The SORCE spacecraft
NASA FACT SHEET
Posted: January 23, 2003
The Sun is the dominant energy source for the entire Earth system, and solar radiation is one of the key climate system variables. Measuring and understanding the total energy output from the Sun or Total Solar Irradiance (TSI) and Spectral Solar Irradiance (SSI) addresses two difficult questions ‹ whether the Sun itself varies and if so, how. For hundreds of years scientists have been observing and learning about our Sun. Empirical models of the TSI based on very early observations (sunspot darkening, and facular and networking brightening) compare favorably with current actual TSI observations. This translates to a better understanding of the Earth's energy balance. For example, ozone is especially sensitive to changes in the Sun's ultraviolet radiation, and both observations and models show a close relationship of their concentrations to solar irradiance. Similarly, water vapor absorbs infrared radiation, and SORCE will provide the first extensive infrared SSI measurements from space.
The spectral measurements of SSI identify the irradiance of the Sun by characterizing the Sun's energy and emissions in the form of colors, i.e. wavelengths, that are absorbed by certain molecules in Earth's atmosphere, land and oceans. Ozone, water vapor and liquid, carbon dioxide, methane, etc, each absorb their own set of characteristic wavelengths. It is important to know how the energy in each wavelength varies, since that tells us how the heating varies in different parts of Earth's atmosphere, land, and ocean. Using the most advanced equipment available, data obtained by SORCE will be used to model the Sun's output and to explain and predict the effect of the Sun's radiation on the Earth's atmosphere and climate.
SORCE will continue a history of TSI measurements with greatly improved accuracy. Monitoring TSI using electrical substitution radiometers (ESRs) from the vantage point of space began with the launch of the Nimbus 7 satellite in November 1978. This was soon followed by an Active Cavity Radiometer Irradiance Monitor (ACRIM) instrument on the Solar Maximum Mission and by the Earth Radiation Budget Experiment. More recently, second and third ACRIM instruments have been launched, in addition to the launch of two instruments on the NASA / ESA Solar Heliospheric Observatory. The various data sets are in basic agreement and show conclusively that variations of TSI track the passage of sunspots across the solar disk with an amplitude of about 0.2%, and that long-term solar cycle variations are only on the order of 0.1%. SORCE will continue these important TSI observations, and will further improve their accuracy to the order of +/- 0.01%.
SORCE also continues earlier SSI measurements carried out in the 1990's. The UARS/ SUSIM and SOLSTICE experiments provided ultraviolet SSI. The SOHO/VIRGO experiment measures SSI at selected wavelengths in the ultraviolet (402 nm), visible (500 nm), and near infrared (862 nm). SORCE will greatly extend the spectral coverage, continuously covering the wavelengths from 1 to 34 nm (XPS) and also from 115 to 2000 nm (SOLSTICE and SIM). SORCE will also improve the SSI accuracy using a similar ESR technique to that described above for the TSI measurements of TIM. Accurate knowledge of the Sun's variations at all wavelengths that are heating important parts of Earth's atmosphere, land and oceans is an essential step to understand, model and predict impacts of the Sun on Earth.
The spacecraft carrying the instruments was developed by the Orbital Sciences Corporation in Dulles, Virginia, under contract to LASP. This spacecraft is three-axes stabilized with a control system to point the instruments at the Sun and the calibration stars. The stowed spacecraft is 39.4 inches (100 centimeters) wide x 63.1 inches (160.3 centimeters) high. With solar arrays deployed, SORCE is 133.6 inches (339.3 centimeters) wide x 63.1 inches (160.3 centimeters) high. The spacecraft, at launch, weighs 632.7 pounds (287 kilograms). The spacecraft is powered by 348 watts of electric power from its solar array.
The accuracy of TIM's readings will allow scientists to observe the subtle changes in solar radiation brought on by the sunspot cycles. They will use these numbers to determine just how much the Sun varies on a day-to-day, a month-to-month, and a year-to-year basis and then compare these variations to changes in the climate. The new readings will also help improve climate models.
Spectral Irradiance Monitor (SIM)
SIM will measure the SSI 4 times per day throughout the SORCE mission. This prism spectrometer uses only a single optical element to both disperse and focus solar radiation. The single optical element minimizes the susceptibility to deterioration of sensitivity on orbit. Sunlight entering this instrument is directed into a prism, which then directs different wavelengths of ultraviolet, visible, and near infrared into separate directions. The separate wavelengths of light will then illuminate five separate detectors including four photodiodes and one absolute detector - a miniaturized version of the TIM electrical substitution radiometer. The photodiodes measure the specific wavelengths of light between ultraviolet radiation and near infrared radiation.
To date, the time history of spectral irradiance variability in the visible and near infrared part of the spectrum is not nearly as complete, or as reliable, as the total solar irradiance and the solar ultraviolet radiation records. Understanding the wavelength-dependent solar variability is of primary importance for long-term climate change studies. By reviewing data from SIM, scientists may be able to tell how the solar cycles affect both visible and near-infrared wavelengths. Combined with improved measurements by ground and by aircraft, they also may be able to discern just how much of this light goes into heating up the lower layers of the Earth's atmosphere and how much goes into the land and oceans. SIM will also aid in efforts to discern exactly how much of the Sun's energy is reflected by industrial aerosols and clouds.
Solar Stellar Comparison Experiment (SOLSTICE)
The calibration for this instrument is unique. The stellar targets establish long-term corrections to the instrument sensitivity, as well as providing a basis for solar-stellar comparison for future generations. As SORCE passes through the nighttime portion of its orbit, SOLSTICE will measure the ultraviolet radiation coming from these selected blue stars. These stars emit spectra that have significant energy in the ultraviolet range measured by SOLSTICE, that are known to be constant in time. So if SOLSTICE's measurements from these stars change over time, then scientists know that the instrument's response has changed. They can then use the knowledge to make adjustments to their data.
Extreme Ultraviolet Photometer System (XPS)
Studies of the solar XUV radiation began in the 1950's with space-based experiments, but the knowledge of the solar XUV irradiance, both in absolute magnitude and variability, has been questionable due largely to the very limited number of observations. With the launch of the Solar and Heliospheric Observatory (SOHO) in 1995, Student Nitric Oxide Explorer (SNOE) in 1998, and Thermosphere-Ionosphere-Mesosphere- Energetics-Dynamics (TIMED) spacecraft in 2001, there is now a continuous data set of the solar XUV irradiance. With this history, significant advances in the understanding of the solar XUV irradiance have begun. The SORCE XPS, which evolved from earlier versions flown on SNOE and TIMED, will continue these XUV irradiance measurements with improvements to accuracy, spectral image, and temporal change.
Science data processing will commence automatically within 24 hours of data reception from the spacecraft. Preview (unfinished) versions of the SORCE data products will be made available to the public within approximately 48 hours, with finished research-grade data products being produced approximately 3 months later, after sufficient in-flight calibration data have been collected. Data products will be delivered to the Goddard Distributed Active Archive Center located at NASA Goddard Space Flight Center for subsequent distribution to the public.
Flight data file
Vehicle: Pegasus XL
Launch date: Jan. 25, 2003
Launch window: 3:10-4:08 p.m. EST (2010-2108 GMT)
Mission staging site: Cape Canaveral, Fla.
Satellite broadcast: AMC 2, Transponder 9, C-band
Launch timeline - Chart with the key events to occur during the launch.
Ground track - Map with the path the rocket will follow into space.
Pegasus - Overview of the air-launched Orbital Sciences rocket.
MISSION STATUS CENTER