[quote=smh;162623]
Nope. 400 miles is sufficiently high that there is not much air resistance. Most of it will continue to orbit for years to come.[/quote]
It is the precise speed and direction required for a near circular
orbit I was considering, not air resistance.
Unless they had near identical velocities, the chances of the debris
having the required velocity after the collision are remote.

Having looked at the diagram you posted, it seems they were travelling
at right angles to each other.
Cheesehead's remark about closing speed is vindicated as well.

Both satellites were orbiting at around 7.5 KM/s. The relative impact speed was about 11 KM/S.

Debris that accelerated will either escape earth orbit or continue in an elliptical orbit with a perigee of about 400 miles. Debris that was slowed down will continue in an elliptical orbit with an apogee of about 400 miles unless the perigee of that orbit is low enough so that the piece of debris slows down by friction of earths atmosphere.

Remember that if you slow down an object in space, it will start falling closer to earth and thus increase it's speed. After half an orbit it will have reached it's lowest point (perigee) and will continue to climb up until it reaches the same point in orbit from where it slowed down (although the surface of the earth has moved underneath the object).

The centre of mass speed is 1/SQR(2) of that of the satellites.
If you consider the debris exploding isotropically (3D) in the C of M
reference frame, not many of the velocities fit your "perigee" bill.

I have to admit at first I was rather impressed that the two satellites managed to hit each other at all. At such high velocities and what with space being quite large it seemed the chance of impact would be truly minuscule. Even if the controllers were trying to make them hit I doubt that they could have achieved such success.

But after a little thought I realised that the chance is only small if we consider only those two satellites and only impacting at that particular time. Once we consider the total combinations all two satellites and the number of opportunities for them the cross each others orbits then I guess the odds come down considerably.

[quote=smh;162623]They both were in a (near) polar orbit. See [URL="http://planetary.org/blog/article/00001840/"]here[/URL] for a diagram.[/quote]I failed to consider that a right-angle collision over Siberia does not necessarily rule out a near-polar orbit for [i]both[/i] satellites.

(There! [I]That[/I]'s why it matters where on the ground was under the location of the collision! :doh!: )

[quote=davieddy;162628]It is the precise speed and direction required for a near circular orbit I was considering, not air resistance. Unless they had near identical velocities, the chances of the debris having the required velocity after the collision are remote.[/quote]
[quote=smh;162629]Debris that accelerated will either escape earth orbit or continue in an elliptical orbit with a perigee of about 400 miles. Debris that was slowed down will continue in an elliptical orbit with an apogee of about 400 miles unless the perigee of that orbit is low enough so that the piece of debris slows down by friction of earths atmosphere.[/quote]One way to minimize confusion when thinking about the result is to consider that, in accordance with a principle posted during discussion of the China/US antisatellite episodes, each particle of collision debris will initially be in a new orbit that will [I]pass through the point at which the collision occurred[/I] but with differing speeds and directions relative to that point. (We're assuming a reference frame where Earth is stationary, for simplicity.)

In computing the resulting orbit, it's not whether a particular piece is accelerated or decelerated, compared to its pre-collision speed (which was either the Cosmos's speed or the Iridium's speed), that matters. (Actually, they're all accelerated, where acceleration has the usual physics definition as any change of velocity and velocity is the combination of speed and direction vector of motion, regardless of the algebraic sign of the change in speed. Acceleration where the speed is reduced is still acceleration in this view.) It's what new velocity (speed + direction) the particle has. Its pre-collision speed, and whether the new speed is greater or less, are irrelevant. The new orbit is whatever results from the combination (new velocity) + (position of collision) + (time of collision).

A particle whose new orbit takes it low enough to encounter significant atmosphere will, usually, burn up the first time it does so. This can happen no matter whether the particle's new speed is lower, higher, or the same, as its pre-collision speed. It can happen either:

(a) within a few minutes, if the particle's new velocity is initially directed so that it heads toward perigee steeply below the collision altitude,

(b) after a longer time (hours, maybe days) if the particle's new velocity is directed so that it proceeds to its new orbit's higher apogee before coming down to touch the atmosphere near perigee, or

(c) farther in the future, if the particle's new orbit has a perigee above significant atmosphere.

The lower the collision altitude, the greater proportion of particles that will encounter the atmosphere without completing their first orbits. (That's why the low-altitude US antisatellite response was less debris-producing than the Chinese higher-altitude test.)

The current projection is for some of the debris to remain in orbit for decades.
Iridium said they had no knowledge. NORAD is saying that they didn't foresee this.

[quote=cheesehead;162640]
The lower the collision altitude, the greater proportion of particles that will encounter the atmosphere without completing their first orbits. (That's why the low-altitude US antisatellite response was less debris-producing than the Chinese higher-altitude test.)[/quote]
In short I was right (abeit only stating the bleeding obvious).

[quote=davieddy;162650]In short I was right (abeit only stating the bleeding obvious).[/quote]Great! I was unsure how to interpret the post I quoted.

The census of orbiting debris I've seen in articles about the collision seem not to include another group, because they're individually too small:

"[FONT=arial][SIZE=2][FONT=arial][FONT=Verdana, Arial, Helvetica][SIZE=2][COLOR=#333333][FONT=arial][SIZE=2][COLOR=#333333][FONT=Arial][SIZE=2][SIZE=3]Tiny spheres of liquid sodium-potassium (NaK) reactor coolant dripped from the former Soviet Union's radar ocean reconnaissance satellites, known as RORSATs[/SIZE].[/SIZE][/FONT][/COLOR][/SIZE][/FONT][/COLOR][/SIZE][/FONT][/FONT][/SIZE][/FONT]"

How'd you like to get drilled by hypervelocity droplets that not only chemically burn your flesh as they penetrate, but also irradiate it once they've stopped tunneling into you? Yuck.

From "Havoc in the Heavens: Soviet-era Satellites Leaky Reactor's Lethal Legacy"

[URL]http://www.space.com/news/mystery_monday_040329.html[/URL] (5 years ago)

[quote=Leonard David][FONT=arial][SIZE=3][FONT=arial][FONT=Verdana, Arial, Helvetica][COLOR=#333333][FONT=arial][COLOR=#333333][FONT=Arial]Old Soviet nuclear powered satellites leaked a trail of menacing radioactive droplets that have become a debris threat to other spacecraft.
[/FONT][/COLOR][/FONT][/COLOR][/FONT][/FONT][/SIZE][/FONT]
[FONT=arial][SIZE=3][FONT=arial][FONT=Verdana, Arial, Helvetica][COLOR=#333333][FONT=arial][COLOR=#333333][FONT=Arial]Tiny spheres of liquid sodium-potassium (NaK) reactor coolant dripped from the former Soviet Unions radar ocean reconnaissance satellites, known as RORSATs. This class of satellite -- no longer launched -- carried a nuclear reactor to power a large radar dish that enabled day/night snooping of Earths oceans.[/FONT][/COLOR][/FONT][/COLOR][/FONT][/FONT][/SIZE][/FONT]

[FONT=arial][SIZE=3][FONT=arial][FONT=Verdana, Arial, Helvetica][COLOR=#333333][FONT=arial][COLOR=#333333][FONT=Arial]After a RORSATs tour-of-duty was over, the reactors fuel core was shot high above Earth into a "disposal orbit". Once at that altitude the power supply unit would take several hundred years before it reentered the Earths atmosphere.[/FONT][/COLOR][/FONT][/COLOR][/FONT][/FONT][/SIZE][/FONT]

[FONT=arial][SIZE=3][FONT=arial][FONT=Verdana, Arial, Helvetica][COLOR=#333333][FONT=arial][COLOR=#333333][FONT=arial][COLOR=#333333]However, in ejecting the core from the main body of a RORSAT, a plumbing problem plagued the satellite design. Faulty seals permitted the NaK coolant to leak, leaving thouands upon thousands of droplets to spill freely in to space. [/COLOR][/FONT][/COLOR][/FONT][/COLOR][/FONT][/FONT][/SIZE][/FONT]

. . .

[FONT=arial][SIZE=3][FONT=arial][FONT=Verdana, Arial, Helvetica][COLOR=#333333][FONT=arial][COLOR=#333333][FONT=arial][COLOR=#333333]There is evidence from ground-based radar measurements that 16 of a total of 31 RORSAT nuclear reactors orbited lost coolant following core ejection into disposal orbits.[/COLOR][/FONT][/COLOR][/FONT][/COLOR][/FONT][/FONT][/SIZE][/FONT]

. . .[SIZE=3]

[/SIZE][FONT=arial][SIZE=3][FONT=arial][FONT=Verdana, Arial, Helvetica][COLOR=#333333][FONT=arial][COLOR=#333333][FONT=arial][COLOR=#333333]Credited with first flagging the Soviet leaking reactor problem was Don Kessler. He has over 40 years of experience in the scientific study of human-made space trash, a large chunk of that time spent at the NASA JSC delving into the problem of space debris.[/COLOR][/FONT][/COLOR][/FONT][/COLOR][/FONT][/FONT][/SIZE][/FONT]

[FONT=arial][SIZE=3][FONT=arial][FONT=Verdana, Arial, Helvetica][COLOR=#333333][FONT=arial][COLOR=#333333][FONT=arial][COLOR=#333333]Kessler is now an orbital debris and meteoroid consultant in Asheville, North Carolina.[/COLOR][/FONT][/COLOR][/FONT][/COLOR][/FONT][/FONT][/SIZE][/FONT]

[FONT=arial][SIZE=3][FONT=arial][FONT=Verdana, Arial, Helvetica][COLOR=#333333][FONT=arial][COLOR=#333333][FONT=arial][COLOR=#333333]There has been a significant change in the RORSAT story since Kessler and his colleagues first reported the problem in 1995. "It is now accepted by all, even by the Russians, as being correct and is now part of all orbital debris models," he said.[/COLOR][/FONT][/COLOR][/FONT][/COLOR][/FONT][/FONT][/SIZE][/FONT]

[FONT=arial][SIZE=3][FONT=arial][FONT=Verdana, Arial, Helvetica][COLOR=#333333][FONT=arial][COLOR=#333333][FONT=arial][COLOR=#333333]NASA's main source in spotting the drifting droplets was the Haystack radar. That facility is operated by Massachusetts Institute of Technologys (MIT) Lincoln Laboratory and has been collecting orbital-debris data for NASA since 1990 under an agreement with the U.S. Air Force. [/COLOR][/FONT][/COLOR][/FONT][/COLOR][/FONT][/FONT][/SIZE][/FONT]

[FONT=arial][SIZE=3][FONT=arial][FONT=Verdana, Arial, Helvetica][COLOR=#333333][FONT=arial][COLOR=#333333][FONT=arial][COLOR=#333333]Kessler led the effort to discover the cause of an abnormally high concentration of objects that was detected by the Haystack radar. These small objects were zipping about the Earth between roughly 530 miles (850 kilometer) and 620 miles (1,000 kilometer) altitude. [/COLOR][/FONT][/COLOR][/FONT][/COLOR][/FONT][/FONT][/SIZE][/FONT]

[B][FONT=arial][SIZE=2][FONT=arial][FONT=Verdana, Arial, Helvetica][SIZE=2][COLOR=#333333][FONT=arial][SIZE=2][COLOR=#333333][FONT=arial][SIZE=2][COLOR=#333333][B][SIZE=3]Something strange going on

[/SIZE][/B][/COLOR][/SIZE][/FONT][/COLOR][/SIZE][/FONT][/COLOR][/SIZE][/FONT][/FONT][/SIZE][/FONT][/B][FONT=arial][SIZE=3][FONT=arial][FONT=Verdana, Arial, Helvetica][COLOR=#333333][FONT=arial][COLOR=#333333][FONT=arial][COLOR=#333333]"Our first clue that something strange was going on," Kessler said, was the high concentration of objects near 600 miles (900 kilometers) above Earth. That intensity of objects could not be explained by an explosion, which would have dispersed the debris over a larger altitude range, he added.[/COLOR][/FONT][/COLOR][/FONT][/COLOR][/FONT][/FONT][/SIZE][/FONT]

[FONT=arial][SIZE=3][FONT=arial][FONT=Verdana, Arial, Helvetica][COLOR=#333333][FONT=arial][COLOR=#333333][FONT=arial][COLOR=#333333]"The concentration was so high that, whatever the source, it represented the most significant impact hazard for spacecraft operating at those altitudes... and still does today," Kessler said. [/COLOR][/FONT][/COLOR][/FONT][/COLOR][/FONT][/FONT][/SIZE][/FONT]

. . .
[SIZE=3]
[/SIZE][FONT=arial][SIZE=3][FONT=arial][FONT=Verdana, Arial, Helvetica][COLOR=#333333][FONT=arial][COLOR=#333333][FONT=arial][COLOR=#333333]Kessler told [I]SPACE.com[/I] that follow-on NASA work has pegged the total mass of leaked NaK as over 360 pounds (165 kilograms) -- greater than he had orginally estimated.[/COLOR][/FONT][/COLOR][/FONT][/COLOR][/FONT][/FONT][/SIZE][/FONT]
[FONT=arial][SIZE=3][FONT=arial][FONT=Verdana, Arial, Helvetica][COLOR=#333333][FONT=arial][COLOR=#333333][FONT=arial][COLOR=#333333]
[/COLOR][/FONT][/COLOR][/FONT][/COLOR][/FONT][/FONT][/SIZE][/FONT][FONT=arial][SIZE=3][FONT=arial][FONT=Verdana, Arial, Helvetica][COLOR=#333333][FONT=arial][COLOR=#333333][FONT=arial][COLOR=#333333]A report on the RORSAT leakage in 1997, led by Alessandro Rossi, a researcher at the Centro Nazionale Universitario di Calcolo Elettronico (Electronic National University Center for Calculation) in Pisa, Italy, pointed out another issue.[/COLOR][/FONT][/COLOR][/FONT][/COLOR][/FONT][/FONT][/SIZE][/FONT]

[FONT=arial][SIZE=3][FONT=arial][FONT=Verdana, Arial, Helvetica][COLOR=#333333][FONT=arial][COLOR=#333333][FONT=arial][COLOR=#333333]Rossi stated that, although the NaK coolant leakage may have been confined to a specific class of satellite no longer launched, "the probability that a debris impact might puncture the radiator of one of the RORSATs still in graveyard orbit, inducing a new leak from the secondary cooling circuit, is far from negligible." [/COLOR][/FONT][/COLOR][/FONT][/COLOR][/FONT][/FONT][/SIZE][/FONT]

. . .
[SIZE=3]
[/SIZE][FONT=arial][SIZE=3][FONT=arial][FONT=Verdana, Arial, Helvetica][COLOR=#333333][FONT=arial][COLOR=#333333][FONT=arial][COLOR=#333333]There is a large population of NaK droplets primarily around 560 miles (900 kilometers) altitude above Earth, Johnson explained. Some of the largest of these are in the roughly 2 inches to 3 inches (5 to 7 centimeters) diameter category. They have been cataloged by the U.S. Space Surveillance Network and are routinely tracked, he said.[/COLOR][/FONT][/COLOR][/FONT][/COLOR][/FONT][/FONT][/SIZE][/FONT]

[FONT=arial][SIZE=3][FONT=arial][FONT=Verdana, Arial, Helvetica][COLOR=#333333][FONT=arial][COLOR=#333333][FONT=arial][COLOR=#333333]The vast majority of the droplets are smaller, down to less than an inch in diameter, Johnson said. "They are not decaying rapidly, although some very small particles have reached lower Earth orbits."[/COLOR][/FONT][/COLOR][/FONT][/COLOR][/FONT][/FONT][/SIZE][/FONT]

. . .

[FONT=arial][SIZE=3][FONT=arial][FONT=Verdana, Arial, Helvetica][COLOR=#333333][FONT=arial][COLOR=#333333][FONT=arial][COLOR=#333333]There is one added element to the RORSAT reactor coolant saga. Are those droplets radioactive?
[/COLOR][/FONT][/COLOR][/FONT][/COLOR][/FONT][/FONT][/SIZE][/FONT]
[FONT=arial][SIZE=3][FONT=arial][FONT=Verdana, Arial, Helvetica][COLOR=#333333][FONT=arial][COLOR=#333333][FONT=arial][COLOR=#333333]Any object, large or small -- a paint fleck or a tiny radioactive sphere -- whizzing about Earth at high speed is troublesome to both piloted and automated spacecraft. Furthermore, eventually those NaK spheres will nose-dive into the upper atmosphere.[/COLOR][/FONT][/COLOR][/FONT][/COLOR][/FONT][/FONT][/SIZE][/FONT]

[FONT=arial][SIZE=3][FONT=arial][FONT=Verdana, Arial, Helvetica][COLOR=#333333][FONT=arial][COLOR=#333333][FONT=arial][COLOR=#333333]There is no doubt that the NaK coolant was radioactive when a RORSAT was running, said Gerald Kulcinski, associate dean for research in the College of Engineering and a professor of nuclear engineering at the University of Wisconsin at Madison. In the process, both Sodium-24 and another isotope, Argon-39 would have been created, he said.[/COLOR][/FONT][/COLOR][/FONT][/COLOR][/FONT][/FONT][/SIZE][/FONT]

[FONT=arial][SIZE=3][FONT=arial][FONT=Verdana, Arial, Helvetica][COLOR=#333333][FONT=arial][COLOR=#333333][FONT=arial][COLOR=#333333]While the radioactive Sodium-24 is short-lived, any Argon-39 released would have a half-life of 270 years, Kulcinski noted. [/COLOR][/FONT][/COLOR][/FONT][/COLOR][/FONT][/FONT][/SIZE][/FONT]

[FONT=arial][SIZE=3][FONT=arial][FONT=Verdana, Arial, Helvetica][COLOR=#333333][FONT=arial][COLOR=#333333][FONT=arial][COLOR=#333333]That being said, however, just how much of that Argon-39 radioactive isotope is encapsulated within a space-frozen NaK coolant droplet is not immediately clear, Kulcinski added. Specific engineering details of how the RORSAT reactor was designed and functioned would be required. Yet another unknown factor is what impurities could have been resident within the NaK coolant, he said.[/COLOR][/FONT][/COLOR][/FONT][/COLOR][/FONT][/FONT][/SIZE][/FONT]

[FONT=arial][SIZE=3][FONT=arial][FONT=Verdana, Arial, Helvetica][COLOR=#333333][FONT=arial][COLOR=#333333][FONT=arial][COLOR=#333333][B]Runaway threshold

[/B][/COLOR][/FONT][/COLOR][/FONT][/COLOR][/FONT][/FONT][/SIZE][/FONT][FONT=arial][SIZE=3][FONT=arial][FONT=Verdana, Arial, Helvetica][COLOR=#333333][FONT=arial][COLOR=#333333][FONT=arial][COLOR=#333333]The NaK droplets represent only the "short-term" issue, Kessler said. He underscored another concern -- terming it a "runaway threshold". That is, collisions in space would increasingly produce large quantities of smaller debris over the next 50 years or so. [/COLOR][/FONT][/COLOR][/FONT][/COLOR][/FONT][/FONT][/SIZE][/FONT]

[FONT=arial][SIZE=3][FONT=arial][FONT=Verdana, Arial, Helvetica][COLOR=#333333][FONT=arial][COLOR=#333333][FONT=arial][COLOR=#333333]"Since we began looking, we have measured debris not only from sources like the NaK, but from solid rocket motors that eject large amounts of centimeter-to-micron-size objects, paint flecks from painted spacecraft surfaces, copper needles from U.S. Air Force communication tests, and more fragments than expected from explosions in space," Kessler said.[/COLOR][/FONT][/COLOR][/FONT][/COLOR][/FONT][/FONT][/SIZE][/FONT]

[FONT=arial][SIZE=3][FONT=arial][FONT=Verdana, Arial, Helvetica][COLOR=#333333][FONT=arial][COLOR=#333333][FONT=arial][COLOR=#333333]"In the long-term, debris resulting from collisions is still the major problem, and will be the most expensive to control," Kessler said. "We are on the threshold, if we have not already exceeded it, of reaching a critical density of objects in low Earth orbit, where collisional fragmentation will cause the debris environment to slowly grow even if all other sources are eliminated."[/COLOR][/FONT][/COLOR][/FONT][/COLOR][/FONT][/FONT][/SIZE][/FONT]
[/quote]Edit:

Hmmm... I just now noticed that the [I]tiny[/I] in "tiny radioactive droplet" only means "down to less than an inch in diameter". Before rereading carefully, I had thought the article was about stuff more like a fine mist, nowhere near as large as BBs (how do standard shotgun pellets match those in size?) except for a few hundred larger ones. That mist size was what I had in mind when I wrote, "How'd you like to get drilled ..."

I'd consider even a half-inch "droplet" to be pretty sizable -- well into the "bullet" range. A half-inch hypervelocity "droplet" isn't going to come to rest before it's completely penetrated you and out the other side of what's left of your body, so maybe the radiation dose would be lower ... not that radiation would be your main concern. Still, probably a closed lead-lined casket at the church-and-chapel visitation. Would-be Harold-and-Maudes ([URL]http://www.imdb.com/title/tt0067185/[/URL], [URL]http://en.wikipedia.org/wiki/Harold_and_Maude[/URL]) better skip that one.

My original insistence on "near circular orbits" overlooked
the point of collision being near the perigee of
an elliptical orbit.

"How satellites could 'sail' home"

[URL]http://news.bbc.co.uk/2/hi/science/nature/8029899.stm[/URL]

[quote]. . .

Extending a sail on an old spacecraft would increase drag and pull it into the Earth's atmosphere to burn up.

Major European space firm EADS Astrium says the scheme has great potential.

"It is an interesting solution, especially for the satellite that has no propulsion system at the end of its life," Brice Santerre told BBC News.

Santerre and colleague Max Cerf have been working on what they call the Innovative DEorbiting Aerobrake System (IDEAS).

The concept involves extending booms and sheeting from spacecraft to increase the amount of drag they experience from the residual air molecules still present at altitudes up to even 750km (470 miles)

. . .[/quote]