Planetary Data: Collection to Use
Since planetary scientists can't easily visit the places they study, they rely on data that was collected by spacecraft like orbiters and rovers. But collecting data in space isn't just a simple click of the camera! Learn more about how we collect data, make it useable, and then use it to study outer space below.
How do we collect planetary data?
Planetary scientists have a unique challenge: how do we study a place we can't go ourselves? Except for a very small number of locations on the Moon where the Apollo astronauts landed, humans have only been able to observe other planetary bodies using data collected by spacecrafts like orbiters and rovers. This type of observation is called remote sensing, and it is an important part of planetary science.
Most remote sensing happens using spacecraft that orbit or fly past a planetary body. Sometimes they observe a planet or moon for many years, like the Mars Reconnaissance Orbiter (MRO) spacecraft shown here. MRO arrived at Mars in 2006 and is still being used today! Others fly past a planetary body and are only able to observe it once, like New Horizons' flyby of Pluto in 2015. Since orbiters and other space-based spacecraft image a surface from far away, they are limited in the amount of detail they can show. But you might be surprised! The High Resolution Imaging Science Experiment (HiRISE) camera on the MRO spacecraft has a resolution of 25 centimeters per pixel (each pixel in the image covers 25 centimeters - roughly the length of a standard sheet of paper - of Mars' surface). That means you can use HiRISE images to see things that are the size of a small table on the surface of Mars!
In some locations, planetary scientists are able to explore using landed spacecraft like rovers. These ground-based spacecraft allow scientists to make detailed observations of rocks and sediments, sometimes down to individual grains and crystals that are the same width as the lead in a pencil!
Ground-based and satellite observations are both very useful in planetary science, and the two work together to give planetary scientists as much information about a planetary body as possible. Spacecraft often carry many different scientific instruments and types of cameras, and the data from each different instrument can help scientists figure out different parts of the puzzle that is the geologic history of a planetary body.
Learn more about some of the ways USGS scientists do remote sensing, including remote sensing of our own Earth, using the links below:
The Perseverance Rover's Mastcam-z
landing site selection for Mars rovers
Observing the Earth with Landsat
How do we make planetary data usable?
Unlike taking pictures with your camera or phone, remote sensing data is not immediately useable. Instead, there are many steps that must happen between the spacecraft and someone using the data.
1) Georeferencing - Georeferencing is the process of putting the pixels in the right place. When data is georeferenced, it is being assigned to a specific location on the surface using coordinates like latitude and longitude. Until georeferencing is completed, there is no location information attached to the data. To georeference data, scientists use information about where the spacecraft was located and how it was oriented (where it was pointing) when it was collected. Once data is georeferenced, it can be viewed in mapping and analysis software, but it's not yet ready to use...
2) Calibration - Before data can be used, it must also be calibrated. Calibration is the process of adjusting the pixel values so they are meaningful and accurate. The data that is sent back to Earth from the spacecraft does not always contain the final, usable pixel values. Instead, it must be calibrated and adjusted once it arrives at Earth. Each instrument has different calibration processes, but the goal is always the same - to make the pixel values mean something to the scientists that will use the data. This may be assigning specific elevations, temperatures, or brightness values. Once data is calibrated, it can be directly compared to other images from the same instrument, and scientists can use the pixel values to see what areas are higher or lower than others. Uncalibrated data can not be directly compared to other images because the pixel values are not yet meaningful.
Sometimes an additional step is completed - orthorectification ("ortho" = straight, "rectification" = make correct). Orthorectification is the process of adjusting an image to remove the effects of topography (shown below). This is needed because spacecraft do not usually observe a planetary body straight down from above (observing something from directly above is called nadir). Instead, they usually collect data at a slight angle to the surface (called off-nadir). This means that what gets imaged will be affected by the terrain, and the image is not spatially accurate. Instead, slopes get shortened or lengthened depending on if they are facing the camera. For example, a camera may be able to see a cliff face that it wouldn't be able to see if it was looking straight down from above. This can be useful for scientists since it allows them to see things they wouldn't be able to see from above. But sometimes scientists need to have an image that is adjusted (orthorectified) so it is spatially accurate. The first step in orthorectification is creating a digital elevation model (DEM). This is a 3D model of the surface that shows all the changes in elevation. An example of a DEM is shown above. Then, the elevation model is used to stretch and shrink the image so it is adjusted for the terrain. This final version of the image is called an orthoimage or orthomosaic.
What kinds of planetary data do we use and what can they tell us?
Because they can’t physically visit their study locations, planetary scientists use many different kinds of data. This allows them to understand the a planetary body in many different ways, not just how they look to the naked eye. Spacecraft carry instruments called spectrometers that collect images using visible light, but also infrared and ultraviolet light, which are all outside the range that is visible to the human eye. These instruments measure the amount of reflected or emitted light at many different wavelengths. This can reveal what minerals and chemicals are present (example shown below), much like the fingerprint of a rock. Infrared data can also measure how quickly a surface heats up and cools off during the day-night cycle (thermal inertia), which can be used to estimate how solid a material is: solid rocks heat up and cool down slowly, while loose materials like sand heat and cool rapidly.
Planetary scientists also use elevation (topography) and radar data. Some spacecraft carry instruments like lasers that can measure the height of the surface and be used to create topographic maps. Topographic data and imagery can also be combined to make 3D models of the surface, which can help planetary scientists better understand how features (for example, different rock units) relate to each other in space, or how materials move across a surface (for example, sand grains down a slope). Radar instruments can be used to study both the surface and the subsurface of a body - the Shallow Radar (SHARAD) instrument on the Mars Reconnaissance Orbiter can actually "see" up to 1 kilometer deep in Mars' crust! Other radar instruments are reflected off the surface, and tell planetary scientists things like how rough a surface is. Since we can't see through Venus' atmosphere, radar is the main way to "see" the surface!
Ground-based spacecraft like rovers can carry other instruments that need to be in contact with or very close to a material to collect data. These instruments can do analyses similar to what can be done in a laboratory on Earth - they can grind rock into powder and analyze it with x-rays, or heat it and observe the gasses or energy that are released. They can also fire lasers at rocks and measure the energy that is released to identify what the rock is made of. They also carry cameras that can take detailed pictures of the surface - from far away hills to individual crystals in a rock.
Explore some planetary data through the USGS Astropedia Data Catalog!
Since planetary scientists can't easily visit the places they study, they rely on data that was collected by spacecraft like orbiters and rovers. But collecting data in space isn't just a simple click of the camera! Learn more about how we collect data, make it useable, and then use it to study outer space below.
How do we collect planetary data?
Planetary scientists have a unique challenge: how do we study a place we can't go ourselves? Except for a very small number of locations on the Moon where the Apollo astronauts landed, humans have only been able to observe other planetary bodies using data collected by spacecrafts like orbiters and rovers. This type of observation is called remote sensing, and it is an important part of planetary science.
Most remote sensing happens using spacecraft that orbit or fly past a planetary body. Sometimes they observe a planet or moon for many years, like the Mars Reconnaissance Orbiter (MRO) spacecraft shown here. MRO arrived at Mars in 2006 and is still being used today! Others fly past a planetary body and are only able to observe it once, like New Horizons' flyby of Pluto in 2015. Since orbiters and other space-based spacecraft image a surface from far away, they are limited in the amount of detail they can show. But you might be surprised! The High Resolution Imaging Science Experiment (HiRISE) camera on the MRO spacecraft has a resolution of 25 centimeters per pixel (each pixel in the image covers 25 centimeters - roughly the length of a standard sheet of paper - of Mars' surface). That means you can use HiRISE images to see things that are the size of a small table on the surface of Mars!
In some locations, planetary scientists are able to explore using landed spacecraft like rovers. These ground-based spacecraft allow scientists to make detailed observations of rocks and sediments, sometimes down to individual grains and crystals that are the same width as the lead in a pencil!
Ground-based and satellite observations are both very useful in planetary science, and the two work together to give planetary scientists as much information about a planetary body as possible. Spacecraft often carry many different scientific instruments and types of cameras, and the data from each different instrument can help scientists figure out different parts of the puzzle that is the geologic history of a planetary body.
Learn more about some of the ways USGS scientists do remote sensing, including remote sensing of our own Earth, using the links below:
The Perseverance Rover's Mastcam-z
landing site selection for Mars rovers
Observing the Earth with Landsat
How do we make planetary data usable?
Unlike taking pictures with your camera or phone, remote sensing data is not immediately useable. Instead, there are many steps that must happen between the spacecraft and someone using the data.
1) Georeferencing - Georeferencing is the process of putting the pixels in the right place. When data is georeferenced, it is being assigned to a specific location on the surface using coordinates like latitude and longitude. Until georeferencing is completed, there is no location information attached to the data. To georeference data, scientists use information about where the spacecraft was located and how it was oriented (where it was pointing) when it was collected. Once data is georeferenced, it can be viewed in mapping and analysis software, but it's not yet ready to use...
2) Calibration - Before data can be used, it must also be calibrated. Calibration is the process of adjusting the pixel values so they are meaningful and accurate. The data that is sent back to Earth from the spacecraft does not always contain the final, usable pixel values. Instead, it must be calibrated and adjusted once it arrives at Earth. Each instrument has different calibration processes, but the goal is always the same - to make the pixel values mean something to the scientists that will use the data. This may be assigning specific elevations, temperatures, or brightness values. Once data is calibrated, it can be directly compared to other images from the same instrument, and scientists can use the pixel values to see what areas are higher or lower than others. Uncalibrated data can not be directly compared to other images because the pixel values are not yet meaningful.
Sometimes an additional step is completed - orthorectification ("ortho" = straight, "rectification" = make correct). Orthorectification is the process of adjusting an image to remove the effects of topography (shown below). This is needed because spacecraft do not usually observe a planetary body straight down from above (observing something from directly above is called nadir). Instead, they usually collect data at a slight angle to the surface (called off-nadir). This means that what gets imaged will be affected by the terrain, and the image is not spatially accurate. Instead, slopes get shortened or lengthened depending on if they are facing the camera. For example, a camera may be able to see a cliff face that it wouldn't be able to see if it was looking straight down from above. This can be useful for scientists since it allows them to see things they wouldn't be able to see from above. But sometimes scientists need to have an image that is adjusted (orthorectified) so it is spatially accurate. The first step in orthorectification is creating a digital elevation model (DEM). This is a 3D model of the surface that shows all the changes in elevation. An example of a DEM is shown above. Then, the elevation model is used to stretch and shrink the image so it is adjusted for the terrain. This final version of the image is called an orthoimage or orthomosaic.
What kinds of planetary data do we use and what can they tell us?
Because they can’t physically visit their study locations, planetary scientists use many different kinds of data. This allows them to understand the a planetary body in many different ways, not just how they look to the naked eye. Spacecraft carry instruments called spectrometers that collect images using visible light, but also infrared and ultraviolet light, which are all outside the range that is visible to the human eye. These instruments measure the amount of reflected or emitted light at many different wavelengths. This can reveal what minerals and chemicals are present (example shown below), much like the fingerprint of a rock. Infrared data can also measure how quickly a surface heats up and cools off during the day-night cycle (thermal inertia), which can be used to estimate how solid a material is: solid rocks heat up and cool down slowly, while loose materials like sand heat and cool rapidly.
Planetary scientists also use elevation (topography) and radar data. Some spacecraft carry instruments like lasers that can measure the height of the surface and be used to create topographic maps. Topographic data and imagery can also be combined to make 3D models of the surface, which can help planetary scientists better understand how features (for example, different rock units) relate to each other in space, or how materials move across a surface (for example, sand grains down a slope). Radar instruments can be used to study both the surface and the subsurface of a body - the Shallow Radar (SHARAD) instrument on the Mars Reconnaissance Orbiter can actually "see" up to 1 kilometer deep in Mars' crust! Other radar instruments are reflected off the surface, and tell planetary scientists things like how rough a surface is. Since we can't see through Venus' atmosphere, radar is the main way to "see" the surface!
Ground-based spacecraft like rovers can carry other instruments that need to be in contact with or very close to a material to collect data. These instruments can do analyses similar to what can be done in a laboratory on Earth - they can grind rock into powder and analyze it with x-rays, or heat it and observe the gasses or energy that are released. They can also fire lasers at rocks and measure the energy that is released to identify what the rock is made of. They also carry cameras that can take detailed pictures of the surface - from far away hills to individual crystals in a rock.
Explore some planetary data through the USGS Astropedia Data Catalog!