How We Study Earth and Other Planets from Space
Late the other night my friend Joan called from the Cachuma Lake campground and asked excitedly what the string of lights was that had just tracked across their sky. Was it a UFO? Luckily, I had heard about Elon Musk’s latest launch of about 60 small satellites as part of Starlink, a satellite-based internet. I later confirmed the launch by checking on the website heavens-above.com, which lists when and where to see satellites (there are several apps that do the same thing).
In addition to the Starlink series, there are currently more than 3,000 active satellites circling Earth. About 2,000 are from the U.S.; NASA runs 16 of them. Satellites run the gamut from communications (including Starlink and other internet relay satellites), to weather watchers, to remote sensing satellites used for research and monitoring of the Earth. There are three main regions where these satellites operate: Geostationary Earth Orbit (GEO), Low Earth Orbit (LEO), and everything in between: Middle Earth Orbit (MEO).
LEO, from about 100 to about 600 miles altitude, is easiest to get to and, being lower, yields a better view for remote sensing satellites. It’s also convenient that, when a satellite dies, it’ll eventually re-enter Earth’s atmosphere and burn up so as not to add to the growing list of space junk. GEO is at the altitude where a satellite’s motion around the Earth is at the same rate as the Earth turns, meaning the satellite appears stationary above the Earth’s equator. That altitude is 22,236 miles.
Some communication and weather satellites are in GEO. Because it’s such a precise altitude, there’s some competition for slots up there. And if a satellite fails way up there, it’ll never come down, so aging satellites are usually moved higher or lower to get them out of the way. There are about 560 active GEO satellites.
In addition to altitude, satellites can circle the Earth either around its equator, over the poles, or in between. Equatorial orbits are easier to attain and so U.S. satellites are typically launched from Cape Canaveral in Florida as the further south you go, the more the Earth’s rotation helps you.
U.S. polar-orbiting satellites are typically launched just up the coast, at Vandenberg Air Force Base because they can launch south over the ocean whereas the Cape has too many inhabited areas in that direction. Polar orbits are popular for monitoring the Earth because you can set your orbit, so it passes over any given place on Earth at the same local time. Also, you can arrange your orbit, so you never go into darkness, yielding more solar power and requiring smaller batteries.
Scientists use satellites to study and monitor different aspects of the Earth’s system. Geologists like myself have used them from the first days of the Landsat series, starting in the early ‘70s. Landsat captured views of the Earth in several different wavelengths of light: blue, green, red, and near infrared. Later, more bands were added as we realized that the colors of different minerals and soils had distinctive features at the different wavelengths. Your eyes do the same thing when you recognize the red of rusted iron or the green of jade. Extending our view into invisible wavelengths made our identifications even better.
Vegetation is also strongly colored, and not just at visible wavelengths. Chlorophyll reflects well at green wavelengths, so leaves are green, but healthy vegetation is even brighter in the near infrared, just beyond our seeing. The health of vegetation can be monitored by watching those near IR wavelengths.
Water is more difficult to monitor as most wavelengths (other than blue) are absorbed. But concentrating on the blue wavelengths allows some degree of monitoring capability and the depth of shallow water can even be detected. Some scientists are using satellite images in the blue part of the spectrum to monitor coral reefs.
The visible and near IR wavelengths we use to study rocks, soil, vegetation, and water are affected strongly by Earth’s atmosphere. The worst culprit is water vapor, which absorbs certain near IR wavelengths so strongly that the Earth’s surface is dark at those wavelengths, making them unusable. Clouds and dust also affect our ability to see the surface from orbit, so sometimes we have to wait for a clear view. But one person’s noise is another’s signal, and atmospheric scientists live for that contaminated data. As different gases absorb different wavelengths of light, these scientists can then track and measure the concentration of those gases in the atmosphere. The temperature of the atmosphere can even be measured using satellites observing in the longer thermal IR wavelengths.
Imaging radar is another remote sensing technique that transmits microwave signals and receives the echoes reflected from the Earth. It provides complementary information about landscapes, the ocean and has the advantage of operating day or night with waves that penetrate through clouds. Some radar systems are used to monitor rain as well.
Since they are less impacted by the atmosphere, radar satellites have proven to be useful for measuring 3D topography. A satellite with two antennas or a single satellite returning to almost the same place in its orbit, can triangulate down to the ground and measure the elevation of the Earth’s surface. That triangulation, though, is measured using the wavelength of the radar (a few inches) so it’s very accurate.
In 2000, I was involved with a space shuttle mission that used two antennas to make a 3D topographic map of almost the whole globe, and in a year or two a satellite will be launched that will use twin antennas to monitor the height of water surfaces like rivers, lakes, and sea level. What’s even better, is that changes in surface elevation can be measured even more accurately using repeated passes of a radar satellite. Changes of as little as a quarter inch have been measured and monitored. One of my projects over the last few years was monitoring the sinking of areas in the Central Valley as groundwater was pumped. We saw up to two feet of subsidence per year in the worst-hit areas.
Lasers can also be used to measure topography by timing pulses sent out from a satellite or aircraft. Aircraft Lidars, a takeoff on the acronym for Radar, are now used routinely to scan areas to produce high-resolution topo maps.
Not only the surface of the Earth can be studied with satellite remote sensing, but also the orbit of the satellite itself can be used to infer something about the internal structure of the planet. Details of how the orbit evolves over time can be used to infer how much of the mass of the planet is concentrated in its core.
But we’ve gone one better over the last 20 years. In 2002, a pair of satellites called GRACE (Gravity Recovery and Climate Experiment) were launched. Rather than looking directly down at Earth, their only measurement was to monitor the distance between each other to an accuracy equivalent to the width of a human hair. And they were 137 miles apart!
As one GRACE satellite chased the other Tom and Jerry style, their separation changed slightly: when the lead satellite was coming up on a denser area of Earth, say a mountain or an area of denser crust, it would feel the attraction and move ahead a little, increasing the distance between the satellites. Then the following satellite would catch up as it felt the mass attraction.
After many revolutions around the Earth, GRACE had made a map of mass variations around the globe and when the effect of mountains was removed, we could see into the crust. But the big news was that as the mission continued, we could see mass changes within the Earth and those changes had to do mostly with the movement of water and ice. GRACE could follow the melting of the Greenland Ice sheet, which has lost about 4 trillion tons of ice since 2002, and changes in the Antarctic ice cap, which hasn’t changed as much.
Even more exciting was that GRACE could see decreases in groundwater as it was pumped faster than it was recharged. Our research team was able to correlate groundwater mass loss in the Central Valley with the subsidence we saw in the radar measurements.
Our capabilities of studying and monitoring the Earth from space have progressed incredibly since the early ‘70s, but there continues to be a tension between trying new things, like the GRACE satellites, and ongoing monitoring with known systems like Landsat, which has been regularly updated for 50 years.
About every 10 years, NASA revisits its priorities for Earth observations from space, with the latest Decadal Survey published in 2017, listing many key objectives for the decade. NASA is also working with the private sector to beef up our monitoring capability. Companies like Planet Labs are lofting small satellites that can provide almost real-time high-resolution images of any place on Earth.
Scientists are also experimenting with tiny “CubeSats” that are only four inches on a side, making for a cheap launch. More than one cube can be coupled together to add capability. Cal Poly San Luis Obispo has lofted several already. The European Space Agency has stepped up with the Copernicus program which promises to continue some of the proven remote sensing systems, and NASA and its partners will likely continue the GRACE series as well as Landsat. Meanwhile, Elon Musk and others will continue to populate the skies with thousands of internet relay satellites to connect us all.