Schwassmann-Wachmann 73P

[Clio]

From a dear friend on the east coast. I thought you would enjoy reading some of his research on this celestial event.

**************************************************************************

The best way to explain the trajectory is to build a visual analogy. Imagine you are in a race car (Earth) and are traveling on a circular racetrack, going counter-clockwise at a constant speed. At the center of the track is the sun.

73P is moving at a higher speed, and is going to pass you on the right. Thinking only in two dimensions for the moment. As 73P overtakes you, it passes in front of you from right to left, almost clipping your front-right bumper as it does. It continues across your lane and off to the left.

Now let's take it to 3-D. The approach of 73P is from slightly above, so you would have to adjust your side-view miror up just a bit to see it coming up on your right side. It is getting lower and lower as it overtakes you but does not get to your level until after it passes you way ahead and on the left.

The view would be like seeing it pass by you on the right and a little above you. As it gets directly in front of you (in your lane), it is about as high as the windshield. It continues to speed away to the left and keeps descending until it gets to ground-level, but that's a ways ahead and also a bit to the left.

To imagine the angles involved here, imagine that you are on a five lane highway, and you are driving down the middle lane. Thinking again in two-dimensions. It would be like 73P begins to move out of the far-right-hand lane about ten car-lengths back. If it wasn't for the fact that it was moving faster than you, it would side-swipe you, but since it is moving faster, it just misses your front bumper. Taking it to 3-D again, it would appear that the 73P is traveling above the ground about 20 feet off the ground. As it overtakes you because of its speed, it is about 10 feet off the ground as it gets even with you, about five feet as it crosses in front of you, and gets to ground level up ahead about ten car-lenghts ahead and in the far left lane.

This is not in one piece, it has broken apart. Instead of a rifling effect of a .22 bullet, it has become the scatter-shot as though fired from a shotgun. Remembering the "spread pattern" produced by a shotgun (remembering the Chaney mishap on the ranch in Texas), the farther it gets from the barrel of the gun, the wider the spread pattern.

If we expand the analogy above to being planes flying in the air, with long banners attached at the tail, then it becomes well within the realm of reason that the banner attached to the passing plane would slap us before it got clear of us.

My Starry Night program lists 73P as being 20km in diameter.

As far as striking the Earth or the moon, let's forget for a moment "why" the comet has fragmented and "why" the comet fragments have seperated from each other a bit. I'll come back to that once I have studied it a bit more.

For now, we know we are looking at several objects coming in with a wider footprint than a single 20 km object. In other words, the path of destruction is wider for anything in its way.

With respect to the timing of the passage, here's what I derived from the software visually.

As 73P moves to overtake the Earth, it gets even with, and slightly above, on May 5. At that time the difference between the two bodies is about 0.1 au.

The moon is also moving in a counter-clockwise motion around the Earth, and at this time the moon is moving up in between the Earth and 73P. Keeping with the race-car analogy, on May 5 you could look out your right window and you would see 73P along side but a bit higher. Coming up quickly in between would be the moon.

By May 12, the moon is basically right outside your right window. This is also the time when the moon will be full, since the sun is always out your left window (remember we are on a circular racetrack representing the Earth's orbit). To add to this, it just so happens that Jupiter is right out your right window as well...way out there, but basically in line. So what I am outlining is a line-up of the sun, Earth, moon, and Jupiter. By May 12-13, 73P has moved ahead a bit and is directly in front of you but a little higher, in what appears to be a successful move to pass you out.

On May 13, 73P is crossing the orbital path of the Earth but slightly higher...so in reality, 73P never actually crosses a point where Earth will travel. On that day the distance between 73P and Earth is 0.079 au.

On May 24, comet 73P has finally reached the Earth's orbital plane. The point where it would "Disappear into the ground" is farther ahead and off to the left. The distance between Earth and 73P is .13 au.

With respect to the moon, the moon is orbiting the Earth counter-clockwise. From May 5 through May 20, the moon is outside the Earth's orbit. Per the analogy, it moves from behind the back bumper, up to the right side, and crosses the front bumper from right to left during this timeframe.

It is only logical to assume then, if some kind of "lagging fragment" is going to cause trouble, it would be more dangerous for the moon, since the moon is a little closer to 73P's path. Although it is only .002 au away from the Earth, it could act in a fashion similar to a "Block" in a game of American football.

From the viewpoint of the center of the sun, looking out directly at Earth, 73P is 5 degrees above Earth's orbital plane on May 4. It takes 20 days for the comet to move down to Earth's orbital plane, crossing it on May 24.

So, during this timeframe 73P moving down towards the Earth's orbital plane at about .25 degrees per day (5 divided by 20).

Using the distance to Earth as a constant (1 au), 5 degrees of seperation from the vantage point of the sun equates to a distance of 0.087156 au. The "Drop per day," then, is 0.004363 au per day.

on or about May 4, 73P is about 13 million kilometers "above" the Earth as seen from the vantage point of the center of the sun...dropping at about 654,500 kilometers per day.

On May 13, about the time the comet crosses to the inside of Earth's orbit (directly in front of our race car, the elevation of the comet will be roughly 7 million kilometers above the Earth. The distance seperating the Earth and 73P at that time will be roughly 12 million kilometers.

****----****----****----****

Combining all the information above, if there are pieces of this comet that are trailing, then the most dangerous pieces will be those that are farther behind, below, and to the right of the comet's published path.

If a fragment is not too far behind, then it is going to have to be a fairly good amount below the published path, or a fair way to the right.

If a fragment was going to hit the Earth or moon, then:

If the fragment is not too far behind, but much lower, impact would occur in the first half of May.

If the fragment is not too far behind, but much to the right, then impact would occur in the second half of May.

If the fragment is a good ways back, but more aligned with the normal path of the comet, then impact would occur between the middle of May and the middle of June.

So, now we can look at the distribution of the fragments to see if any of the above scenarios are worth studying.

**************************************************************************

I find his work interesting, and am running it past one of my nuclear astro - physicist friends to get his take on the orbital mechanics that my friend is using.

I thought there might be some here interested in this topic.

[jasd]

The above might be considered timely material! Such an occurrence together with unwelcome meteoric impacts precipitated our own Dark Ages. That said,

although it seems unlikely that we’re in danger of experiencing an event at this time, one has to be concerned that such a fly-by as described in your post was attended by a train of particulate matter that, conceivably, could be ‘left behind’ by gravitational or [electro]magnetic attraction – the subsequent haze in our upper atmospheres could prove, ummm, disturbing… to say the least.

Any feedback from your physicist friends?

[Clio]

All of them have directed me to NASA's and the JPL's web page for NEO's, where one finds this fascinating bit of information:

Object

Close-Approach Date

(TDB)

YYYY-mmm-DD HH:MM ± D_HH:MM Miss Distance

Nominal

(LD/AU) Miss Distance

Minimum Ascending

(LD/AU) V

relative

(km/s) V

infinity

(km/s) N

sigma H

(mag)

73P/Schwassmann-Wachmann 3-BD 2006-May-11 21:53 ± 11:11 33.7/0.0867 0.04/0.00010 14.79 14.78 3 n/a

The important information is two fold. First, the Minimum miss distance, which is for a sigma 3 (which this is, note the lone digit 3 in the second line) is 0.04 / 0.00010 or 4/10ths of the distance from the center of the earth to the moon.

That's 6300 miles from earth surface.

(Minimum miss is what they DON'T think will happen... about a 2% chance but they can't rule it out)

Then the Nominal miss is 33.7 / 0.0867 which is what they think will happen, but again they're not sure.

How can you tell they're not sure?

Simple... Most minimum and nominal miss distances are within a tenth of a percentage point. These are not. And the numbers have not gotten any tighter with an additional one week of observation since the VLT's got powered up in Arecibo, Paranal, and Hubble the week of April 30th.

In essence... the experts are not sure... It's most likely just going to be an AWESOME light show... but it might get really, really awesome on the 11th... or maybe the 12th.... because they're not real certain about the time of fly-by either... giving it a + / - 11:11 or sometime within a 22 hour window.

It's just one of those fascinating things....