Solar Stellar Irradiance Comparison Experiment

Solar Stellar Irradiance Comparison Experiment. It was a pioneering scientific instrument launched aboard the Gamma Ray Observatory in 1991. Designed to measure the total radiative output of the Sun with unprecedented precision, it also compared this output to that of other stars. The experiment provided critical long-term data for understanding solar variability and its influence on Earth's climate.

Overview

The experiment was a key component of the scientific payload on the Gamma Ray Observatory, a major satellite in NASA's Great Observatories program. Its development was led by scientists at the Goddard Space Flight Center, building upon earlier solar monitoring work from missions like the Solar Maximum Mission. The primary goal was to obtain a continuous, highly accurate record of the Total Solar Irradiance, a fundamental parameter in heliophysics and climatology. By also observing a set of bright, stable stars, it aimed to place the Sun's behavior in a broader stellar evolution context.

Instrumentation and Design

The core of the instrument was an active cavity radiometer, a highly stable device that absorbed sunlight to precisely measure its heating effect. This design was an evolution of technology flown on earlier missions like the Nimbus 7 satellite and the Earth Radiation Budget Experiment. For stellar observations, it utilized a dedicated telescope and photometer system. The entire instrument was mounted on the exterior of the Gamma Ray Observatory, requiring precise pointing and thermal control systems managed by the Goddard Space Flight Center team. Its calibration was traceable to national standards maintained by institutions like the National Institute of Standards and Technology.

Scientific Objectives and Operation

The primary objective was to monitor the Total Solar Irradiance over a significant portion of a solar cycle to understand variations linked to phenomena like sunspots and solar flares. A concurrent goal was to observe stable stars such as Sirius and Vega to determine if solar-like variability was common. Following its deployment from the Space Shuttle Atlantis during the STS-37 mission, the instrument began near-continuous operation. Its data was integrated into a broader climate record that included measurements from subsequent missions like the Solar and Heliospheric Observatory and the Solar Radiation and Climate Experiment.

Data and Key Findings

The experiment collected nearly a decade of precise data, covering the declining phase of Solar cycle 22 and the rise of Solar cycle 23. It confirmed that the Total Solar Irradiance varies in step with the sunspot cycle, decreasing during solar minimum. The stellar comparison data suggested that many stable stars showed very low levels of variability, making the Sun a relatively stable star over short timescales. These findings were crucial for models of Earth's energy budget and were cited in major assessments by the Intergovernmental Panel on Climate Change. The dataset was later cross-calibrated with measurements from the Variability of Solar Irradiance and Gravity Oscillations instrument on the Solar and Heliospheric Observatory.

Legacy and Related Missions

The experiment established a critical link in the continuous space-based record of Total Solar Irradiance that began with the Nimbus 7 mission. Its success paved the way for more advanced dedicated satellites like the Solar Radiation and Climate Experiment launched in 2003. The methodology of comparing solar and stellar irradiance influenced the design of instruments on later Great Observatories program satellites, including the Hubble Space Telescope. The long-term climate record it helped build remains a foundational resource for ongoing research at institutions like the National Oceanic and Atmospheric Administration and the World Meteorological Organization.

Category:Spacecraft instruments Category:Solar space missions Category:NASA programs