Student Nitric Oxide Explorer (SNOE ("snowy"), also known as Explorer 72, STEDI-1 and UNEX-1), was a

NASA The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeeding t ...

small scientific satellite which studied the concentration of

nitric oxide Nitric oxide (nitrogen oxide or nitrogen monoxide) is a colorless gas with the formula . It is one of the principal oxides of nitrogen. Nitric oxide is a free radical: it has an unpaired electron, which is sometimes denoted by a dot in its che ...

in the

thermosphere The thermosphere is the layer in the Earth's atmosphere directly above the mesosphere and below the exosphere. Within this layer of the atmosphere, ultraviolet radiation causes photoionization/photodissociation of molecules, creating ions; the ...

. It was launched in 1998 as part of NASA's

Explorer program The Explorers program is a NASA exploration program that provides flight opportunities for physics, geophysics, heliophysics, and astrophysics investigations from space. Launched in 1958, Explorer 1 was the first spacecraft of the United Stat ...

. The satellite was the first of three missions developed within the Student Explorer Demonstration Initiative (STEDI) program funded by the NASA and managed by the

Universities Space Research Association The Universities Space Research Association (USRA) was incorporated on March 12, 1969, in Washington, D.C. as a private, nonprofit corporation under the auspices of the National Academy of Sciences (NAS). Institutional membership in the assoc ...

(USRA). STEDI was a pilot program to demonstrate that high-quality space science can be carried out with small, low-cost ( The satellite was developed by the

University of Colorado Boulder The University of Colorado Boulder (CU Boulder, CU, or Colorado) is a public research university in Boulder, Colorado. Founded in 1876, five months before Colorado became a state, it is the flagship university of the University of Colorado syst ...

Laboratory for Atmospheric and Space Physics The Laboratory for Atmospheric and Space Physics (LASP) is a research organization at the University of Colorado Boulder. LASP is a research institute with over one hundred research scientists ranging in fields from solar influences, to Earth' ...

(LASP) and had met its goals by the time its mission ended with reentry in December 2003.

Overview

SNOE was the 72nd mission of the Explorer program by NASA dedicated to the scientific investigation of the space environment of the

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...

. SNOE was the first of three projects developed within the university satellite program (STEDI) whose objective is to reach students in the development of satellites with limited means in the context of the strategy of "faster, better, cheaper" promoted by then-

NASA administrator The Administrator of the National Aeronautics and Space Administration is the highest-ranking official of NASA, the national space agency of the United States. The administrator is NASA's chief decision maker, responsible for providing clarity to ...

Daniel Goldin Daniel Saul Goldin (born July 23, 1940) served as the 9th and longest-tenured Administrator of NASA from April 1, 1992, to November 17, 2001. He was appointed by President George H. W. Bush and also served under Presidents Bill Clinton and Georg ...

. The program was funded by NASA and managed by the

. The mission, developed by the

in 1994, was selected among 66 proposals to be one of the six pre-selected satellites of the program. In February 1995, the satellite was selected along with

TERRIERS Terrier (from Latin ''terra'', 'earth') is a type of dog originally bred to hunt vermin. A terrier is a dog of any one of many breeds or landraces of the terrier type, which are typically small, wiry, game, and fearless. Terrier breeds vary gr ...

Boston University Boston University (BU) is a private research university in Boston, Massachusetts. The university is nonsectarian, but has a historical affiliation with the United Methodist Church. It was founded in 1839 by Methodists with its original campu ...

and CATSAT of the

University of Leicester , mottoeng = So that they may have life , established = , type = public research university , endowment = £20.0 million , budget = £326 million , chancellor = David Willetts , vice_chancellor = Nishan Canagarajah , head_labe ...

in the United Kingdom. SNOE was built and operated entirely by the

of the university.

Mission

The objective of the mission was the detailed study of variations in the concentration of nitrogen monoxide in the thermosphere. Nitric oxide, though a minor component of this region of space, has a significant impact on the composition of ions in the ionosphere and in the heat of the thermosphere. The detailed objectives are: * Detailing how the variations of X-ray radiation from the sun affects the density of

in the lower layer of the thermosphere * How auroral activity increases the amount of nitric oxide in the

polar regions of Earth The polar regions, also called the frigid geographical zone, zones or polar zones, of Earth are the regions of the planet that surround its geographical poles (the North Pole, North and South Poles), lying within the polar circles. These high l ...

Spacecraft

SNOE was a compact hexagonal structure, approximately high and across it widest dimension, weighing a maximum of . It was spin-stabilized at five

revolutions per minute Revolutions per minute (abbreviated rpm, RPM, rev/min, r/min, or with the notation min−1) is a unit of rotational speed or rotational frequency for rotating machines. Standards ISO 80000-3:2019 defines a unit of rotation as the dimensionl ...

, and its axis of rotation was perpendicular to the orbital plane. The exterior of the satellite was covered with

solar cell A solar cell, or photovoltaic cell, is an electronic device that converts the energy of light directly into electricity by the photovoltaic effect, which is a physical and chemical phenomenon.

s that provide 37

watt The watt (symbol: W) is the unit of power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantify the rate of energy transfer. The watt is named after James Wa ...

Launch

It was launched, on 26 February 1998 at 07:07 UTC by an

Orbital Sciences Corporation Orbital Sciences Corporation (commonly referred to as Orbital) was an American company specializing in the design, manufacture, and launch of small- and medium- class space and launch vehicle systems for commercial, military and other governmen ...

's ''

Stargazer Stargazer may refer to: * an observational astronomer, particularly an amateur Aerospace * Stargazer (aircraft), a Lockheed L-1011 airliner used to launch the Pegasus rocket * Orbiting Astronomical Observatory 2, nicknamed Stargazer, the first s ...

'' and a

Pegasus-XL Pegasus is an air-launched launch vehicle developed by Orbital Sciences Corporation (OSC) and now built and launched by Northrop Grumman. Capable of carrying small payloads of up to into low Earth orbit, Pegasus first flew in 1990 and remai ...

launch vehicle A launch vehicle or carrier rocket is a rocket designed to carry a payload (spacecraft or satellites) from the Earth's surface to outer space. Most launch vehicles operate from a launch pad, launch pads, supported by a missile launch contro ...

, into a Sun-synchronous circular orbit, along with the Teledesic T1 satellite, at

altitude Altitude or height (also sometimes known as depth) is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The exact definition and reference datum varies according to the context ...

and 97.70°

inclination Orbital inclination measures the tilt of an object's orbit around a celestial body. It is expressed as the angle between a Plane of reference, reference plane and the orbital plane or Axis of rotation, axis of direction of the orbiting object ...

. It span at 5

rpm Revolutions per minute (abbreviated rpm, RPM, rev/min, r/min, or with the notation min−1) is a unit of rotational speed or rotational frequency for rotating machines. Standards ISO 80000-3:2019 defines a unit of rotation as the dimensionl ...

with the spin axis normal to the orbit plane and carried three instruments: an ultraviolet spectrometer to measure nitric oxide altitude profiles, a two-channel auroral photometer to measure auroral emissions beneath the spacecraft, and a five-channel solar soft X-ray photometer. SNOE also carried a

GPS The Global Positioning System (GPS), originally Navstar GPS, is a Radionavigation-satellite service, satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of t ...

receiver for accurate orbit and attitude determination. The SNOE spacecraft and its instrument complement were designed, built, and operated entirely at the Laboratory for Atmospheric and Space Physics (LASP) of the University of Colorado Boulder. The spacecraft functioned normally until in December 2003.

Instruments

SNOE was equipped with three scientific instruments: * A two-channel Auroral Photometer, which performs measurements of auroral emissions beneath the satellite * A five-channel Solar X-ray Photometer, which measures the soft X-ray emissions by the Sun * An Ultraviolet Spectrometer, which performs a vertical profile of the concentration of nitric oxide

Auroral Photometer (AP)

The auroral

photometer A photometer is an instrument that measures the strength of electromagnetic radiation in the range from ultraviolet to infrared and including the visible spectrum. Most photometers convert light into an electric current using a photoresistor, ph ...

(AP) is a two-channel broad-band instrument that is used to determine the energy deposited in the

upper atmosphere Upper atmosphere is a collective term that refers to various layers of the atmosphere of the Earth above the troposphere and corresponding regions of the atmospheres of other planets, and includes: * The mesosphere, which on Earth lies between the ...

by energetic auroral

electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no kn ...

s. It is similar to airglow photometers developed by LASP and flown on OGO-5 and OGO-6 in the late 1960s. Each channel consists of a Hamamatsu phototube detectors, a UV filter, and a field of view limiter (circular, 11° full-cone). The combination of a

Caesium iodide Caesium iodide or cesium iodide (chemical formula CsI) is the ionic compound of caesium and iodine. It is often used as the input phosphor of an X-ray image intensifier tube found in fluoroscopy equipment. Caesium iodide photocathodes are hi ...

(CsI)

photocathode A photocathode is a surface engineered to convert light (photons) into electrons using the photoelectric effect. Photocathodes are important in accelerator physics where they are utilised in a photoinjector to generate high brightness electron be ...

and a

Calcium fluoride Calcium fluoride is the inorganic compound of the elements calcium and fluorine with the formula CaF2. It is a white insoluble solid. It occurs as the mineral fluorite (also called fluorspar), which is often deeply coloured owing to impurities. ...

(CaF₂) filter produces a bandpass from 125 to 180 nm for channel A, allowing a combined measurement of the LBH bands, the OI doublet at 135.6 nm, and the OI triplet at 130.4 nm. For channel B a

barium fluoride Barium fluoride (BaF2) is an inorganic compound with the formula BaF2. It is a colorless solid that occurs in nature as the rare mineral frankdicksonite. Under standard conditions it adopts the fluorite structure and at high pressure the PbCl2 s ...

(BaF₂) filter is used producing a 135 to 180 nm bandpass and providing a measurement of the LBH bands and the OI doublet at 135.6 nm with the exclusion of the OI triplet at 130.4 nm. The sensitivity of channel A at 130.4 nm is 23 counts/second/Rayleigh and the sensitivity of channel B at 135.6 nm is 26 counts/second/Rayleigh. The AP is mounted with its optical axis perpendicular to the spacecraft spin axis. The AP produces continuous data with an integration time of 183 ms, but only the downward-looking part of each spin will be stored.

Solar X-ray Photometer (SXP)

The solar X-ray photometer (SXP), measures the

solar irradiance Solar irradiance is the power per unit area (surface power density) received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument. Solar irradiance is measured in watts per square metre (W/ ...

at wavelengths from 2 to 35 nm. Each of the five photometer channels contains a

silicon photodiode A photodiode is a light-sensitive semiconductor diode. It produces current when it absorbs photons. The package of a photodiode allows light (or infrared or ultraviolet radiation, or X-rays) to reach the sensitive part of the device. The packag ...

; wavelength selection is accomplished by thin metallic films deposited onto the diode surface. Coatings are selected so that overlapping bandpasses can be used to isolate key parts of the spectrum at low resolution:

Tin Tin is a chemical element with the symbol Sn (from la, stannum) and atomic number 50. Tin is a silvery-coloured metal. Tin is soft enough to be cut with little force and a bar of tin can be bent by hand with little effort. When bent, t ...

(Sn): 2-8 nm;

Titanium Titanium is a chemical element with the symbol Ti and atomic number 22. Found in nature only as an oxide, it can be reduced to produce a lustrous transition metal with a silver color, low density, and high strength, resistant to corrosion in ...

(Ti): 2-16 nm;

Zirconium Zirconium is a chemical element with the symbol Zr and atomic number 40. The name ''zirconium'' is taken from the name of the mineral zircon, the most important source of zirconium. The word is related to Persian '' zargun'' (zircon; ''zar-gun'', ...

/Titanium (Zr/Ti): 5-20 nm;

Aluminum Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It has ...

Carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with o ...

(Al/C): 15-35 nm. The field of view is 70° full cone. The SXP takes 12 measurements per spin, centered on the zenith, with a 63 seconds integration time. Thus, it obtains an integrated solar measurement once per orbit, when the Sun is near the

zenith The zenith (, ) is an imaginary point directly "above" a particular location, on the celestial sphere. "Above" means in the vertical direction (plumb line) opposite to the gravity direction at that location (nadir). The zenith is the "highest" ...

Ultraviolet Spectrometer (UVS)

The objective of the ultraviolet spectrometer (UVS) is to measure the density of nitric oxide in the terrestrial upper atmosphere (thermosphere) by observing the (1,0) and (0,1) gamma bands. The UVS design is similar to instruments flown on the

Solar Mesosphere Explorer The Solar Mesosphere Explorer (also known as Explorer 64) was a NASA spacecraft to investigate the processes that create and destroy ozone in Earth's upper of the atmosphere of Earth. The mesosphere is a layer of the atmosphere extending from ...

(SME),

Pioneer Venus Orbiter The Pioneer Venus Orbiter, also known as Pioneer Venus 1 or Pioneer 12, was a mission to Venus conducted by the United States as part of the Pioneer Venus project. Launched in May 1978 atop an Atlas-Centaur rocket, the spacecraft was inserted into ...

, and several launch vehicles. It consists of an Ebert-Fastie spectrometer, an off-axis telescope, and two Hamamatsu phototube detectors. The combination of the spectrometer and the detectors produces a spacing of 22 nm between the two channels and the exit slits are sized to give each detector a 3.7 nm bandpass. The grating in the spectrometer is set to place the (1,0) gamma band (215 nm) on one detector and the (0,1) gamma band (237 nm) on the other detector. Both channels have a sensitivity of 450 counts/second/ kiloRayleigh. The UVS is mounted with its optical axis perpendicular to the spin axis of the spacecraft. Its telescope images the entrance slit of the spectrometer on the limb with the long axis of the slit parallel to the horizon. The image of the slit on the limb is high, which determines the fundamental altitude resolution of the instrument. The integration time is 27 ms.

Selected science results

The limb-scanning Ultraviolet Spectrometer on SNOE observed

polar mesospheric clouds Polar may refer to: Geography Polar may refer to: * Geographical pole, either of two fixed points on the surface of a rotating body or planet, at 90 degrees from the equator, based on the axis around which a body rotates *Polar climate, the cli ...

, finding that PMCs occur more frequently in the northern latitudes than in the southern, but that they otherwise conform well to the standard model of

cloud formation In meteorology, a cloud is an aerosol consisting of a visible mass of miniature liquid droplets, frozen crystals, or other particles suspended in the atmosphere of a planetary body or similar space. Water or various other chemicals may com ...

. SNOE also helped to map the effect of global X-rays on the atmosphere. Enhanced fluxes of solar soft X-rays were detected by SNOE. Solar soft X-ray irradiance was measured by the spacecraft's Solar X-ray Photometer (SXP) between 2- and 20-nm, and covered irradiance levels outside of solar minimum and maximum conditions. In the 2- to 7-nm interval the irradiance levels ranged from , while in the 6- to 19-nm interval the range was observed to be . These values were a factor of four times higher than those predicted by the Hinteregger, ''et al.'' (1981) empirical model.

Atmospheric entry

SNOE re-entered the atmosphere on 13 December 2003 at 09:34 UTC (± 6 minutes), descending over 2.9° South, 273.8° East, on orbit 32248, after 5 years and 290 days.

References

External links

SNOE website
by the University of Colorado Boulder
SNOE entry
at ESA's eoPortal {{Explorer program Atmospheric sciences Explorers Program NASA satellites Student satellites Spacecraft launched by Pegasus rockets Spacecraft launched in 1998 Spacecraft which reentered in 2003