 Explorer 9 before launch | |
| Names | S-56A Explorer IX NASA S-56A |
|---|---|
| Mission type | Air density research |
| Operator | NASA |
| Harvard designation | 1961 Delta 1 |
| COSPAR ID | 1961-004A |
| SATCATno. | 00081 |
| Mission duration | 3 years (achieved) |
| Spacecraft properties | |
| Spacecraft | Explorer IX |
| Spacecraft type | Air Density Explorer |
| Bus | S-56 |
| Manufacturer | Langley Research Center |
| Launch mass | 7 kg (15 lb) |
| Dimensions | 3.66 m (12.0 ft) diameter |
| Power | solar cells and rechargeable batteries |
| Start of mission | |
| Launch date | 16 February 1961, 13:05:00GMT |
| Rocket | Scout X-1 (ST-4) |
| Launch site | Wallops Flight Facility,LA-3 |
| Contractor | Vought |
| Entered service | 16 February 1961 |
| End of mission | |
| Decay date | 9 April 1964 |
| Orbital parameters | |
| Reference system | Geocentric orbit[1] |
| Regime | Medium Earth orbit |
| Perigee altitude | 545 km (339 mi) |
| Apogee altitude | 2,225 km (1,383 mi) |
| Inclination | 38.91° |
| Period | 118.6 minutes |
| Instruments | |
| Satellite Drag Atmospheric Density | |
Explorer Program | |
Explorer 9, known asS-56A before launch, was aNASA satellite which was launched in February 1961 to study the density and composition of the upperthermosphere and lowerexosphere.[2] It was a reflight of the failedExplorer S-56 mission, and consisted of a 7 kg (15 lb), 3.66 m (12.0 ft)balloon which was deployed into amedium Earth orbit.[3] The mission was conducted byNASA'sLangley Research Center.
The spacecraft consisted of alternating layers ofaluminium foil andMylarpolyester film. Uniformly distributed over the aluminium surface were 5.1 cm (2.0 in)-diameter dots of white paint for thermal control. The sphere was packed in a tube 21.6 cm (8.5 in) in diameter and 48.3 cm (19.0 in) long and mounted in the nose of the fourth stage of itsScout X-1 launch vehicle.[4]
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 special report n. 100,Cambridge, Massachusetts, July 1962.[5] This experiment resulted in the successful determination of reasonable density values until the satellite reentered the Earth's atmosphere on 9 April 1964.[6]
Explorer 9 was launched fromLaunch Area 3 at theWallops Flight Facility (WFF), atop aScout X-1 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 anapogee of 2,225 km (1,383 mi), aperigee of 545 km (339 mi), 38.91° ofinclination and aperiod of 118.6 minutes.[7] It was assigned theHarvard designation 1961 Delta 1.[8]
Upon separation of the fourth stage, the sphere was inflated by anitrogen 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 mWbeacon was carried for tracking purposes, but the beacon failed on the first orbit and the SAOBaker-Nunn camera network had to be relied upon for tracking. Power was supplied bysolar cells andrechargeable batteries.[1]
The second of six identicalair density 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.[4]
Careful and continuous photographic observation of the satellite allowed scientists to conduct an unprecedented survey of the Earth'sexosphere. The satellite first confirmed the daily bulge in theupper atmosphere caused by the Sun's heating the air during the day and verified the model of exospheric temperature developed according to other satellite data.[9] It was later discovered that the temperature of the exosphere increased after geomagnetic disturbances in theionosphere, peaking around five hours after each event.[10] With three years of data, scientists were able to discern a seasonal variation to theexosphere's density, with an increase of 25% observed in winter over summer at a referencelatitude of 39°. The delay in the temperature increase resulting from magnetic disturbances was further refined to 5.2 ± 0.4 hours.[11]
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.[1]
A replica of the spacecraft, possibly a flight backup, is currently located in theSmithsonian Institution'sNational Air and Space Museum, although it is not on display.[12]
This article incorporates text from this source, which is in thepublic domain. This article incorporates text from this source, which is in thepublic domain. This article incorporates text from this source, which is in thepublic domain.
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