SUMMARY: Researchers from the University of Michigan are predicting that when asteroid 99942 Apophis (2004 MN4) swings past the Earth in 2029, it will get so close that astronomers should learn a tremendous amount about how the Earth's gravity can shift asteroid orbits. The researchers are hoping that a space agency will put instruments on the surface of the asteroid to measure seismic data, similar to the way seismologists use earthquakes to probe the Earth's interior.

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First, it is difficult to overemphasize how close Apophis will come to Earth during 2029. For example it will pass between many communication satellites and the Earth, causing short outages. It may even be necessary to move some satellites in Molyna, sun synchronous, or synchronous equatorial orbits to avoid collision. (Or more likely to maneuver some obsolete satellites into collision orbits as a way of getting them out of orbit. ;-)

However, what I wanted to talk about is that something will need to be done about Apophis, if not in 2029, later. There is a big difference between the low-probability that Apophis (2004 MN4) will hit Earth sometime in this century--which is very low--and the total probability that Apophis will eventually collide with the Earth, which is very high.

Once Apophis has its very near encounter with Earth in 2029, it will keep returning to virtually the same spot in its orbit at nearly the same time as Earth. Eventually this will result in a collision with the Earth or the moon, unless Apophis gets into an orbit where it will pass close to some other planet--relatively unlikely.

Of course, if the human race intervenes, they can favor the collision with the moon, or better move it to a stable orbit around the Earth. How difficult would this be? Not very. The cheapest approach in terms of applied delta V would be to cause Apopsis to brush the top of the atmosphere losing just enough velocity to trap Apophis in the Earth-Moon a close pass to the Moon could then put it on course for L4 or L5. It would probably take more final delta V to guarantee it stayed there that would be provided by colliding with the dust currently trapped by the stable Lagrange points, but the total externally provided delta V should be in the hundreds of feet per second range. Difficult, but doable, especially with decades to prepare. And as explained above some of this delta V could come from collisions with satellites in Earth orbit, either obsolete satellites, or truckloads of material put in orbit especially for the purpose. One advantage of the data that can be collected in 2029, is that it will show how sturdy Apophis is, so that the size of colliding objects can be kept to those that won't cause fracturing. (A tank of water may be the best choice for this. You get not only the merger of the two velocity vectors, there will be additional thrust as the water boils out of the resulting crater.)

There is an even wilder seeming idea. Build a beanstalk cable on Apophis, and release it on one of Apophis' near misses of Earth. You have to have great faith in weather predictions and engineering skills, but if you do, the free end of the cable can be fastened to say a large float, and once the system stabilizes you have a beanstalk from someplace in say the Atlantic Ocean out to Apophis a few Earth diameters out.

Has anyone noticed that the near-miss (which isn't a certain thing, by the way; there is a 1 in 30 probability of impact) will occur on friday the 13th?

It will not be Good Friday, by the way, at least not according to the western method of computing Easter. I checked. But it might very well be exactly 2000 years after the event commemorated on Good Friday.

In this connection I should perhaps mention that according to our present (Gregorian) calender, all weekdays start to fall on the same dates again after 400 years. January 1st, 1601 was a monday, and so was january 1st, 2001.

And the Gregorian calender would of course have been in step with the Julian calender during Roman times.

Well, that is interesting, and I was always told AD meant `In the year of our lord,' and that it logically started the day Jesus was born. Its not much of a leap of interpretation to change that around to starting at the date of the resurrection I guess, which you can be sure any and all millennialists will soon be doing.

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"All truth passes through three stages. First, it is ridiculed, second it is violently opposed, and third, it is accepted as self-evident." Arthur Schopenhauer - renowned 19th Century German philosopher.

Quite true; A.D. or Anno Domini means In the year of the Lord, i.e. the number of years after the Lord's birth, not His death and/or resurrection. Minus one, of course; someone in his 25th year, say, has yet to reach his 25th birthday.

By the way, there is some uncertainty whether the A.D. count of years is correct. Officially, we are now living in 2005 A.D., but actually it might be 2009 A.D. or even somewhat later. This has to do with the death of king Herod, which could not have taken place before 1 A.D. Therefore I prefer to write 2005 C.E. or Common Era, meaning the count of years which is actually being used.

So, an asteroid due to hit or miss the Earth of friday the 13th of april 2029 C.E. might actually approach us on friday the 13th of april 2033 A.D.

Interestingly, it's closest approach to the Earth will be right above Western Europe. If it were to hit after all, it would wipe out the European Union, destroying the most secularized part of the world, and leaving a world whose politics would be dominated by the U.S.A. And perhaps a world where it would be feasible to set up some kind of "millennial kingdom".

I can't help wondering how long this asteroid has been in its present orbit. Orbits are usually computed into the future, not into the past, but if one were to do so, would one find that this asteroid should have hit the Earth in the eighties, and in the fifties, and in the twenties, over and over again?

They say there's a one in 45,000 chance of a collision in 2036, however, are they taking into account gravitational nudges from the Earth in its close flyby in 2029?

How it comes out of the flyby in 2029 will go a long way in determining whether it will hit in 2036.

How big was the rock that created Meteor Crater in Arizona?

How big will Apophis appear in the night sky at closest passage? About a tenth the diameter of the moon?

Barringer Crater's meteor estimate: It was 150 feet across, weighed roughly 300,000 tons, and was traveling at a speed of 28,600 miles per hour (12 kilometers per second) according to the most recent research.

Apophis will appear about 2 arcseconds and will still appear pointlike unless magnified. The Moon is about 30 arcseconds.

Yes. If Earth didn't alter its path in 2029, it would miss in 2036. The one-in-45,000 chance is the odds of hitting the crucial spot in 2029 that leads to a 2036 impact.

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Shouldn't that be arcminutes? For both the Moon and Apophis?
Two arcminutes would be twice the angular diameter of Venus at its
closest to Earth, I believe, so an obvious dot when fully illuminated,
rather than a starlike point.

-- Jeff, in Minneapolis

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Definitely for the Moon. (I must avoid trying to use metric degrees...)

I'll stick with approximately 2 arcseconds for Apophis. While it will be close it won't be that close, and its diameter is tiny, hundreds of meters, while the moon is thousands of kilometers.

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it will pass within the orbit of Geosync satellites, within 20,000 miles, so two arcminutes seeems plausible when the Moon covers 30 arcminutes and the asteroid will be ten times closer than the Moon.

assuming the rock is half a km in diameter that would make it one 7000th the diameter of the Moon

500m at 36,000km works out at a bit less than 3 arcseconds.

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Actually, if it gets close enough to hit, it'll go into at least a partial orbit first, more likely a full orbit, so saying where it'll hit is not currently predictable.

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No. Apophis is tiny. It's closer to 2 arcseconds for Apophis. Not 2 arcminutes.

Again, Wikipedia: Apophis:

Here's how they would compare if Apophis were 2 arcseconds (and much closer) and the moon 30 arcminutes (and normally distant), a factor of about 900 to 1:

Apophis-Moon.png

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Can’t they use computer models to predict the amount of rotations of the earth to get 90% predictable location?

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We came in peace for all mankind…
The moon no longer as any astronomical importance! Thanks to NASA for defacing it!