Explorer 9

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
SATCAT no.| 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,
13:05:00 GMT
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 S-56

Explorer S-45 ->

Explorer 9, known as S-56A before launch, was a NASA satellite which was launched in February 1961 to study the density and composition of the upper thermosphere and lower exosphere.[2] It was a reflight of the failed Explorer S-56 mission, and consisted of a 7 kg (15 lb), 3.66 m (12.0 ft) balloon which was deployed into a medium Earth orbit.[3] The mission was conducted by NASA's Langley Research Center.

## Contents

* 1 Spacecraft
* 2 Experiment
* 2.1 Satellite Drag Atmospheric Density
* 3 Launch
* 4 Mission results
* 5 Legacy
* 6 See also
* 7 References

## Spacecraft

The spacecraft consisted of alternating layers of aluminium foil and Mylar polyester 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 its Scout X-1 launch vehicle.[4]

## 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 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]

## Launch

Launch of Explorer 9 on a Scout X-1 (ST-4) on 16 February 1961.

Explorer 9 was launched from Launch Area 3 at the Wallops Flight Facility (WFF), atop a Scout 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 an apogee of 2,225 km (1,383 mi), a perigee of 545 km (339 mi), 38.91° of inclination and a period of 118.6 minutes.[7] It was assigned the Harvard designation 1961 Delta 1.[8]

Upon separation of the fourth stage, the sphere was inflated by a nitrogen 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 was carried for tracking purposes, but the beacon failed on the first orbit and the SAO Baker-Nunn camera network had to be relied upon for tracking. Power was supplied by solar cells and rechargeable batteries.[1]

The second of six identical air 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]

## Mission results

Careful and continuous photographic observation of the satellite allowed scientists to conduct an unprecedented survey of the Earth's exosphere. The satellite first confirmed the daily bulge in the upper 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 the ionosphere, peaking around five hours after each event.[10] With three years of data, scientists were able to discern a seasonal variation to the exosphere's density, with an increase of 25% observed in winter over summer at a reference latitude 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]

## Legacy

A replica of the spacecraft, possibly a flight backup, is currently located in the Smithsonian Institution's National Air and Space Museum, although it is not on display.[12]

## See also

* Explorer S-56
* Explorer program

## References

1. ↑ 1.0 1.1 1.2 "Display: Explorer-9 1961-004A". NASA. 28 October 2021. https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1961-004A. This article incorporates text from this source, which is in the public domain.
2. ↑ Smith, Woody. "Explorer Spacecraft Series". NASA. https://history.nasa.gov/explorer.html. This article incorporates text from this source, which is in the public domain.
3. ↑ Wade, Mark. "S-56". Encyclopedia Astronautica. http://www.astronautix.com/craft/s56.htm.
4. ↑ 4.0 4.1 Krebs, Gunter (8 April 2020). "AD A, B, C / (S-56) / Explorer S-56, 9, 19, 24, 39, 57". Gunter's Space Page. https://space.skyrocket.de/doc_sdat/explorer_ad.htm.
5. ↑ Jacchia, Luigi G.; Slowey, Jack (1963). "Accurate drag determinations for eight artificial satellites: atmospheric densities and temperatures". Smithsonian Contributions to Astrophysics 8 (1): 1–99. doi:10.5479/si.00810231.8-1.1. ISSN 0081-0231.
6. ↑ "Experiment: Satellite Drag Atmospheric Density". NASA. 28 October 2021. https://nssdc.gsfc.nasa.gov/nmc/experiment/display.action?id=1961-004A-01. This article incorporates text from this source, which is in the public domain.
7. ↑ McDowell, Jonathan. "Satellite Catalog". Jonathan's Space Report. http://planet4589.org/space/log/satcat.txt.
8. ↑ McDowell, Jonathan (21 July 2021). "Launch Log". Jonathan's Space Report. https://planet4589.org/space/log/launchlog.txt.
9. ↑ Jacchia, L. G.; Slowey, J. (1962). "Preliminary Analysis of the Atmospheric Drag of the Twelve-Foot Balloon Satellite (1961 δ1)". Sao Special Report 84. Bibcode: 1962SAOSR..84.....J. http://cdsads.u-strasbg.fr/cgi-bin/nph-iarticle_query?1962SAOSR..84.....J. Retrieved 21 December 2018.
10. ↑ "Analysis of the Atmospheric Drag of the Explorer IX satellite from Precisely Reduced Photographic Observations". http://articles.adsabs.harvard.edu/pdf/1963SAOSR.125.....J.
11. ↑ Roemer, M. (1966). "Atmospheric Densities and Temperatures from Precisely Reduced Observations of the Explorer IX Satellite". Sao Special Report 199. Bibcode: 1966SAOSR.199.....R. http://cdsads.u-strasbg.fr/cgi-bin/nph-iarticle_query?1966SAOSR.199.....R. Retrieved 21 December 2018.
12. ↑ "Satellite, Explorer 9, Balloon Replica". National Air and Space Museum. https://airandspace.si.edu/collection-objects/satellite-explorer-9-balloon-replica.

* v
* t
* e

Explorers Program

List of Explorers Program missions

Missions|

| 1958–1992|

* Explorer 1
* 2†
* 3
* 4
* 5†
* S-1†
* 6 (S-2)
* 7 (S-1A)
* S-46A†
* 8
* S-56†
* 9 (S-56A)
* S-45†
* 10
* 11 (S-15)
* S-45A†
* S-55†
* 12 (EPE-A)
* 13 (S-55A)
* 14 (EPE-B)
* 15 (EPE-C)
* 16 (S-55B)
* 17 (AE-A)
* 18 (IMP-A)
* 19 (AD-A)
* S-66A (BE-A)†
* 20 (IE-A)
* 21 (IMP-B)
* 22 (BE-B)
* 23 (S-55C)
* 24 (AD-B)
* 25 (Injun 4, IE-B)
* 26 (EPE-D)
* 27 (BE-C)
* 28 (IMP-C)
* 29 (GEOS-A)
* 30 (Solrad 8)
* 31 (DME-A)
* 32 (AE-B)
* 33 (IMP-D)
* 34 (IMP-F)
* 35 (IMP-E)
* 36 (GEOS-B)
* 37 (Solrad 9)
* 38 (RAE-A)
* 39 (AD-C)
* 40 (Injun 5)
* 41 (IMP-G)
* 42 (Uhuru, SAS-A)
* 43 (IMP-H)
* 44 (Solrad 10)
* 45 (SSS-A)
* 46 (MTS)
* 47 (IMP-I)
* 48 (SAS-B)
* 49 (RAE-B)
* 50 (IMP-J)
* 51 (AE-C)
* 52 (Hawkeye 1)
* 53 (SAS-C)
* 54 (AE-D)
* 55 (AE-E)
* DADE-A†
* DADE-B†
* 56 (ISEE-1)
* 57 (IUE)
* 58 (HCMM)
* 59 (ICE)
* 60 (SAGE)
* 61 (Magsat)
* 62 (DE-1)
* 63 (DE-2)
* 64 (SME)
* 65 (CCE)
* 66 (COBE)
* 67 (EUVE)

|

MIDEX|

* 69 (RXTE)
* 71 (ACE)
* 77 (FUSE)
* 78 (IMAGE)
* 80 (WMAP)
* FAME
* 84 (Swift)
* 85–89 (THEMIS)
* 92 (WISE)
* 95 (TESS)
* 96 (ICON)
* SPHEREx

SMEX|

* 68 (SAMPEX)
* 70 (FAST)
* 73 (TRACE)
* 74 (SWAS)
* 75 (WIRE)†
* 81 (RHESSI)
* 83 (GALEX)
* 90 (AIM)
* 91 (IBEX)
* 93 (NuSTAR)
* 94 (IRIS)
* GEMS
* IXPE
* PUNCH
* TRACERS

UNEX/MO/I|

* HETE-1†
* 72 (SNOE)
* 76 (TERRIERS)†
* 79 (HETE-2)
* INTEGRAL
* 82 (CHIPSat)
* CINDI
* Suzaku
* TWINS
* Hitomi†
* NICER
* GOLD
* XRISM
* GUSTO
* CASE

Proposals|

| Proposals|

* FINESSE
* Arcus
* OHMIC
* ASTRE
* EXCEDE

|

* Green titles indicates active current missions
* Grey titles indicates cancelled missions
* Italics indicate missions yet to launch
* Symbol † indicates failure en route or before intended mission data returned

* v
* t
* e

Orbital launches in 1961

Samos 2 | Tyazhely Sputnik | Venera 1 | Explorer 9 | Discoverer 20 | Discoverer 21 | Transit 3B * LOFTI-1 | S-45 | Korabl-Sputnik 4 | Korabl-Sputnik 5 | Explorer 10 | Discoverer 22 | Discoverer 23 | Vostok 1 | Mercury-Atlas 3 | Explorer 11 | S-45A | Discoverer 24 | Discoverer 25 | Transit 4A * SOLRAD 3 * Injun 1 | S-55 | Discoverer 26 | TIROS-3 | Midas 3 | Discoverer 27 | Discoverer 28 | Vostok 2 | Explorer 12 | Ranger 1 | Explorer 13 | Discoverer 29 | Samos 3 | Discoverer 30 | Mercury-Atlas 4 | Discoverer 31 | Discoverer 32 | Midas 4 * Westford 1 | Discoverer 33 | DS-1 No.1 | Mercury-Scout 1 | Discoverer 34 | Discoverer 35 | Transit 4B * TRAAC | Ranger 2 | Samos 4 | Mercury-Atlas 5 | Zenit-2 No.1 | Discoverer 36 * OSCAR 1 | DS-1 No.2 | FTV-2203

Payloads are separated by bullets ( * ), launches by pipes ( | ). Crewed flights are indicated in bold text. Uncatalogued launch failures are listed in italics. Payloads deployed from other spacecraft are denoted in brackets.

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*[MIDEX]: Medium-Class Explorers
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