johnnydeep wrote: ↑
Sat Aug 14, 2021 8:07 pm
Ok, so I haven't thought much about meteor showers before, but now have two questions:
1. The text mentions that this meteor was caused by a piece of dust travelling at 60 km/s through the rarefied upper atmosphere. A mere spec of dust - perhaps sand grain sized as suggested by the title - can really appear this spectacular? Also, how do we really know how big these particles are? I assume they range in size.
There is a reasonably reliable relationship between luminous flux and mass. So as a rule, if we have a photometric measurement we can estimate mass. And if we know what it is made of, we can go from mass to size. In fact, this meteor produced a persistent train, so it was almost certainly larger than a grain of sand. Probably more like the size of a marble.
2. Here comes my lack of 3D visualization skill again: Why do these always appear to radiate from a point in Perseus? Is it always the exact same point on the celestial sphere or is it moving over time? I would think that since the cause of these meteors is the Earth passing through the Sun-orbiting debris left behind after the passage of comet Swift-Tuttle, that the point where Earth crossed its path wouldn't remain fixed. Hmm, thinking some more, I guess orbits are
pretty fixed in space after all (except for slow precession over time - https://en.wikipedia.org/wiki/Apsidal_precession
) so that if two orbits cross once, they will continue to cross periodically for quite a while.
Indeed, the orbit of the particles for any given shower is quite stable. The particles are cleared from the orbit, either towards or away from the Sun, by various forces over a period of a few thousand years. If there is still an active comet in the orbit, the particles are replenished near each perihelion. But comets aren't active forever, either. So showers are short lived in astronomical terms- typically no more than a few thousand years.
The radiant is a vanishing point illusion. The meteors are all moving parallel to each other, coming from the direction of their radiant (actually, from an apparent direction that depends both on their actual vector and the vector of the Earth's motion). Think of the appearance of snow in the headlights as you drive down the road. Each snowflake is traveling in the same direction as all the other snowflakes, but you see them radiating outward from a point ahead of the car.