The following was originally published on the Stellaris forums by Murmeldjuret as a guest post on my series. Reproduced here with permission. Thanks, Murmeldjuret.
Today I will be continuing TheBeautifulVoid’s astroknowledge series.
In 1728 Newton first described a hypothetical cannonball that would orbit the earth like the moon did. An orbit, like the moon, but artificially constructed. In 1903 Konstantin Tsiolkovsky calculated the exact speeds and dynamics needed for artificial satellites, and proposed using liquid fuel rockets to lift them there. Since the launch of Sputnik, United States Space Surveillance Network has tracked 26000 objects in space, of which now only 8000 remain, the rest succumbing to the fiery grave of reentry. Today, there are roughly 1000 operational satellites orbiting our planet.
There are many types of satellites, the four main being:
- Communication satellites (satellite tv, mobile phones, internet etc)
- GPS satellite array
- Astronomical satellites looking outward towards the stars
- Earthward satellites looking down on earth
Up until around 600km altitude, atmospheric drag is significant enough to decay orbits and eventually send the satellite into reentry. Therefore most LEO satellites are in the 700-800km altitude region. The most common orbit for earthward satellites is sun-synchronous orbit, in a semi-polar orbit. This means that they are above the same spot on earth at the same time of day. Useful if you want to take images of the ground in broad daylight, when the sunlight is the brightest and you get the best readout on your instruments.
The main method used to monitor things is namely passive remote sensing, using the light of sun reflecting off what you want to see. Our eyes are passive remote sensors. This differs from active sensing that sends out its own signal to be reflected, such as RADAR or laser inferometry. Regardless, the atmosphere between the satellite and the ground has to be filtered out. The same atmosphere that makes ground to space imaging difficult makes space to ground imaging difficult. This is however, an easier problem as we know what we are looking at. Unlike ground to space that are placed above the cloudline, what we want to see here is usually below the cloudline, and we will thus not get any data for most wavelengths.
Geographic information system (GIS) is the term for mapping the appearance, distance, and topography of the planet’s surface. All have problably seen the various map applications today. Look at google maps of a major city or university town and you can probably tilt the view to see an approximate 3D view inferred from taking multiple pictures at multiple angles. You can actually see anything that is big enough to be above noise including heights of all trees, cars, and hedges. While a more time-consuming method than just flat images, it is remarkably effective. It is even possible to show what the city would look like from the ground, even though it is taken at an altitude of above 600km. Here is the Lund Observatory as approximated in 3D shape as seen from space.
Photosynthesis absorbs a specific band in the visible while green leaves reflect a specific band in near-infrared. The ratio between these forms the basis of something called vegetation index. An improved version, the Enhanced Vegetation Index, or EVI is being used more and more to determine the health of plants, farmland, forests, grass, and even plankton. See the top left of image below for Vegetation Index. Bottom left has a simple contrast false colour between absorbed visible and reflected NIR for plants. Satellites are better at telling us what is alive and what isn’t than standing 1m from it and looking at it with only our eyes.
(Image from DigitalGlobe, pretty worthwhile site if you want to see commercial satellite imaging)
With spaceborn interferometers you can measure phase shift from distance between orbit and surface. This is a remarkably exact height measure. It is used to measure glaciers, mountains, and the overall topography. Even more interesting is seeing them vary with time. How much does a glacier shrink every summer to grow back every winter, and how is the mean varying over the years?
The above image is a famous inferometer measurement of the Izmit earthquake in 1999. It measures the total change in height in a wavelike pattern from 0-2.8cm. The line in the middle is the faultline. Since any measurements from the surface would also be from an unknown height it has previously been hard to precisely measure how earthquakes change topography. But a satellite orbit cares nothing for earthquakes, and suddenly we have invariant measurements of earthquake dynamics.
Just like you can measure atmospheres of exoplanets with transit, you can do the exact same thing with earth. Measure our atmosphere’s spectrum by having it between a satellite and the sun/star. Since we can position a satellite with a specific altitude of atmosphere between it and the lightsource we can determine the atmospheric composition of different altitudes and locations. The ozone hole, the carbon dioxide concentrations over cities, the methane from cow farts, the water evaporation from warm sunny days, it can all be measured with both height and geographic location.
One of the weirdest data I have seen is a system that measures the age of snowfall. Freshly fallen snow is loosely packed with pockets that can reflect light internally before emitting it to space, whereas older snow loses these pockets. Measuring the ratio of directly vs indirectly reflected light will tell you quite reliably how old snowfall is. Daily maps of ski-resorts telling the quality and type of snow for the resort might actually be a thing in the future.
Can you detect crime from space? Yes you can. Using vegetation index from before, you can track forestry, and thus also illegal deforestation. Both smoke plumes from burning fields and a lack of photosynthesis can be seen from space, and has been used as evidence. Big Brother is watching.
Can you see religion from space? To some degree, you actually can. With muslim attendance day being friday, jewish saturday, and christian sunday, you can map CO2 emissions over denser cities and regions over many weeks, and the statistical deviation on friday-sunday actually corresponds to religious denomination of the region and is statistically significant. (Now I don’t want a religious debate of any kind in this thread so please refrain from starting one.)
There is an apocryphal story that during the recent conflicts in Afghanistan a military satellite tracked the footprints of an ambush group back to their base. While I doubt the credibility it is actually not unthinkable. Satellites are getting to that level, and what they can tell us is surprisingly much. It is cheaper to buy an image of your town from one of several satellite companies than it is to send out a helicopter or plane to take it from the air. For those who dislike agriculture growing dependent on a more and more complicated web, how is satellite imaging as a strand in that web? 100 minerals for +1 food anyone?