A Summary of the Upper Atmosphere Research Satellite (UARS)
Facts | Significant Scientific
Achievements | Top Eight Science
Highlights | Other Notable Achievements
Constituents, Dynamics, Particles, and
NASA officially announced its intent to develop the Upper Atmosphere
Research Satellite (UARS) in 1979. Nine instruments and a group of
theoretical investigators were chosen by an open proposal selection
process. These instruments and a brief description of their
capabilities are given in Table 1. An additional instrument, ACRIM, was
given a flight of opportunity on the UARS spacecraft. Due to funding
delays and the Challenger accident, UARS was not launched until 1991.
UARS is considered the first of the Mission to Planet Earth series of
- Charles Jackman, PS
- Anne Douglass, Deputy PS
UARS was deployed from the shuttle Discovery into a 585 km, 57 degree
inclination orbit on September 15, 1991. After deployment, all UARS
instruments began checkout and operation. There were no initial
instrument failures or major problems. The solar and MLS instruments
began to take data very soon after deployment, the infrared limb
sounding instruments were turned on later after to allow for instrument
The UARS solar array extends off to one side of the spacecraft, and
rotates to track the sun. During one of the monthly yaw maneuvers, the
rotation of the solar array is stopped. Then the observatory is yawed
around and the array is restarted, rotating in the opposite direction.
On June 2, 1992, the rotating UARS solar array began to exhibit motion
anomalies. All instruments were turned off while the problem was
diagnosed. The array was later restarted on June 17 using a work around
procedure. The motion anomalies arose from the drive clutch system.
Engineering model tests on the clutch suggested that metal debris is
generated during the periods when the array is started and stopped. The
accumulation of this debris prevents the clutch spring from engaging
the array drive shaft. If the clutch does not fully grip the drive
shaft, slippage occurs which wears the shaft and generates more
debris. After the restarting on June 17, the array functioned
nominally until September 1993 when the clutch began to slip again on
the drive shaft. After some effort, the backup drive was engaged. The
clutch system operated nominally until and Earth sensor failure in
March 1995. Complications resulting from this failure led to
additional solar array drive problems, and a decision was made in May
1995 stop the solar array and operate the instruments on a reduced
UARS has been operating in this mode for over a year now
and all working instruments have gotten enough time to continue
UARS experienced a further reduction in power in June 1997 with the loss
of one of three batteries. In October 1999, the UARS experienced
difficulties with its remaining tape recorder. Use of only 25% of a tape
recorder is now possible and the UARS is in a "real-time" operation
through communication using two Tracking and Data Relay Satellites (TDRS's).
The two TDRS's allow capture of about two-thirds of measurements
in a given orbit.
The priority instruments in the reduced power and power-sharing mode are
the HALOE, SOLSTICE, and SUSIM. Other instruments are operated as
the available power allows. Power is plentiful during the "day" part
of the UARS orbit and two instruments (HRDI and PEM) take advantage of
this situation by also operating in a "daytime-only" mode.
UARS has been operating in this mode for awhile now and all working
instruments have gotten enough time to continue producing data.
After functioning from turn on October 28, 1991 the ISAMS chopper wheel
ceased to rotate in mid-January, 1991. Attempts to restart the wheel
were unsuccessful until it suddenly began to rotate again two months
later. The mechanism failed again in July 1992. The ISAMS instrument
contains the first spaceborn Sterling cycle cooler. This cooler
continues to operate as part of the lifecycle test even though the
instrument does not take data.
In April 1993, the CLAES cryogen expired on schedule and the instrument
ceased taking data. The MLS 183 GHz radiometer failed about the same time.
This channel measured stratospheric water vapor and mesospheric ozone.
The 63 and 205 GHz radiometers continue to operate measuring T, ClO,
stratospheric ozone and lower stratospheric HNO3. The Particle
Environment Monitor (PEM) instrument has experienced failures
(October 1991 and May 1999) and degradations (June 1994 and
November 1998) in all of its low-energy proton sensors
(<35 keV); however, its proton sensors detecting energies
greater than 35 keV remain operational as well as its electron
and X-ray channels. All other instruments are fully operational.
UARS data is processed at the Central Data Handling Facility (CDHF), at
Goddard Space Flight Center. UARS data is transferred to Goddards
Distributed Active Archive Center (DAAC) for public distribution
It is impossible to rank the major scientific discoveries from UARS in
order of importance. Each of the 10 instruments has produced high quality
data and important unique or complementary results.
Here are ten
significant scientific achievements by UARS instruments.
[ Top ]
- Seasonal mapping of chlorine radicals and reservoirs in the lower
A few months after launch, MLS was able to map ClO (chlorine monoxide -
an ozone destroying radical) within the Arctic vortex showing the
extent of ClO formation and its close association with polar
stratospheric cloud formation temperatures. Not only was this was an
important confirmation of earlier aircraft results, but it also showed
the extent of the zone of elevated ClO. Since these initial
observations UARS has continued to monitor both the Arctic and
Antarctic late winter-spring ozone depletions. The northern hemisphere
depletion in January-March 1996 was the largest ever.
- Containment of polar vortex chemistry within the vortex region.
At the time of the launch of UARS some scientists speculated that the
ozone destroying chemicals within the polar vortex would leak to
mid-latitudes. Other scientists argued from a dynamical perspective
that containment of the chemicals must occur. It was not until the
launch of UARS and the mapping of trace species by UARS instruments
that containment could be demonstrated.
- Descent in the center of the polar vortex.
HALOE scientists were the first to notice very low concentrations of
the long lived trace gas methane (CH4) in the center of the spring
Antarctic polar vortex. Analysis showed that very low values of methane
exist within the mid and upper stratosphere in late fall and these
values descend to the lower stratosphere by late spring, a net change
12-15 km over six months. This amount of descent is remarkable in any
part of the atmosphere, and was later confirmed by measurements by
CLAES (N2O, and CH4) and ISAMS (N2O and CO).
- Infrared mapping of aerosols and PSCs.
Mt. Pinatubo erupted on June 15, 1991 injecting up to 20 megatons of
sulfur dioxide directly into the stratosphere. Reaction of SO2 with
stratospheric OH produces sulfuric acid which condenses into aerosols
at stratospheric temperatures and pressures. UARS observations by the
instruments CLAES, HALOE and ISAMS were used to track the aerosol cloud
from its IR emissions. This is the first near synoptic mapping of
volcanic aerosol layers.
- First direct measurement of winds from space.
Both WINDII and HRDI on UARS were built to measure winds from space.
Although the techniques are different, both rely on the Doppler shift
of an oxygen emission line in the mesosphere. HRDI, additionally
detects daytime stratospheric winds using the Doppler shift of an
oxygen absorption line in the stratosphere. These are the first remote
space born wind sounders. HRDI and WINDII both have been able to give
the first complete global picture of the atmospheric tide. HRDI has
also been able to measure the tropical quasibiennial oscillation winds
in the stratosphere.
- First global maps of chlorofluorocarbons and their products from
Some people outside the scientific community believe that
chlorofluorocarbons are not responsible for ozone loss at the poles.
They argue that chlorofluorocarbons are heavy molecules and will never
rise into the stratosphere. High levels of observed stratospheric
chlorine are due to volcanic activity, they argue.
CLAES has detected both CFCl3 (F11) and CF2Cl2 (F12) in the
stratosphere. Both are found to decrease strongly with altitude above
the tropopause. As the chlorofluorocarbons are breakdown in the
stratosphere, they release Cl and F which form HF and HCl. HF is a
long lived trace gas with no important natural sources. HALOE has
detected HF and Hcl in the stratosphere and found that both increases
with altitude as the CFCs decrease and are increasing with time.
- Tropical transport in the stratosphere.
The long lifetime of the UARS mission has lead to some remarkable trace
gas trend information. One of the more dramatic observations has been
the vertical transport of water vapor upward in the tropical
stratosphere. The amount of water vapor entering the stratosphere
changes throughout the year as the tropical tropopause gets colder and
warmer. These variations in water vapor ascend slowly into the
stratosphere and appear coherent to about 30 km (from 16 km). The water
vapor observations tell us that the tropical region must be quite
isolated from the rest of the stratosphere or these variations would be
- Measurement of the UV and Visible component of solar variability.
ACRIM records the total solar irradiance while SOLSTICE and SUSIM
measure the UV flux from Lyman alpha (~121.6 nm) up to around 400 nm.
SOLSTICE uses stars, while SUSIM uses onboard calibration lamps to
correct for instrument changes over time. UARS was launched near the
end of the maximum of solar cycle 22 and now the sun has reached the
minimum between solar cycles 22 and 23. A comparison of energy change
over this period by these instruments shows that the UV variation
accounts for a significant 40% of the change in the total solar
- The role of energetic particles in stratospheric chemistry.
Energetic particle observations by PEM have shown that most of the
relativistic electrons observed at geosynchronous altitudes (by GOES,
for example) are trapped. Only about 1- 10% of the relativistic
electron precipitations (REPs) measured at geosynchronous altitudes
actually precipitate into the earths atmosphere. These measurements
thus show that REPs have a relatively small global impact on
stratospheric odd nitrogen which was a controversy before the UARS
- Upper tropospheric water vapor in the presence of clouds.
The MLS team noticed after launch that they were getting some spectral
interference from water vapors. Further analysis showed that they
could use the MLS instrument to actually measure upper tropospheric
water. Since MLS is a microwave emission instruments, the measurements
could be made even if ice clouds are present. This new water vapor
measurement is currently being used to study how cirrus clouds impact
In January 2000, UARS investigators re-examined science highlights during
the UARS mission. Here is a list of those highlights, grouped by the
Project Science Office. They are given in categories of the "Top Eight
Science Highlights from UARS" and "Other Notable Science Achievements from
UARS," which are provided in the four categories of Chemical Constituents,
Dynamics, Particles, and External Influences.
Top Eight Science Highlights from UARS
[ Top ]
- The direct correlation between three-dimensional distributions of
observed ozone depletion and reactive chlorine was established.
- The dominance of human-made chlorofluorocarbons in the chlorine
and fluorine amounts in the stratosphere was clearly demonstrated.
- Using several years of observations the movement and mixing of air
in the stratosphere and mesosphere were derived globally and seasonally.
- The longest time series (in excess of 8 years) of absolutely calibrated
solar ultraviolet spectral irradiances over both maximum and minimum
levels of solar activity were measured.
- Methods were developed for identification of stratospheric particle
formation and composition through analysis of spectral signatures.
- The evolution of reactive chlorine over the winter and its role
in the springtime polar ozone destruction has been confirmed and
quantified for both southern and northern hemispheres over several
- The seasonal variation in water vapor transport from the tropical
troposphere to the stratosphere indicated the link between the amount
of water entering the stratosphere and the seasonal cycle in the upper
tropospheric tropical temperature.
- Observations of water vapor and methane in the tropics show the
coherent upward propagation of the seasonal cycle for time scales
longer than one year, indicating that horizontal mixing between the
tropics and middle latitudes is limited.
Other Notable Science Achievements from UARS
(provided in the four categories of Chemical
Constituents, Dynamics, Particles, and
External Influences) [ Top
| [ Top ]
- Mesospheric chlorine and fluorine amounts are generally consistent with
ground-based measurements of chlorine and fluorine source gases when
factoring in a five year lag for transport of these constituents from
the ground to the mesosphere.
- Global and seasonal mapping of chlorine radicals and reservoirs
in the stratosphere has provided much insight into the formation
and partitioning of the chlorine family constituents.
- The first global maps of chlorofluorocarbons and their products
were provided showing that chlorofluorocarbons do reach the stratosphere
and their altitude and latitude distributions are consistent with the
production of more active forms of chlorine.
- The first demonstration of the dramatic difference between hemispheres
of the seasonal evolution of the major nitrogen and chlorine reservoirs
was provided which results from the hemispheric meteorological differences.
- The variations in stratospheric water vapor and methane
have been quantified over the eight years of the mission.
- Upper tropospheric water vapor in the presence of clouds was
measured and is being used to study how cirrus clouds impact the
- Observations in the troposphere are serving as a very valuable test
data set for applications of the retrieval codes being developed for
future satellite missions.
- Ozone measurements quantified the effect of aerosols on SAGE II satellite
ozone retrievals especially during the recovery phase from the Mt. Pinatubo
Dynamics | [ Top
- Chemical constituents appear to be contained in the polar vortex during
the wintertime in both poles.
- The descent of chemical constituents in the center of the polar vortex
has been quantified over several years at both poles.
- Atmospheric tides, the filtering of gravity waves as they propogate
upwards, and the effects of gravity wave breaking upon the establishment
of upper atmospheric temperature fields were measured globally and
seasonally over several years.
- The barriers between the tropics and middle latitudes and middle
and polar latitudes were clearly demonstrated.
- Direct observations of the mesospheric and thermospheric winds
were used to characterize the elusive diurnal tide.
- Planetary scale disturbances were found to be very regular and extremely
large in the mesosphere and lower thermosphere.
- The diurnal tide and planetary scale disturbances were shown to lead
to major perturbations in the global scale distribution of atomic oxygen
which can be greater than that from the photochemistry.
Particles | [ Top
- A quantification of the decline in stratospheric sulfate aerosol
abundance after the 1991 Mt. Pinatubo volcanic eruption to the near
background levels in 1999 was provided.
- A quantification of the variation in chemical constituents caused
by stratospheric sulfate aerosol changes was provided.
- Daily or near daily maps of water and nitric acid have been used to
identify processes which affect polar stratospheric cloud formation.
- Global maps of aerosols and polar stratospheric clouds in both
vertical and areal extent were derived.
- The retrievals of aerosol extinction at multiple wavelengths
in the infrared has shown that the wavelength dependence of the extinction
differs for particles of different composition and size.
| [ Top ]
- A time series of the solar Mg II core-to-wing ratio index, an important
proxy for solar spectral irradiance originating in the chromosphere,
transition region, and upper photosphere, is being produced which can
be used to bridge the gap in the time series of past and future measurements
of a Mg II index by the SBUV and SBUV/2 instruments aboard other satellites.
When combined, the composite time series will span more than two solar cycles.
- The effects of electron precipitation on various chemical constituents
minor species appear to be organized in geomagnetic coordinates rather
than geographic coordinates.
- The influence of relativistic electrons on the middle atmosphere has
been more firmly established -
a) Although relativistic electrons deposit most of their energy in the
mesosphere, a study of the largest relativistic electron precipitation
event during the UARS time period failed to find any related mesospheric
b) Most of the energy deposited in the atmosphere by electron
precipitation (and corresponding bremsstrahlung radiation) was shown
to come from the steady long duration diffuse auroral precipitation rather
than from relativistic electron precipitation events.
As is evident from the discussions in the previous sections, UARS has
provided an enormous amount of new information on the middle and upper
atmosphere. As of this writing, over seven hundred refereed scientific
papers have been written on the results from UARS. In every sense, the
mission has been a success and continues to be successful.
Some useful facts:
- Original UARS mission lifetime was 18 months.
- UARS cost about $750 million to build.
- It is the largest Mission to Planet Earth spacecraft ever launched or
now planned to be launched.
- Dr. Paul Crutzen - co-winner of the 1995 Noble Prize in Chemistry - is a member
of the UARS Science Team.