Space archaeology, also known as satellite remote sensing, is the use of satellites or aircraft to take pictures of Earth's surface that show subtle hints of buried features.
The methods employed are:
- Aerial photography - which got its start in France with balloonist Gaspard-Felix Tournachon, and which really came into its own during WW I.
- Multispectral and hyperspectral sensors — multispectral Scanners (MSS) were first included in the Landsat Program in 1972; hyperspectral sensors obtain the electromagnetic spectrum for each pixel in an image.
- Thermal Infrared Multispectral Scanners (TIMS) — were also first introduced in the Landsat Program, one has been included on each of the last five Landsat satellites.
- Color Infrared Film (CIR) — far-infrared refers to thermal imaging, while near-infrared uses wavelengths ranging from 700 nm to 900 nm.
- Microwave radar — uses radio waves to determine the heights of terrain.
The history of space archaeology
The roots of space archaeology are in the cold war, with the U.S.'s KH-11 Program of the 1970s. It involved satellites with cameras that used optoelectronics instead of film. They were able to view the Earth in gamma rays, x-rays, ultraviolet, and infrared, besides visible light.
When the KH-11 photos were declassified in the 1990s, archaeologists were able to view them, and one of those viewing them was the University of Alabama at Birmingham anthropology professor Sarah Parcak.
Using satellite images, Parcak and her team have found over 3,000 ancient Egyptian settlements, more than a thousand lost tombs, and over a dozen pyramids.
However, Parcak's premier discovery is the mapping of the lost city of Tanis, which was the capital of Egypt in 1,000 BCE. Yes, that's the same city in which Indiana Jones finds the Ark of the Covenant in the movie Raiders of the Lost Ark.
In 2010, only a small portion of Tanis had been mapped. Then, Parcak got satellite imagery that was taken during a wet time of the year. The mud-brick with which Tanis had originally been built, absorbed the water, and when Parcak looked at a satellite image, the entire city was revealed.
Parcak has also been able to use satellite imagery to determine that Egypt's Old Kingdom, which existed between 2,575 BC and 2,150 BC, ended due to a prolonged drought. She saw evidence of small settlements having been abandoned in favor of larger settlements.
This discovery has implications for today's climate change: in the face of drought, rural inhabitants will be forced to abandon their homes and move to larger cities.
In 2016, Parcak won the $1 million TED prize to build a website where the lay public can participate in making space archaeology discoveries.
In 2017, Parcak launched the Global Explorer website, which provides high-resolution satellite images to the public. Users then search for evidence of looting, besides searching for potential archaeological sites.
Parcak's work has been featured in three BBC documentaries, one featuring her discoveries in Egypt, one on her discoveries in ancient Rome, and the third on her attempt to find Viking settlements in Canada's Newfoundland.
In July 2019, Parcak released a book, Archaeology from Space: How the Future Shapes Our Past. In it, she describes how each pixel on a computer screen represents an actual area on the Earth's surface, and how everything on Earth's surface has its own distinct chemical signature that determines the light it reflects.
The future of space archaeology
Currently, the highest resolution satellite imagery is 0.3 meters or about one foot. However, resolutions down to 0.1 meters, or a centimeter, are coming.
Remote sensing experts assign what's known as "false color" to the features on images, such as vegetation, soil, water, or structures. These colors highlight the different classes of surface features.
While features sometimes show up in visible light, they are often more easily seen in other wavelengths of the electromagnetic spectrum. For example, near-infrared is better at spotting small differences in vegetation.
Another space archaeology resource is LiDAR, which stands for Light Detection and Ranging. It is a surveying method that measures the distance to a target by illuminating the target with laser light, then measuring the reflected light. Differences in return times and wavelengths are used to create a 3D representation of the target.
LiDAR is what was used to discover thousands of new Mayan sites in the Guatemalan jungle. In early 2018, the results of a LiDAR aerial survey of 800 square miles (2,100 square kilometers) of northern Guatemala were released.
They revealed over 60,000 previously undiscovered structures, including pyramids and entire cities.
Synthetic Aperture Radar (SAR) uses a moving radar antenna over a target region that provides finer spatial resolution than can be achieved with conventional beam-scanning radars. The data is then used to create 2D or 3D reconstructions of objects, such as a landscape.
Interferometric SAR (InSAR) uses two or more synthetic aperture radar (SAR) images to generate digital elevation maps by using differences in the phase of the waves returning to the satellite or aircraft.
INSAR is used in the monitoring of earthquakes, volcanoes, landslides, and structural engineering.
The cutting edge of space archaeology is drones, which can see at a resolution of a couple of centimeters. However, they are banned in places like Egypt.