when satellites collide, it's really a drag

The NASCAR season has started, which means I got to go down to Daytona a couple weeks ago.  Daytona is a 2.5 mile track and banked at 31 degrees, which means that speeds can get high enough that the cars become aerodynamically unstable.  To limit speed, engine power is artificially restricted, which means that minimizing drag - the friction between air molecules and the car's surface - becomes incredibly important.

Unfortunately, I didn't get to watch much of the Daytona 500, since we were back home finishing taping a "Science of NASCAR" project. One of the fun parts of that weekend was dragging The Rocket Scientist (TRS) out to Texas Motor Speedway, where he explained to me in-between takes that drag played an important role in another scientific event that happened earlier in the week.

On Feb 10th, a privately owned US Iridium satellite collided with a defunct Russian military satellite.  The Iridium satellite is one of 66 satellites in a communications network.  The Russian satellite is "space junk" - stuff that is not longer used, but is still in orbit.  Newer satellites are equipped with enough fuel to "make them commit suicide", as The Rocket Scientist puts it poetically, by burning up on re-entry.  Otherwise, you have to resort to something like shooting down misbehaving satellites with missiles.

You would think that someone -- like NORAD, whose job it is to predict possible satellite collisions -- would have noticed there might be a problem, but there apparently were no warnings before the impact.  The Pentagon uses optical telescopes and radar to track more than 18,000 objects. You can find some of the information on space-track.org.  Aerospace analyst T S Kelso, using the Pentagon's public data, says the available data shows the two satellites should have missed each other by 584 meters. 

NASA's chief scientist estimates that there are about 19,000 objects in low Earth orbit and higher Earth orbits.  Only 10% of those objects are useful satellites.  The rest is space junk.  That count includes an estimated 1,000 objects larger than 10 centimeters (4 inches) created by this months' satellite collision. 

In 2004, Dr. Hugh Lewis and collaborators calculated what would happen if an Iridium satellite hit a 1-kg piece of space junk.  They calculated the probability of a collision as one in a few million.  Of course, with all the new debris -- and the fact that the remaining 65 Iridium satellites move in circular orbits that cross each other at the poles (the collision happened over Siberia) -- those odds have probably increased by a few orders of magnitude.

A spacecraft would need 40 joules of energy per gram of its mass to be totally obliterated.  The Chinese missile test was estimated at 250 joules per gram.  The energetics in the satellite collision are even larger.  The two satellites that hit each other were going 42,120 km/h. The Iridium satellite was 1,234 lbs and the Russian satellite was 1,984 lbs. That adds up to about 50,000 joules of energy per gram of satellite.  I'm guessing the insurance adjuster is going to total both satellites.  (Iridium actually has 'spares' in alternative orbits that they can press into service to replace the destroyed satellite, so they won't need to hassle with the insurance company over a loaner.) 

Just because I am compelled to give The Rocket Scientist a hard time whenever possible, I asked him how his "peeps" had managed to let this happen.  His answer?

Sunspots.

Well, they've been correlated to everything from crop yields and energy use to the Dow Jones Average and skirt length, so why not?

I was heartened to learn that sunspots have to do with magnetism (something I actually think I understand); however, these are some pretty large magnets.  A typical sunspot has a diameter of about 37,000 km. The intense magnetic activity of a sunspot affects convection - the rise and fall of gases or liquids.  The inhibited convection on the surface decreases the sunspot temperature to around 4,000-4,600 K.  The photosphere (the outer layer of the Sun) ranges from about 6,000 Kelvin at the inner boundary to 4,500 K at the outside, with an effective temperature of 5,800 K, so sunspots are actually cold areas on the Sun.

Irradiance is the power per area incident on a surface.  If you had a 60-W light bulb shining on a 1 square meter area, the irradience would be 60 W/m².  Sunspots are lower temperature than the rest of the surface, so you would think that sunspots would decrease the irradience.  Surprisingly, the opposite happens:  The irradience increases.  More sunspots means hotter, fewer sunspots mean colder.

Sunspots are cyclic with about an 11.1-year period.  The period is related to the fact that the solar system doesn't revolve around the Sun.  The Sun is slightly displaced from the solar system's center of gravity (CG), getting up to a million kilometers from the CG at times.  The motion of the Sun relative to the CG affects turbulence in the photosphere and that transition from turbulent to not affects the number of sunspots.

The Rocket Scientist explained to me that sunspots change latitude during the course of a solar cycle.  They form a "butterfly pattern".  Sunspots usually occur in a belt of plus and minus 35 degrees from the equator.  At the start of a new solar cycle, the sunspots form at high latitudes.  As more and more sunspots appear, they start to form at lower latitudes.  During the minimum, they are closest to the equator.  A plot of latitude vs. date looks like butterfly wings.

The 11.1 year cycle is superposed on the total number of sunspots.  There have been periods where the overall number of sunspots was very small.  The most famous of these is the 70-year period called the  Maunder minimum, which was from 1645 to 1715.  During one period where you'd expect 40,000-50,000 sunspots, there were only about 50. 

We're in another low period right now, with 2008 having been declared the "Blankest Year of the Space Age" in terms of sunspot activity.  The minimum in the sunspot cycle probably happened sometime late summer or early Fall of 2008.

Drag becomes important because sunspots affect the temperature and the density of the atmosphere.  During sunspot maximum, drag increases and when drag increases, it slows down satellites and their orbits become smaller.  During a sunspot minimum, drag decreases.

That, The Rocket Scientist thinks, is why the models used to predict satellite collisions may have been off.  The abnormal sunspot activity - or lack of sunspot activity - is great for communications.  But it causes a host of other effects, one of which may have been the satellite collision.

Altitude makes a difference in racing.  The higher the altitude, the less dense the air.  That decreases drag, but it also decreases how much oxygen gets into the engine per volume, which decreases horsepower.

Putting a racetrack at the top of Mount Kilomanjaro, The Rocket Scientist says, would make for interesting racing.  Of course, he's never tried getting out of the Pocono racetrack, or he'd know that mountains are definitely not the best thing when it comes to post-race traffic. 

Comments

[Professor R]

Super post - I've been trying to find info on the satellite collision..Regards Cormac

[Dave]

No, no, no! You don't want to even think about shooting an obsolete satellite down with a missile. That'll produce as much (or even more!) space junk than a collision between satellites will. The reason it sometimes is done (and mostly works) is because the satellite in question is about to re-enter, and busting it up makes the pieces less aerodynamic, and thus controls where the larger chunks end up reentering at (e.g., over the ocean, instead of over/into a city). But, for high altitude satellites, all a missile collision would do is create LOTS of pieces of high speed debris, something you certainly don't want.

There really isn't any way, short of a (unfunded) space tug, that obsolete satellites can be recovered and deorbited. Even the never satellites that are designed for handling obsolescence don't necessarily deorbit themselves. For geosynchronous satellites, it takes a rather huge delta-V to get them to reenter the atmosphere. So, a lot of these are merely moved to a slightly higher (or lower) parking orbit where they will free up space for a new geosynchronous satellite (and, a huge delta-V translates into an incredible expense for a satellite).

Of course, not all satellites are geosynchronous. There are quite a few LEOs (Low Earth Orbit satellites), such as the Iridium satellite involved in the collision. The reason for this is that geosynchronous satellites have to orbit at a specific altitude (23,000 miles, roughly) over the equator, and that's a long way for a radio signal to propagate (Remember that the strength of a radio signal drops off as the square of the distance.). So, a satellite at, oh, say 400 miles altitude will have a signal about 3300 times as strong as one in geosynchronous orbit (or, conversely, it can receive signals 3300 times weaker, and needs 3300 times less transmitter power)! Of course, the down side of a LEO is that it can't see very far, so the ground access footprint is quite limited, resulting in the need for an array of satellites to cover an area.

Toss into this the fact that quite a few of the Soviet/Russian communications satellites are in what are called Molniya orbits, which are polar, highly eccentric orbits, and you have the makings of an imminent collision between such a satellite and some of the other LEOs in lower inclination orbits (The molniya orbits are useful since they allow coverage of high latitudes, which a large amount of Russia is at.).

Now, for some real fun, since the energy of a satellite collision depends not upon the speed of a satellite, but the velocity [1], consider what will happen when a retrograde satellite hits a non-retrograde satellite.

[1] Velocity is a vector which accounts for the direction, while speed is only a scalar, and does not account for direction.

Fortunately, for all of us, Project Westford was a startling failure, and wasn't repeated.

Dave

P.S. I'm not a rocket scientist, but I play one... :*)

[Diandra Leslie-Pelecky]

Wow, Dave - you should have written this blog entry. The Rocket Scientist was telling me about Molniya orbits, but I didn't have time to mention them. Thanks!

[agm]

Low sunspot counts don't cause an increase in drag. It's probably better to say that the processes that lead to more or fewer numbers of sunspots also affect the energy density output of the sun, which is ultimately how the sun affects how compressed the magnetosphere gets. The sunspots are more like the speedometer than the driver.

And having established that mental image, I'll commit the cardinal sin of immediately abandoning it in favor of another. Imagine an inner tube mounted on a pole in a stream of flowing water. In fact, imagine a whole series of them nested like Russian dolls. If you make the flow stronger, it compresses the donuts in the direction facing the flow, the way that squeezing balloon animals distorts their shape. If make the flow weaker, external pressure drops, and the donuts can undo some of the shape distortion.

In this case the nested donuts are the earth's magnetosphere and ionosphere. When solar wind reaches the earth, it pushes on the magnetosphere/ionosphere system similar to how the water flow pushes on the nested donuts. If you increase the ram pressure of the solar wind (i.e., it flows faster, or the particle density increases), the magnetosphere shrinks, as does the ionosphere since there are coupling mechanisms that keep the two related. If the ram pressure drops, the opposite happens. If the sun puts out more energy, then ionospheric density rises since more particles are being ionized, and the atmosphere as a whole grows, like when you heat the air in a hot air balloon. (This, coincidentally, is why density drops on the nightside of the earth compared to the dayside. No ionizing sunlight, so ions recombine into neutral atoms and molecules. In fact, no energy input, so atmospheric density drops at any given altitude since temperature drops.)

During solar quiet times, which we know from things like ACE data and counting sunspots and so on, the pressure typically drops compared to active solar periods, the magnetosphere expands, allowing the ionosphere/atmosphere to expand, which means there is an increase in atmospheric density at a given altitude. Greater density, greater drag. I'm told that this is a pretty accurate explanation for why Skylab's orbit decayed significantly faster than was planned, and that a fair amount of what was figured out as a result of learning what happened to Skylab. It also is why the shuttle has been used a number of times to boost ISS into higher-radius orbits.

[eingram]

I know this is off topic, but I have a physics question.
In the game of computer solitaire, does dragging a stack of cards require more energy than dragging one?
At first glance this seems like a frivolous question, but it seems to me that amount of electrons emitted might be different.
I realize there wouldn't be a huge difference.
Thanks for considering this in advance.

[Partially Deflected]

Plenty of satellites are in Polar Declining orbit. Most weather satellites and some intelligence satellites are polar. A satellite in polar orbit covers the entire Earth by "flying" directly from pole to pole while the Earth revolves under it.

For U.S. launches it's easy, if it's launched from Vandenberg AFB then it's going into polar orbit; launches from Kennedy Space Center are going into equatorial orbit (geosynchronous or LEO): if it is actually launched from Cape Canaveral AFS then it's a military bird and probably is not going to be covered in the news.

Your local and national news anchor have no clue and refer to KSC as "Cape Canaveral". The shuttle does not take off from or land at Cape Canaveral. There have been no manned launches from the Cape since Gemini.

[Partially Deflected]

oops - I meant rotates, not revolves.