by Keldor314 » Sun Dec 03, 2006 3:51 pm
Nereid wrote:lewishb wrote:why cant our telescopes use the near earth objects to obstruct the light from a star so we can see its planets
![Question :?:](./images/smilies/icon_question.gif)
Thanks, Lewis
In principle, they can.
In practice, it won't work.
Why? Either because the NEOs move too quickly or the exoplanets are too faint (or both).
In more detail: the time you would have to image (or detect) an exoplanet is no more than the time the NEO is occulting (blocking) the star's disc AND not blocking the planet. Taking just the first part: assume (for now) that the star's disc is a point; then the time available is the time it takes the NEO to travel the chord the star makes ... which depends on the NEO's apparent size (in arc seconds, say) and its speed (in arcsecs/sec, say).
AFAIK, most NEOs are quite small, and, being close, move quite quickly across the sky .... so the max time available is no more than a minute, and may be as little as a second.
An exoplanet will have an apparent magnitude of what, 15? 25? And the NEO will have an apparent magnitude of what, 10?
So we would be trying to detect an object at least 5 mags fainter than the NEO, in a window of less than a minute (and maybe only a second) ...
Imagine a rock the size of your car whizzing past you at 15 miles per second. As it goes past, it happens to occlude the star in question. Now, the time it occludes the star is the length of time it takes the car sized rock to move from where the "front bumper" first covers the star until where the "back bumper" reveals it again. This works out to simply the width of the rock, say 15 feet. Now, if its moving at 15 miles per second, and it blocks the star over a distance of 15 feet, that works out to 1/5280 of a second, a rather short time for a deep exposure.
Now, even the largest NEOs are only a few miles across at most, so even then they will only occlude the star for less than a second.
It is an interesting approach though - you may even be able to get something out of it if you used something larger and more distant (like Pluto, for example) as your occluder.
[quote="Nereid"][quote="lewishb"]why cant our telescopes use the near earth objects to obstruct the light from a star so we can see its planets :?: Thanks, Lewis[/quote]
In principle, they can.
In practice, it won't work.
Why? Either because the NEOs move too quickly or the exoplanets are too faint (or both).
In more detail: the time you would have to image (or detect) an exoplanet is no more than the time the NEO is occulting (blocking) the star's disc AND not blocking the planet. Taking just the first part: assume (for now) that the star's disc is a point; then the time available is the time it takes the NEO to travel the chord the star makes ... which depends on the NEO's apparent size (in arc seconds, say) and its speed (in arcsecs/sec, say).
AFAIK, most NEOs are quite small, and, being close, move quite quickly across the sky .... so the max time available is no more than a minute, and may be as little as a second.
An exoplanet will have an apparent magnitude of what, 15? 25? And the NEO will have an apparent magnitude of what, 10?
So we would be trying to detect an object at least 5 mags fainter than the NEO, in a window of less than a minute (and maybe only a second) ...[/quote]
Imagine a rock the size of your car whizzing past you at 15 miles per second. As it goes past, it happens to occlude the star in question. Now, the time it occludes the star is the length of time it takes the car sized rock to move from where the "front bumper" first covers the star until where the "back bumper" reveals it again. This works out to simply the width of the rock, say 15 feet. Now, if its moving at 15 miles per second, and it blocks the star over a distance of 15 feet, that works out to 1/5280 of a second, a rather short time for a deep exposure.
Now, even the largest NEOs are only a few miles across at most, so even then they will only occlude the star for less than a second.
It is an interesting approach though - you may even be able to get something out of it if you used something larger and more distant (like Pluto, for example) as your occluder.