Juno I


Juno I
Juno-1 explorer-2.jpg
Juno I satellite launch vehicle carrying Explorer 2. (USAF)
FunctionOrbital launch vehicle
ManufacturerChrysler for the ABMA
Country of originUnited States
Height21.2 m (70 ft)
Diameter1.78 m (5 ft 10 in)
Mass29,060 kg (64,070 lb)
Payload to LEO
Mass11 kg (24 lb)
Launch history
Launch sitesLC-5 and 26A, Cape Canaveral Missile Annex, Florida
Total launches6
First flightJanuary 31, 1958
(First orbit: Explorer 1 January 31, 1958)
Last flightOctober 23, 1959
First stage – Redstone (stretched)
Engines1 Rocketdyne A-7
Thrust42,439 kgf (416.18 kN; 93,560 lbf)
Specific impulse235 s (2.30 km/s)
Burn time155 seconds
Second stage – Baby Sergeant cluster
Motor11 Solid[1]
Thrust7,480 kgf (73.4 kN; 16,500 lbf)
Specific impulse220 s (2.2 km/s)
Burn time6 seconds
FuelPolysulfide-aluminum and ammonium perchlorate (Solid)
Third stage – Baby Sergeant cluster
Motor3 Solid
Thrust2,040 kgf (20.0 kN; 4,500 lbf)
Specific impulse236 s (2.31 km/s)
Burn time6 seconds
FuelPolysulfide-aluminum and ammonium perchlorate (Solid)
Fourth stage – Baby Sergeant
Motor1 Solid
Thrust680 kgf (6.7 kN; 1,500 lbf)
Specific impulse249 s (2.44 km/s)
Burn time6 seconds
FuelPolysulfide-aluminum and ammonium perchlorate (Solid)

The Juno I was a four-stage American booster rocket that launched America's first satellite, Explorer 1, in 1958. A member of the Redstone rocket family, it was derived from the Jupiter-C sounding rocket. It is commonly confused with the Juno II launch vehicle, which was derived from the PGM-19 Jupiter medium-range ballistic missile.


The Explorer Project began as a U.S. Army proposal (Project Orbiter) to place a "civilian" artificial satellite into orbit during the International Geophysical Year. The proposal was based on the Redstone missile vehicle. Although that proposal was rejected in favor of the U.S. Navy's Project Vanguard, which made the first sub-orbital flight Vanguard TV0 in December 1956, the Soviet Union's launch of Sputnik 1 on 4 October 1957 (and the resulting "Sputnik crisis") and the failure of the Vanguard 1 launch attempt resulted in the Army program being funded to match the Soviet space achievements.

The rocket family is named for the Roman goddess and queen of the gods Juno for its position as the satellite-launching version of the Jupiter-C. The name was proposed by JPL Director Dr. William Pickering in November 1957. The September 1956 test launch of a Jupiter-C for the Army Ballistic Missile Agency could have been the world's first satellite launch. Had a fourth stage been loaded and fueled, the nose cone would have overshot the target and entered orbit. Such a launch did not occur until early 1958 when a Juno 1 successfully launched the first United States satellite, Explorer 1 after the Soviet Union's Sputnik 1 in October 1957.[2]

Juno-I launched the Explorer 1 satellite on 31 January, 1958, becoming the first U.S. satellite, as well as discovering the Van Allen radiation belt.


The Juno I consisted of a Jupiter-C first stage, based on the Redstone missile; with three additional solid fuel stages based on the Sergeant missile to provide the added impulse to achieve orbit. The fourth stage was mounted on top of the "tub" of the third stage, and fired after third-stage burnout to boost the payload and fourth stage to an orbital velocity of 8 kilometres per second (29,000 km/h; 18,000 mph), with an acceleration of 25–51 g. The tub along with the fourth stage were set spinning while the rocket was on the launch pad to provide gyroscopic force in lieu of a guidance system that would have required vanes, gimbals, or vernier motors. This multi-stage system, designed by Wernher von Braun in 1956 for his proposed Project Orbiter, obviated the need for a guidance system in the upper stages. It was the simplest method for putting a payload into orbit but having no upper-stage guidance, the payload could not achieve a precise orbit. Both the four-stage Juno I and three-stage Jupiter-C launch vehicles were the same height (21.2 meters), with the added fourth-stage booster of the Juno I being enclosed inside the nose cone of the third stage.

Launch history

Following the successful launch of Explorer 1 on 31 January 1958, the first U.S. satellite, Juno I made five more launches before being retired in favor of Juno II. Although Juno I's launch of the Explorer 1 satellite was a huge success for the U.S. space program, only two of its remaining five flights were successful, Explorer 3 and Explorer 4,[1] giving the Juno I vehicle a mission total success ratio of 50%. The Juno I vehicle was replaced by the Juno II in 1959.

The American public was happy and relieved that America had finally managed to launch a satellite after the launch failures in the Vanguard and Viking series. With the relative success of the Juno I program, von Braun developed the Juno II, using a PGM-19 Jupiter first stage, rather than a Redstone.

Flight No. Date / time (UTC) Rocket,
Launch site Payload Payload mass Orbit User Launch
1 February 1, 1958
Juno I LC-26A Explorer 1 22 kg LEO ABMA Success
Maiden launch of Juno I. First American satellite launched. Explorer 1 ceased transmission of data on May 23, 1958 when its batteries died, but remained in orbit for more than 12 years. It made a fiery reentry over the Pacific Ocean on March 31, 1970.
2 March 5, 1958
Juno I LC-26A Explorer 2 23 kg LEO ABMA Failure
Fourth stage did not ignite.
3 March 26, 1958
Juno I LC-5 Explorer 3 23 kg LEO ABMA Success
Down June 28, 1958.
4 July 26, 1958
Juno I LC-5 Explorer 4 29 kg LEO ABMA Success
Down October 23, 1959.
5 August 24, 1958
Juno I LC-5 Explorer 5 29 kg LEO ABMA Failure
Booster collided with second stage after separation, causing upper stage firing angle to be off.
6 October 23, 1958
Juno I LC-5 Beacon 1 23 kg LEO ABMA Failure
Second stage separated prematurely from booster.



  1. ^ a b J. Boehm, H.J. Fichtner, and Otto A. Hoberg, EXPLORER SATELLITES LAUNCHED BY JUNO 1 AND JUNO 2 VEHICLES, NASA Report.
  2. ^ Bello, Francis (1959). "The Early Space Age". Fortune. Archived from the original on November 3, 2013. Retrieved June 5, 2012.