Mars Solar Conjunction Explained: Why NASA Goes Silent Every 26 Months

Every 26 months, NASA stops talking to Mars. For about two weeks the rovers are on their own, running pre-loaded instructions while the Sun sits squarely between the two planets and scrambles every radio signal. It is called Mars solar conjunction, it happened again in January 2026, and it has quietly produced one of the most precise tests of Einstein’s relativity ever made. Here is what actually happens — and what doesn’t.

Conjunction: Earth, the Sun and Mars in a line — with the Sun in the middle.

The five things to know

26 monthsBetween Mars solar conjunctions. The last was 9 January 2026; the next is around February 2028.

~2 weeksThe “commanding moratorium”: NASA sends no new instructions while the Sun is within about 2° of Mars in the sky.

Not safe modeRovers keep working on pre-loaded plans — weather, radiation and camera tasks — and send home status pings.

0.002%The precision to which the Cassini probe confirmed Einstein’s general relativity in 2002, by measuring how the Sun delayed its radio signal during conjunction.

Two kindsMars conjunction puts the planet behind the Sun. The famous black-dot “transits” are Mercury and Venus passing in front — a different event.

What solar conjunction actually is

A planet is in solar conjunction when, seen from Earth, it lines up with the Sun. For the outer planets — Mars, Jupiter, Saturn — that always means the planet is on the far side of the Sun. For Mercury and Venus, which orbit inside Earth’s path, there are two kinds: superior conjunction (behind the Sun) and inferior conjunction (between Earth and the Sun). Only inferior conjunctions can produce a transit, the rare event where Venus or Mercury appears as a black dot crossing the solar disc. Mars never transits the Sun from Earth, and no planet “casts a shadow” on the Sun.

12 facts about Mars solar conjunction

1. It happens every 26 months because Earth laps Mars on the inside track; the same cycle sets launch windows, which is why Mars missions leave in bunches.
2. The Sun doesn’t block the signal — its atmosphere scrambles it. Radio waves passing through the hot, charged corona get distorted. A garbled command could send a rover the wrong way, so NASA simply stops sending any.
3. The blackout is about two weeks. In November 2023 the moratorium ran from the 11th to the 25th; the 2021 one from 2 to 16 October. The January 2026 conjunction followed the same pattern.
4. Rovers do not enter safe mode. Engineers upload two weeks of instructions in advance. Perseverance and Curiosity park, monitor weather and radiation, and take pictures of dust devils; orbiters keep observing.
5. Mars still talks to us — quietly. Spacecraft send short “heartbeat” tones so controllers know they are alive; the backlog of science data is sent once the Sun moves out of the way.
6. Ingenuity sat out two conjunctions. The Mars helicopter was grounded during each blackout rather than risk a flight nobody could supervise.
7. Conjunction has tested Einstein better than almost anything. General relativity predicts a radio signal grazing the Sun is delayed by about 200 microseconds — the Shapiro delay. In 2002 the Cassini probe measured it during its conjunction and matched the prediction to 1 part in 50,000.
8. Scientists use the blackout to study the Sun. The way the signal is distorted reveals the density and turbulence of the corona — “radio sounding” — a measurement no instrument on Earth can make.
9. Every Mars mission has lived through it. Opportunity survived seven conjunctions in 14 years; Curiosity has now passed six since 2012.
10. It also silences Mars from Earth telescopes. Around conjunction Mars is lost in the Sun’s glare for weeks — the opposite of opposition, when Mars is closest and brightest (next: February 2027).
11. Timing matters for people, not just robots. Future crewed missions will need to plan around a fortnight each cycle when Earth can’t respond in real time — one reason NASA is testing laser communication, which is less affected by the corona.
12. Other planets have them too. Jupiter and Saturn pass behind the Sun roughly every 13 months — which is why the Juno and Cassini teams also planned around blackouts.

Recent and upcoming Mars conjunctions

Conjunction	Commanding pause (approx.)
8 October 2021	2–16 October 2021
18 November 2023	11–25 November 2023
9 January 2026	Late December 2025 – mid-January 2026
February 2028	Roughly two weeks around the date

Myth busters

Three solar-conjunction “facts” the internet keeps getting wrong

✗During conjunction a planet passes between Earth and the Sun and casts a shadow on it.
That is a transit, and only Mercury and Venus can do it. Mars conjunction puts Mars behind the Sun. Nothing casts a shadow on the Sun.

✗Time runs slower for the spacecraft because it is closer to the Sun.
Mars doesn’t move closer to the Sun at conjunction; only its signal does. The real relativity effect is the Shapiro delay on the radio signal — about 200 microseconds.

✗Conjunction turns the Sun into a gravitational lens for finding exoplanets.
The Sun’s gravitational-lens focus is 550 times farther out than Earth. No telescope near Earth can use it, and conjunction has nothing to do with exoplanet detection.

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Sources: NASA JPL – Mars fleet during solar conjunction (2023) · NASA JPL – 2021 conjunction · NASA – Mars solar conjunction explained · Spaceflight Now – Cassini tests Einstein during conjunction (2003) · Tests of General Relativity: A Review (arXiv)

Last fact-checked: October 2026.