Expert: Tom Whiting - 4/9/2005

Question
If there was a large comet or asteroid on a collision coarse with earth, would we know before it hit? Coild astronomers tell were it will hit so they can evacuate the area?

Answer
Hi Jocelyn,
Sorry I didn't answer sooner but we've been down at
Cherry Springs State Park observing all weekend, as the
weather was so clear and nice, and I even stayed over
a couple of extra days, and just got home this afternoon.

But to your question.....Perhaps, and perhaps not.
It all depends on... from which direction it is coming.....if it's coming in from the sunward side, like the last one, the Tunguska  Event of June 30, 1908 over Siberia...then no, we won't see it at all before impact.  They didn't see that one
coming in.
But that approach course is pretty rare.  From any other direction other than in the direction of the sun, then yes, we would know ahead of time....and the time before impact would vary depending upon how bright the object was and how soon it was detected, and how fast it is moving relative
to the Earth.

Yes, once the orbit is determined, (takes 3 individual sightings to triangulate an orbit)  then astronomers could
tell almost exactly where the impact point is, + or - 5 miles,  just like we put down Rovers on Mars within a 10 mile circle of the spot that they were shooting for, and even on Titan
they put the spacecraft down within 10 miles of desired
point of landing.

The odds on a water impact is 7 in 10  as 3/4 of the world
is ocean covered.  Evacuation is only an option if the body
is small, say 1 mile in diameter or less.  You'd have to
evacuate the coast lines bordering the impact.
Greater than 3 or 4 miles in diameter, no evacuation is
necessary because it would cause a nearly world-wide
mass extinction of life anyway- a global disaster  - perhaps 70% of the lifeforms on this planet would 'bite the dust' (no pun intended) similar to the Cretaceous Event 62 million years ago.   (The so called 'dinosaur' extinction).

Realize though....the odds depend strictly on how big
an object you are considering....we get pelted 24/7 with
dust, golf-ball sized meteoroids, all the time.
Even baseball sized meteorids perhaps once a day....we call them meteors and fireballs...we see them all the time.
Even a basketball size perhaps once a month; perhaps a
car-sized piece of space junk once a year.  But none of these
are dangerous, as they mostly burn up, or hit and make a
thud or crater 3 feet in diameter.
The bigger you go up from there, the odds really change
exponentially, fortunately....they figure a 100 yard (football sized) rock is like once per century, and a 1 mile wide rock is like once every one million years.
A global disaster rock of 3-10 miles in diameter is like
once every 30-100 million years.
(So don't lose any sleep over it).

We, the Earth, is a fast moving, very small target to hit....
kind of like hitting a running rabbit from a Cessna light
aircraft in a full screaming dive, and you are hanging out
the door with a rifle, diving on the running rabbit at 200 mph, and you get only one shot per decade at hitting that rabbit under those adverse conditions....good luck!
(That's why we get a lot of near misses by bodies say,
1-10 feet in diameter).  The odds are you in that aircraft
are going to score a lot of near-misses before you can
score a hit.
But going back to the Earth, the Earth is moving at 18.5 miles
per second around the sun.....that means we move our
own diameter every 7.2 minutes.  So a body has to not only
be crossing the orbit (ecliptic) at the right time, that is be in
the right place (and remember, space is 3-dimensional)
but it would also have to be "on time" during that 7.2
minute period.  If it's either slightly inside or outside of
our orbit, then it would near-miss either inside or outside
of the Earth.....and if its ahead or behind those 7.2 minutes,
then it either would pass across our orbit ahead of the
Earth or behind the Earth.  The bottom line being, we are
a very, very difficult, small fast-moving target to hit with a big object.
(So again, no sleep lost over it.)

Hope all this helps,
Clear Skies,
Tom Whiting
Erie,  PA