January 23, 2015 – The Soil Moisture Active Passive (SMAP) mission, scheduled for launch on January 29, will measure the moisture in Earth’s soil with greater accuracy and higher resolution than any preceding mission, producing a global map of soil moisture every three days. Here are five quick facts about the spacecraft and what it studies.
1. Soil moisture is a tiny fraction of water with a big punch. Only 0.001 percent of Earth’s total water is lodged in the top few feet of soil. That tiny percentage, however, affects all living things on land and plays an important role in moving water, carbon and heat between land and atmosphere.
2. Soil moisture can compound water risks. A flood follows a heavy rainfall — but only if the ground cannot soak up the rain. Waterlogged soil makes a region more flood-prone. Going to the opposite extreme, a drought can parch soil to such an extent that plants are unable to grow even after a few rains have fallen. Knowing soil moisture allows hydrologists to make better decisions related to the risk of flooding and drought, such as how much water to retain in reservoirs.
3. Soil moisture controls the on-off switch for carbon dioxide cleanup. The world’s vast northern forests remove carbon dioxide from the air as they grow, helping to clean up our emissions from burning fossil fuels. But when the ground freezes, that process switches off. Carbon dioxide builds up in the atmosphere until the ground thaws in the spring and plants begin growing again. Knowing where and for how long the ground is frozen or thawed is an important part of understanding the role of the northern forests in reducing greenhouse warming. SMAP will map frozen and thawed soils north of 45 degrees north latitude (about the latitude of Minneapolis), around the globe.
4. SMAP is a twofer. The spacecraft’s radiometer produces an accurate reading of how much moisture is in the top two inches (five centimeters) of soil, but it has low spatial resolution, that is, one measurement covers a large area. A radar instrument produces an image with higher spatial resolution, but it can’t measure soil moisture as accurately as a radiometer. Through sophisticated data processing, SMAP combines observations from the two instruments into a very accurate measurement with high spatial resolution.
5. SMAP has a huge, folding, spinning antenna. At 19 feet 8 inches (6 meters) in diameter, SMAP’s rotating mesh antenna dwarfs the size of the instruments and spacecraft and is the largest rotating antenna of its kind that NASA has yet deployed. But the entire dish furls into a cylinder one foot (diameter) by four feet (30 by 120 centimeters) to fit inside the rocket’s fairing for launch, and it weighs only 128 pounds (about 58 kilograms).
SMAP will launch from Vandenberg Air Force Base on a United Launch Alliance Delta II rocket and maneuver into a 426-mile (685-kilometer) altitude, near-polar orbit that repeats exactly every eight days. The mission is designed to operate at least three years.
SMAP is managed for NASA’s Science Mission Directorate in Washington by the agency’s Jet Propulsion Laboratory (JPL) in Pasadena, California, with instrument hardware and science contributions made by NASA’s Goddard Space Flight Center in Greenbelt, Maryland. JPL is responsible for project management, system engineering, radar instrumentation, mission operations and the ground data system. Goddard is responsible for the radiometer instrument. Both centers collaborate on science data processing and delivery to the Alaska Satellite Facility, in Fairbanks, and the National Snow and Ice Data Center, at the University of Colorado in Boulder, for public distribution and archiving. NASA’s Launch Services Program at the agency’s Kennedy Space Center in Florida is responsible for launch management. JPL is managed for NASA by the California Institute of Technology in Pasadena.