Asteroid Collision

Brian Tung wrote:

Gavin Whittaker wrote:

In uk.sci.astronomy Gareth Slee writted:
: Million to One chance of a collision apparently.
: http://news.bbc.co.uk/1/hi/uk/3200019.stm

from which:

"The chances of a catastrophe are likely to become even slimmer once more
measurements of the asteroid's orbit have been made."

If this is true, shouldn't astronomers increase the number of
measurements? If we look REALLY hard, we might be able to push it into
Jupiter.


A possible interpretation: The current odds are 1 in a million, based
on very tentative measurements. Once better measurements are recorded,
it turns out that the odds will become either

1. One in a billion, since the reduced error in measurements are
now significantly less than the predicted "miss"; or

2. Around 99.9 percent, since the predicted "miss" is essentially
zero.

If we think there's a one in a million chance of outcome 2, and the
rest of the time, outcome 1 happens, then our best estimate of the odds
right now is

(999,999/1,000,000)*(1/1,000,000,000) + (1/1,000,000)*(999/1,000)

which is very close to one in a million. Note that it is very likely
that the odds of a collision will drop a lot, but that is balanced by
a tiny probability that it will become a virtual certainty.

I believe the proper way to calculate impact odds is cross-sectional
area of the earth divided by the cross-sectional area of the
error-bounded predicted location of the asteroid at closest approach,
adjusted for statistical density distribution within the error-bounded
predicted location.

lal_truckee wrote:
I believe the proper way to calculate impact odds is cross-sectional
area of the earth divided by the cross-sectional area of the
error-bounded predicted location of the asteroid at closest approach,
adjusted for statistical density distribution within the error-bounded
predicted location.

That's true, but a separate question. The original poster seemed to
be asking not how is the probability calculated, but why that probability
is "likely" to drop once better elements are obtained for the asteroid.
In particular, if it always drops, why isn't the initial probability
lower than it might be?

The answer is that it only drops *most* of the time. A tiny fraction of
the time, the Earth is still within an error's breadth of the recomputed
path, and since the error is smaller, the probability of impact is
correspondingly larger.

As an analogy, consider the rock thrown at my head. Initially, I only
know the point of impact to within, say, 10 m, and if I'm within that
10 m radius circle, there is a probability of impact with my head,
although that probability is small. Once the point of impact is known
to an accuracy of 1 m, the chances that my head is still in the circle
is small, so the probability of impact drops to near zero in 99 percent
of the cases. One percent of the time, though, my head is still within
the circle, and since the circle is smaller, the probability of impact
is larger.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

"Paul Schlyter" wrote in message
...
In article ,
Gavin Whittaker wrote:

In uk.sci.astronomy Gareth Slee writted:
: Million to One chance of a collision apparently.
: http://news.bbc.co.uk/1/hi/uk/3200019.stm

from which:

"The chances of a catastrophe are likely to become even slimmer once
more measurements of the asteroid's orbit have been made."

If this is true, shouldn't astronomers increase the number of
measurements?

WELL, WHAT DO YOU THINK THEY'RE DOING? Do you think they'll never
ever make another observation of this object?



If you re-read the post, you'll see that Gavin was making a joke.


Duh!


Don't you mean Doh?

In article ,
Stuart Mulligan wrote:

"Paul Schlyter" wrote in message
...
In article ,
Gavin Whittaker wrote:

In uk.sci.astronomy Gareth Slee writted:
: Million to One chance of a collision apparently.
: http://news.bbc.co.uk/1/hi/uk/3200019.stm

from which:

"The chances of a catastrophe are likely to become even slimmer once
more measurements of the asteroid's orbit have been made."

If this is true, shouldn't astronomers increase the number of
measurements?

WELL, WHAT DO YOU THINK THEY'RE DOING? Do you think they'll never
ever make another observation of this object?

If you re-read the post, you'll see that Gavin was making a joke.

I did, but I failed to see the smiley indicating it was a joke.... :-)


Duh!

Don't you mean Doh?

Actually, I don't know the difference between them (remember that
English is not my native language). Care to elaborate a bit?

--
----------------------------------------------------------------
Paul Schlyter, Grev Turegatan 40, SE-114 38 Stockholm, SWEDEN
e-mail: pausch at stockholm dot bostream dot se
WWW: http://www.stjarnhimlen.se/
http://home.tiscali.se/pausch/

In article , Brian Tung wrote:

OK, I'm going to do the straight line on this one.

Paul Schlyter wrote:
I did, but I failed to see the smiley indicating it was a joke.... :-)

No smiley, but it seemed pretty obvious it was a joke, even without the
smiley. Lots of people have written subtle jokes in books that don't
have a single smiley in them.

I know -- but there's a risk in doing so, since the absence of the
smiley will make a number of people take your joke seriously. IRL
that would be less likely to happen, because the mimics of your face
and/or the tone of your voice would indicate it was a joke. But that
extra information gets lost in electronic communication.


Duh!

Don't you mean Doh?

Actually, I don't know the difference between them (remember that
English is not my native language). Care to elaborate a bit?

"Duh" is a
..........................
"Doh" is a
..........................

Thanks for your explanation!


--
----------------------------------------------------------------
Paul Schlyter, Grev Turegatan 40, SE-114 38 Stockholm, SWEDEN
e-mail: pausch at stockholm dot bostream dot se
WWW: http://www.stjarnhimlen.se/
http://home.tiscali.se/pausch/

"Paul Schlyter" wrote in message
...
the absence of the
smiley will make a number of people take your joke seriously. IRL
that would be less likely to happen, because the mimics of your face
and/or the tone of your voice would indicate it was a joke. But that
extra information gets lost in electronic communication.

Smilies have only been around a few years, the printed or scribed book (with
no face or tone of voice) has been around for thousands of years.

So why get excited about it now?

"Brian Tung" wrote in message
...
OK, I'm going to do the straight line on this one.

Paul Schlyter wrote:
I did, but I failed to see the smiley indicating it was a joke.... :-)

No smiley, but it seemed pretty obvious it was a joke, even without the
smiley. Lots of people have written subtle jokes in books that don't
have a single smiley in them.

Duh!

Don't you mean Doh?

Actually, I don't know the difference between them (remember that
English is not my native language). Care to elaborate a bit?

"Duh" is a slang expression indicating, in contexts like the above,
something that it so obvious that it goes without saying (and yet
someone went ahead and said it anyway). It dates from no later than
the 1940s and may have had its origin in cartoon blockheads who said
"duh" when pausing in putative thought.

"Doh" is a slang expression indicating that the speaker (or someone
the speaker is referring to)

Isn't "Doh" an expression used by the speaker only in reference to himself?
Thats how I understood (and meant) it anyway.

In uk.sci.astronomy Paul Schlyter writted:
: In article ,
: Stuart Mulligan wrote:

: If you re-read the post, you'll see that Gavin was making a joke.
:
: I did, but I failed to see the smiley indicating it was a joke.... :-)

You have a point, it was my omission.
Coincidentally, we have all been given small cards at work this week
with smileys on, for this very reason. It's a real pain having to pass it
round when you make a joke in a large group, but it saves anyone
misunderstanding.
I gather Billy Connolly now uses an industrial-sized one that was made in
the Clyde shipyards for live performaces.

: Duh!
:
: Don't you mean Doh?

...a deer, a femail deer...


Gavin

Stuart Mulligan wrote:
"Doh" is a slang expression indicating that the speaker (or someone
the speaker is referring to)

Isn't "Doh" an expression used by the speaker only in reference to himself?
Thats how I understood (and meant) it anyway.

Initially, I suppose. However, it's now common (in our parts, anyway)
for someone to "comment" on a companion's boneheaded move by calling out
"Doh!":

Fred: "I just accidentally sent my e-mail to a spammer's mailing list."

Barney: "Doh! Major, major doh!"

Mo.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

In article , Henry wrote:
"Paul Schlyter" wrote in message

"Paul Schlyter" wrote in message
...
the absence of the
smiley will make a number of people take your joke seriously. IRL
that would be less likely to happen, because the mimics of your face
and/or the tone of your voice would indicate it was a joke. But that
extra information gets lost in electronic communication.

Smilies have only been around a few years, the printed or scribed book (with
no face or tone of voice) has been around for thousands of years.

So why get excited about it now?

A book is a monologue -- usenet is a dialogue.

A book is also almost always much longer than a usenet post. By reading
a book, you get familiar with the author's writing style. Reading a
usenet post is not enough to get familiar with someone's writing style.

Finally, a book is usually more carefully written than a usenet post.
And that's because there's a review process in publishing a book.
Your publisher won't let you publish any garbage -- but usenet will
allow you to post any garbage.

--
----------------------------------------------------------------
Paul Schlyter, Grev Turegatan 40, SE-114 38 Stockholm, SWEDEN
e-mail: pausch at stockholm dot bostream dot se
WWW: http://www.stjarnhimlen.se/
http://home.tiscali.se/pausch/