Upper Atmosphere Research Satellite

The Upper Atmosphere Research Satellite (UARS) is an orbital observatory whose mission was to study the Earth’s atmosphere, particularly the protective ozone layer.

The 5,900 kg (13,000 lb) satellite was deployed in 1991 from Space Shuttle Discovery mission STS-48. The original mission life was to be three years. UARS originally orbited at an operational altitude of 375 mi (604 km) with an orbital inclination of 57 degrees. In June 2005, six of the ten instruments were still operational.

On October 26, 2010 the International Space Station performed a debris avoidance maneuver in response to a conjunction with this satellite. The satellite is expected to fall from orbit some time between September 22 and September 24, 2011.

Instruments

Chemical studies

Cryogenic Limb Array Etalon Spectrometer (CLAES)

File:Claes.gif

Cutaway view of the CLAES instrument

CLAES was a spectrometer that determined the concentrations and distributions of nitrogen and chlorine compounds, ozone, water vapor and methane. It did this by inferring the amount of gases in the atmosphere by measuring the unique infrared signature of each gas.

In order to differentiate the relatively weak signature of trace gases from the background radiation in the atmosphere, CLAES had to have high resolution and sensitivity. To achieve this, the instrument combined a telescope with an infrared spectrometer. The whole instrument was cryogenically cooled to keep heat from the instrument from interfering with the readings. The cryogenics system consisted of an inner tank of solid neon at −257°C (−430°F) and an outer tank of solid carbon dioxide at −150°C (−238°F). As the neon and carbon dioxide evaporated, they kept the instrument cool. The final cryogens evaporated from the instrument on May 5, 1993 and the instrument warmed up, ending its useful life.

The instrument looked sideways out of the UARS platform to allow the instrument to look through the stratosphere and the lower mesosphere. CLAES produced a 19-month global database showing the vertical distributions of important ozone-layer gases in the stratosphere and their variation with time of day, season, latitude, and longitude.

Improved Stratospheric and Mesospheric Sounder (ISAMS)

Cutaway view of the ISAMS

ISAMS is an infrared radiometer for measuring thermal emission from the Earth’s limb (the line of the horizon as seen from UARS), on both sides of the spacecraft. It used the pressure-modulation technique to obtain high spectral resolution, and innovative stirling-cycle coolers to achieve high detector sensitivity. The specific objectives of ISAMS were (i) To obtain measurements of atmospheric temperature as a function of pressure, from the tropopause to the mesopause, with good accuracy and spatial resolution, and hence to study the structure and dynamics of the region, (ii) To investigate the distribution and variability of water vapour in the middle atmosphere, to determine its role in the atmospheric general circulation, and its sources and sinks in the middle atmosphere, (iii) To measure the global distribution of oxides of nitrogen and hence to investigate their origins and their roles in catalytic cycles which control the amount of ozone in the stratospheric ozone layer. It also made extensive observations of volcanic aerosols and polar stratospheric clouds in the middle atmosphere. The instrument operated from September 1991–July 1992.

Microwave Limb Sounder (MLS)

The MLS instrument before installation in the UARS spacecraft

The MLS detected naturally occurring microwave thermal emissions from Earth’s limb to create vertical profiles of atmospheric gases, temperature, pressure and cloud ice. MLS looks 90° from the angle of UARS’ orbit.

Thermal radiation enters the instrument through a three-mirror antenna system. The antenna mechanically scans in the vertical plane through the atmospheric limb every 65.5 seconds. The scan covers a height range from the surface up to 90 km (55 miles). Upon entering the instrument, the signal from the antenna is separated into three signals for processing by different radiometers. The 63-GHz radiometer measures temperature and pressure. The 183-GHz radiometer measures water vapor and ozone. The 205-GHz radiometer measures ClO, ozone, sulfur dioxide, nitric acid and water vapor.

As of June 2005, the 63- and 205-GHz radiometers are operational. The 183-GHz radiometer failed after 19 months of operation.

Halogen Occultation Experiment (HALOE)

Diagram of the HALOE instrument

HALOE uses solar occultation to measure simultaenous vertical profiles of Ozone (O₃), Hydrogen Chloride (HCl), hydrogen fluoride (HF), Methane (CH₄), Water Vapor (H₂O), Nitric oxide (NO), Nitrogen Dioxide (NO₂), Temperature, Aerosol Extinction, Aerosol composition and size distribution versus atmospheric pressure at the Earth’s limb. The measurements are done at eight different wavelengths of infrared across a 1.6 km (1.0 mile) wide field of view of Earth’s limb.

A vertical scan of the atmosphere was obtained by tracking the sun during occultation. The scan will measure the amount of solar energy absorbed by gases in the atmosphere.

In order to support scanning, the instrument came in two parts, the optics unit on a two-axis gimbal and a fixed electronics unit. The optics unit contains a telescope that collects solar energy as well as the gas detectors. The electronics unit handles data, motor control and power for the instrument.

Dynamics

High Resolution Doppler Imager (HRDI)

Diagram of the HRDI instrument

HRDI observed the emission and absorption lines of molecular oxygen above the limb of the Earth, uses the Doppler shift of the lines to determine horizontal winds and uses the line shapes and strengths to obtain information about temperature and atmospheric make-up.

The instrument consists of two parts, the telescope and the interferometer which consists of an optical bench and support electronics.

The telescope used a narrow field of view to prevent Doppler shift variation across the field of view from distorting the results. Input from the telescope is fed to the processor via a fiber optic cable.

HRDI conducted scientific operations from November 1991 until April 2005.

Wind Imaging Interferometer (WINDII)

Diagram of the WINDII instrument

The WINDII instrument measured wind, temperature and emission rate from airglow and aurora. The instrument looked at Earth’s limb from two different angles, 45 degrees and 135 degrees off the spacecraft’s angle of motion. This allowed the instrument to read the same areas of the sky from two angles within a few minutes of the previous reading.

The instrument consists of an interferometer which feeds to a CCD camera. The two telescopes (45 degrees and 135 degrees) each have a one meter long baffle tube to reduce stray light during daytime viewing. The input from the telescopes is positioned side-by-side on the CCD so both views are imaged simultaneously.

Energy inputs

Solar Ultraviolet Spectral Irradiance Monitor (SUSIM)

Diagram of the SUSIM instrument

SUSIM measured ultraviolet (UV) emissions from the sun. The observations are made both through vacuum and through occultations of the sun through the atmosphere. This allowed a comparison of the amount of UV light that reaches the earth and the amount absorbed by the upper atmosphere.

Because of the energy of UV, instrument degradation is a major issue. To help with this problem, the instrument contained two identical spectrometers. One was used almost continuously during the daylight portion of UARS’ orbit. The second was used infrequently to verify the sensitivity of the first.

Solar Stellar Irradiance Comparison Experiment (SOLSTICE)

The Solar Stellar Irradiance Comparison Experiment was designed to measure solar radiation. The instrument used a novel approach to calibration: instead of calibrating against an internal reference lamp, the instrument regularly took measurements of bright blue stars, which have theoretically very stable emissions over intervals on the order of the spacecrafts’ operational lifetime. The instrument’s input slit was configurable for solar or stellar modes, to accommodate for the vast difference in target brightness. In addition to stars, SOLSTICE also took occasional measurements of targets of opportunity, including the moon and other objects in the solar system. The instrument’s science team is at the Laboratory for Atmospheric and Space Physics (LASP) at the University of Colorado at Boulder. UARS SOLSTICE’s science mission has been carried on by a SOLSTICE instrument on the Solar Radiation and Climate Experiment (SORCE) spacecraft.

Active Cavity Radiometer Irradiance Monitor II (ACRIM2)

Photo of the UARS/ACRIM2 Total Solar Irradiance monitoring instrument

The ACRIM2 instrument on the UARS satellite measured the total solar irradiance (TSI), the total solar radiant energy reaching Earth, continuing the climate change database begun in 1980 by the ACRIM1 experiment on the Solar Maximum Mission (SMM). The ACRIM1 experiment's results provided the first discoveries of intrinsic variations in the TSI and their relationships to solar magnetic activity phenomena. ACRIM experiments have confirmed TSI variation occurs on virtually every timescale from their 2 minute observation cadence to the decades-long length of the TSI record to date. A precise knowledge of the TSI and its variation over time is essential to understanding climate change. Recent findings indicate that instrinsic TSI variation has had a much larger role (up to 50 %) in global warming during the industrial era than previously predicted by global circulation models (GCM’s). The profound sociological and economic implications of understanding the relative climate change contributions of natural and anthropogenic forcings makes it essential that the TSI database, a critical component of climate change research, be carefully sustained into the forseable future. The UARS/ACRIM2 experiment was an important part of providing the long term TSI database.

A bright pass of UARS photographed from the Netherlands on 16 June 2010 (photo M. Langbroek)

End of Mission and Re-Entry

Orbit Lowering Burn

UARS was decommissioned in 2005 and a final orbit lowering burn, followed by the passivation of the satellite's systems was performed in early December.

Re-entry

On September 7, 2011, NASA announced the impending uncontrolled re-entry of UARS and noted that there is a small risk to the public. As of September 15, 2011, the orbit of UARS was 235 km (146 mi) by 265 km (165 mi). Re-entry is projected for 23 September 2011. Some debris may survive to reach the surface.

Further reading

• Goddard Space Flight Center (1987). The UARS Instruments. In UARS Project Data Book, pp. 4–1–4-63. NASA.

External links

• UARS project science office page
• UARS summary from NASA Earth Science
• View and download HALOE data
• UARS Re-Entry Updates
• NASA's Satellite Upper Atmosphere Research Satellite (UARS) Fall Back To Earth On Friday