Upper Atmosphere Research Satellite
Encyclopedia
The Upper Atmosphere Research Satellite (UARS) was a NASA
-operated orbital observatory whose mission was to study the Earth’s atmosphere, particularly the protective ozone layer
. The (13,000-lb) satellite was deployed from Space Shuttle Discovery
during the STS-48
mission on September 15, 1991. It entered orbit at an operational altitude of 600 kilometres (372.8 mi), with an orbital inclination of 57 degrees.
The original mission duration was to be only three years, but was extended several times. When the mission ended in June 2005 due to funding cuts, 14 years after the satellite's launch, six of its ten instruments were still operational. Nonetheless, UARS
was decommissioned and a final orbit-lowering burn was performed in early December 2005. On October 26, 2010, the International Space Station
performed a debris-avoidance maneuver in response to a conjunction with UARS.
The decommissioned satellite re-entered the atmosphere on September 24, 2011, with considerable media attention, caused by NASA predictions that substantial parts of it could reach the ground. In the event, it landed in a remote area of the Pacific.
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 (−430°F) and an outer tank of solid carbon dioxide
at (−238°F). As the neon and carbon dioxide evaporated, they kept the instrument cool for a planned 19 months. 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.
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. ISAMS uses 7 gas cells for 6 different gases : CO2 (times 2), CO, CH4, N2O, NO2 and H2O. The CO2 cells also allow measurement of ozone (O3), nitric acid (HNO3) and dinitrogen pentoxide (N2O5)
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.
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 (55 miles). Upon entering the instrument, the signal from the antenna is separated into three signals for processing by different radiometers. The radiometer measures temperature and pressure. The radiometer measures water vapor and ozone. The radiometer measures ClO
, ozone, sulfur dioxide
, nitric acid
and water vapor.
As late as June 2005, the 63 and 205 GHz radiometers remained operational, but the 183 GHz radiometer failed after 19 months of operation.
HALOE uses solar occultation
to measure simultaneous 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 (0.994196378639691 mi) 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.
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.
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.
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.
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 foreseeable future. The UARS/ACRIM2 experiment was an important part of providing the long term TSI database.
of the satellite's systems in early December of that year.
of UARS was 175 km (108.7 mi) by 185 km (115 mi). Some debris was expected to survive to reach the surface. It was expected to fall sometime during the late evening of September 23 or the early morning of September 24. Of the 26 pieces expected to survive reentry and strike the surface, the estimated mass of the largest piece was 158.3 kg (349 lb), possibly reaching the surface at a velocity of 44 metres per second (144.4 ft/s) (44 m/s (98.4 mph; 158.4 km/h)). Smaller pieces were expected to strike the surface at up to 107 m/s (107 m/s (239.4 mph; 385.2 km/h)).
On September 24, at 07:46 UTC, an updated statement on the NASA UARS website read: "The Joint Space Operations Center at Vandenberg Air Force Base
in California said the satellite penetrated the atmosphere over the Pacific Ocean", a precise re-entry time and location was not yet determined. According to Nicholas Johnson, chief orbital debris scientist at NASA’s Johnson Space Center, "We don’t know where the debris field might be... We may never know." However, the Joint Space Operations Center later announced that the reentry took place at 04:00 UTC on September 24, at 14.1°S 170.1°W; according to NASA, the debris field would extend between positions 300 miles (482.8 km) and 800 miles (1,287.5 km) downrange
, or generally northeast, of this position.
Stretching long and in diameter, UARS was among the largest spacecraft to plummet uncontrollably through the atmosphere, although it is a slim cousin to NASA's 75-tonne Skylab
station, which crashed to Earth in 1979. Russia's last space station, the 135-tonne Mir
, crashed into the Pacific Ocean in 2001, but it was a guided descent. NASA now plans for the controlled re-entry of large spacecraft, but it did not when UARS was designed.
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
-operated orbital observatory whose mission was to study the Earth’s atmosphere, particularly the protective ozone layer
Ozone layer
The ozone layer is a layer in Earth's atmosphere which contains relatively high concentrations of ozone . This layer absorbs 97–99% of the Sun's high frequency ultraviolet light, which is potentially damaging to the life forms on Earth...
. The (13,000-lb) satellite was deployed from Space Shuttle Discovery
Space Shuttle Discovery
Space Shuttle Discovery is one of the retired orbiters of the Space Shuttle program of NASA, the space agency of the United States, and was operational from its maiden flight, STS-41-D on August 30, 1984, until its final landing during STS-133 on March 9, 2011...
during the STS-48
STS-48
-Mission parameters:*Mass:**Orbiter landing with payload: **Payload: *Perigee: *Apogee: *Inclination: 57.0°*Period: 96.2 min-Mission highlights:...
mission on September 15, 1991. It entered orbit at an operational altitude of 600 kilometres (372.8 mi), with an orbital inclination of 57 degrees.
The original mission duration was to be only three years, but was extended several times. When the mission ended in June 2005 due to funding cuts, 14 years after the satellite's launch, six of its ten instruments were still operational. Nonetheless, UARS
UARS
UARS may refer to:*Upper Atmosphere Research Satellite, an orbital observatory whose mission was to study the Earth’s atmosphere, particularly the protective ozone layer, and which crashed on Earth in 2011...
was decommissioned and a final orbit-lowering burn was performed in early December 2005. On October 26, 2010, the International Space Station
International Space Station
The International Space Station is a habitable, artificial satellite in low Earth orbit. The ISS follows the Salyut, Almaz, Cosmos, Skylab, and Mir space stations, as the 11th space station launched, not including the Genesis I and II prototypes...
performed a debris-avoidance maneuver in response to a conjunction with UARS.
The decommissioned satellite re-entered the atmosphere on September 24, 2011, with considerable media attention, caused by NASA predictions that substantial parts of it could reach the ground. In the event, it landed in a remote area of the Pacific.
Cryogenic Limb Array Etalon Spectrometer (CLAES)
Spectrometer
A spectrometer is an instrument used to measure properties of light over a specific portion of the electromagnetic spectrum, typically used in spectroscopic analysis to identify materials. The variable measured is most often the light's intensity but could also, for instance, be the polarization...
that determined the concentrations and distributions of nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
and chlorine
Chlorine
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...
compounds, ozone
Ozone
Ozone , or trioxygen, is a triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic allotrope...
, water vapor
Water vapor
Water vapor or water vapour , also aqueous vapor, is the gas phase of water. It is one state of water within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice. Under typical atmospheric conditions, water vapor is continuously...
and methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
. It did this by inferring the amount of gases in the atmosphere by measuring the unique infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
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
Cryogenics
In physics, cryogenics is the study of the production of very low temperature and the behavior of materials at those temperatures. A person who studies elements under extremely cold temperature is called a cryogenicist. Rather than the relative temperature scales of Celsius and Fahrenheit,...
cooled to keep heat from the instrument from interfering with the readings. The cryogenics system consisted of an inner tank of solid neon
Neon
Neon is the chemical element that has the symbol Ne and an atomic number of 10. Although a very common element in the universe, it is rare on Earth. A colorless, inert noble gas under standard conditions, neon gives a distinct reddish-orange glow when used in either low-voltage neon glow lamps or...
at (−430°F) and an outer tank of solid carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
at (−238°F). As the neon and carbon dioxide evaporated, they kept the instrument cool for a planned 19 months. 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
Stratosphere
The stratosphere is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. It is stratified in temperature, with warmer layers higher up and cooler layers farther down. This is in contrast to the troposphere near the Earth's surface, which is cooler...
and the lower mesosphere
Mesosphere
The mesosphere is the layer of the Earth's atmosphere that is directly above the stratosphere and directly below the thermosphere. In the mesosphere temperature decreases with increasing height. The upper boundary of the mesosphere is the mesopause, which can be the coldest naturally occurring...
. 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)
Thermal energy
Thermal energy is the part of the total internal energy of a thermodynamic system or sample of matter that results in the system's temperature....
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
Stirling engine
A Stirling engine is a heat engine operating by cyclic compression and expansion of air or other gas, the working fluid, at different temperature levels such that there is a net conversion of heat energy to mechanical work....
to achieve high detector sensitivity. ISAMS uses 7 gas cells for 6 different gases : CO2 (times 2), CO, CH4, N2O, NO2 and H2O. The CO2 cells also allow measurement of ozone (O3), nitric acid (HNO3) and dinitrogen pentoxide (N2O5)
The specific objectives of ISAMS were: (i) To obtain measurements of atmospheric temperature as a function of pressure, from the tropopause
Tropopause
The tropopause is the atmospheric boundary between the troposphere and the stratosphere.-Definition:Going upward from the surface, it is the point where air ceases to cool with height, and becomes almost completely dry...
to the mesopause
Mesopause
The mesopause is the temperature minimum at the boundary between the mesosphere and the thermosphere atmospheric regions. Due to the lack of solar heating and very strong radiative cooling from carbon dioxide, the mesopause is the coldest place on Earth with temperatures as low as -100°C...
, 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 detected naturally occurring microwaveMicrowave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...
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 (55 miles). Upon entering the instrument, the signal from the antenna is separated into three signals for processing by different radiometers. The radiometer measures temperature and pressure. The radiometer measures water vapor and ozone. The radiometer measures ClO
Chlorine
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...
, ozone, sulfur dioxide
Sulfur dioxide
Sulfur dioxide is the chemical compound with the formula . It is released by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide unless the sulfur compounds are removed before burning the fuel...
, nitric acid
Nitric acid
Nitric acid , also known as aqua fortis and spirit of nitre, is a highly corrosive and toxic strong acid.Colorless when pure, older samples tend to acquire a yellow cast due to the accumulation of oxides of nitrogen. If the solution contains more than 86% nitric acid, it is referred to as fuming...
and water vapor.
As late as June 2005, the 63 and 205 GHz radiometers remained operational, but the 183 GHz radiometer failed after 19 months of operation.
Halogen Occultation Experiment (HALOE)
Occultation
An occultation is an event that occurs when one object is hidden by another object that passes between it and the observer. The word is used in astronomy . It can also refer to any situation wherein an object in the foreground blocks from view an object in the background...
to measure simultaneous vertical profiles of ozone (O3), hydrogen chloride
Hydrogen chloride
The compound hydrogen chloride has the formula HCl. At room temperature, it is a colorless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric humidity. Hydrogen chloride gas and hydrochloric acid are important in technology and industry...
(HCl), hydrogen fluoride
Hydrogen fluoride
Hydrogen fluoride is a chemical compound with the formula HF. This colorless gas is the principal industrial source of fluorine, often in the aqueous form as hydrofluoric acid, and thus is the precursor to many important compounds including pharmaceuticals and polymers . HF is widely used in the...
(HF), methane (CH4), water vapor (H2O), nitric oxide
Nitric oxide
Nitric oxide, also known as nitrogen monoxide, is a diatomic molecule with chemical formula NO. It is a free radical and is an important intermediate in the chemical industry...
(NO), nitrogen dioxide
Nitrogen dioxide
Nitrogen dioxide is the chemical compound with the formula it is one of several nitrogen oxides. is an intermediate in the industrial synthesis of nitric acid, millions of tons of which are produced each year. This reddish-brown toxic gas has a characteristic sharp, biting odor and is a prominent...
(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 (0.994196378639691 mi) 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
Gimbal
A gimbal is a pivoted support that allows the rotation of an object about a single axis. A set of two gimbals, one mounted on the other with pivot axes orthogonal, may be used to allow an object mounted on the innermost gimbal to remain immobile regardless of the motion of its support...
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.
High Resolution Doppler Imager (HRDI)
Doppler effect
The Doppler effect , named after Austrian physicist Christian Doppler who proposed it in 1842 in Prague, is the change in frequency of a wave for an observer moving relative to the source of the wave. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from...
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)
Airglow
Airglow is the very weak emission of light by a planetary atmosphere. In the case of Earth's atmosphere, this optical phenomenon causes the night sky to never be completely dark .-Development:The airglow phenomenon was first identified in 1868 by Swedish scientist...
and aurora
Aurora (astronomy)
An aurora is a natural light display in the sky particularly in the high latitude regions, caused by the collision of energetic charged particles with atoms in the high altitude atmosphere...
. 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
Charge-coupled device
A charge-coupled device is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example conversion into a digital value. This is achieved by "shifting" the signals between stages within the device one at a time...
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.
Solar Ultraviolet Spectral Irradiance Monitor (SUSIM)
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
(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.Active Cavity Radiometer Irradiance Monitor II (ACRIM2)
Orbit-lowering burn
UARS was decommissioned in 2005, and a final orbit-lowering burn was followed by the passivationFeedback passivation
In nonlinear control, feedback passivation is the use of feedback to convert a nonpassive dynamical system into a passive one....
of the satellite's systems in early December of that year.
Re-entry
On September 7, 2011, NASA announced the impending uncontrolled re-entry of UARS and noted that there was a small risk to the public. As of 01:30 UTC September 23, 2011, the orbitApsis
An apsis , plural apsides , is the point of greatest or least distance of a body from one of the foci of its elliptical orbit. In modern celestial mechanics this focus is also the center of attraction, which is usually the center of mass of the system...
of UARS was 175 km (108.7 mi) by 185 km (115 mi). Some debris was expected to survive to reach the surface. It was expected to fall sometime during the late evening of September 23 or the early morning of September 24. Of the 26 pieces expected to survive reentry and strike the surface, the estimated mass of the largest piece was 158.3 kg (349 lb), possibly reaching the surface at a velocity of 44 metres per second (144.4 ft/s) (44 m/s (98.4 mph; 158.4 km/h)). Smaller pieces were expected to strike the surface at up to 107 m/s (107 m/s (239.4 mph; 385.2 km/h)).
On September 24, at 07:46 UTC, an updated statement on the NASA UARS website read: "The Joint Space Operations Center at Vandenberg Air Force Base
Vandenberg Air Force Base
Vandenberg Air Force Base is a United States Air Force Base, located approximately northwest of Lompoc, California. It is under the jurisdiction of the 30th Space Wing, Air Force Space Command ....
in California said the satellite penetrated the atmosphere over the Pacific Ocean", a precise re-entry time and location was not yet determined. According to Nicholas Johnson, chief orbital debris scientist at NASA’s Johnson Space Center, "We don’t know where the debris field might be... We may never know." However, the Joint Space Operations Center later announced that the reentry took place at 04:00 UTC on September 24, at 14.1°S 170.1°W; according to NASA, the debris field would extend between positions 300 miles (482.8 km) and 800 miles (1,287.5 km) downrange
Downrange
Downrange is the horizontal distance traveled by a spacecraft, or the spacecraft's horizontal distance from the launch site. More often, it is used as an adverb or adjective specifying the direction of that travel being measured in a horizontal direction....
, or generally northeast, of this position.
Stretching long and in diameter, UARS was among the largest spacecraft to plummet uncontrollably through the atmosphere, although it is a slim cousin to NASA's 75-tonne Skylab
Skylab
Skylab was a space station launched and operated by NASA, the space agency of the United States. Skylab orbited the Earth from 1973 to 1979, and included a workshop, a solar observatory, and other systems. It was launched unmanned by a modified Saturn V rocket, with a mass of...
station, which crashed to Earth in 1979. Russia's last space station, the 135-tonne Mir
Mir
Mir was a space station operated in low Earth orbit from 1986 to 2001, at first by the Soviet Union and then by Russia. Assembled in orbit from 1986 to 1996, Mir was the first modular space station and had a greater mass than that of any previous spacecraft, holding the record for the...
, crashed into the Pacific Ocean in 2001, but it was a guided descent. NASA now plans for the controlled re-entry of large spacecraft, but it did not when UARS was designed.