Post by michaelmorris on May 22, 2013 17:17:32 GMT
I've recently started using a Samsung SDC-435 with a 2.1mm board lens as an all sky camera with some success picking up meteors and spacecraft. almiraobservatory.awardspace.com/OBSconstruction16.html When it's clear, I find it pretty easy to distinguish a plane from a meteor. However, if there is a bit of cloud, I sometimes find it hard to distinguish between a fast aeroplane and slow meteor. What is the slowest speed (degrees per second) I would expect for a meteor? Thanks
There are a few things to take into consideration when determining the lowest angular speed for your particular setup: the actual speed (km/s) of the meteoroid, the angle between where the meteor was observed and the radiant point and the distance between the observer and the meteor.
1. The lowest possible speed (km/s) for a meteoroid is 11.2km/s. This lower limit is related to Earth's escape velocity and is the velocity an object would acquire if it simply fell from deep space towards the Earth under the Earth's gravitational attraction alone. For reference, the maximum speed is 72.8km/s - this corresponds to a head on collision between the earth at perihelion, where its orbital velocity is 30.3km/s, and a meteoroid having the solar system escape velocity of 42.5km/s at 0.98AU (the Earth's perihelion distance) from the Sun.
2. Angular distance from the radiant point - If a meteoroid's trajectory takes it along a path from the radiant directly towards the observer then, from the observer's perspective, the meteoroid is heading straight towards them - the angular speed will therefore be zero. It is more likely that the trajectory will be at some angle away from the observer and the maximum angular speed (and maximum observed trail length) will be when the meteor is observed 90 degrees from the radiant. In between these two values the observed velocity will be lower due to the path being foreshortened.
3. Distance between the meteor and the observer - the greater the distance. the lower the angular speed and the shorter the observed path length. Meteors typically have an altitude circa 100km - if observed at an altitude of 45 degrees then the distance from the meteor to the observer would be 141.42km.
Taking all this into account and using basic trigonometry, a meteor at a position on the sky that is 45 degrees from the meteor's radiant travelling at 11.2km/s would have an apparent speed of cos45*11.2km/s = 7.92km/s
Assuming a distance from the observer to the meteor of 141km, the difference in observed position from one second to the next would be given by tan(angle) = 7.92 / 141.42
Solving this yields an angular speed of 3.2 degrees / second
Welcome indeed! Looks like you've got a fine system there.
I use Handy AVI myself but not for meteor work. Off the top of my head I'd have said a couple of deg/sec but William has explained it nicely. But the exact orientation of the flight with respect to you might change the apparent velocity due to perspective and foreshortening. It's a tricky problem!
The recordings on Youtube look good.
The book William mentioned is good as it elaborates on such things more than the usual general meteor material.
The International Meteor Organisations' Observers Handbook is also an excellent and detailed read. It's currently being re printed but check out www.imo.net/imo/publications
Post by michaelmorris on May 23, 2013 13:00:02 GMT
Thanks for the speedy and helpful replies and the links to publications. I've just ordered Martin Beech's book. From what you have said, it seems plausible that some of the 'debateable' videos are meteors. However, this now raises another question. For how long would I expect a (non-fireball) meteor to be visible?
As you've probably come to expect, the answer begins with "It depends ..."
Some meteorids enter the earth's atmosphere at a very shallow angle and skim along the top of the atmosphere. There's enough atmosphere to make them glow / cause atmospheric ionisation (ie make them visible as meteors) but the atmospheric density isn't sufficiently high to cause a dramatic reduction in velocity or (as is the fate of most visible meteors), fragmention and complete ablation. These "earth grazers" can take a few seconds to cross the Field of View - I've had some in excess of 8 seconds.
This website: www.flightradar24.com/ can be useful, particularly the "playback" option, if you want to know what commercial air traffic was overhead at the time of the observation. And over course www.heavens-above.com/ will tell you about satellites brighter than magnitude +5