbit of the earth. They have a semi-major axis < 1.0 AU and aphelion (just) over 1 AU.
The aphelion values I get for the approximate fireball orbit, are in the range 1.0 - 1.15
A wide variation in entry azimuth (I tried between 60 and
I need to alter the trajectory direction to values significantly larger than entry from a direction of 120 degrees (well past due east) to get aphelion values that start to get well beyond 1.15 AU and semi-major axis values > 1.0 AU.
For the current very preliminary nominal trajectory solution (entry azimuth
[editted table 15:25 UT to reflect new calculations/correction of error]
Vini q Q a e i
12.0 0.82 1.00 0.91 0.10 6.5
15.0 0.46 1.02 0.74 0.39 15.0
18.0 0.31 1.04 0.67 0.55 20.7
20.0 0.24 1.05 0.65 0.62 24.8
25.0 0.16 1.09 0.62 0.76 37.4
27.0 0.13 1.11 0.62 0.79 43.7
30.0 0.11 1.14 0.62 0.83 54.5
Vini is the initial speed (in km/s), q the perihelion distance (in AU), Q the aphelion distance (in AU), a the semi-major axis (in AU), e the eccentricity, i the inclination.
These values should be taken with caution and only as rough indications. There are (still) large uncertainties in the trajectory and entry angle, as well as the speed of the fireball. They do show however (as well as variations on the trajectory not listed here) that an Aten-orbit is the implied solution.
The Earth encountered the meteoroid close to the meteoroid's aphelion, when it was moving almost in parallel with the Earth.
NOTE / UPDATE 26/09/2012, 19:25 UT: There is some confusion on the web regarding my analysis and the "retrograde"/ "prograde" character of this object.
The "retrograde"character is only true for an earth-centered orbit (i.e., an object orbiting the earth, such as an artificial satellite). An east-west movement in that case means it is "retrograde" (against the motion of the earth's rotation).
This is not necessarily the case for a sun-centered orbit however. An east-west moving object then can be (and is, in this case!) in a normal, "prograde" orbit (=moving in the same direction around the sun as the planets). The difference is the frame of reference: earth-centric versus sun-centric.
So beware: the "retrograde" orbit refers to what the orbit would be for an earth-orbiting satellite (which this object was not). The Aten heliocentric orbit presented here, is however prograde.