Sun sets for a NASA solar monitoring spacecraft

August 11, 2014
Artist's rendering of the AcrimSat spacecraft. Credit: NASA

After 14 years of monitoring Earth's main energy source, radiation from the sun, NASA's Active Cavity Radiometer Irradiance Monitor satellite has lost contact with its ground operations team at NASA's Jet Propulsion Laboratory, Pasadena, California, and its mission has been declared completed.

AcrimSat's ACRIM 3 instrument was the third in a series of satellite experiments that have contributed to a critical data set for understanding Earth's climate: the 36-year, continuous satellite record of variations in total solar radiation reaching Earth, or total . The three ACRIM instruments have supplied state-of-the-art data during more than 90 percent of that time. Three other satellite instruments launched in 1995, 2003 and 2013 continue to monitor total solar irradiance.

Launched on Dec. 21, 1999, for a planned five-year mission, AcrimSat went silent on Dec. 14, 2013. Attempts since then to reestablish contact have been unsuccessful. The venerable satellite most likely suffered an expected, age-related battery failure.

The sun puts out a fairly stable amount of energy compared with many other stars. "That's where the term 'solar constant' comes from," said AcrimSat project manager Sandy Kwan of JPL, referring to a standard unit of measurement in astronomy. Over the sun's 11-year cycle, the average variation in visible light is about one-tenth of one percent—a change so small that scientists only discovered it when they were able to observe the sun from satellites above our light-scattering atmosphere. Kwan pointed out that AcrimSat's grandfather, the ACRIM 1 instrument on the Solar Maximum Mission satellite launched in 1980, was the first instrument to show clearly that solar irradiance does vary.

Although the percentage of change is minuscule, the energy it represents can have important effects on Earth. Scientists believe that sustained changes of as little as 0.25 percent in over periods of decades to centuries caused significant climate change in Earth's distant past. Today, as greenhouse gases accumulate in the atmosphere, it's critical to understand the relative contributions of variations in solar irradiance and human-produced greenhouse gases to changes in Earth's climate. To gain that knowledge, a long, continuous series of solar observations is an essential tool.

"The data record from the ACRIM series remains valuable for studying solar variability," said Greg Kopp, a senior research scientist at the University of Colorado's Laboratory of Astrophysics and Space Physics in Boulder. "This more than three-decade-long data series exceeds the duration of any other irradiance instruments."

Richard Willson, ACRIM principal investigator, has used the ACRIM data set to study cycles in the sun's variations. With co-investigator Nicola Scafetta of Duke University, Durham, North Carolina, Willson has been able to attribute some regular cycles of variation in irradiance to the alignment of planets and their gravitational tug on the sun. "The sun, Earth and Jupiter are aligned in their orbits every 1.09 years, and we see a bump in solar irradiance every year at that time," Willson explained. "That's just one of many cycles we have found. People have guessed at these effects for 150 years, but finding these frequencies in ACRIM data made it possible to pin down the effects for the first time."

Willson noted that the cycles have been connected with past changes in climate through analyses of air trapped for centuries in glacial ice. "Our measurements have contributed significantly to understanding the sun's effect on climate on time scales up to half a million years."

AcrimSat was built at a cost of $26 million, equivalent to about $45 million today. Kwan noted that the ACRIM 3 instrument was still working perfectly when the satellite lost contact and that AcrimSat's batteries had far exceeded their shelf life.

The spacecraft, built by Orbital Sciences Corporation in Dulles, Virginia, remains safely in orbit about 435 miles (700 kilometers) above Earth and is expected to stay aloft for another 64 years.

Explore further: Improved measurements of sun to advance understanding of climate change

Related Stories

SORCE satellite: A Decade in the Sun

April 1, 2013

NASA's Solar Radiation and Climate Experiment (SORCE) satellite has been providing data on the sun's irradiance for 10 years. SORCE measures electromagnetic radiation produced by the sun and the power per unit area of that ...

New solar instrument reaches orbit

November 21, 2013

An instrument that measures the sun's energy output is in orbit after it was launched last night on the U.S. Air Force's Space Test Program Satellite-3. The satellite was aboard a Minotaur I rocket that launched at 8:15 p.m. ...

NASA-funded mission to study the sun's energy

July 11, 2014

On July 14, 2014, a sounding rocket will be ready to launch from White Sands Missile Range, New Mexico a little before noon local time. Soaring up to 180 miles into Earth's atmosphere, past the layers that can block much ...

Recommended for you

New Horizons team selects potential Kuiper Belt flyby target

August 29, 2015

NASA has selected the potential next destination for the New Horizons mission to visit after its historic July 14 flyby of the Pluto system. The destination is a small Kuiper Belt object (KBO) known as 2014 MU69 that orbits ...

Prawn Nebula: Cosmic recycling

September 2, 2015

Dominating this image is part of the nebula Gum 56, illuminated by the hot bright young stars that were born within it. For millions of years stars have been created out of the gas in this nebula, material which is later ...

Image: Hubble sees a youthful cluster

August 31, 2015

Shown here in a new image taken with the Advanced Camera for Surveys (ACS) on board the NASA/ESA Hubble Space Telescope is the globular cluster NGC 1783. This is one of the biggest globular clusters in the Large Magellanic ...

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.