Juno I

Juno I is a Redstone missile with three tubs of solid motors stacked on it. NASA’s Historical Data Book gives the total height with payload as 21 metres, the diameter as 1.8 metres, and the first stage thrust as 369,184 newtons, which matches the Rocketdyne A-7 engine’s stated 83,000 pounds almost exactly. Payload capacity is 18 kilograms to a 555 kilometre orbit. Do not use the data book’s 474,601 newton total: that is the arithmetic sum of all four stages, not a liftoff figure.

The upper stages were eleven scaled-down Sergeant solid motors, then three, then one, spun for stability and fired in sequence. The last motor never separated, which is why Explorer 1 is often quoted at 30.66 pounds when only 18.35 pounds of that was instrumentation. The rest was a dead rocket casing bolted to the science.

It could have flown much earlier. On 20 September 1956 a Jupiter-C carried a dummy fourth stage to 682 miles altitude and 3,335 miles downrange, thirteen months before Sputnik. Von Braun’s team was not permitted to make that payload live. NASA’s own history preserves the instruction he was given, in the plainest possible terms: "Wernher, now listen. There is much talk about satellites these days. Satellites are not for you!"

Sputnik ended that argument, and on 31 January 1958 at 10:48 p.m. Eastern time, 03:48 GMT on 1 February, the first Juno I put Explorer 1 into orbit. The instrument that flew found something nobody had ordered: the radiation belts around the Earth. Six Juno I vehicles flew in all and three succeeded, which is a mediocre record for a vehicle that opened American spaceflight, and an unremarkable one for 1958.