42 Years of Measuring the Sun, the Earth and the Energy in Between
By Denise Lineberry
On January 31, 1958, Explorer 1 became the first satellite launched by the United States. Its main scientific instrument, a cosmic ray detector, was designed to measure the radiation environment in Earth’s orbit. Although its last transmission was in May 1958, it continued to circle the Earth more than 58,000 times. While these looping orbits continued, NASA was busy building other revolutionary instruments to observe and better understand Earth’s systems.
In 1975, just five years after Explorer 1 burned as it entered Earth’s atmosphere, NASA’s first Nimbus instrument was launched, providing the world’s first direct observations of the amount of solar radiation entering and leaving Earth. This confirmed and improved early climate models and laid the foundation for NASA’s Earth Radiation Budget Experiment (ERBE).
In the 1970s, the ERBE team began planning the next phase of Earth radiation budget measurements. Retired ERBE experiment scientist Bruce Barkstrom recalled that the very first meeting of the ERBE science team required an entire day spent trying to determine exactly where the top of the atmosphere was. After much debate, they commissioned someone at NASA’s Langley Research Center in Hampton, Virginia, to develop the figure, which ultimately ended up being about 30 kilometers above the sphere that forms Earth.
“This was the level of detail we needed to address as a science team,” Barkstrom said.
In October 1984, ERBE was launched aboard NASA’s Earth Radiation Budget Satellite (ERBS) from the Space Shuttle Challenger (STS-41G).
“We had to get up at 3:30 in the morning to watch the ERBS launch at 7:30, and what I remember about that morning was that we had an overcast sky. And when the shuttle turned on, it was such a bright exhaust that it lit up the whole sky from below,” Barkstrom recalled. “And then, sure enough, the shuttle went through the clouds, and the light faded, and probably about a minute later the sky lit up again because the sun was reflecting off the exhaust.
“It’s impossible for me to describe this without getting a little emotional.”
For 10 years, ERBE has provided invaluable data to scientists studying energetic interactions between the Sun, clouds and Earth. Its satellite measurements provided new information about Earth’s radiation at the top of the atmosphere, including the significant radiative effects of clouds on incoming and outgoing energy in the overall process.
In the late 1980s, satellite instruments provided the first direct observation of the cooling of Earth’s climate by clouds. Former CERES Principal Investigator Bruce Wielicki developed an algorithm to apply to the Nimbus and ERBE models to help quantify cloud forcing – the difference between the components of the radiation budget for average cloudy and cloud-free conditions.
Armed with new knowledge about the important role clouds play in Earth’s energy budget, the science team was eager to collect more data. In 1997, the first in a new series of instruments, the Cloud and Earth Radiant Energy System (CERES), was launched, expanding the important ERBE measurements.
Six other CERES instruments have since been activated in space to measure solar energy reflected by Earth, the heat emitted by the planet and the role of clouds in this process.
“The CERES instrument is small, it’s very sleek, it’s probably the most precise radiometry NASA has ever used,” said Kory Priestley, CERES principal investigator. “We are now trying to build the next generation of instruments to meet the same requirements.”
The seventh and final CERES instrument was launched aboard NOAA’s Joint Polar Satellite System (JPSS)-1 in November 2017. It has since been activated and first light is expected in January 2018.
For 42 years, NASA has been observing the Earth’s energy balance. The NASA Langley Earth Radiation Budget Science Team is the only group producing ERB data globally. Although our understanding of Earth’s energy budget and the technology used to collect data has made considerable progress since Explorer 1 and Nimbus, this understanding is constantly evolving.
“With Earth observations, you never complete your understanding, so you’re always at the mercy of someone discovering new things,” Barkstrom said. “If you work in observational science, you will never have that final escape route to absolute certainty where you never have to change things.”
According to Barkstrom, attempts to understand the radiation budget began around 1880. Earth’s energy budget is a metaphor for the delicate balance between energy from the Sun and energy radiated to space. Continuous, stable and accurate data records over several decades are essential to understanding the Earth’s energy budget.
The data collected improves models that provide seasonal and longer-term forecasts, which help industry and policymakers better plan for the future.
NASA’s Total and Spectral Solar Irradiance Sensor (TSIS)-1 is currently on the International Space Station on a mission to measure the Sun’s energy input to Earth.
Various satellites have captured a continuous record of this solar energy input since 1978. The TSIS-1 sensors advance previous measurements, allowing scientists to study the Sun’s natural influence on the ozone layer, atmospheric circulation, clouds and Earth’s ecosystems.
These observations are essential to the scientific understanding of the effects of solar variability on the Earth system.
