RASC Calgary Centre - The Atmosphere, Astronomy and Green Lasers

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Part 1 - The Atmosphere and Green Laser Pointers First, let me say something about green laser pointers used by astronomers. These devices are dangerous! Green laser pointers are not a toy, should never be aimed at an aircraft, and should never be used by children. The light output from green lasers is extremely bright and can permanently damage the retina of a person's eye. In addition, they cast a beam that is at least several miles long. Several thousand aircraft pilots have complained that they have been blasted by lasers while in the air or on their landing approach. People in countries all over the world have been arrested for interfering with the operation of an aircraft. There are rumours that green lasers (and the new blue lasers) will be treated the same as offensive weapons, and if used against aircraft, the RCMP, FBI or U.S. Homeland Security agencies will probably get involved. The United States recently made it a Federal offense, punishable by fines up to $250,000 or up to 5 years in prison, to point a laser at an aircraft - see http://thomas.loc.gov/cgi-bin/cpquery/z?cp109:hr250: If you want to stay out of jail we suggest that you never, ever point any type of laser at an aircraft or let children play with these lasers ! If you would like to read more about airline pilot concerns surrounding this dangerous activity and arrests already made - just Google "green laser aircraft arrest". Think it won't happen in Calgary? - Think again and see: http://www.cbc.ca/canada/calgary/story/2008/07/07/laser-pointer.html and then see: http://www.cbc.ca/canada/calgary/story/2010/08/17/calgary-hawc-laser-pointer-beam-arrest.html and in July 2011,Man fined $5,000 for pointing laser at three Calgary aircraft and Calgary man fined $5000 for distracting aircraft with a green laser and Our Response According to the CBC: "The penalty if convicted of aiming a laser into a cockpit is $100,000 and/or five years in prison under the Aeronautics Act, according to Transport Canada's website. In the last two years, there have been 198 events involving lasers and aircraft in Canada." Going to Australia? It is now illegal to import these laser pointers into the country without a permit. see: http://www.customs.gov.au/site/page.cfm Unfortunately, in January 2011 there was another incident of a person (not an RASC member) using a green laser to flash several aircraft in Calgary. The individual was subsequently caught and charged. Jan 7, 2011 http://www.calgaryherald.com/Laser+beam+pointed+incoming+aircraft+Calgary/4067494/story.html Jan 29, 2011 http://www.calgaryherald.com/news/Charges+laid+after+planes+over+Calgary+with+laser/4187367/story.html The following is the letter that some of the RASC Calgary Centre Executive sent to the Calgary Herald outlining our position on the misuse of green lasers: RASC_Letter_to_the_Editor_February_2011.pdf Unfortunately, in August 2013 there was another incident of a person (not an RASC member) using a green laser to flash a police helicopter in Calgary. The individual was subsequently caught and charged. See http://www.660news.com/2013/08/01/teen-facing-charges-after-pointing-laser-at-police-helicopter/ and http://calgary.ctvnews.ca/man-charged-after-shining-laser-pointer-at-hawc2-1.1393277 What a green laser hit looks like through the window of an airplane cockpit: Would YOU like to try to land an aircraft full of passengers while blinded like this? Image credit: Unknown Image credit: Transport Canada UPDATE - February 27, 2020 The problem of people blinding airline pilots with green lasers has become so common that several companies now manufacture special glasses to stop the light during landings. https://www.laserpointersafety.com/laserglasses/laserglasses.html Green Laser Pointers: The maximum legal limit for power output from any laser pointer in the USA is a Class IIIa of 5 mW (milliwatt). There may also be more restrictive local regulations and the maximum wattage is lower in some other countries). I do not know what the law is in Canada, but any RASC member traveling to the U.S. should be aware of their regulations. If the light output is only 5 milliwatts, why are people concerned about blinding other people? The danger arises from the fact that the 5 milliwatts is concentrated in a very thin coherent light beam, and that the beam travels virtually unchanged for many miles. So what? - It's still only 5 milliwatts. Well, most people are familiar with 40, 60 and 100 Watt light bulbs but the "wattage" of a conventional bulb is mostly converted to heat. A 100 Watt incandescent light bulb puts out only about 5 to 7 Watts of visible light. (The new compact fluorescent bulbs can emit the same amount of light but at much lower wattage because they generate much less heat.) Also - the light from a "light bulb" is spread out in all directions (i.e. into a sphere). So let's convert that paltry 5 milliwatt beam to the equivalent wattage of an incandescent bulb. I measured the spot size my Green laser makes over a distance of 2 metres (see diagram below). The size of the spot was approximately 4mm in diameter (2mm radius). This is approximately the size of a person's pupil under daytime lighting conditions. The area of the spot = π R 2 = 12.6mm2 Note the diagram below. The surface area of a sphere is given by the equation: Area = 4 π R 2 where "R" is the radius of the sphere (in this case: 2m or 2000mm). For a sphere of this radius, the surface area is 50,265,482mm2. The sphere size is therefore 50,265,482mm2 / 12.6mm2 = approximately 4 million times the size of the spot. Assuming the entire 5 milliwatts of light was concentrated in an area of only 12.6mm2 this means the equivalent "light bulb" wattage would be 4,000,000 * 5 milliwatts = 20 thousand Watts! ** BUT ** Since an incandescent bulb produces about 7W of light for a 100W consumption, then we would rate a bulb that produces 20,000 watts of light as a "280,000 Watt" bulb. Ever looked straight into a 280,000 Watt light bulb from 2 metres? - Don't! At greater and greater distances the beam diverges and the spot size increases beyond the size of a person's pupil thereby limiting the amount of laser light that can enter the eyeball. However, it is still exceedingly bright at great distances and can easily distract or temporarily blind pilots several miles away. Part 2 - "Why does the beam from a green laser appear to "end" in the sky as compared to just going on forever?" DSLR photo of the beam from my green laser taken at SSSP 2006 - 15 sec exposure at f5, ISO 800, 18.5mm lens: The coherent collimated beam from a laser pointer travels many kilometres and can create a "spot" on very distant objects. In fact, several observatories use laser retroreflectors left on the Moon by the Apollo astronauts to reflect laser pulses from the Earth to the Moon (a round-trip distance of 800,000 km) giving the actual distance from the Earth to the Moon to within a few centimetres. [1] [2] [3] Lasers are also used to determine distances to satellites as well as military targets. The best colour to use to point out astronomical objects at night is green since the wavelength of modern green laser pointers based on Diode Pumped Solid State (DPSS) laser technology is 532 nanometres which is very near the peak of human visual sensitivity (555 nm). Thus, a 5 mW green laser pointer produces nearly the brightest beam allowable by law. In a vacuum, the laser beam itself would be invisible - regardless of colour. However, as the beam passes through Earth's Atmosphere some of the photons encounter large airborne particles which reflect some of the light back to an observer. However, this creates intermittent tiny bright flashes of light or "knots" in the beam (as can be seen on the left side of the photo above near the triangle of stars) - it is not why we can see the beam itself. It is extremely small airborne particles called aerosols [2] [3] having a diameter significantly less than the wavelength of the light that causes the beam to become visible. The effect of minute particles scattering light is called Rayleigh scattering [1] [2] and it's most noticeable effect is to turn the daytime sky blue. Rayleigh scattering causes photons to be scattered in a roughly spherical manner around these particles. Some of the light is scattered forward (in the direction of the beam), a lesser amount is scattered to the sides and about the same amount that is scattered forward is scattered backwards towards the light source. This backwards scattering is why the beam is more visible to people standing near the astronomer using it, than people standing some distance to the side. The more of these minute particles there are in the atmosphere, the more Rayleigh scattering there is. So: we let our eyes get fully dark-adapted so we can see dim light in the dark we choose a colour (green) to which our night-vision is the most sensitive (versus red to which we are least sensitive) we get the brightest laser pointer allowable by law we go outside and point the laser upwards at an astronomical object therefore we should see the beam going on-and-on forever - right? Answer: No - The beam seems to travel only a few hundred metres at most! Many people at star parties have commented to me on the "Luke Skywalker Light Saber" effect - that the beam is bright, yes, that it's green, yes, that it's highly visible, yes - but that it "stops" as a light saber would (even though it's a lot longer than Luke's). There are several reasons why the visible portion of the beam might "stop" even though we know that the actual laser beam itself travels upwards essentially unchanged forever: The inverse square law - light intensity drops off at the square of the distance from the source The thickness of the Earth's atmosphere (100 km) after which there are no atmospheric aerosols to scatter the light back to us Atmospheric layering where each layer has different physical and chemical properties Nope - none of these are the real reason - all these possibilities occur at much greater distances for a coherent collimated laser beam. The Planetary Boundary Layer The Planetary Boundary Layer is the lowest part of the Troposphere that is directly influenced by the presence of the Earth's surface. As in fluid dynamics, the air (a fluid) that is in direct contact with the Earth's surface moves very little (ignoring local winds). The air above can be in significant motion (e.g. the jet stream). When two fluids are in proximity where one is not moving and the other is moving a boundary layer must develop to accommodate the transition from one state to the other. Therefore, the portion of the atmosphere directly above the surface of the Earth is "capped" by a boundary layer. The thickness of the Planetary Boundary Layer changes from day-to-night and from day-to-day depending on local terrain (water, desert, earth, mountains) and local weather. This thickness varies from a few hundred to a few thousand metres (i.e. it is very thin compared to the thickness of the whole Troposphere.) The NASA DIAL project (Differential Absorption Lidar) uses lasers in the measurement of air greenhouse gases such as ozone and NO2 by combination of absorption and scattering of laser light. In the NASA DIAL technique, the atmospheric gas concentration profile is determined by analyzing the Lidar backscatter signals for laser wavelengths tuned 'on' and 'off' at an absorption line of the gas of interest. The absorption regions are in the Ultraviolet near 300 nm for ozone and in the Infrared near 727 or 815 nm for water vapor. Dr. Edward V. Browell, the Head of the NASA Lidar Applications Group at the NASA Langley Research Center explained the phenomena as follows: "We transmit high-power laser beams in the zenith on many occasions and observe the same optical effect you describe. The cause of this is the enhancement of aerosols (atmospheric particles) in the planetary boundary layer (PBL) which causes enhanced scattering of the laser beam back to your eyes. Above the PBL, which can be very low at night (<100 m), the amount of aerosols is very low compared to within the PBL, and as a result the scattering of the laser beam appears to end abruptly at the top of the PBL. More sensitive detectors, such as we use in Lidar, can continue to sense the scattering from aerosols and molecules well above the PBL demonstrating that the beam does not just stop there. If you are interested, you can see from our airborne lidar images that are posted on our web site (http://asd-www.larc.nasa.gov/lidar/lidar.html) how the aerosols (and scattering) vary across the atmosphere." The following image linked in from the NASA DIAL project illustrates the aerosol concentration in the lowest 10km of the atmosphere during one African overflight mission using airborne Lidar. Note the low altitude at which the sudden dropoff occurs: (click on the image to see a larger version on the DIAL website) (Image from the NASA DIAL projectweb site. So, does this only happen with hand-held laser pointers? Answer: Nope - see the following images: (clicking on these images will take you to the web page on which they reside) Part 3 - The Earth's Atmosphere The following links are excellent references regarding the makeup of the Earth's atmosphere: http://en.wikipedia.org/wiki/Earth's_atmosphere http://www.srh.noaa.gov/srh/jetstream/atmos/layers.htm http://liftoff.msfc.nasa.gov/academy/space/atmosphere.html Composition of the Atmosphere: The Earth's atmosphere contains: 78.084% nitrogen 20.947% oxygen 0.934% argon 0.033% carbon dioxide and trace amounts of other gases as well as about 3% water vapor, and suspended dust, spores, bacteria, aerosols, and pollutants Near the Earth's surface the nitrogen and oxygen are in molecular form: we symbolize them as N2 and O2. Near the top of the atmosphere these gases are in atomic form, N1 and O1, because the energy of sunlight breaks each molecule into two atoms. The argon of the atmosphere is always in atomic form, and its atomic weight and atomic motions are such that the Earth's gravity does not permit its escape from the Earth into interplanetary space. At sea-level, one cubic inch (1 inch x 1 inch x 1 inch) (16.39 cm3) of "air" contains approximately 400 billion billion (4*1020) air molecules, each moving at about 1600 km/hr (1000 miles/hr), and colliding with other molecules and anything else they come into contact with about 5 billion times per second. This is the reason for "air pressure". Layering of the Atmosphere: (Diagram by Roland Dechesne.) The Earth's atmosphere is generally classified into layers (based on physical characteristics such as temperature changes. chemical composition, movement, and density). From the highest to the lowest, these layers are called: Exosphere - from "outer space" down to about 500 km thermopause - the transition layer between the layer above and the layer below Thermosphere - from the Exosphere down to about 80-90 km and it includes: - the Ionosphere, the thin layer of sodium atoms used by observatories to create "artificial stars" for adaptive optics, - the area where the aurora form, - and also, about 60 to 65 miles (97 - 105 km) above Earth's surface, very high altitude winds at speeds of 200 to 300 miles per hour in a little understood region of Earth's atmosphere called the upper-atmosphere jet stream. mesopause Mesosphere - from the Thermosphere down to about 50 km, and is where very high altitude noctilucent clouds form stratopause Stratosphere - from the Mesosphere down to about 10km and it includes the "Ozone Layer" which protects us from the Sun's ultraviolet radiation situated approximately 20 to 30 kilometres (12 to 19 mi) above Earth, though the thickness varies seasonally and geographically. tropopause Troposphere - from the Stratosphere down to the Earth's surface and it includes the 'Planetary Boundary Layer' and the commonly known "Jet Stream" at about the height that airliners fly. Temperature and Pressure Profile: Atmospheric pressure at sea level is defined as "one Atmosphere". Other equivalent measurements are: 1,013 millibars, 760 mm or 29.92 inches of Hg (mercury), or 14.7 pounds/square inch (psi). This pressure decreases rapidly with altitude, dropping by a factor of 10 for every increase of 16 km (10 miles). The pressure is 1 atmosphere at sea level, 0.1 atmosphere at 16 km, and only 0.01 atmosphere at 32 km. The density of air also decreases rapidly with altitude. At 3 km (2 miles) air density has decreased by 30% and, for those that are not accustomed to it, can cause breathing difficulties or High Altitude Pulmonary Edema (HAPE) when the lungs fill up with fluid as a result of the body adapting to high elevation. HAPE can occur above 2.5 km. The highest permanent human settlements are at about 4 km (3 miles). Although the pressure and density of air decreases exponentially with altitude, the Temperature Profile of the atmosphere has a unique shape which is the result of the cooling due to the decrease in pressure and density of each layer as well as the warming effects from the Earth's surface below and Solar Radiation from above. The graph above was produced from elements of the "Standard Atmosphere Model" [1] [2]. Actual temperatures and pressures and the specific heights of the layers - especially those lower in the atmosphere - depend greatly on Solar and local conditions as well as local weather. The Troposphere - Where the Atmosphere Begins and Where We Live varies in thickness begins at the Earth's surface and extends to 6 km (4 miles) high at the poles and up to 20 km (12 miles) high at the equator The summit of Mount Everest at 8.848 km or 5.498 miles) is a little less than 1/2 the height of the Troposphere. at +/-50° Latitude it is about 8.9 km (5.5 miles) thick Almost all weather occurs in this layer of the atmosphere. The gases become thinner with height and the temperature also decreases from "surface temperature" to about -51°C (-60°F). The transition boundary between the troposphere and the layer above is called the tropopause. Both the tropopause and the troposphere are known as the lower atmosphere. 50% of the atmosphere by mass is below an altitude of 5.6 km. 90% of the atmosphere by mass is below an altitude of 16 km. The common cruising altitude of commercial airliners is about 10 km. The commonly known "Jet Stream" varies in altitude between 7-16 km. Where does the Atmosphere end? 99.99997% of the atmosphere by mass is below 100 km The highest airplane flight - by the X-15 in 1963 - reached an altitude of 354,300 ft or 108 km i.e. just into "outer space" Typical small meteorites burn up in the Mesosphere at heights from 80 to 100 km Therefore, 100 km is usually given as the thickness of the atmosphere (it varies), although in the rarefied region above this there are auroras and other atmospheric effects. The following diagram shows the Earth and its 100 km atmosphere to scale: Original image of the Earth from the Earth and Moon Viewer: http://www.fourmilab.ch/earthview/ Part 3 - The Atmosphere and Astronomy How the Atmosphere Affects Light and Other Radiation: It's a pretty thin layer compared to the size of the Earth, but it still affects the view we get of astronomical objects due to pollution, turbulence and diffraction. See my page on Atmospheric "Seeing" to understand how turbulence affects views. The Earth's atmosphere blocks most high-energy (high frequency) radiation - Gamma rays, X-rays, and a lot of UV rays - from reaching the surface. Some UV rays do get through prompting the posting of the UV index on weather channels so that people can take measures against sunburn and UV-induced skin cancer when it exceeds safe levels. The atmosphere is essentially transparent to wavelengths in the visible light spectrum. At longer wavelengths it blocks most incoming low-infrared and microwave energies. Fortunately or unfortunately it also blocks a lot of outgoing infrared energy causing the "greenhouse" planetary warming effect. At wavelengths longer than 10 -2m (1 centimeter) the atmosphere becomes transparent again for high-frequency radio transmissions. However, the "Ionosphere" - several layers of ionized air (having electrons knocked free of atoms by solar UV and X-ray radiation) high in the Thermosphere can reflect radio waves which permits long distance "over the horizon" radio communications. At wavelengths longer than approximately a meter, the atmosphere again becomes increasingly opaque. The Thickness of the Atmosphere Depends on Viewing Angle If we want to look through the "thinnest" amount of air we can: get higher up - i.e. on a mountain top look straight up rather than at objects nearer to the horizon Unfortunately, most objects of interest never pass "directly overhead" and so we find ourselves looking at them at an angle through varying amounts of air. The following scale diagram shows how the amount of air we look through varies from the vertical (the Zenith) at point "A" to the horizontal at point "B". In the diagram I am assuming we can ignore our height above the ground since it is usually very much lower than 100 km. (I am also ignoring the effects of atmospheric refraction as explained on my Horizon page): So if 100 km is the thickness viewing straight up, what's the thickness of the atmosphere viewing horizontally? The following diagram which is not to scale will help illustrate the solution: The distance d from point "A" to point "B" is one side of a right triangle. The other two sides are of length RE and RE+100km From the Pythagorean Theorem we know: The square of the hypotenuse is equal to the sum of the squares of the other two sides So: (RE+100)2 = (RE)2 + (d)2 Solving for d: (d)2 = (RE+100)2 - (RE)2 And: d = square root( (RE+100)2 - (RE)2 ) Substituting RE = mean radius of the Earth = 6371 km we get: d = square root(1284200) d = 1133 km That's 11.33 times more atmosphere than just looking "up"! So, how does this change with the viewing angle to the object from the horizon to the Zenith (called the angular altitude of an astronomical object)? Well - it's a little complicated, but it involves solving for the length of the hypotenuse of a triangle situated between a horizontal line and a curved line representing the outer edge of the atmosphere. The following diagrams which are not to scale will help illustrate the solution: For every angle α from 0° (towards the horizon) to 90° (i.e. the Zenith) the Thickness of the Atmosphere (T) is the length of the hypotenuse of a right triangle where the height (h) and the width (d) are both varying to fit under a curved line of radius RE+100 km. T is found by solving a "non-right-angle triangle" knowing only one angle α and the length of two sides RE and RE+100. The proof involves using the "Law of Cosines" and the positive solution for a "Quadratic Equation" to get T as a function of the angle α. The solution is that T increases slowly as the angle moves downward from the Zenith and then increases rapidly as the angle approaches the horizon. The clearest viewing and photography occurs when objects are at the highest possible angular altitude above the horizon, i.e. towards the bottom left of the diagram below: Graph of T = ( 2*RE*cos(90+α) + square root { (2*RE*cos(90+α))2 - 4*(RE2-(RE-100)2) } ) / 2 The shape of the curve is very similar to that of Atmospheric Refraction (the change in an object's Apparent Position) versus the object's angle from the Zenith as shown on my Horizon page. From Calgary, the Celestial Equator (0° Declination) appears at an maximum angular altitude of 90°-Latitude = 39° above the southern horizon. Due to the tilt of Earth's axis, the Ecliptic appears highest in winter at 23.4° above the Celestial Equator at 62.4° altitude, and it appears lowest in the summer at 23.4° below the Celestial Equator at 15.6° altitude. This changing position is explained on my Right Ascension and Declination page. Therefore items along the Ecliptic such as planets, asteroids, and comets are best seen (and photographed) during the winter months. The Moon's orbit is tilted 5.16° to the plane of the ecliptic and so the Moon can appear anywhere within 10.32° of the ecliptic. Winter Full Moons appear brighter and clearer than Summer Full Moons because during the summer months the light from the Moon is travelling through 2½ to 4½ times as much atmosphere.