Earth 1

4 October 1957

Sputnik

83.6 kg of aluminium that put the first human-made star in the sky.

215 km

altitude

0.00 km/s

speed

1 / 1440

orbit

Real orbit: perigee 215 km, apogee 939 km, period 96.1 min. Earth and orbit drawn to the same scale.

The numbers

Launched
4 October 1957, 19:28:34 UTC
From
NIIP-5, Tyuratam (later Baikonur), Kazakh SSR
Rocket
R-7 Semyorka, modified 8K71PS
Mass
83.6 kg
Body
Polished aluminium-alloy sphere, 58 cm across
Antennas
Four whips, two of 2.4 m and two of 2.9 m
Radio
20.005 MHz and 40.002 MHz, 1 W, pulses of about 0.3 s
Power
Three silver-zinc batteries, 51 kg of the total mass
Orbit
215 × 939 km, 65.1° inclination
Period
96.2 minutes
Transmitted
22 days, until the batteries ran out on 26 October 1957
Fell
4 January 1958, burning up after about 1,440 orbits

Why it stayed up

Sputnik was falling the whole time. At about 8 km/s it fell around the curve of Earth as fast as the ground curved away beneath it. Kepler's third law gives the time for one lap:

T = 2π √(a³ / μ)

  • T: time for one orbit.
  • a: the average of the closest and farthest distance from Earth's centre, 6,371 km + (215 + 939) / 2 = 6,948 km.
  • μ: Earth's pull, its mass times the gravitational constant, 398,600 km³/s².

Put in the numbers and T comes out at 96 minutes, within a few seconds of what the trackers measured. The orbit was an ellipse, so the speed changed: fastest near 215 km, slowest near 939 km. Drag from the thin upper air at the low point shrank the orbit a little every lap until it fell in January 1958.

The machine

Two aluminium half-shells sealed with 36 bolts and filled with dry nitrogen. A fan moved the gas to keep the batteries and two radio transmitters at working temperature. The surface was polished to reflect sunlight and stay cool.

The beep carried data. A pressure switch and two thermal switches changed the length of the pulses, so a puncture by a micrometeoroid or a failing fan would show up on the ground. Engineers chose 20 and 40 MHz so that radio amateurs across the world could hear it on ordinary short-wave receivers.

How listening to it made GPS

At the Johns Hopkins Applied Physics Laboratory, William Guier and George Weiffenbach recorded Sputnik's signal and watched its pitch slide down as it passed, the Doppler effect:

f = f₀ (1 − v / c)

  • f: the pitch heard on the ground.
  • f₀: the pitch sent, 20.005 MHz.
  • v: how fast the satellite moves away from the listener (negative while it approaches).
  • c: the speed of light.

From one pass they could work out the whole orbit. Their colleague Frank McClure turned it round: if you know the orbit, the same curve tells you where you are. That became Transit, the US Navy's satellite navigation system from 1964, and the idea behind GPS.

Timeline

  1. 1954 – 55

    Mikhail Tikhonravov's group argues for a satellite. The International Geophysical Year (July 1957 to December 1958) invites nations to launch one; the US announces Vanguard in July 1955, the USSR answers days later.

  2. 30 Jan 1956

    The Soviet government approves Object D, a 1,000+ kg science laboratory. Its instruments fall behind schedule.

  3. 21 Aug 1957

    After failed tries in May and July, the R-7, the world's first intercontinental ballistic missile, flies 6,000 km to Kamchatka.

  4. Sep 1957

    Sergei Korolev, chief designer, swaps Object D for the Prosteishiy Sputnik (PS-1), the simplest satellite, so the USSR flies first. Object D later flies as Sputnik 3.

  5. 4 Oct 1957

    Launch. The core stage reaches orbit too, at 26 m long it is the bright moving star people see. Sputnik itself is near the limit of the naked eye.

  6. 5 Oct 1957

    Pravda runs a short TASS bulletin. Within days the beep is on radio stations worldwide and in every newspaper.

  7. 3 Nov 1957

    Sputnik 2 carries Laika, the first animal in orbit. She dies within hours from overheating; the craft was never meant to return.

  8. 6 Dec 1957

    America's Vanguard TV-3 rises about a metre and explodes on live television. Papers call it Flopnik.

  9. 4 Jan 1958

    Sputnik 1 re-enters and burns up.

  10. 31 Jan 1958

    Explorer 1, built by JPL on a von Braun Juno I, becomes the first US satellite. James Van Allen's Geiger counter finds the radiation belts named after him.

  11. 7 Feb 1958

    The Pentagon creates ARPA, later DARPA, whose ARPANET became the internet.

  12. 29 Jul 1958

    Eisenhower signs the National Aeronautics and Space Act. NASA opens on 1 October 1958.

  13. 2 Sep 1958

    The National Defense Education Act pays for science, maths and language teaching across the US.

  14. 12 Apr 1961

    Yuri Gagarin orbits Earth on Vostok 1, on a descendant of the same R-7.

What came next

Every satellite in the sky now, from the GPS and weather constellations to thousands of Starlinks, follows the same law Sputnik did. See them live on the space tracker, and everyone who has followed Gagarin up on the astronauts page.

Sources