Voyager 1 is now so distant that every exchange with Earth begins with patience. A command sent by NASA takes more than 23 hours to arrive, and any reply needs another 23 hours to come home.

The probe is still returning measurements from beyond the heliopause, nearly 49 years after its launch on 5 September 1977. As of NASA’s April 2026 update, two science instruments remained active: a magnetometer and a plasma-wave detector.

One part of the title needs a literal-minded clarification. Voyager does not carry one conventional battery the physical size of a car battery. It has three radioisotope thermoelectric generators, each about 40 centimetres across and 58 centimetres long. Together they are substantially larger and heavier. The car-battery comparison is best understood as shorthand for the modest scale of the electrical supply, not the dimensions of the complete power system.

I think the verified engineering is more interesting than the simplified comparison.

The delay has grown throughout the mission

NASA’s live Voyager position page says Voyager 1 will reach a distance of one light-day from Earth in November 2026. At that point, a radio signal will need 24 hours to cross the gap in one direction.

The exact Earth-to-spacecraft distance changes slightly as Earth moves around the Sun, but the long-term trend is outward. Voyager 1 is travelling away from the Sun at about 17 kilometres per second. No spacecraft has gone farther.

Light speed does not make communication instantaneous. It only sets the fastest possible travel time. When engineers send a diagnostic instruction, they cannot see whether it worked until the command reaches Voyager, the spacecraft executes it and telemetry makes the entire return trip. A basic command-and-response cycle now takes roughly two days before analysis time is added.

That delay changes troubleshooting. The team must plan command sequences conservatively because there is no real-time correction if the ageing spacecraft responds unexpectedly.

“Still sending data” now means two instruments, not pictures

Voyager 1’s cameras were switched off after the probe took its final Solar System portrait in 1990. The data coming back today are measurements rather than photographs.

On 17 April 2026, engineers shut down the low-energy charged-particle experiment to conserve power. NASA’s announcement says the instrument had operated almost continuously since launch and had measured ions, electrons and cosmic rays beyond the heliosphere.

The remaining magnetometer measures magnetic fields, while the plasma-wave subsystem detects oscillations in the thin ionised material around the spacecraft. These instruments do not send back a cinematic view of interstellar space. They return small streams of numbers describing an environment no other working probe has sampled at that distance.

Voyager’s communications hardware is equally restrained by modern standards. NASA’s spacecraft description gives a normal science telemetry rate of 160 bits per second through a 3.7-metre high-gain antenna. Earth receives the extremely weak signal through the large antennas of the Deep Space Network.

The power source is heat, not a rechargeable battery

Solar panels are not useful this far from the Sun. Voyager instead carries three Multi-Hundred Watt Radioisotope Thermoelectric Generators, or MHW-RTGs, mounted end-to-end on a boom.

Plutonium-238 inside the generators releases heat as it decays. Thermocouples turn a small fraction of the temperature difference into electricity, without a turbine or other moving machinery. The process is continuous: there is no charging cycle and no switch that can pause radioactive decay for later.

At launch, each generator produced about 158 watts of electricity, giving the spacecraft roughly 470 watts in total. A NASA technical history of radioisotope power systems lists each Voyager unit at 39.73 centimetres in diameter, 58.31 centimetres long and 37.69 kilograms in mass.

Voyager therefore launched with three generators weighing about 113 kilograms altogether. That is plainly not one car battery. What is striking is that a few hundred watts supported the computers, radio, heaters and instruments of an outer-planet probe, and that a declining portion is still supporting a much smaller set of functions today.

Four watts disappear from the budget each year

The plutonium decays predictably, while the thermoelectric material also loses efficiency. NASA says each Voyager has about four fewer watts available every year.

Four watts sounds trivial until the remaining power margin is only a few watts. Mission controllers have progressively turned off heaters, instruments and other loads so that demand does not exceed supply. This is why the latest instrument shutdown extended the mission rather than representing an isolated failure.

NASA’s Voyager FAQ says the probes may remain within communication range of the Deep Space Network until about 2036, depending on how much power remains for the transmitter. Science collection may end earlier as instruments are retired one by one.

There is no single expiration date. A component can fail unexpectedly after nearly five decades, or careful power management can preserve a useful signal longer than current estimates.

Interstellar space does not mean beyond all solar influence

NASA concluded that Voyager 1 crossed the heliopause in August 2012. That boundary separates the bubble dominated by the solar wind from the interstellar plasma beyond it. In that specific and scientifically useful sense, Voyager is operating in interstellar space.

It has not escaped everything gravitationally associated with the Sun. The distant Oort Cloud is thought to contain icy bodies still bound to the Solar System. NASA estimates Voyager 1 will take about 300 years to reach its inner edge and perhaps 30,000 years to travel beyond it.

Both statements can therefore be true: Voyager is beyond the heliosphere and measuring interstellar material, yet it remains far from crossing the outermost gravitational boundary people sometimes mean by “leaving the Solar System”.

The signal is the achievement

Voyager 1 was built before the first space shuttle flight, the World Wide Web and the personal computer era. Its original planetary mission carried it past Jupiter in 1979 and Saturn in 1980. The interstellar work is an extension conducted with hardware designed in the early 1970s.

The mission’s longevity is not evidence that the probe is somehow untouched by age. It is evidence of redundancy, careful design and repeated compromises. Every watt saved and every instrument retired narrows the mission while keeping its remaining measurements possible.

More than 23 hours after Voyager transmits, a faint radio signal reaches Earth carrying a tiny stream of data from beyond the Sun’s plasma bubble. The numbers are modest. The fact that there are still numbers at all is what deserves attention.