My colleague 'Yoda' leaned forward with a wry smile, as if he were about to spill a secret. "I couldn't reach the Rocket Scientist," he said of our common colleague who was on August vacation, "I hope he's not going to be too upset when he finds out his instrument is about to fly really close to the space shuttle exhaust plume." I mentally wrote this off to some type of space science joke that I clearly wasn't in on, but I was sitting Monday morning in a session at the American Geophysical Union meeting in San Francisco and darned if the whole session -- entitled "Active Experiments in the Ionosphere Using Chemical Releases From the Space Shuttle and Rockets" -- didn't mention the exact same incident. I went because I was wondering whether the conspiracy theorists were aware that researchers were releasing chemicals in the atmosphere and I wondered what the science behind it was. It turns out that rocket exhaust - and not just that from the space shuttle - is a useful tool in studying the upper reaches of the Earth's atmosphere.
I learned a number of interesting things from the session, including the fact that space scientists consider 'really close' to mean 87 kilometers.
I had to look up a little background information to fully understand the talks. I didn't have to go deep, but I did have to go high. The rest of us call the gas surrounding the Earth the 'atmosphere', but specialists divide the atmosphere into layers. The atmosphere has a mass of about 5 trillion trillion kilograms, but the molecules making up the atmosphere are not distributed uniformly with height - the density decreases the further you move away from the Earth's surface. Three-quarters of the atmosphere's mass is concentrated within a ring of about 11 kilometers (7 miles) away from the surface. The troposphere extends from the surface up about 10 km (6 miles). Continuing to move away from the Earth's surface, we have the stratosphere (which contains the ozone layer) and the mesophere. Finally, between roughly 80 km and 1000 km above the surface of the earth lurks the ionosphere, which includes the thermosphere and the exosphere. The actual numbers that define these regions vary from source to source, and of course, nature doesn't give us lines of demarcation. Regardless of where exactly it starts, the ionosphere is important to all us non-space-scientists because of its effect on communications.
Radio waves are electromagnetic waves and electromagnetic waves travel in straight lines. Since the Earth is curved, radio waves shouldn't be able to travel large distances, like from Newfoundland to England. The Earth's curvature should result in the waves traveling out into the atmosphere. But in 1901, Guglielmo Marconi (another notable scientist who was less than a stellar pupil in school) was in Newfoundland, using a kite to support a 500-foot antenna and received a Morse code representation of the letter 'S' that was sent from Cornwall, England. Although this is widely heralded as the first trans-Atlantic communication, Marconi was a little sloppy documenting his achievement. Rivals questioned whether he had actually communicated across the sea or just succeeded in picking up a lot of noise. So Marconi organized another, better documented, demonstration to silence his critics. In 1902, he took a ship from Great Britain and recorded signals sent daily from England to the ship as it traveled away from England.
In addition to demonstrating long-distance radio transmission (up to about 2100 miles), Marconi found a significant difference between nighttime transmissions and daytime transmissions. Medium and long-wavelength transmissions traveled further at night than they did during the day. We know now that communications also change during periods of unusual 'space weather', like sunspot activity.
The key to how the atmosphere affects communication lies in understanding how its composition changes with height and conditions. Dry air near the Earth (the stuff we breathe) contains about 78% nitrogen and 21% oxygen. The remaining 1% are made of miscellaneous gasses: Mostly argon, followed by carbon dioxide (CO2). Water vapor is present on a variable basis, depending on location and weather conditions.
The atmosphere gets less and less dense the further and further you move from the surface of the Earth. At some point, there are so few molecules that the gravitational force kind of sorts the molecules by mass. Heavier gases, like oxygen and nitrogen, reside around 100 km, while the higher altitudes are primary occupied by hydrogen.
A neutral molecule has the same number of protons and electrons, so neutral molecules have no net positive or negative charge. Ionizing a molecule (giving electrons to or taking electrons from the molecule) gives that molecule a positive charge (on losing electrons) or a negative charge (on gaining electrons). An oxygen molecule (O2) that loses an electron become a positively charged oxygen molecule (O2+) a.k.a. an ion.
Molecules are constantly being ionized. The densest regions of the atmosphere contain lots of ionized molecules and lone electrons. Since molecules prefer to be neutral, the ions and electrons pair up again as soon as possible, so the lower levels of the atmosphere are primarily neutral. As you rise in height to around 85 km above the surface of the Earth, the atmosphere is thin enough that the probability of an ion and an electron meeting up is pretty small and this creates a soup of positive molecular ions and negative free electrons called a plasma. The negative electrons and the positive ions are attracted to each other by the electromagnetic force, but they are too energetic to stay tied together in an electrically neutral molecule.
The ability of charge -- and variations in charge -- in the ionosphere to affect the propagation (transmission) of electromagnetic waves is characterized by the dielectric constant. The dielectric constant depends on the type and density of ionized particles the wave must pass through. Depending on the frequency of the wave and the dielectric constant of the region of space it is traversing, the molecules can absorb, reflect or refract the electromagnetic waves.
The truly specialized scientists divide the ionosphere into layers according to (roughly) what types of molecules are present, their density and the temperature. For example, the D layer (60-90 km above the Earth's surface), contains a lot of ionized nitric oxide. Above the D layer are the rather un-creatively named E and F layers. The ionization process is due primarily to the Sun, so as the intensity of the radiation from the Sun changes, the dielectric constant changes. The maximum ionospheric plasma density (approximately one million electrons per cubic centimeter) occurs at noon in the F region, around 250-300 km in altitude. When the Sun goes down, the degree of ionization can change significantly, since only cosmic rays are available to ionize the gas molecules. In fact, the D layer mostly disappears at night, which is a big advantage because the D layer is responsible for partially absorbing AM radio waves. This is why you can pick up some radio stations in the evening that you can't pick up during the day, and why some radio stations have to cut back their power at night. If the D layer is in the way, AM radio waves are reflected back to the ground; however, at night, the D layer disappears and the waves can travel further up to the E layer and be reflected there, returning to the Earth with greater strength than they would have had if they had to travel through the intensity-mooching D layer. AM radio waves, incidentally are in the range of hundreds of kilohertz: 890 on the AM dial is a frequency of 890 kilohertz or kHz. More importantly than pre-empting Coast-to-Coast AM, the ionosphere and it's 'space-weather'-related changes impact things like the global positioning system (GPS) and military communications.
The space shuttle engines we usually talk about are the ones that launch it into orbit, but the space shuttle also has an orbital maneuver subsystem, consisting of two engines that allow the astronauts to change the shuttle's orbit and position itself for returning to Earth. Each of the two hypergolic engines use nitrogen tetroxide (N2O4) to oxidize monomethylhydrazine (CH3(NH)NH2) and produce over 6,000 lbs of thrust. Such firings are visible from Earth and when they happen unexpectedly, as with a failed Russian rocket launch a week or so ago, people do make UFO reports.
P.A. Bernhardt of the Naval Research Laboratory noted that a ten-second burn of the orbital maneuver subsystsem engines on the Space Shuttle deposits over one gigajoule of energy into the atmosphere, which is about the amount of energy contained in the average lightening bolt. Bernhardt is the principal investigator (PI) of the SIMPLEX (Shuttle Ionospheric Modification with Pulsed Localized EXhaust) program, which uses ground-based radio waves of different frequencies to study the impact of space shuttle engine firings on the ionosphere. Bernhardt's paper gave an overview of SIMPLEX and other missions that utilize the space shuttle exhaust to study the ionosphere.
The idea of using rocket exhaust began when ionospheric irregularities were observed during the launch of Skylab in 1973. The space shuttle exhaust is mostly carbon dioxide (CO2), water vapor (H2O), nitrogen (N2) and hydrogen (H2) gases. The 10 kilograms of gases released each second interact with ambient O+ ions in the F-region, producing a lot of molecular ions that then recombine with the electrons in the plasma. (Remember that the lone electrons exist because they normally don't have a lot of molecules around them to interact with.) The density of electrons thus decreases, sometimes quite significantly, and the local depression in the plasma can persist for minutes to hours, depending on the specific conditions and location.
The decrease in free electrons changes the dielectric constant, but the sudden injection of high speed, much denser and colder gases (gases coming out a nozzle are cold, but that's another blog) into the ambient plasma can introduce large-scale persistent instabilities. For example, when a rocket fires on the underside of the ionosphere near the equator, a Rayleigh-Taylor instability can be initiated due to plasma interchange instabilities. Terrestrially, the Rayleigh-Taylor instability is responsible for things like weather inversions. These instabilities can have very long-lasting effects on the ionosphere as well.
Frank D. Lind of MIT Haystack Observatory reported on a study that investigated the impact of the exhausts at different angles relative to the local magnetic field in the ionosphere. (Electrically charged particles can be steered with a magnetic field.) They found that interactions at specific angles can create large-amplitude ion acoustic waves. Ion acoustic waves are like sound waves (where the molecules move parallel to the direction of wave propagation), but they are made of ions instead of air molecules. The amplitude of a sound wave is proportional to how loud the sound is, so a large-amplitude ion acoustic wave is the ionospheric equivalent to shouting in the library. Just as shouting in the library can stimulate librarian instabilities, the ion acoustic waves generated by the shuttle exhaust can stimulate plasma instabilities.
J.L. Baumgardner of Boston University was the one that really put the issue in perspective for me. Naturally occurring ionospheric storms can induce significant enough drops in the electron density to affect GPS. So can rocket engines. Baumgardner reported on one of the last launches of the Titan rockets in 2005. This giant produces about 100 times more exhaust per second than the space shuttle, depositing over 30,000 kilograms of exhaust over altitudes between 273 kilometers and 494 kilometers. Thirty-thousand kilograms is approximately equal to three really good sized blue whales worth of exhaust. Their group found, as I recorded in my notebook, 'a big ole hole' of reduced electron density (reduced to 75% of its original value) that persisted until the sun rose, by which time the exhaust molecules had dispersed and the ionizing process could re-start. Baumgardner used the phrase "man-made space weather", which was what got me really thinking given all the talk at this meeting about climate change. As if it weren't enough to worry about man-made climate change here on Earth, it turns out perhaps we need to start thinking about the impact of space travel on the atmosphere.
Marine navigation, highway traffic systems, commercial aviation, emergency systems, military systems and even the in-car GPS you use everyday depend on high-precision GPS services. Predicting space weather is even more challenging than predicting Earth weather, but a study of Dehel, et. al. suggested that sudden changes within short times could produce 14 meters (about 40 feet) of error in identifying positions. That might not make it likely that you can't find your hotel in a new city, but if you're the military targeting a smart bomb, that amount of error translates to a miss. Even more concerning for ordinary folk, having that amount of error would change how closely, for example, the FAA can have planes following each other when taking off and landing, which could greatly slow down the airport. So the ionosphere turns out to be much more important than I ever knew.
As for the Rocket Scientist's satellite flying within 87 km of the space shuttle exhaust, UTD is responsible for the CINDI (Coupled Ion Neutral Dynamics Investigations) aboard the CNOFS satellite. CINDI can measure the charged ions and the neutral molecules pressure, temperature and velocity, which means that it can map out where the molecules were going before the exhaust was emitted and where they were going afterward. He was actually happy to find that CINDI was able to fly "really close" to the space shuttle exhaust and I am going to catch up with him when we return to find out if he's learned anything from the results.