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HelenofBorg
2010-Jun-02, 09:17 PM
Here (http://en.wikipedia.org/wiki/99942_Apophis) is a wikipedia article about the asteroid Apophis. According to the information, it will come within the orbit of geosynchronous satellites around Earth. As the diagram below (from the website) shows below, it is well within the orbit of the moon, and will be very close.

http://upload.wikimedia.org/wikipedia/en/3/34/Apophis_pass.svg

I don't doubt for a second that the main bulk of the asteroid will miss Earth in 2029, just as the current scientific data says it will. That said, it's proximity is worrying because thinking about it I can't help but wonder if the Earth's gravity might pull away any loose chunks of rock on the asteroid which might be large enough to cause us harm, even if not large enough to be of apocalyptic proportions. I have not been able to find any data on this possibility so I don't know any more than the information I've linked to here, and my own speculation.

Does anyone else on this forum have any more information concerning any possible dangers associated with this asteroid, and if it does pose such a threat, is it possible to nudge Apophis out of the way?

Hungry4info
2010-Jun-02, 09:57 PM
Are we 100% certain that the asteroid Apophis is no threat to Earth? No. But in much the same respect, I am not 100% certain the Moon isn't either.

There are odds and probabilities about it. No one cay say with absolute certainty that the asteroid will not hit Earth. But it is extremely, extremely likely that it will miss the planet.

Go here (http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=99942) and click on "[Close-approach data]" near the bottom. A column states impact probability. As you might imagine, this asteroid has been tracked carefully so its orbital motion has been pinned down pretty well... well enough to have an impact probability so low that it may as well be 0 (as is stated in the column). The orbital elements are given with 1σ uncertainty, so you can plug that into Celestia and toy around with it all you like. The Wikipedia article you linked states:

As of October 7, 2009, refinements to the precovery images of Apophis by the University of Hawaii's Institute for Astronomy, the 90-inch Bok Telescope, and the Arecibo Observatory have generated a refined path that reduces the odds of a April 13, 2036 impact to about 1 in 250,000 1 in 250,000. That's a 0.0004% chance of impact. That was from Oct 2009 as well. The odds of impact generally drop sharply with time.

If Apophis had a non-negligable impact probability, the major world goverments would probably have considered doing something about it.

In summary, the odds of the asteroid hitting us are extremely low. There is no reasonable cause for alarm.

HelenofBorg
2010-Jun-02, 10:10 PM
The info you provided fits in well with what the wikipedia article says, that the asteroid will miss, as the impact probability column is zero throughout. I was more worried about Earth's gravity pulling any loose pieces of the asteroid away from it's main body which could hit Earth, not about the asteroid itself hitting Earth because the data says that the whole thing hitting us will not happen. That's what made me wonder if there were any plans in progress to divert it.

Swift
2010-Jun-02, 10:12 PM
HelenofBorg,

First, welcome to BAUT (funny user name).

Second, I'm not sure why this thead was in ATM (Against the Mainstream). You are asking a question, and this is the best forum for answers, so I've moved it to Q&A.

I see Hungry4Info is already supplying some answers

DrRocket
2010-Jun-02, 10:25 PM
I don't doubt for a second that the main bulk of the asteroid will miss Earth in 2029, just as the current scientific data says it will. That said, it's proximity is worrying because thinking about it I can't help but wonder if the Earth's gravity might pull away any loose chunks of rock on the asteroid which might be large enough to cause us harm, even if not large enough to be of apocalyptic proportions. I have not been able to find any data on this possibility so I don't know any more than the information I've linked to here, and my own speculation.


Except for extremely an extremely small gradient ("tidal forces) the Earth's gravitational attraction for the asteroid is essentially uniform, so it will not tend to pull it apart. So either the entire asteroid hits us or nothing does. Ditto for the moon.

HelenofBorg
2010-Jun-02, 11:21 PM
HelenofBorg,

First, welcome to BAUT (funny user name).

Second, I'm not sure why this thead was in ATM (Against the Mainstream). You are asking a question, and this is the best forum for answers, so I've moved it to Q&A.

I see Hungry4Info is already supplying some answers

I wasn't entirely sure which subforum to post into. I thought that because the theory that the asteroid could potentially cause harm even though it won't hit us might be a bit controversial so I thought it might belong in there but I wasn't totally sure. Thanks for moving it to the right location.

As to my username, my given name is Helen and I am a Star Trek fan. In Star Trek, when Captain Picard got assimilated by the Borg he became Locutus of Borg. I just thought that 'HelenofBorg' sounded cool, hence it stuck.

[/hijacking my own thread]

DrRocket, Thanks for your explanation, I didn't know whether asteroids were uniform or not, and obviously, for it to break up it would have to be NOT uniform, or at the very least have a weak spot. If Apophis is uniform then we have absolutely nothing to worry about, because like you said, all of it will hit us or none of it will, and the current data linked to by hungry4info says that it won't. It might be interesting to watch with a telescope though, I'm imagining it will look like a giant shooting star if it comes within range of the upper atmosphere. I'm tempted to buy a little telescope just for that event. although I might buy one anyway to look at the craters on the moon.

WayneFrancis
2010-Jun-03, 12:40 AM
As it has been pointed out nothing is 100% but I'm pretty sure I'll wake up ever morning and still be bound to the ground by the same gravitational law that I've experience for the last 39 years.

01101001
2010-Jun-03, 01:48 AM
Don't worry. The earth won't pull your feet off your body. Nor pull pieces off our cute little Apophis friend.

formulaterp
2010-Jun-03, 06:59 AM
Let's also keep in mind that Apophis isn't one of your typical Texas sized doomsday threats. Current estimates give it a diameter of just 270 meters. A small chunk of Apophis isn't something we really need to worry about.

DrRocket
2010-Jun-03, 07:45 AM
DrRocket, Thanks for your explanation, I didn't know whether asteroids were uniform or not, and obviously, for it to break up it would have to be NOT uniform, or at the very least have a weak spot. If Apophis is uniform then we have absolutely nothing to worry about, because like you said, all of it will hit us or none of it will, and the current data linked to by hungry4info says that it won't. It might be interesting to watch with a telescope though, I'm imagining it will look like a giant shooting star if it comes within range of the upper atmosphere. I'm tempted to buy a little telescope just for that event. although I might buy one anyway to look at the craters on the moon.

It is not that the asteroid is uniform. It is that the gravitational field is uniform.

Another way to look at it is that the asteroid is essentially in free fall.

Jeff Root
2010-Jun-03, 08:36 AM
I want to point out something that doesn't bear directly on the
question asked, but is closely related: The probability of Apophis
hitting Earth is uncertain primarily because of uncertainty in its
position and velocity when it was observed. A tiny error in the
measurements can give a large difference in its later position.
The second contributor is uncertainty in how its orbit will change
due to the gravitational pulls of other bodies, solar light pressure,
and so forth. This uncertainty is very small compared to the first,
and is largely due to uncertainty in the orbit: If you don't know the
orbit of Apophis precisely, you can't calculate precisely how much
the gravity of other planets will affect that orbit.

Apophis is most likely rocky. It is most likely not very strongly
held together. However, it would have to get very much closer
to Earth than the predicted distance in order for Earth's gravity
to pull it apart. If it did get that close, it would be pulled apart
in very much the same way comet Shoemaker-Levy 9 was
pulled apart by Jupiter's gravity in 1992. The pieces were in
orbit around Jupiter, and all twenty-one impacted Jupiter over
a five-day period in July 1994. That comet must have been
considerably larger than Apophis.

If Apophis is about 270 metres in diameter, it is large enough to
do significant damage when impacting Earth. If it got through
the atmosphere in one piece, it would make a crater much larger
than Meteor Crater in Arizona.

If Apophis were to pass close enough to the Earth for it to be
pulled apart, the pieces could not hit Earth until the next pass,
on a later orbit. Apohis is small enough that if it were broken
into many pieces before hitting Earth's atmosphere, each piece
could be too small to do much damage. The smallest would
burn up entering the atmosphere.

-- Jeff, in Minneapolis

CrazyJesse
2010-Jun-03, 08:57 AM
Awww man, if it takes out DirectTV, people are gonna riot, and that is a threat.

Hungry4info
2010-Jun-03, 12:06 PM
It is not that the asteroid is uniform. It is that the gravitational field is uniform.

Another way to look at it is that the asteroid is essentially in free fall.

Remember that the comet SL9 was tidally disrupted by Jupiter before it impacted. The gravitational field can be uniform, but since Fg gets exponentially higher as you approach a body, the planet-facing part of the asteroid will experience a higher gravitational attraction with the planet than the anti-facing part of the asteroid. The difference between the two increases as distance decreases. Naturally, it's a function of the mass of the planet, and the radius of the asteroid.

Jupiter's pretty massive and SL9 seems to have been larger than Apophis. The fact that Jupiter did tidally disrupt SL9 suggests that the comet was held together very poorly (If I recall, a tidal disruption was not expected). So unless Apophis is extremely fragile, or a very loose rubble pile, then I'm sure it won't be disrupted as it passes by Earth.

jfribrg
2010-Jun-03, 01:00 PM
One thing that has been implied in the above posts but not explictly stated is that if the asteroid does break up due to tidal forces from Earth, the pieces will all continue moving in essentially the same orbit. You won't get one piece hurling toward Earth while the rest of the pieces continue along the predicted line. To redirect a piece of the asteroid toward Earth would require a very large input of energy, and there isn't any energy source to do that ( the energy sources of the gravity of the Earth, Moon, Sun, etc. has already been taken into account in arriving at that 1:250000 probability. As already stated, the reason the probability isn't zero is that there is uncertainty in the measurements, but as more measurements are made this uncertainty diminishes. At some point, the probability will be known to be zero. I'm not sure when that might be.

antoniseb
2010-Jun-03, 04:43 PM
... Another way to look at it is that the asteroid is essentially in free fall.

I want to spell out for HelenOfBorg something implicit in DrRocket's message here. The Earth won't pull something onto itself from this asteroid as it zooms by. The asteroid and the rocks resting on it are all on the same trajectory zooming past the Earth. Now, IF Apophis were to pass within the Roche limit of the Earth, loose rocks would separate from it, and depart the Earth vicinity in slightly different orbits from the main rock... but it won't get that close.

joema
2010-Jun-03, 05:26 PM
Here (http://en.wikipedia.org/wiki/99942_Apophis)....I can't help but wonder if the Earth's gravity might pull away any loose chunks of rock on the asteroid which might be large enough to cause us harm...
As has already been explained, that's not how gravity works. For ages people thought gravity affected lighter and heavier objects differently. In his famous "Tower of Pisa" experiment, Galileo proved otherwise. Historians currently debate whether Galilieo actually did the experiment, but the result is valid nonethless and has been repeated many times, including on the moon: http://en.wikipedia.org/wiki/Galileo's_Leaning_Tower_of_Pisa_experiment

Whether falling objects on the earth's surface, on the moon, or in outer space, the same principle applies. So your concern about the earth's gravity pulling away a loose chunk of asteroid won't happen.

It's true at much closer distances tidal forces can puts stress on the object, but that doesn't pull chunks away from it. I doubt Apophis will be close enough for tidal forces to be a factor.

slang
2010-Jun-03, 10:56 PM
antoniseb mentions the Roche Limit (http://en.wikipedia.org/wiki/Roche_limit). That, and some of the links, is a great entry point to do some reading about how asteroids 'n stuff behave around bigger objects, like planets. If HelenofBorg looks anything like Seven of Nine, I'm a fan. :)

DrRocket
2010-Jun-04, 12:03 AM
I want to spell out for HelenOfBorg something implicit in DrRocket's message here. The Earth won't pull something onto itself from this asteroid as it zooms by. The asteroid and the rocks resting on it are all on the same trajectory zooming past the Earth. Now, IF Apophis were to pass within the Roche limit of the Earth, loose rocks would separate from it, and depart the Earth vicinity in slightly different orbits from the main rock... but it won't get that close.

Yes. Absolutely correct, and you get the point.

The important words are loose, slightly and IF within the Roche limit.

The Roche limit of the Earth is a bit under 6400 km, and if the asteroid comes that close the predictions are quite erroneous. The are satellites well outside that limit.

Even if the asteroid were to come withing the Roche limit the dispersion in trajectors of loose pieces woudl be fairly small. at least initially.

And it is important to recognize that the Roche limit is the point at which the tidal gradient of the Earth's gravity overcomes the self-gravity of the asteroid, which is pretty small. The point is that the Roche limit considers only gravitational forces within the asteroid, an no other cohesive forces. So "loose" means really loose, like dry sand rather than wet sand, or powder snow rather than packed snow.

Some others have noted differential light pressure, which does exist, and is also miniscule -- completely irrelevant except over much longer time spans that what applies to a near-term encounter with the Earth.

If one wants one can nitpick anything, as there are all sorts of negligible effects. No doubt someone will bring up quantum tunneling.

But the bottom line is either the predictions are wrong and Apophis will hit us, or they are right and it will zoom by, close but not close enough. There is no appreciable chance of a middle ground where some of it hits us and the rest passes by.

Jeff Root
2010-Jun-04, 06:44 AM
One of the main things that could and should be inferred from
my post above is that the uncertainty in the orbit is due to limited
observations, and that additional observations do not necessarily
change the predicted orbit, but may just reduce the uncertainty
in the prediction. With measurements over only a small part of
one or two orbits, the prediction might be that Apophis will be at
a particular location at a particular time, with an uncertainty of
plus or minus so many miles and so many minutes. With more
measurements over a larger portion of more different orbits,
prediction of the location and time might be unchanged, but the
uncertainties in that location and time are greatly reduced.
That can reduce the probability of collision from large to zero.

-- Jeff, in Minneapolis