Rock on the Ground or Not?
Posted March 31st, 2008 by Admin
Lasse wrote >>Hi all; If one sees a meteor fall with a brightness of -4 or brighter, it's duration is +10 sec. What is the chances of finding a small rock where it suppose to have landed? Is it big enough considering the brightness and length of fall. It needs to be said that the angle of the fall was around 75-80 degrees, allmost straight down.
Chris wrote >>That's a difficult question to answer. The majority of meteorites come from meteors that are not terribly bright. But mag -4, while technically
a fireball, isn't much of one, and is very common. It's fair to say that a -4 meteor is a candidate for meteorite production, but I don't know
how to quantify that chance.
If the information you are providing is purely from your own observation, there's not enough information to make any estimates. From a single vantage point, the velocity and fall angle of the event are impossible to determine. It may have been heading nearly towards you, several hundred kilometers away, or it may have been dropping straight down 50 km away. There's no way to tell. Finally, from a single
viewpoint you have no way of estimating where the actual fall might be. Even with many witnesses and camera views, it's usually not possible to narrow a fall zone to less than many square miles.
The meteors that are most likely to produce meteorites have some common characteristics. They are slow, usually less than 20 km/s which is most likely in early evening meteors). Their entry angles are shallow, which provides lots of time to slow down while high in the atmosphere, before the air becomes so thick that a fast object can ablate rapidly ablate away, or fragment into small pieces that burn up.
Lasse wrote >>Hi Chris; Thanks for your answer. Well, this one pointed straight towards me. While bright and when it got weaker it did not move at all. I could see this because there were two stars just beside it. I am an amteur astronomer, so observing is nothing new to me. It did not deviate during the fall. I first thought it to be a GRB since it was so stationary, but then I realized that that was out of the question, so the only thing left was a meteorite. It showed no sign of break up. Checking the star map, it held an angle of 76 degrees. It's brightness could be a lot higher than -4. It's hard to tell.
Chris wrote >>Ah. So you're saying that you observed a stationary meteor, that didn't appear to move across the sky at all? If so, that's the one special case where you can accurately estimate the fall angle from a single viewpoint. If the meteor's apparent altitude was +76°, then that is indeed the fall angle (which is too steep to make this a promising candidate for meteorite production).
Being so high in the sky also lets you narrow down both the distance and the speed. The long duration suggests a slow speed, and a final height of 30km would be a reasonable estimate. So that would place the end of the meteor around 7km away from you, on the azimuth of the event. If something survived, however, it would have continued to fall at a low speed for several minutes, subject to the effects of the wind. Any meteorites that landed could have been in any direction from the retardation point, including behind it (that is, farther away from you). If you have good wind information, you can make some rough estimates.
My own intuition is that the combination of steep descent angle and lack of terminal explosion suggests that the meteoroid simply ablated away, and probably didn't produce any meteorites.
Lasse wrote >Hi Chris; That was bad news. I hoped for a small rock out there. I calculated 3 km, but then I used 15 km as terminal height.
Rob wrote >>Hi Chris and Lasse, There is another possibility to consider: Lasse may not have seen a fireball at all.
Consider:
1. So-called "point meteors" are very rare; they are rarer still
when the radiant is close to the observer's zenith.
2. The 10-second duration is probably too long for a point meteor at such high elevation angle. Even at the slowest possible initial entry velocity (11.2 km/sec), a meteor only 14 degrees from zenith cannot maintain a velocity above 3 km/sec for that length of time.
I offer an alternate explanation: Lasse may have observed a glinting satellite, perhaps an Iridium satellite. This is easy enough to check, knowing the date, time and location of the observation. On March 28th and 29th there were high elevation Iridium flares for Sweden in the early evening in the eastern sky -- in the vicinity of the bowl of the Big Dipper. --Rob
Chris wrote >>Hi Rob- It's certainly possible. I've seen three head-on meteors, and recorded a few dozen on cameras. But I'm an astronomer, and spend a lot of time looking.
I don't know how accurate the 10-second time estimate was. If accurate, it's not only too long for most meteors, but also a bit long (but not impossibly) for an Iridium flare. More to the point, however, is that Lasse was comparing the event location to surrounding stars. Iridiums move pretty fast, and a 10-second flare would move significantly. It sounds like he was enough in tune with the stars to notice something like that.
Another possibility, depending on the time and sun angle, would be a reflection from a weather balloon (or its payload). I've seen them do some pretty odd things, and they appear stationary for a long time.
Lasse wrote >>Hi Chris and Rob; Thanks for your replies. I can say with certainty that it was not a satellite. These are moving objects and I've seen plenty of those as well as iridium flares. I'm also an amateur astronomer and have seen my share of these man made objects. I've also seen a few meteors that almost took aim at me before, but slightly diverted, all fast and short ones. I could be mistaken on the time. It's hard to estimate time when things happen fast. It could well be a lot less instead. I have tried to go through the event in my head and the shortest time I've come up with is 5-6 sec but that feels a bit short. But it may be more correct. It's based on the fact that the light caught my eye, I turned my head towards it and stared at it and concluded that it was out of place and should not be there. A second or two later it started to get weaker and disappeard. The weaening was gradual, not like turning a light bulb off. So it might be somewhere in the order of 5-7 sec.
Thing is, observing is quite easy but estimating time during an observation is a lot harder especially if all attention is concentrated towards the pure visual part.
The weather balloon theory sounds interesting. I've never seen anything like that. But then, it has to be very high to reflect the sun this time
of the year an hour from midnight.