I’ve been puzzling over Asteroid DA-14′s close approach a week back, wondering whether any of the big fireballs (Chelyabinsk, San Francisco, Cuba, Japan) might have been part of a cloud of rocks accompanying DA-14.
NASA says that that the Chelyabinsk fireball had nothing to do with DA-14. Someone called William Cooke posted on a NASA blog about the Chelyabinsk fireball, in which he wrote:
The meteor hit the atmosphere at a speed of 18 km/s (11.2 miles per second or 40,300 mph). It was moving at a shallow entry angle (less than 20 degrees) and broke apart some 15-25 km above the Russian city. Most of the damage was caused by the shock wave produced when the meteor disrupted.
Several thousand meteors enter Earth’s atmosphere each day. The vast majority of these, however, occur over the oceans and uninhabited regions, and a good many are masked by daylight. Those that occur at night also are rarely noticed by people. Due to the combination of all of these factors, only a handful of witnessed meteorite falls occur each year. The Russia meteor was one of those rare instances.
Included was a description of the Chelyabinsk asteroid’s orbit, which was markedly different from that of DA-14.
The principal problem with this blog post is the date on which it was written: 15 Feb 2013. That was the same day as the closest approach of DA-14, and the Chelyabinsk fireball. Comments under the article also complain that the Chelyabinsk asteroid was not travelling S – N, but E- W.
On Feb 18, 2013 03:22:43 PM sceptic .
This is really silly. We know that the meteorite var travelling from east to west, and it was clear from videos the same morning. NASA is wrong, but they still pretend the trajectory was north to south. And there is no possibility they could calculate the orbit above with any accuracy.
So it looks like NASA scientists did a quick back-of-envelope calculation based on seeing the asteroid pass left-to-right in front of the rising sun, and assumed (not unreasonably) that it was travelling N – S. They then figured out a likely orbit to explain this, and this became the Official Orbit of the Chelyabinsk Asteroid, and the principal explanation of why it had nothing to do with DA-14.
Also, it was pointed out that DA-14 came up from below the Earth’s south pole, and it would have been very difficult for any accompanying fragments to land in the northern hemisphere.
Anyway, I was interested enough to dig out and dust off the orbital simulation model I wrote 4 or 5 years back, and got hold of DA-14′s vectors (position and velocity at given times) from JPL, and plugged them into my model. And I found that DA-14 passed more or less exactly where it was reported to, and at the reported time. I then started adding accompanying fragments, and wrote about it last week.
Today I spent a while finding one of these fragments that would have headed straight for the centre of the Earth. And then I looked to see where, relative to the Earth, it was coming from, and it was roughly 22 degrees from the Earth’s south pole, much as had been said. And this meant that most of the Earth’s northern hemisphere would have been on the opposite side from where a DA-14 fragment cloud would have been coming.
There would have been no way that DA-14 fragments travelling at 20 km/sec (the typical sort of figure being given for the speed of the Chelyabinsk asteroid) could have landed in the northern hemisphere.
But then I used my earth-bullseye DA-14 fragment to find out what its speed relative to the Earth was, and I found that it was moving at just 6.9 km/sec as it was about to strike the Earth. And this is quite near one report of DA-14′s speed at its closest approach. Nothing like 20 km/sec. And this was because DA-14 was slowly overtaking the Earth:
The asteroid passes will speed by at about 7.8 kilometers per second (17,400 MPH, 4.8 miiles per second) – or about 8 times the speed of a rifle bullet.
And this is pretty darn slow. As an illustration of how slow it is, we can consider the Hubble Space Telescope, which is in a low earth orbit (559 km altitude) and travelling at 7.5 km/sec. That’s the speed that something has to be moving if it’s going to stay in orbit at that altitude. And the speed rises the nearer the satellite gets the Earth. It has to be going at over 7.5 km/sec if it’s at an altitude of 100 km. Conversely, geostationary satellites 36,000 km out are only moving at 3 km/sec.
So if a DA-14 fragment travelling at 7.0 km/sec passed between the Hubble telescope and the Earth’ surface, it would be moving too slowly to skim past and back out into space. It would have fallen to Earth, perhaps after going round it a few times. Such fragments, if large enough, would have produced fireballs that were travelling near-horizontally in the high atmosphere.
If there are no reports so far of any impacts in the southern hemisphere, it may be because most of the Earth’s land mass is concentrated in the northern hemisphere, and so any fragments that fell there came down in the oceans. Or on Antarctica.
I’ll continue to investigate. I’ll see if I can create some DA-14 asteroid fragments that come in a few hundred km above the Earth’s surface, and find what speed they’re doing, and what happens to them in the northern hemisphere. I’ll probably have to also consider what happens when one of them starts travelling through the high atmosphere, and begins to slow and heat up.
Anyway, at the moment it’s looking quite likely that DA-14 fragments could indeed have come down in the northern hemisphere.
Update: Although the fragment cloud around DA14 has the same initial speed as it, and approach the Earth at about 6 km/sec, fragments that pass close to the Earth are travelling about twice as fast.
The screen capture below shows the Earth seen looking down onto the ecliptic plane, with the Hubble space telescope orbit as a broken red ring around it, and the paths of two DA14 fragments coming in below the Earth, one of them hitting the Earth in between Antarctica and Australia (where its path changes from red to green). At the point of impact, this fragment was travelling at about 13 km/sec.
If all of the fragments are travelling at these sorts of speeds (Earth’s escape velocity is 11.2 km/sec)), it’s very unlikely that they’d come down in the northern hemisphere.
But this assumes that the fragments in a cloud around DA14 are all travelling at the same velocity. More likely is that the fragment cloud would be spinning around its centre of gravity, and this might change the outcome.