Explorer 9, known as S-56A before launch, was a

NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the federal government of the United States, US federal government responsible for the United States ...

satellite which was launched in February 1961 to study the density and composition of the upper

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 ...

and lower

exosphere The exosphere is a thin, atmosphere-like volume surrounding a planet or natural satellite where molecules are gravitationally bound to that body, but where the density is so low that the molecules are essentially collision-less. In the case of ...

. It was a reflight of the failed Explorer S-56 mission, and consisted of a ,

balloon A balloon is a flexible membrane bag that can be inflated with a gas, such as helium, hydrogen, nitrous oxide, oxygen, or air. For special purposes, balloons can be filled with smoke, liquid water, granular media (e.g. sand, flour or rice), ...

which was deployed into a

medium Earth orbit A medium Earth orbit (MEO) is an geocentric orbit, Earth-centered orbit with an altitude above a low Earth orbit (LEO) and below a high Earth orbit (HEO) – between above sea level.

. The mission was conducted by

Langley Research Center The Langley Research Center (LaRC or NASA Langley), located in Hampton, Virginia, near the Chesapeake Bay front of Langley Air Force Base, is the oldest of NASA's field centers. LaRC has focused primarily on aeronautical research but has also ...

Spacecraft

The spacecraft consisted of alternating layers of

aluminium foil Aluminium foil (or aluminum foil in American English; occasionally called tin foil) is aluminium prepared in thin metal leaves. The foil is pliable and can be readily bent or wrapped around objects. Thin foils are fragile and are sometimes ...

and

Mylar BoPET (biaxially oriented polyethylene terephthalate) is a polyester film made from stretched polyethylene terephthalate (PET) and is used for its high tensile strength, chemical stability, dimensional stability, transparency reflectivity, an ...

polyester Polyester is a category of polymers that contain one or two ester linkages in every repeat unit of their main chain. As a specific material, it most commonly refers to a type called polyethylene terephthalate (PET). Polyesters include some natura ...

film. Uniformly distributed over the aluminium surface were -diameter dots of white paint for thermal control. The sphere was packed in a tube in diameter and long and mounted in the nose of the fourth stage of its

Scout X-1 Scout X-1 was an American expendable launch system and sounding rocket which was flown seven times between August 1960 and October 1961. Four orbital and three suborbital launches were made, with four of the launches resulting in failures. The ...

launch vehicle.

Experiment

Satellite Drag Atmospheric Density

Because of its symmetrical shape, Explorer 9 was selected for use in determining upper atmospheric densities as a function of altitude, latitude, season, and solar activity. Density values near perigee were deduced from sequential observations of the spacecraft position, using optical ( Baker-Nunn camera network) and radar tracking techniques. A good discussion of the general techniques used to deduce density values from satellite drag data can be found in L. G. Jacchia and J. Slowey, "Accurate drag determination for eight artificial satellites of atmospheric densities and temperatures",

Smithsonian Astrophysical Observatory The Smithsonian Astrophysical Observatory (SAO) is a research institute of the Smithsonian Institution, concentrating on Astrophysics, astrophysical studies including Galactic astronomy, galactic and extragalactic astronomy, cosmology, Sun, solar ...

special report n. 100,

Cambridge, Massachusetts Cambridge ( ) is a city in Middlesex County, Massachusetts, United States. It is a suburb in the Greater Boston metropolitan area, located directly across the Charles River from Boston. The city's population as of the 2020 United States census, ...

, July 1962. This experiment resulted in the successful determination of reasonable density values until the satellite reentered the Earth's atmosphere on 9 April 1964.

Launch

Explorer 9 was launched from Launch Area 3 at the

Wallops Flight Facility Wallops Flight Facility (WFF) is a rocket launch site on Wallops Island on the Eastern Shore of Virginia, United States, just east of the Delmarva Peninsula and approximately north-northeast of Norfolk, VA, Norfolk. The facility is operated ...

(WFF), atop a

launch vehicle with the serial number ST-4. It was the first spacecraft launched from Wallops Island to achieve orbit, with one previous attempt having failed. The launch occurred at 13:05:00 GMT on 16 February 1961, and resulted in Explorer 9 being deployed into an orbit with an

apogee An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. The line of apsides (also called apse line, or major axis of the orbit) is the line connecting the two extreme values. Apsides perta ...

of , a

perigee An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. The line of apsides (also called apse line, or major axis of the orbit) is the line connecting the two extreme values. Apsides perta ...

of , 38.91° of

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

and a period of 118.6 minutes. It was assigned the Harvard designation 1961 Delta 1. Upon separation of the fourth stage, the sphere was inflated by a

nitrogen Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...

gas bottle, and a separation spring ejected it out into its own orbit. The two hemispheres of aluminium foil were separated with a gap of Mylar at the spacecraft's equator and served as the antenna. A 136 MHz, 15 mW

beacon A beacon is an intentionally conspicuous device designed to attract attention to a specific location. A common example is the lighthouse, which draws attention to a fixed point that can be used to navigate around obstacles or into port. More mode ...

was carried for tracking purposes, but the beacon failed on the first orbit and the SAO

Baker-Nunn camera A Schmidt camera, also referred to as the Schmidt telescope, is a catadioptric Astrophotography, astrophotographic Optical telescope, telescope designed to provide wide Field of view, fields of view with limited Aberration in optical systems, abe ...

network had to be relied upon for tracking. Power was supplied by

solar cell A solar cell, also known as a photovoltaic cell (PV cell), is an electronic device that converts the energy of light directly into electricity by means of the photovoltaic effect.

s and

rechargeable batteries A rechargeable battery, storage battery, or secondary cell (formally a type of energy accumulator), is a type of electrical battery which can be charged, discharged into a load, and recharged many times, as opposed to a disposable or prima ...

. The second of six identical

air density The density of air or atmospheric density, denoted '' ρ'', is the mass per unit volume of Earth's atmosphere at a given point and time. Air density, like air pressure, decreases with increasing altitude. It also changes with variations in atmosph ...

research satellites to be launched, Explorer 9 was the first to successfully reach orbit. It was still operational when the next satellite in the series, Explorer 19, was launched, allowing simultaneous readings to be taken and compared.

Mission results

Careful and continuous photographic observation of the satellite allowed scientists to conduct an unprecedented survey of the Earth's

. The satellite first confirmed the daily bulge 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 th ...

caused by the Sun's heating the air during the day and verified the model of exospheric temperature developed according to other satellite data. It was later discovered that the temperature of the exosphere increased after geomagnetic disturbances in the

ionosphere The ionosphere () is the ionized part of the upper atmosphere of Earth, from about to above sea level, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays ...

, peaking around five hours after each event. With three years of data, scientists were able to discern a seasonal variation to the

's density, with an increase of 25% observed in winter over summer at a reference

latitude In geography, latitude is a geographic coordinate system, geographic coordinate that specifies the north-south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from −90° at t ...

of 39°. The delay in the temperature increase resulting from magnetic disturbances was further refined to 5.2 ± 0.4 hours. Explorer 9 was the first spacecraft placed in orbit by an all-solid launch vehicle and the first spacecraft successfully launched into orbit from Wallops Island. The spacecraft reentered the Earth's atmosphere on 9 April 1964.

Legacy

A replica of the spacecraft, possibly a flight backup, is currently located in the

Smithsonian Institution The Smithsonian Institution ( ), or simply the Smithsonian, is a group of museums, Education center, education and Research institute, research centers, created by the Federal government of the United States, U.S. government "for the increase a ...

National Air and Space Museum The National Air and Space Museum (NASM) of the Smithsonian Institution is a museum in Washington, D.C., in the United States, dedicated to history of aviation, human flight and space exploration. Established in 1946 as the National Air Museum, ...

, although it is not on display.

References

{{Orbital launches in 1961 Spacecraft launched in 1961 Spacecraft which reentered in 1964 Passive satellites Explorers Program Balloon satellites Satellites in medium Earth orbit Geospace monitoring satellites