Experiments 16

One Light-Day

Voyager 1 is about to become the first thing people have made to sit a full light-day from Earth. This page takes it apart: the spacecraft, the power it has left, its three computers rebuilt in Rust and running here, and the record it carries.

Rust · WASM · Canvas 3D · Web Audio · JPL HorizonsView source 

Launched September 5, 1977

Voyager 1.

The farthest thing people have ever made. Still switched on.

Kilometers from Earth
25,750,390,489
Light takes
23 h 51 m 34 s

1 / 8

High-gain antenna

  • 3.7 m dish · X-band

2 / 8

Magnetometer

  • MAG
  • 13 m boom

3 / 8

Plasma wave

  • PWS
  • PRA
  • Two 10 m antennas

4 / 8

Three RTGs

  • Plutonium‑238 heat

5 / 8

Scan platform

  • ISS
  • IRIS
  • UVS
  • PPS

6 / 8

Particles

  • CRS
  • LECP
  • PLS

7 / 8

Golden Record

  • 30.5 cm · gold-plated copper

8 / 8

Bus

  • Ten bays · six computers

Built for a five-year trip to Jupiter and Saturn. Still working, forty-nine years later.

Ten instruments. Two still on.

  • High-gain antenna: 3.7 m dish · X-band
  • Magnetometer: 13 m boom, MAG on
  • Plasma wave: Two 10 m antennas, PWS on, PRA off
  • Three RTGs: Plutonium‑238 heat
  • Scan platform: ISS off, IRIS off, UVS off, PPS off
  • Particles: CRS off, LECP off, PLS off
  • Golden Record: 30.5 cm · gold-plated copper
  • Bus: Ten bays · six computers

It is looking at you. Its dish has to hold Earth inside a radio beam just 0.6° wide.

Distance

Almost a light‑day away.

No one has ever sent anything this far. Every number here comes from JPL’s trajectory and ticks in real time.

One light-day in

51days10hours16minutes07seconds

November 18, 2026 at 10:16 UTC, as NASA times it. A signal sent then takes a full day to arrive.

Your bits leave at 16 per second and arrive in 23 h 51 m.

  1. Moon 1.3s
  2. Sun 8m 19s
  3. Jupiter 43m
  4. Saturn 1h 19m
  5. Neptune 4h 10m
  6. Heliopause, 2012 16h 51m
Voyager 1 today 23h 51m
Light time from Earth, on a log scale. Planets at their average distance.
  • From the Sun

    171.878AU

    Earth sits at 1.

  • Speed from the Sun

    60,774km/h

    About 3.6 AU a year.

  • Receding from you

    39.85km/s

    Swings by 24 as Earth orbits.

  • Downlink

    160bit/s

    Commands go up at 16.

Power

470 watts at launch. 214 today.

Sunlight is far too weak for solar panels out here, so the power comes from the heat of decaying plutonium‑238. As the heat fades, something has to be switched off.

Year

1977

Power
470W
−7.6 W a year
From the Sun
  1.0AU
Instruments onRunning
10of 10

Drag the chartDrag

  1. PPSPhotopolarimeter, switched off Jan 29, 1980
  2. ISSImaging Science, switched off Feb 14, 1990
  3. IRISInfrared Interferometer, switched off Jun 3, 1998
  4. PLSPlasma Science, switched off Feb 1, 2007
  5. PRAPlanetary Radio Astronomy, switched off Jan 15, 2008
  6. UVSUltraviolet Spectrometer, switched off Apr 19, 2016
  7. CRSCosmic Ray Subsystem, switched off Feb 25, 2025
  8. LECPLow-Energy Charged Particles, switched off Apr 17, 2026
  9. MAGMagnetometer, still on
  10. PWSPlasma Wave, still on

Computers

Six computers. 68 kilobytes between them.

Three kinds, two of each, so a spare can take over. Two of their jobs are rewritten below in Rust, compiled to WebAssembly and running on this page.

Memory, to scale

69,632 bytes in all

  • CCS · Computer Command Subsystem

    Runs the stored sequences, decodes commands from Earth and watches for faults.

    2 × 4,096 words · 18-bit · plated wire

  • AACS · Attitude and Articulation Control Subsystem

    Keeps the dish on Earth with a Sun sensor, a star tracker and hydrazine thrusters.

    2 × 4,096 words · 18-bit · plated wire

  • FDS · Flight Data Subsystem

    Collects instrument and engineering data and formats it into telemetry frames.

    2 × 8,192 words · 16-bit · CMOS

The 3D model on this page is 1,301,972 bytes, 18.7 times the memory of all six computers.

All six computers. Each square is 69,632 bytes.

Holding Earth in the beam.

The dish’s X-band beam is 0.6° wide. Voyager lets its pointing drift inside a deadband and fires a tiny hydrazine pulse whenever it reaches the edge. A wider deadband saves fuel and wear on the thrusters. Too wide, and Earth slips out of the beam.

Earth, seen from the dish
Error against rate, one axisError vs rate
  • Thruster pulses a dayPulses a day

    –

    Voyager 1 in 2024: about 40

  • Worst signal lossSignal loss

    0.0 dB

    0 dB is dead center

  • Earth

    In the beamIn beamSlipping outSlipping

    The deadband sits inside the beam.The deadband is wider than the beam.

voyager.rs
Running in WebAssembly
34pub fn step(&mut self, dt: f64) -> Pulse {35    self.error += self.rate * dt;36    if self.error > self.deadband && self.rate > 0.0 {37        self.fire(-KICK_DEG_S);38        return Pulse::Minus;39    }40    if self.error < -self.deadband && self.rate < 0.0 {41        self.fire(KICK_DEG_S);42        return Pulse::Plus;43    }44    Pulse::Coast45}
47fn fire(&mut self, kick: f64) {48    self.rate += kick;49    self.pulses += 1;50}
56pub fn signal_db(&self) -> f64 {57    -12.0 * (self.error / beamwidth()).powi(2)58}

voyager.rs

Holding Earth in the beam

The chip that died.

In November 2023, one memory chip in the Flight Data Subsystem failed and the telemetry turned to nonsense. There was no spare chip. JPL cut the stranded code into pieces, found room for each one elsewhere and fixed every address that pointed at them, from a light-day away.

  1. Telemetry turns into a repeating pattern of ones and zeros.

  2. Engineers send a “poke” asking the FDS to read back its memory.

  3. The readout shows one chip, about 3% of FDS memory, has failed.

  4. The first relocated code goes up, split across other addresses.

  5. Engineering data comes back readable.

  6. All four working science instruments return data.

Telemetry turns into a repeating pattern of ones and zeros.

FDS healthy. Telemetry decoding.

Chip 21 holds the four routines that build every frame.

Downlink                  

    voyager.rs
    Running in WebAssembly
    189pub fn relocate(&mut self, routine: usize, base: usize) -> Result<u32, String> {190    let len = LAYOUT[routine].2;191    if base + len > WORDS || !self.fits(base, len) {192        return Err(format!("{} words do not fit at {base:#06x}", len));193    }194    let old = self.bases[routine];195    for word in old..old + len {196        self.owner[word] = FREE;197    }198    self.bases[routine] = base;199    self.load(routine);200    Ok(self.patch())201}
    203fn fits(&self, base: usize, len: usize) -> bool {204    (base..base + len).all(|w| self.owner[w] == FREE && !self.is_dead(w))205}
    207fn patch(&mut self) -> u32 {208    let mut patched = 0;209    for &(from, at, to) in CALLS.iter() {210        let site = self.bases[from] + at;211        let call = CALL | self.bases[to] as u16;212        if self.memory[site] != call {213            self.memory[site] = call;214            patched += 1;215        }216    }217    patched218}
    231pub fn frame(&mut self, watts: f64, nanotesla: f64) -> Vec<u16> {232    self.count += 1;233    if !self.healthy() {234        return vec![STUCK; FRAME_WORDS];235    }236    let mut frame = vec![0; FRAME_WORDS];237    frame[..2].copy_from_slice(&SYNC);238    frame[2] = self.count as u16;239    frame[3] = (watts * 10.0) as u16;240    frame[4] = (nanotesla * 1000.0) as u16;241    for word in frame.iter_mut().skip(5) {242        *word = self.noise() as u16;243    }244    frame245}

    voyager.rs

    The Rust running on this page

    The Golden Record

    A message with instructions.

    A 12-inch gold-plated copper record with a stylus, and a cover engraved with how to play it, in units of the hydrogen atom.

    Pick a drawing to read itThe record under the cover

    How fast to spin it.

    1. Read the marks around the record. A tick is 1, a dash is 0.
    2. That is 5,113,380,864 in binary.
    3. Count it in hydrogen’s tick, 0.704 billionths of a second: 3.60 s for one turn.
    4. 60 ÷ 3.6 = 16⅔ turns a minute. The drawing turns at that speed now, and each mark lights as it passes the top.
    • Pictures

      115

      After the calibration circle

    • Languages

      55

      Greetings, Akkadian to Wu

    • Music

      90 min

      27 pieces, Bach to Chuck Berry

    • Sounds of Earth

      35

      Surf, thunder, a kiss

    Tech specs

    By the numbers.

    Figures from NASA and JPL. The gold ones are live.

    • Launch

      Sep 5, 1977

      12:56 UTC on a Titan IIIE–Centaur

      Launch Complex 41, Cape Canaveral

    • Mass

      815 kg

      At launch, with hydrazine

      105 kg of it science instruments

    • Size

      3.7 m dish

      Ten-sided bus, 1.78 m across and 47 cm tall

      A 13 m magnetometer boom and two 10 m antennas

    • Power

      214 W today

      From three plutonium‑238 RTGs

      470 W at launch, losing about 4 W a year

    • Computers

      69,632 bytes

      Six computers, two each of CCS, AACS and FDS

      CCS and AACS: 4,096 18‑bit words each

      FDS: 8,192 16‑bit words each

    • Storage

      328 m of tape

      Eight-track digital tape recorder

      About 500 million bits

    • Radio

      160 bit/s down

      16 bit/s up, on S‑band at 2.1 GHz

      X‑band down at 8.4 GHz, to the Deep Space Network’s largest dishes

    • Pointing

      16 thrusters

      Hydrazine, about 40 pulses a day in 2024

      Sun sensor and star tracker

    • Instruments

      2 of 10 still on

      Magnetometer and plasma wave

      The rest are off, most recently LECP in April 2026

    • Distance

      25,750,390,489 km

      From Earth, live from JPL Horizons

      In interstellar space since August 25, 2012

    • Speed

      16.9 km/s

      Away from the Sun, about 3.6 AU a year

    • Carrying

      12 in record

      The Golden Record, gold-plated copper

      With a stylus, a cartridge and instructions

      Decode the cover

    Obituary

    Filed early.

    Newspapers write obituaries ahead of time. Voyager 1 should drift out of the Deep Space Network’s reach around 2036, so here is its obituary, on file and hoping not to be needed.

    Not needed yet.

    Voyager 1 is 23 h 51 m 34 s of light away and still talking. NASA expects the Deep Space Network to hear it until about 2036, so the obituary waits on file with its date still to come.

    Obituaries[DATE TK], 2036

    Voyager 1, the Farthest Thing People Ever Made, Falls Silent at 58

    It visited two planets, photographed home from six billion kilometers away and kept talking for decades on 69,632 bytes of memory and the warmth of a little plutonium.

    Voyager 1, the spacecraft that carried a golden record, a stylus and instructions for both farther than anything else people have ever made, fell silent on [DATE TK], 2036, about 31 billion kilometers from home. It was 58 [CHECK AGE]. The cause was a long decline in power.

    It was born at Cape Canaveral, Fla., on Sept. 5, 1977, 16 days after its twin, and was named first anyway because it was going to reach Jupiter first. It did. In March 1979 it found volcanoes erupting on Io, the first seen anywhere but Earth. At Saturn it swung close past the moon Titan, a detour that flung it up and out of the plane of the planets. It never visited another world, and it never seemed to mind.

    On Feb. 14, 1990, six billion kilometers out, it turned around and photographed Earth, a pale blue dot smaller than a pixel. Thirty-four minutes later it switched off its cameras. It never looked at anything again.

    It kept listening. It became the most distant human-made object in 1998 and, in August 2012, the first to reach interstellar space. It did all of it with 69,632 bytes of memory, on power that faded by about four watts a year. When a memory chip failed in 2023, engineers a light-day away rewrote its code around the hole. In 2025 it went back to roll thrusters that had been given up for dead in 2004. It was, by any measure, hard to kill.

    In its last years it gave up its senses one at a time to stay warm and awake: its cosmic ray telescopes in 2025, its particle detector in 2026 and, eventually, the rest.

    It is survived by its twin, Voyager 2, which left home 16 days earlier and is still, somehow, not as far away; by the record it carries, with greetings in 55 languages, a kiss, a humpback whale and Chuck Berry; and by several generations of engineers at NASA’s Jet Propulsion Laboratory. It was predeceased by its cousins Pioneer 11, in 1995, and Pioneer 10, in 2003.

    It is not stopping. In about 40,000 years it will pass within 1.7 light-years of a star called Gliese 445, still carrying the record and the needle to play it.

    In lieu of flowers, the family asks that you look toward Ophiuchus, the Serpent Bearer, where it was last heard from. Its final message went out at 160 bits per second and took more than a day to arrive.

    Still listening

    One-way light time: 23h51m34s

    Anything you send now arrives about this time tomorrow.

    Uplink · 16 bits per second

    Notes on how it works

    Notes

    The spacecraft

    The 3D model is NASA’s own Voyager, made by NASA’s Visualization Technology Applications and Development team (VTAD) for Eyes on the Solar System. A build script splits it into the parts this page takes apart and scales it so the dish is 3.66 m across. It is drawn in the browser with three.js, using physically based materials, a single Sun and self-shadowing. The exploded positions are for legibility, not how the parts sit in flight.

    Voyager 3D model: NASA Visualization Technology Applications and Development (VTAD), via NASA’s Eyes on the Solar System (NASA/JPL-Caltech). This page is not affiliated with or endorsed by NASA.

    Distance

    Distances come from JPL Horizons (target −31, geocentric), sampled weekly from 2024 to 2031 and interpolated with the range rate between samples. The countdown uses NASA’s announced moment, November 18, 2026 at 10:16:07 UTC. Horizons’ own geocentric light time reaches 1,440 minutes about 16 hours later, around 02:00 UTC on November 19. The speed in Specs is Horizons’ heliocentric range rate for the same moment.

    A command you send in the sheet does not reach the Deep Space Network. The page records when you pressed send and uses Horizons’ light time for that moment. The opcode, operand and parity layout is made up. The 16 bits per second uplink rate is real.

    Power

    The wattage curve is an exponential fitted through 470 W at launch and 225 W in 2023, which gives about 3.5 W lost a year now, close to NASA’s “about 4 watts.” Instrument switch-off dates are NASA’s. Anything after today is the fit, not a mission plan. The instrument count in Specs follows the same switch-off dates.

    The Rust

    Voyager’s computers were programmed in assembly for 18-bit plated-wire machines (CCS and AACS) and a 16-bit CMOS machine (FDS). None of this is flight code. It’s a small Rust module, voyager.rs, written to show the same ideas. The site’s WebAssembly package compiles it, and the code on the page is extracted from that file.

    The pointing demo runs two axes of a deadband controller. Each thruster pulse flips the drift rate, and the default settings give 40 to 50 pulses a day, close to the figure NASA gave for 2024. Signal loss uses the usual beam approximation, −12 × (error ÷ 0.6°)² dB.

    The memory demo is 8,192 words in 32 chips of 256. NASA said the failed chip held about 3% of FDS memory. The routine names, sizes and free gaps are invented. The two constraints come from NASA’s account: no single gap could hold the stranded code, and every reference to it had to be updated.

    The Golden Record

    The cover is redrawn from NASA photograph GPN-2000-001978, and its binary was transcribed from the photo. Every value is decoded with hydrogen’s hyperfine period, 0.70402 ns. The rim gives 3.60 s a turn, the side view 53.8 minutes, and the video diagram 8.34 ms a line. Pulsar periods and angles are W. R. Johnston’s reading of the map. Line lengths were measured from the photograph.

    The picture player turns a 512 × 384 image into one line of sound every 8.34 ms, with a sync pulse at the start of each line. It then decodes that sound back into the picture. The real record interlaces its lines and puts pictures on both stereo channels.

    The obituary

    Newspapers keep obituaries of famous people on file and update them until they run. This one is written that way, so it is fiction about the future. What it says about the past comes from the sources below. The date, the age and the cause are placeholders, marked TK and CHECK AGE the way an editor would leave them. 2036 is NASA’s rough estimate of how long the Voyagers can stay in touch, not a scheduled date.

    Sources