Imagine observations of our home planet, worldwide, being continuously updated – without satellites. Is that even possible? Our planet is changing – although the changes are tiny -; the human population is both growing and changing in a way that causes large resources to be needed in few dense areas. By using satellites as our eyes in space, we can measure changes hard to observe even in our back yard thanks to new technology. Observations are going from local to global – and local!
Today we’re able to monitor glaciers calving as well as areas with a great amount of phytoplankton drifting across the sea. We can keep an eye on bush-fires, see volcanos erupt and monitor small changes in the Earths surface due to an earthquake. And there is more to come!
The eyes of a satellite are designed to capture certain wavelength of ElectroMagnetic Radiation, EMR, just like human eyes. But when every human is doomed to see just the wavelengths between 380 and 750 nanometers there are few restrictions designing eyes for a satellite.Sensors can be designed to see EMR in the infrared or ultraviolet spectrum. You can even go further away from what we call “visible light”. When you have an idea for a mission, it’s important to find out what wavelengths are to be used for the best result.
The sensors in a satellite can be either passive or active. A passive sensor just receives EMR. The sun is a hot sphere of plasma with a “surface”-temperature around 6000 Kelvin. According to “black body radiation” an object as hot as the sun emits EMR in a wavelength our eyes are designed to detect. The Earth, on the other hand, is a quite cool object with an average surface temperature of 300 K. A consequence from that is that the EMR emitted by the Earth is around 20 times the wavelength of visible light, typically 10-11 microns. Such wavelengths, thermal infrared, are important to Earth Observation Satellites. By detecting EMR emitted by the Earth´s oceans we can monitor the temperature on the surface of the oceans, crucial for our climate.
Active sensors generate their own ElectroMagnetic Radiation. The satellite sends pulses to the Earth and measures what comes back. It works pretty much like a radar. By this technique it’s, for example, possible to monitor the height of the ocean surface. Often satellites are equipped with a mixture of active and passive sensors.
Earth Observation Satellites, EOS, are to be found in different orbits, depending on the mission. A Meteosat in geostationary orbit is can “look” at Europe and Africa continuously but can never give us information about America. The altitude of a satellite in GEO also affects the picture. The resolution of an image taken by Meteosat could be three to five kilometers per pixel. Sentinel-2A, orbiting Earth at an altitude of 786 km and equipped with a high-resolution imager gives us images with a resolution of ten meters per pixel.
To be continued…
Jan teaches mathematics and interdisciplinary science to pupils 13-16 years of age at Sursik School, Pedersöre, Finland. Space-related science often gives some sort of answer to the question “Why?”, a question quite common in math class. It also triggers curiosity, one key component in progress.