DEPARTURE
A FIELD GUIDE TO VOYAGER & PIONEER
Ten chapters on the four machines that used the outer planets as stepping stones out of the Solar System — and the two messages they carry for whoever finds them. Scroll to begin the mission.
BEGIN — CHAPTER 01 →Only four machines have ever done this
Orbiting the Sun takes one speed. Escaping Earth takes more. Escaping the Sun's gravity entirely — leaving on a trajectory that never comes back — takes the third cosmic velocity, more than the rockets of the early 1970s could give these small probes. All four in this guide borrowed the rest from the planets they flew past.
Pioneer 10, Pioneer 11, Voyager 1 and Voyager 2 are on permanent solar-escape trajectories today. A fifth, New Horizons, joined them in 2006. Their speeds below are for 10 October 2026, from JPL's Horizons ephemerides [1].
FIG. 1.1 — SPEED RELATIVE TO THE SUN ON 10 OCT 2026, KM/S [1]. BARS SCALED TO 20 KM/S.
FIG. 1.2 — THE ESCAPE CHECK
After its last planetary flyby each probe coasts with only the Sun pulling it back. Energy is conserved, so its speed at any distance follows from today's speed and distance [1]. That speed keeps falling — but towards a floor, the speed it keeps forever (v∞), not towards zero.
Escaping means staying above the local escape speed, √(2GM/r), which shrinks faster with distance than the probe slows down: the probe's speed levels off at v∞ while escape speed keeps falling as 1/√r. Pick a probe and slide it outward from its last flyby; the curve and the readouts are computed, not drawn.
Model: v(r)² = v∞² + 2GM/r, with today's speed and distance from JPL Horizons [1] and GM of the Sun = 1.327 × 10¹¹ km³/s². It ignores the planets' small pulls after the last flyby; against Horizons it is within 0.01 km/s from 1995 to 2036.
Stealing momentum from a planet
A planet in orbit is itself moving fast. Send a spacecraft round the planet's trailing side and gravity turns its path forward, along the planet's motion. Relative to the planet the craft leaves exactly as fast as it came; relative to the Sun it leaves faster — like a ball bouncing off the front of an oncoming train. The planet pays with an immeasurably small slowing of its own.
Pass in front instead and the same turn slows the craft down. Why the side matters, how close a pass has to be, and why this is not free energy is worked out with live instruments in SLINGSHOT; the escape check in chapter 1 shows what Jupiter and Saturn left these four with.
FIG. 2.1 — THE GRAND TOUR: A ONCE-IN-175-YEARS ALIGNMENT
In the late 1970s Jupiter, Saturn, Uranus and Neptune lined up so that a single spacecraft could use each planet's gravity to reach the next — an alignment that comes round about once every 175 years [2]. Voyager 2 is the only spacecraft ever to take it, passing all four between 1979 and 1989 [3].
The first to leave the inner Solar System
Pioneer 10 launched on 3 March 1972 (UTC; the evening of 2 March in Florida), the first spacecraft to cross the asteroid belt and the first to reach Jupiter, on 4 December 1973 [4]. Pioneer 11 followed in April 1973, flew past Jupiter in December 1974 and became the first spacecraft to reach Saturn, on 1 September 1979 [5]. Simple, spin-stabilised and built years before Voyager, they proved the outer Solar System was survivable.
FIG. 3.1 — SCHEMATIC (NOT TO SCALE). SELECT A COMPONENT.
Built to work alone for fifty years
Voyager 2 launched on 20 August 1977; Voyager 1 followed on 5 September, on a faster path that overtook its twin [3][8]. Both carry three radioisotope generators, no repair option and computers that run about 8,000 instructions per second, with roughly 68 kilobytes of memory on each spacecraft — less than a single photo on a modern phone [7]. Select a component.
FIG. 4.2 — THE POWER BUDGET, 1977–2036
Each Voyager left Earth with about 470 watts from its three plutonium-238 generators [8]. The fuel alone fades slowly — its half-life is 87.7 years [9] — but the thermocouples that turn its heat into electricity age as well. By the start of 1997 output was down to about 335 W [8], and since then each spacecraft has lost about 4 W a year [7][13]. The model joins those published figures with straight lines: between them it interpolates, after 2026 it extrapolates.
Every few watts has cost something. Voyager 2's plasma instrument was switched off on 26 September 2024 [10]; Voyager 1's cosmic-ray instrument on 25 February 2025 and Voyager 2's low-energy particle instrument on 24 March 2025 [11][13]; Voyager 1's low-energy particle instrument on 17 April 2026 [12]. In August 2026 engineers swapped some of Voyager 2's powered devices for lower-power ones, buying its three remaining instruments at least another year; the same swap is planned for Voyager 1 [13].
Solid line: the model through 470 W at launch, 335 W at the start of 1997 and −4 W per year since [7][8][13]. Dashed line: what plutonium decay alone would leave, 470 W × ½^(t / 87.7 yr) [9]. Dots: published values. Instrument counts start in 2024, when the current shutdown sequence began.
What was expected, and what was found
Messages with almost no chance of arriving
Both Pioneers carry a small engraved plaque [4]; both Voyagers carry a 12-inch gold-plated phonograph record [14]. Neither was expected to ever be found. Both were made anyway.
Turning the camera back around
On 14 February 1990, about 6 billion kilometres — 40 AU — from the Sun and with its planetary mission complete, Voyager 1 turned its camera back towards the Sun at Carl Sagan's suggestion and took 60 frames for a family portrait of the Solar System [15][1]. In one of them Earth is a point of light about one pixel across, caught in a scattered ray of sunlight. Minutes later the cameras were switched off for good, to save power for the long cruise [15].
Where the Sun's influence ends
The Sun blows a bubble of charged particles — the heliosphere — far past the planets. Its edge is not a wall but a change of plasma: Voyager 2 measured the denser, colder interstellar plasma directly when it crossed [19]; Voyager 1, whose plasma instrument no longer worked, revealed the same jump through the frequency of plasma waves [18]. Step through the boundary layers.
Two still speaking, two gone silent
Positions are for 10 October 2026, from JPL's Horizons ephemerides [1]. Pioneer 10 and 11 can no longer be heard [4][5]; the Voyagers still answer, with two and three instruments switched on [12][13].
FIG. 9.1 — HOW FAR IS “NOW”?
Far from every planet, each Voyager now coasts almost in a straight line, slowed only slightly by the Sun, so its position on any date follows from where it is and how fast it moves today [1]. The distance from Earth adds a yearly swing of up to about 0.8 AU either way as Earth circles the Sun — about seven minutes of light time, added to or taken from every command.
Voyager 1 first sits one light-day from Earth in mid-November 2026; measured from the Sun it is not until early February 2027. The Deep Space Network may be able to hear the Voyagers until about 2036 [7] — slide through those ten years. How faint their signal is by then is the subject of LINK.
Model: motion under the Sun's gravity alone, starting from the Horizons position and velocity of 10 October 2026 [1], plus Earth on its slightly elliptical orbit. Against Horizons it stays within a minute of light time, and its one-light-day dates within a day, through 2036.
What we chose to say about ourselves
In about the year 40,272 Voyager 1 will pass within 1.7 light-years of the star AC+79 3888; Voyager 2 comes as close to Ross 248 in about 40,000 years [7]. Whatever they carry will still be legible long after everyone who built them is gone.
Sources cited throughout this guide
Bracketed numbers in the text above correspond to the sources below.