A day in the life of a ray of light
Scroll to follow a ray that leaves the Sun right now.
speed of light299,792.458 km/sthat is, 1.08 billion km per hour, almost 7.5 times around the Earth in one second
Why the distance does not grow evenly
Voyager 1 is moving away from the Sun at almost 17 km/s, but Earth races along its orbit at 29.8 km/s. For a few months every year, we close in on the probe faster than it flies away.
Projected onto the direction of Voyager 1, which lies some 35° above the plane of Earth's orbit, Earth's velocity swings over a year between about +24 and −24 km/s. Added to the probe's steady flight, this gives a speed of recession from us that varies between about −7.6 and +41 km/s. The minus sign is not a mistake: from the second half of January until around the end of April, the distance actually shrinks.
Because of this oscillation, the mark could have been crossed several times, back and forth. That does not happen: after 18 November the distance keeps growing until 21 January 2027, then shrinks until 24 April, but it stops at 1.0010 light-days, above the mark. So Voyager 1 crosses the mark only once.
Measured from the Sun, the one-light-day mark is reached only on 3 February 2027, because in November Earth is on the side of the Sun opposite the probe, and the distance to us is about three quarters of an astronomical unit greater than the distance to the Sun.
NASA uses the geometric distance, that is, where the probe is at that moment. If you measure where the probe was when the light now reaching us left it, the mark comes about 16 hours later, on 19 November, at around 02:18 UTC.
Between 28 September 2026 and 2 November 2027, the values come straight from the JPL Horizons ephemerides; outside that period, from the page's model, fitted to the same data (difference below 40 km). The two charts share the same time axis: when the speed at the bottom drops below zero, the curve at the top drops too.
The Grand Tour, rebuilt
In the late 1970s, Jupiter, Saturn, Uranus and Neptune lined up in a configuration that recurs about once every 175 years. A probe could hop from one planet to the next, using the gravity of each as a slingshot.
Gary Flandro, then a doctoral student working at JPL, noticed in 1965 that the alignment would allow a complete tour of the outer planets. The original “Grand Tour” programme was cancelled for budget reasons, and two more modest probes flew instead, initially meant only for Jupiter and Saturn. Voyager 2's trajectory, however, was chosen so that the way to Uranus and Neptune would stay open.
Voyager 2 left first, on 20 August 1977, but Voyager 1, launched on 5 September on a faster trajectory, overtook it as early as December. Without the gravity assists, the journey to Neptune would have taken about 30 years; Voyager 2 got there in 12.
Voyager 1 had one priority: the flyby of Titan, Saturn's moon with a dense atmosphere. The flyby flung it northwards, out of the plane of the planets, by some 35°. Voyager 2 passed over Neptune's north pole, and the planet's gravity bent its path southwards. Switch to the “From the side” view to see the two probes part ways.
Playback slows down by itself near each flyby and event. The arcs between planets are rebuilt by solving Lambert's problem: one conic around the Sun between the planets' positions on the flyby dates. The trajectory after the last flyby is a hyperbola around the Sun, fitted to the JPL Horizons ephemerides for 2026–2027. As a check, the model gives 121.6 AU for Voyager 1 at the heliopause (25 August 2012) and 119.0 AU for Voyager 2 (5 November 2018), in agreement with the values published by NASA.
The Pale Blue Dot
On 14 February 1990, at 04:48 GMT, Voyager 1 turned its cameras towards home. It had passed Saturn almost ten years earlier and was climbing above the plane of the planets. It took 60 frames, a “family portrait” of the Solar System with six planets and the Sun. In one of them, caught in a ray of sunlight scattered in the camera's optics, Earth can be seen.
The idea was Carl Sagan's, and he pushed for it for years. The team feared that pointing the cameras so close to the Sun could damage them, and engineering resources were limited; NASA Administrator Richard Truly finally approved the sequence. At 05:22 GMT, 34 minutes after the photograph of Earth, Voyager 1's cameras were switched off for good, to save power for the long journey.
- distance from Earth
- ~6 billion km (40.5 AU)
- above the ecliptic
- ~32°
- size of Earth in the frame
- 0.12 of a pixel
- camera and filters
- narrow-angle; blue, green, violet
Sagan gave the image's name to his 1994 book. His point, in short: on that dot lived everything we love, everyone we have ever heard of, all of our history, with its wars and quarrels over a fraction of it. Seen from out there, none of it looks large, and the only reasonable conclusion is to deal more kindly with one another and to cherish the only home we have ever known.
In 2020, for the photograph's 30th anniversary, JPL engineer Kevin M. Gill reprocessed the original data with modern techniques, together with two of the people who had planned the images, Candy Hansen and William Kosmann. There too, Earth appears as a faintly bluish dot in a ray of light.
What Voyager 1 saw that day
The simulation uses the same trajectory as the rest of the page. The Sun is at the centre, and the distance from the centre is the angle on the sky, on a logarithmic scale. From 6 billion km, Earth was only 1.4° from the Sun, about three times the diameter of the full Moon as seen from here. Venus was even closer, which is why both came out in rays of scattered light.
Mercury was too close to the Sun to be photographed, and Mars could not be made out because of the sunlight scattered in the camera's optics. Pluto, too small and too far away, did not make it into the portrait either.
The Golden Record
Mounted on the body of each probe is a gold-plated copper phonograph record, a message for anyone who might ever find it. Its cover explains, without a single word, how to play it and where it comes from.
What is on the record
The record is 30 cm across and made of gold-plated copper. It sits in an aluminium case mounted on the body of the probe, together with a cartridge and a stylus for playing it. The contents were chosen by a committee chaired by Carl Sagan of Cornell University, and the producer was the writer Timothy Ferris.
It holds 115 images encoded in analogue form, sounds of Earth (waves, wind, thunder, birds, whales), greetings in 55 ancient and modern languages, written messages from US President Jimmy Carter and UN Secretary-General Kurt Waldheim, and 27 pieces of music from different cultures and eras, from Bach and Beethoven to Chuck Berry. Music takes up about three quarters of the record.
A bottle cast into the cosmic ocean
Sagan knew that the chance of the record ever being found is tiny. In his words: “The spacecraft will be encountered and the record played only if there are advanced spacefaring civilizations in interstellar space. But the launching of this ‘bottle’ into the cosmic ‘ocean’ says something very hopeful about life on this planet.”
And the ocean is vast. Only in about 40,000 years will Voyager 1 pass within some 1.7 light-years of another star, Gliese 445, which is itself approaching the Sun.
Listen to the record
The links open in a new tab; NASA's pages have audio players.
- Greetings in 55 languagesFrom languages of ancient Mesopotamia, such as Sumerian and Akkadian, to modern languages, with a translation of each greeting.
- Sounds of EarthWaves, wind, thunder, birds, whales, footsteps, heartbeats, laughter.
- The music, 90 minutesClassical pieces from East and West and traditional music from around the world.
- The 115 imagesFrom mathematical definitions to people from every corner of the world.
- The greetings, with subtitlesThe audio file on Wikimedia Commons; the Akkadian greeting starts at 0:19.
The instruments that went dark one by one
Each Voyager left with the same ten science instruments. Today Voyager 1 still uses two of them and Voyager 2 three. The rest were switched off either because they failed or, mostly, to leave enough power for the others.
Power on board: heat turned straight into electricity
Plutonium-238 decays by emitting alpha particles, which stop inside the material and heat it. One gram of plutonium oxide gives off about half a watt of heat, continuously, with no moving parts: enough to make a thermally insulated pellet glow red. Each Voyager generator holds 24 spheres of plutonium-238 oxide, about 4.5 kg of plutonium in all, which at launch produced roughly 2,400 W of heat.
Plutonium-238 is not the isotope used in nuclear weapons (that is plutonium-239). It is chosen because it decays fast enough to give plenty of heat, but slowly enough to last for decades, and its alpha radiation is easy to stop.
The heat flows outwards through 312 silicon-germanium thermocouples. Each one is a junction between two different semiconductors: when one end is hot and the other cold, a current flows through them (the Seebeck effect). At launch, the hot side was at about 1,000 °C and the cold side at about 300 °C. The finned beryllium casing radiates the rest of the heat into space.
The efficiency is low, around 6.5%: from 2,400 W of heat came about 157 W of electricity per generator, so about 470 W for the whole spacecraft, at 30 V direct current. The power drops for two reasons: plutonium-238 has a half-life of 87.7 years, and the thermocouples age. Together they make up the loss of about 4 W a year.
- year
- —
- electrical power available
- —
- heat from the plutonium
- —
- plutonium-238 remaining
- —
- efficiency
- —
- loss in that year
- —
- instruments switched on
- —
The curve is an approximation: it starts at 470 W at launch and passes through the values reported by JPL, below 270 W in 2011 and about 220 W for Voyager 1 in 2024. The remaining instruments use about 4 W each; most of the current goes to the computers, the transmitter and the heaters. For the present, the curve gives a loss of about 3.5 W a year; NASA, rounding, speaks of 4 W. NASA estimates that the probes could keep sending engineering data until around 2036.
How much each instrument uses
The bar shows all of the spacecraft's electrical power in the year chosen above. Each instrument that is switched on has its own share; the rest goes to the computers, the radio transmitter, the heaters and the power electronics. Whatever is left unused is turned into heat by a regulator and radiated into space.
Each instrument's consumption is its design value, from NASA's “Voyager Backgrounder” booklet (1980): the cameras together 41.9 W, LECP 3.9 W in cruise and 4.2 W at the planets, PWS 1.4 W in normal mode and up to 18 W with the waveform receiver, used during flybys. The booklet gives no value for IRIS, so it is left out of the sum. The power available is the estimate above. JPL says the remaining instruments now use about 4 W each.
Cold is as dangerous as running out of power
The probes must be kept warm enough that the hydrazine in the thruster lines does not freeze. Switching off an instrument saves power, but it also means one less source of heat. That is why the science and engineering teams decided years ago on the order in which the instruments are switched off, and at every shutdown they have to make sure the spacecraft does not get too cold.