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Q: Asteroid and Comet Detection ( No Answer,   9 Comments )
Subject: Asteroid and Comet Detection
Category: Science > Astronomy
Asked by: hardy678-ga
List Price: $75.00
Posted: 20 Apr 2006 18:59 PDT
Expires: 28 Apr 2006 10:58 PDT
Question ID: 721147
Hello Google Researchers!
Subject: Asteroid Detection
I have read a couple sites about the latest technology in Asteroid and
Comet detection. I have learned that large asteroids are mapped and
tracked and none are going to hit the earth anytime soon, however I
also read that many smaller asteroids and comets go unmapped. I am
wondering that if a smaller asteroid that wasnt being tracked was
going to hit earth, would we have radar or some sort of sensor or
another way to realize an asteroid was coming our way or would it hit
undetected? I want to be sure that if an asteroid or comet was coming
for my town we would have time to evacuate. If you could, please tell
me more about the whole asteroid and comet detecting process, and what's the
difference between an asteroid and a comet? Can both be tracked? Also,
I am wondering if I should be worrying alot about getting hit by an
asteroid, I live in West Chester, PA. I am 16 years old, is it likely
an asteroid will hit in my lifetime. Also, with the 2012 end of the
world prophecies, aren't the ones about an asteroid hitting the earth
false since nobody has found any asteroids en route to earth? I am a
really big worryer and I just need some reassurance that I will be
alright, and that my nation will protect me. Please get back to me,
Thanks alot.

Clarification of Question by hardy678-ga on 26 Apr 2006 10:41 PDT
Also: are there any HUGE asteroids that go undetected and can hit
earth? Or would we definately see one of those big ones coming?

Clarification of Question by hardy678-ga on 27 Apr 2006 12:34 PDT
Also, what is a dark asteroid? Can a HUGE dark asteroid hit the earth
without being noticed before impact? Or can we still see them?
There is no answer at this time.

Subject: Re: Asteroid and Comet Detection
From: myoarin-ga on 21 Apr 2006 14:11 PDT
The chances of something else upsetting your life are much, much
greater  - at your age, starting with having a driver's license. 
Worry about the things that are possibly within your control.  There
are just so many things that could happen that are  - will be - 
beyond your control and not even on your list of dangers.
Asteroids and comets are the least likely problem, and one that none
of us will be able to do anything about.

By the way, your question was locked for a while because "Google" was
in the text, not because a Researcher was pondering on it.

Regards, Myoarin
Subject: Re: Asteroid and Comet Detection
From: qed100-ga on 21 Apr 2006 19:51 PDT

   Ok: do objects collide with Earth? Yes. It's a common fact that
tiny bits of dust & rock intersect Earth all the time. Go out on any
clear night and you can see these objects plowing through the upper
atmosphere. Any old night of the year you can see several each hour.
During meteor showers you can see several per minute. They are of
course routinely vaporised in the act of passing, at high speed,
through the air. They are disintegrated into even smaller
constituents, and are no threat whatsoever. Do any make it to the
ground? Yes. Enough larger meteors make it to the ground that there is
an entire industry in the sale of meteorites to collectors.

   What about much larger objects, massive enough to make it to the
ground and do some real damage? Yes, there's plenty of evidence that
this happens. On a very small number of recorded occasions, meteorites
have damaged several peoples' cars, houses, livestock, and even one or
two humans. But there are no documented cases to my knowledge that
anyone in modern times has been killed by a direct hit.

   Very massive objects do occasionally smack into Earth. There are
many known impact craters on this planet. I personally live less than
a hundred miles from an eight-mile diameter crater (Kentland), though
it's millions of years old, and the only part still above ground is
the central peak.  The most famous one is Barringer Crater, near
Winslow, Arizona. It's in pristine condition, it's magnificent, and
you ought to visit it sometime. It's only a few tens of thousands of
years old. From known properties of asteroids, their typical
velocities, etc., it's estimated that the asteroid which blasted out
this crater was about big enough to fill a football field.

   The known craters on Earth are all older than recorded human
civilisation. What about more recent times? Well, at least once in the
last century (1908) an impact occurred which wreaked incredible
destruction. It happened over the Tunguska region of Siberia, and when
it happened it created a blindingly bright fireball and fell pine
trees in all directions for a radius of many miles. Those dead trees
are still there, laying on their sides to this day. Incidentally,
evidence tends to point to this impact being with a small chunk of
comet, rather than an asteroid-type object.

   Incidentally, evidence of asteroid impacts isn't just found on
Earth. The Moon is very plainly pocked up by millions of craters. Many
planetary objects have been photographed up close by space probes. All
rocky bodies which have been visited are seen to carry impact craters.
Even the asteroids themselves have impact craters from collisions with
other asteroids.

   One interesting planetary collision was just a few years ago,
within your lifetime. In the summer of 1994 comet Shoemaker-Lavy 9
crashed into the planet Jupiter. It was the first such powerful
collision to be predicted ahead of time & directly observed by
astronomers. The comet blew a hole in the planet's cloudy atmosphere
so large that Earth could have fit into it. Had that comet impacted
our planet, it might conceivably have wiped out all human life.

   Alright, it's a fact that planetary collisions occur. But what is
the threat, the risk, to you in Pennsylvania? The likelihood of you
being killed, or even slightly hurt, by an incoming space object is
very small. For one thing, the only ones which even *can* hit you, or
your town, need to be not less than a certain size, otherwise they
burn to bits in the upper atmosphere. But- the bigger the object, the
more rare it is. Earth may be hit by many thousands of bits of dust
each day, each of which gets destroyed, but only a handful of rocks
the size of your fist may make it to the ground. And it's empirically
obvious that really destructive impacts hardly ever happen, since we'd
know about it if a crater the size of the Pentagon were to be created.
Impacts with such large objects are so rare that they practically
never happen as far as any one person chosen at random is concerned.

   The bigger the rocks, the fewer there are of them, and the less
likely it is for it to coincidentally find itself where Earth also
happens to be. They are few & far between. The Tunguska event was the
only devastating impact in modern history. The Shoemaker-Levy 9 impact
was a major opportunity for scientists exactly because it was a once
in many lifetimes event.

   Also, when there is an impact, it's not most likely to happen near
to any given city or town. Why is this? It's because cities occupy
only a very, very tiny fraction of Earth's total surface area. An
impact could occur at any spot on the globe. But with most of this
planet's surface being uninhabited (especially out in the oceans,
which amount to more than half of Earth's exposed surface) it's
proportionately more probable that wilderness will be struck by a
space object than any people-filled region. (All those movies with
asteroids crashing into Earth are funny. They have it that just about
the *only* places which get hit from space are the busiest cities.)
Again regarding the Tunguska event: it happened in one of the most
poorly inhabited (by humans) wildernesses on this planet.

   But what about all those zillions of overlapping craters on the
Moon? Don't they show that impacts are likely? Well, for one thing,
the Moon gets clobbered by a broader variety of sized objects because
it has no significant atmosphere. Another thing is that, for the very
same reason (no atmosphere, no weather), the craters on the Moon stay
in like-new condition for a lot longer than on Earth. What I'm getting
at here is that nearly all the visible craters on the Moon are
ancient. They are leftover from an earlier age, billions of years ago,
when there was a lot more loose asteroidal junk orbiting about the
solar system. That junk is comparatively scarce nowadays; it's all
been "eaten" by the larger planetary objects by way of impacts.

   But what about near passes by small asteroids? Aren't the reports
of such transients on the rise? After all, years ago they were never
seen to pass so close to Earth. But nowadays they are found rather
frequently. Does this mean that the incidence of near misses is
elevating? No. What this means is that, until just a few years back,
people had neither the means nor the inclination to survey the
asteroid population to such a degree as to even uncover all these
small, grazing objects. Probably Earth has been in close company with
such near objects all along. It's only at this time in history that
they've finally been revealed. (And I must stress, that even though
we've very likely been accompanied by these rocks for eons, we haven't
also been frequently clobbered by them.)

   In short, it's vastly more likely for you to be hurt in a car
accident than by a meteorite impact. Yet, you've no plans to abandon
riding in cars, do you?

   As for the objects in question, you asked, "what's the difference
between an asteroid and a comet?" Their differences lie in their
orbits and their compositions (what they are made of). I think it's a
prudent idea to simply point you to a good website. For instance this
one is a fine start:

It'll give you a good overview of the major objects in our solar
system, including asteroids & comets.

   You also asked, "Can both be tracked?" Well, yes, of course. All
bodies are subject to the same principles of motion. They are all
masses which have velocity, inertia, and which gravitate toward each
other. The laws of planetary motion are well understood, and once a
few key parameters have been measured for an orbiting object, it's
future travels can be rather precisely calculated for many years into
the future. And of course, these objects can be kept under surveilance
directly, with regular telescopic sightings.

   Now, even though I advise you to worry about anything but asteroids
& comets, does this mean that it's a big waste of time and money to
count them and keep track of them? No, it's not at all a wasteful
thing to do. It's relatively inexpensive for a modern society to
maintain such a program. It can be done with just a few people, and
not with the very largest of telescopes. And even though devastating
impacts are rare, once such an event does happen, it could be one of
the worst things ever to happen to human kind.

   I think it's wise to have an ongoing program of asteroid
surveilance. This is the first age in human history that we've had the
ability to actually do something to evade such a potential
catastrophe. Prior to just a few decades ago there was no rocket
travel. Now space travel is well enough established that it's possible
to be able to visit asteroids. (In fact, small probes already have
done just that several times.) If the orbit of an asteroid were to be
determined to be on a collision-course with Earth, we as a
civilisation might do something proactive to alter that fact. Of
course, this could only be accomplished with enough advanced warning,
presumably at least ten  or twenty years. That's the main purpose of
asteroid surveys; to provide enough years of warning to give us the
needed time.
Subject: Re: Asteroid and Comet Detection
From: myoarin-ga on 22 Apr 2006 03:38 PDT
Quod est demonstrandum!   Great answer!
Subject: Re: Asteroid and Comet Detection
From: neilzero-ga on 22 Apr 2006 17:29 PDT
The asteroids that have orbits slightly shorter than Earth's orbit,
but otherwise about the same as Earth's orbit are most likely to hit
with minutes instead of days warning. Worse you probably won't be told
where it will hit until seconds before impact. We think there are
several hundred of this kind, big enough to total a city. Several of
them will likely hit Earth sometime in the coming thousand years, but
they will likely hit in the ocean, or a thinly populated location, so
there are about 10,000 ways you are more likely to be killed or
To find these sneeky asteroids we need a telescope something like The
Hubble space telescope, but in solar orbit similar to these asteroids.
Such a telescope would also help find comets, which typically have
long orbits that reach farther than Mars, but come closer than Earth
(is to the Sun) for days or months. The asteroid hazzard is thought to
be about ten times greater than the comet hazzard.   Neil
Subject: Re: Asteroid and Comet Detection
From: thither-ga on 22 Apr 2006 19:23 PDT
I would preface my comments with:

"God, grant me the serenity to accept the things I cannot change, the
courage to change the things I can, and the wisdom to know the

It's good advice (regardless if you are religious or not).

Here are the statistics:

"Scientists develop asteroid risk statistics by estimating the total
number of objects that exist and by studying evidence of past
encounters -- big holes in the ground called impact craters.
From these clues, they say your chances of death by asteroid are about
the same as dying in a plane crash, roughly 1-in-20,000 during your
lifetime. You're more liable to be electrocuted to death (1-in-5000
chance), succumb to skin cancer or be killed in a car crash.
Yet asteroids pose more risk than tornadoes (1-in-60,000 chance),
rattlesnake bites or food poisoning."

"the average American stands about a 1-in-100 or 1-in-200 chance of
dying in an automobile."

I can live with those (asteroid) odds. If you can't, maybe you should
become involved:

"Today, astronomers scan the skies in both hemispheres looking for
rogue asteroids and comets. The fact that these efforts are woefully
under-funded -- and that some rely on private funding -- says how
little the bean-counters in governments appreciate the value of this

Perhaps you have found your calling.

I would also want to add that splashdown in ocean might not be an
acceptable result:

"When the possible consequences of asteroid impacts on Earth are
discussed, people seem to be comforted by the fact that two thirds of
the planet?s surface is ocean. In fact, we should worry more: an ocean
impact can multiply damage by triggering the mother of all tsunamis."

And finally, if you really want to freak yourself out, reading the
opening chapters of Arthur C. Clarke's Rendezvous with Rama.
But then finish the book - you will replace your fear with wonderment.

Have a good day.
Subject: Re: Asteroid and Comet Detection
From: brix24-ga on 25 Apr 2006 13:46 PDT

You will notice the generally reassuring tone of the comments. One
number that surprised me, though, was ?they say your chances of death
by asteroid are about the same as dying in a plane crash, roughly
1-in-20,000 during your lifetime.? It surprised me because there have
been many deaths from airplane crashes in the last 50 years but no
documented case of a death caused by an asteroid or comet. I suspect
that the people quoted are multiplying numbers of deaths (say, a
couple of billion) from a very rare, but large asteroid impact to get
part of the risk.

You asked about the asteroid and comet-detecting process. This is done
at a number of sites in the northern hemisphere using optical
telescopes and automatic detection of movement of faint objects. Radar
telescopes seem to be more useful in determining an orbit more exactly
once an asteroid has been detected by an optical observation.

The following site indicates that NASA is using optical telescopes for
detection and a radar telescope to better define an orbit once a ?near
earth asteroid? is detected:

The following site describes the method used to detect near earth objects:

As you know, the goal of the current programs is to discover 90% of
the asteroids that pose a global hazard by the end of 2008.  As of
April 23, 2006, 832 of an estimated 1000 - 1100 of these have been

?As of April 23, 2006, 4030 Near-Earth objects have been discovered.
832 of these NEOs are asteroids with a diameter of approximately 1
kilometer or larger. Also, 768 of these NEOs have been classified as
Potentially Hazardous Asteroids (PHAs).?

Your concern is about the asteroids smaller than 1 km (100 m to 1 km),
but which can still cause tremendous local damage if they hit the
earth. The current program detects some of these; the numbers just
quoted show that 3198 of the 4030 near-earth objects already found are
less than 1 km in size (though there are still many more to be

NASA has a site that shows the recent and upcoming closest approaches
by asteroids; you might note the sizes of the asteroids listed and
their closest approaches in terms of lunar distance.

For the sets listed today, 4/25/06, the minimum closest approach of an
asteroid was twice as far away as the moon. Occasionally, though, a
tracked asteroid will come closer; there is mention of a really close
one in an NPR interview with the NASA scientist, Donald Yeomans:

and there is a graph of a projected close approach by a tracked asteroid in 2029:

If you scroll down to the third color figure at
you will see that the number of >1km near-earth objects discovered is
starting to level off, but the number of smaller ones is still
increasing rapidly.

In the NPR interview, Donald Yeomans mentions a proposal to track the
smaller asteroids that you are concerned about.

This proposal seems to be part of a House bill that was passed by both
houses of Congress on December 21, 2005; however, I don?t know enough
law to be sure that this program has actually been approved (or how
subject to change it might be). You can find this information near the
end of a long site
at the point beginning, ?HOUSE NASA AUTHORIZATION BILL (DECEMBER 17,
2005) Following is NEO language from the House NASA Authorization
Bill, which is presumably consistent with the final NASA Authorization
Bill passed by both Houses of Congress on December 21:
(d) Near-Earth Object Survey-
SURVEY PROGRAM- The Administrator shall plan, develop, and implement a
Near-Earth Object Survey program to detect, track, catalogue, and
characterize the physical characteristics of near-Earth objects equal
to or greater than 100 [apparently changed to 140] meters in diameter
in order to assess the threat of such near-Earth objects to the Earth.
It shall be the goal of the Survey program to achieve 90 percent
completion of its near-Earth object catalogue (based on statistically
predicted populations of near-Earth objects) within 15 years after the
date of enactment of this Act.?

Earlier in the same this site, there is a news release with an
interview of Don Yeomans that discusses both asteroids and comets. It
MSNBC, 1 November 2005.?
?Well, the big ones, the ones that cause global problems, are the
easiest ones to find. And that is NASA's goal now, to find the ones
that are larger than a kilometer. We're doing well, actually: We're up
to 807 out of a total population of about 1,100 asteroids of that
Once we find them, they're no longer a problem, because we can track
them for 100 or 200 years in the future, and keep an eye on them. None
of them are a problem, but it's the smaller ones - and there are
hundreds of thousands of them that we haven't been able to deal with
yet, because we don't have the telescope apertures yet.?


?You mentioned the comets. Now, they're the wild cards. The bad news
is, if they come in from the outer solar system, they don't start
outgassing until they get well inside the orbit of Jupiter, and thus
don't show themselves. It only takes 9 months to get from Jupiter's
orbit to Earth's orbit. So that's the bad news.
The good news is, in terms of numbers, there are 100 times more
asteroids than comets in the near-Earth space. So asteroids are really
the major problem, and we can deal with them. Comets are 100 times
less of a problem, in terms of numbers, and we can't really deal with
them anyway at the moment. So we're concentrating on the asteroids at
the moment.
It would take a lot more highly developed technologies to discover
comets early enough to do something about them. We'd have to have
sensors out beyond the asteroid belt, near Jupiter's orbit, looking
for these things. And even if we found them, the orbit determination
at that distance would be so poor, we couldn't predict precisely
whether they would be a threat until they got in a lot closer anyway.
So at the moment there's no clear path for dealing with long-period
comets. But they're a 1 percent problem, compared to the 99 percent
problem of the asteroids.?

Other information:
In ?Workshop on Near Earth Objects: Risks, Policies and Actions
January 20-22, 2003,? there is a concise table listing damage expected
to be associated with asteroids or comets according to their size. In
addition, there is an estimate of the probability of occurrence in
this century.

Note that you should compare the area affected with the total area of
the earth if you want to estimate the probability that an event would
affect you directly.

Some general information on comets and asteroids is in this 1998 article:

There, the damage area of the asteroid/comet damage in Siberia in 1908
is given as ?It flattened about 700 square miles of trees.? Although
this is a large area when we think of a person?s immediate
environment, it is 3.6 millionth of the surface area of the earth, so
a repeat event is unlikely to affect you directly. (Earth?s area is
from the CIA factbook: and the
Google search bar was used to convert 700 sq mi to sq km: 700 square
miles = ? square km).

NASA also answered a generic question on collisions with earth in 1998 and 1999:

Some information is known about the short-period comets; a 12/20/2005
Jet Propulsion Laboratory document covers 286 short-period comets and
has a table showing the 40 closest approaches to earth.

The definition of a short-period comet is somewhat variable. (Google
search strategy: define:short-period comet) One definition is ?Comet
with an orbital period around the Sun less than 20 years.? Another is
?"Comets that orbit mainly in the inner solar system. Usually these
objects orbit the Sun in less than 200 years. Halley?s comet is an
example of a short-period comet." I suspect the first definition has a
typo, but I?m not expert on that.

And, just for fun:

It took me a while to get used to navigating the site, specifically,
to use the tabs when they appeared.

My conclusion: you really don?t have to worry (but the others already
told you that).
Subject: Re: Asteroid and Comet Detection
From: brix24-ga on 26 Apr 2006 17:53 PDT
The larger the asteroid, the more light it reflects and the easier it
is to detect. The largest know near-earth asteroid is 1036 Ganymed. It
was discovered in 1924; such an early discover hints at the ease of
detecting the largest asteroids.

The larger the asteroid, the easier it is to detect and the more
warning we would have if one were on a collision course with the

For more general information, you might be interested in this site.

It has a number of questions you might be interested in:

Can radar be used to search for NEOs? 
(explains why radar isn't best for initial searching)

What will we do if we find an asteroid or comet on a collision course with Earth? 
(The lead-time mentioned probably refers to the larger near-earth asteroids.)

Would I be told? 

What is the largest asteroid?
(Note that this refers to an asteroid that stays between Mars and
Jupiter and is not a "near-earth asteroid." It (Ceres) poses no danger
to us. Consistent with ease of detectability with increasing size was 
the discovery of Ceres in 1801; note again that the large asteroids
are those more easily detected - in this case, this asteroid was
detected even though it doesn't come anywhere as near to us as the
near-earth asteroids.

How many NEOs are there? 
(Note that the number of asteroids drops dramatically as size
increases; there should be very few greater than 10 km if you continue
the progression mentioned.)

On a related note, testimony before Congress has this statement:
"Their numbers increase by a factor of about 220 for a diameter that
is reduced by a factor of 10."

Going in the other direction, the number of near-earth asteroids
should decrease by a factor of 220 for a diameter that is increased by
a factor of 10. Using the current estimate of about 1100 near-earth
asteroids with a size greater than 1 km, there should be (very
roughly) about 5 with a size greater than 10 km.

I have not readily found a table of sizes for all the near-earth
asteroids; most of the time, astronomers list the near-earth asteroids
in terms of their absolute magnitude, which I believe roughly reflects
their size, but also has contributions from their shape and their
ability to reflect light.
Subject: Re: Asteroid and Comet Detection
From: brix24-ga on 26 Apr 2006 18:27 PDT
Sorry, this was a typo:

"The largest know near-earth asteroid is 1036 Ganymed."

It should be

The largest known near-earth asteroid is 1036 Ganymed.

I also forgot to mention its size: 32 km.
Subject: Re: Asteroid and Comet Detection
From: brix24-ga on 27 Apr 2006 18:33 PDT
Dark asteroids can be, and are, detected. Asteroids are sometimes
classified as dark asteroids, medium asteroids, and light asteroids.
They differ in their albedo, or fraction of incident light that they
reflect. According to this somewhat technical document

dark asteroids reflect about 6% of the incident light, medium
asteroids reflect about 16%, and light asteroids reflect about 40%.
(Note that a dark asteroid with 3 times the diameter of a light
asteroid has 9 times the cross-section and will reflect more total
light than the light asteroid. However, I don't know enough about the
telescopes and detection devices to know if the area makes a
difference in detection.)

Further down, the article has this statement: 

"It is interesting to note that even for dark asteroids, any object
passing through Earth?s orbit at a reasonably accessible solar
elongation ? say greater than 60 degrees ? is readily detectable by
Pan-STARRS. Since this is a third of the circumference of Earth?s
orbit, and the objects have periods of   1 ? 2 year, this tells us
that Pan-STARRS will reach a high detection probability for such
objects after a few orbits."

This Pan-STARRS telescops is currently under construction with
progress pictures here:

Here is a statement on the Pan-STARRS capability:
"By reaching objects 100 times fainter than those currently observed
in the NEO surveys, Pan-STARRS should quickly help finish off the
Congressional mandate to find and determine orbits for the 1-km (and
larger) threatening NEOs. Further, we will be able to push the
detection limit for a complete (99%) sample down to objects as small
as 300 meters in diameter."

Here is a picture of light material under the surface of the dark
asteroid, Asteroid Eros (the second-largest near-earth asteroid) taken
in 2001.

Here are two additional Pan-STARRS sites with various other information:

Technical stuff:
The definition of albedo was gotten by typing


in the Google search bar.

The definition of dark, medium, and light asteroids was derived from
section 2 "H-magnitudes" of the very first article I listed. These
statements were used:
"and pV is the albedo."
"For a dark asteroid (PV ~= 0.06), the absolute magnitude for 300m
diameter is H ~= 21.28. In a
30s integration with a V +R filter, Pan-STARRS can detect a point
source with V ~= 24.3 at 5-sigma
(assuming FWHM=000.6).
This detection limit corresponds to  ?m ~= 3.0. Medium (PV ~= 0.16)
and light (PV ~= 0.40) albedo
class objects can be detected to  ?m ~= 4.1 and  ?m ~= 5.1 respectively."

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