Explorer 1 at 50

50 years ago today, at 10:48 pm Eastern Standard Time on January 31, 1958, the U.S.’ first satellite was launched. Here’s a brief (just short of 2 minutes long) clip of old newsreal footage about the event:

A slightly longer (2:43) clip is also available, this one showing the pre-launch spin-up of the turntable holding the upper stages of the Juno 1 rocket:

If you’ve been listening to SOS podcast episodes (in particular, the podcast episode on Sputnik), you’ve heard of the events that led up to the launch. As a refresher, here’s a quick bulletized version of that history:

  • 1952 — International Council of Scientific Unions (ICSU) establishes July 1, 1957 – December 31, 1958 as the International Geophysical Year (IGY)

  • June, 1954 — U.S. Army starts Project Orbiter, to explore launching an Earth satellite

  • October, 1954 — ICSU calls for artificial satellites to be launched during the IGY

  • August, 1955 — Stewart Committee cancels Army’s Project Orbiter, and Air Force’s Project Score, in deference to the Navy’s (less-blatantly-military) Project Vanguard

  • 20 September, 1956 — Army personnel set altitude record with a Jupiter C rocket (essentially the same as the vehicle that would later launch Explorer 1, but with sand-filled 4th stage)

  • September, 1957 — U.S.S.R. announces an impending satellite launch

  • 4 October, 1957 — U.S.S.R. launches Sputnik 1

  • 3 November, 1957 — U.S.S.R. launches Sputnik 2, carrying a dog named Laika

  • 8 November, 1957 — Army / JPL team authorized to “prepare” a satellite launch

  • 6 December, 1957 — Vanguard launch attempt results in on-pad explosion

  • 31 January, 1958 — Explorer 1 launches

  • 3 March, 1958 — Explorer 2 launch fails when 4th stage does not ignite

  • 17 March, 1958 — First successful Vanguard launch

  • 26 March, 1958 — Explorer 3 launches

Since big-round-number anniversaries tend to draw a fair amount of attention, I suspect various sources on the web will be covering the anniversary and the events leading up to it. Some items, though, seem to escape mention in many discussions of Explorer 1, so I’d like to discuss a few bits of lesser-known Explorer 1 history.

  • While later dubbed “Explorer 1,” the satellite’s original (if unimaginative) name was “Satellite 1958 Alpha.” The renaming occurred while the U.S. government pondered how to announce the successful launch.

  • Explorer’s Juno-1 launch vehicle had four stages. The first stage was an upgraded Redstone liquid propellant rocket. The second, third, and fourth stage rockets consisted of eleven, three, and one scaled-down Sergeant solid propellant rocket motors (respectively). Since the upper stages of the rocket were unguided, they were kept on their desired path in ascent by spinning them up to 750 RPM before launch. You can see this spin up take place in the second video, above (the upper stages are nested inside one another, in the cylindrical section atop the nose of the launch vehicle).

    This design inadvertently led to the discovery of a basic principle of dynamics — given even a minimal amount of energy dissipation, a spinning body in space will eventually re-orient itself to a “flat spin” since that is the body’s minimum energy state. This surprised the operators of Explorer 1 when days after launch they realized it was tumbling end over end (rather than spinning along its long axis, as it was at launch). The flexing of the spacecraft’s whip antennas had helped Explorer 1 transition itself to a flat spin.

  • Explorer 1’s instrumentation consisted of a Geiger-Mueller detector (to measure charged particle radiation), five temperature sensors, a micrometeorite impact microphone, and a ring of micrometeorite erosion guages. The scientific instruments were hosted in the striped section at the front, which stayed attached to the empty scaled-down Sergeant fourth-stage rocket casing. The external skin of the instrument section was painted in strips of white and dark green to provide passive temperature control of the satellite (i.e., the relative proportions of the light and dark stripes were selected to yield a desired range of temperatures in the spacecraft’s desired orbit).

    Explorer cutaway
  • The Explorer I instruments used transistors in their electronics, the first documented use of transistors in space.

  • Explorer 1 weighed 14 kg (30.66 pounds), of which the instrument section accounted for 8.4 kg (18.35 pounds). The spacecraft was 203 cm (80 inches) long, and 15.9 cm (6.25 inches) in diameter. Compare these numbers to Sputnik’s 83.6 kg (183.9 pounds) weight in a 58.5 cm (23 inches) diameter sphere.

  • Data from the spacecraft’s instruments were sent to the ground by two transmitters — a “high-power” (60-milliwatt) transmitter operating on 108.03 MHz and a “low-power” (10-milliwatt) transmitter operating on 108.00 MHz. The transmitters used antennas consisting of two fiberglass slot antennas in the body of the satellite and four flexible whips forming a turnstile antenna. Electrical power was provided by two chemical batteries, which operated the high power transmitter for 31 days, and the low-power transmitter for 105 days (until May 23, 1958). The spacecraft itself stayed in orbit until March 31, 1970 after completing more than 58,000 orbits.

  • Explorer 1 was injected into an elliptical orbit — with a perigee altitude of 354 km (220 miles), an apogee altitude of 2515 km (1,563 miles), and an inclination (w.r.t. the equator) of 33.24 degrees). The high apogee altitude inadvertently resulted in the spacecraft flying through belts of trapped radiation around the Earth, but more on this later.

  • cnt_rate.gif

    Explorer 1 is often given credit for discovering the Van Allen belts, intense regions of trapped charged particles girdling the Earth. This is essentially true, if oversimplified. Since the spacecraft was designed and built in a hurry, there was no time to redesign a tape recorder to fit in the compact instrument compartment — instrument data was only available in real-time, while the satellite was within range of a ground station. Data from the Geiger counter behaved as expected at low altitudes, but was non-existent at high altitudes. Since nobody could explain how a radiation-free zone might exist around the Earth, after discarding failure scenarios, researchers theorized that the equipment was saturated by passage through a high-radiation zone.

    Explorer 2 failed to reach orbit, but Explorer 3 was successful — and since it carried a tape recorder, was able to show that the Geiger counter saw increasing activity with altitude until the instrument reached its maximum value, then dropped abruptly to zero. Later in the orbit, the maximum value would again be seen, followed by a drop to lower levels. This confirmed the existence of radiation belts around the Earth — later dubbed the Van Allen belts, named after the principal investigator of Explorer’s science package (James Van Allen).

  • The Soviets missed their chance to discover the Van Allen belts — Sputnik 1 carried no radiation detector, but Sputnik 2 did. While an increase in radiation with altitude was noticed, the implications of this data were not realized until after the discovery of the Van Allen belts was announced.

Plenty of other sites will likely be adding their takes on the anniversary over the course of the next day or two. I plan on adding links to them as I come across them, so make sure you come back to this post if you’d like to read more. And of course, if you find any other interesting Explorer 1 coverage you’d like to share with other readers, either leave it in a comment, or drop me a line — I’ll happily link to anybody else’s page on the anniversary.

Links of interest:

JPL’s Explorer 1 pages

Explorer I – Wikipedia

Space Exploration History: Explorer 1

NASA History: Explorer Spacecraft Series

Smithsonian Institution datasheet on Explorer 1 and its launch vehicle

About.com — Brief History of the Explorer 1 Mission

A Brief History of Magnetospheric Physics During the Space Age

NASM Space Artifacts: Explorer 1

SPACE.com — Explorer I Launch Team Recalls America’s First Satellite

SPACE.com — 50 Years Later: First U.S. Satellite’s Souvenirs Still Circle the Earth

Jan. 31, 1958: Explorer I Makes It Official — There’s a Space Race

NASA – A Moment in Time: Explorer 1

Fantastic four remember Explorer 1/Jupiter-C launch on 50th Anniversary of vital mission

Star Talk: 50 years ago this week, U.S. provided answer to Sputnik

Explorer I Resolution Introduced to Commemorate 50th Anniversary of the Birth of the U.S. Space Program | SpaceRef – Your Space Reference

Build your own Explorer 1 from cardstock

Build your own Juno 1 from cardstock

Other anniversary pages:

AIA Press Release | First U.S. Satellite a Triumph of American Ability and Vision

Astroprof’s Page » Explorer 1

The Reply at Asymptotia

Bad Astronomy Blog » 50 years after Explorer 1

America’s space age turns 50 – Cosmic Log – msnbc.com

dailywireless.org » Golden Anniversary for Explorer 1

Greg Laden’s Blog : Happy Birthday The Space Age

Matt’s Sci/Tech Blog: 50th anniversary of Explorer 1

New Scientist Space Blog: US space programme born 50 years ago today

Spaceports: Explorer-1 Golden Anniversary: 10:48 pm on January 31, 2008

Okay, NOW the space age turns 50 – Tech_Space – USATODAY.com

Tom’s Astronomy Blog » Our First Satellite (w00t!)

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