PDA

View Full Version : The Perseids: Why is There a Meteor Shower?



Fraser
2011-Aug-11, 02:30 AM
Every year from late July to mid-August, the Earth encounters a trail of debris left behind from the tail of a comet named Swift-Tuttle. This isnít the only trail of debris the Earth encounters throughout the year, but it might be one of the most notorious as it is responsible for the annual Perseid meteor [...]

More... (http://www.universetoday.com/88094/the-perseids-why-is-there-a-meteor-shower/)

Perikles
2011-Aug-11, 07:34 AM
I can understand why there is a shower, but not why they all come from the direction of Perseus at a speed of 22,500 km/hr. Are all the dust particles describing more or less the same elliptical orbit as the comet? What is a typical velocity for such particles relative to the sun? If the earth is travelling at a speed of around 108,000 km/hr towards some point in the ecliptic, I find it hard to see how the velocity of dust particles relative to Earth is so small. Could somebody explain?

grapes
2011-Aug-11, 09:44 AM
I can understand why there is a shower, but not why they all come from the direction of Perseus at a speed of 22,500 km/hr. Are all the dust particles describing more or less the same elliptical orbit as the comet? What is a typical velocity for such particles relative to the sun? If the earth is travelling at a speed of around 108,000 km/hr towards some point in the ecliptic, I find it hard to see how the velocity of dust particles relative to Earth is so small. Could somebody explain?A "typical" velocity is 108,000 km/hr. :)

If such particles were coming from "behind" us, their velocity relative to the earth would be 0 km/hr, if they were coming right at us, their velocity relative to earth would be twice that, or 216,000 km/hr, right? Depending on their orientation, the relative velocity could be anything in between those two values.

Perikles
2011-Aug-11, 10:40 AM
A "typical" velocity is 108,000 km/hr. :)

If such particles were coming from "behind" us, their velocity relative to the earth would be 0 km/hr, if they were coming right at us, their velocity relative to earth would be twice that, or 216,000 km/hr, right? Depending on their orientation, the relative velocity could be anything in between those two values.Right. I'm just beginning to grasp the idea that the plane of the ellipse containing the dust particles lies at an acute angle to the plane of the Earths orbit, so that where they intersect and Earth is at that point, the particles colliding with Earth are seen to be moving from one direction, viz. Perseus. I don't know why I originally assumed the two planes were at right angles. Thanks.

grapes
2011-Aug-11, 12:22 PM
This list at wiki shows the variation in velocities:
http://en.wikipedia.org/wiki/List_of_meteor_showers

I remember the Leonids as being pretty fast, and it looks like they lead the list at 71 km/s, which is 256,000 km/hour. The slowest looks like 14 km/s, or a fifth of that.

Hmmm, it lists the Perseids as 59 km/s, which is 212,000 km/hour. Did someone drop a decimal point somewhere?

ETA: The UT article uses a range, "11 km/sec (25,000 mph), to 72 km/sec (160,000 mph)", so that's ballpark I guess.