The only two space agencies in the world to land and successfully deploy robotic rovers on Mars, NASA and the China National Space Administration (CNSA), have no choice but to place their Red Planet vehicles on “safe mode,” and shut down research.

The reason is undeniable: the sun is about to pass between the Earth and Mars in an event called a “Mars solar conjunction,” which prevents all direct communications until the two planets regain line-of-sight positions once more, according to a recent blog post from NASA.

While this will only last from October 2 to 16, it also raises concerns about maintaining communications with probes, or even crewed missions to the Red Planet, the outer planets, and beyond.

The sun is in the way

If NASA engineers attempt to signal its Mars rovers through the sun’s ionizing rays, communications could become disrupted or even corrupted. Obviously, rovers like Perseverance require highly exacting commands to perform actions, and any corrupted signals could potentially cause them to take dangerous actions. Consequently, NASA has elected to refrain from sending anything at all to its Mars rovers from October 2 to 16. For the same reasons, the CNSA told China’s state-run Global Times that its Tianwen-1 space probe and Zhurong rover will enter safe mode, halting all scientific work until the Mars solar conjunction has passed.

But just because NASA and CNSA will be out of touch for a few weeks, doesn’t mean both agencies’ robotic probes won’t have plenty of homework to carry out in the interim. “Though our Mars missions won’t be as active these next few weeks, they’ll still let us know their state of health,” said Roy Gladden of NASA’s Jet Propulsion Laboratory, where he works as the manager of the Mars Relay Network, in the agency post. “Each mission has been given some homework to do until they hear from us again.”

For example, Curiosity and Perseverance won’t stop recording the Martian weather, but multiple onboard instruments will shut down. And the InSight lander will also keep listening for Marsquakes, while NASA’s orbiting assets will relay messages to Earth whenever possible. To be clear, this isn’t a case of a solar flare or superflare ravaging the surface of the planet and frying all hardware and hypothetical life in or around the Red Planet. Put simply, the sun is in the way, which means we can’t communicate clearly to anything on Mars for a few weeks. If NASA or CNSA tried to send signals through anyway, it could put a quick end to a mission.

Any four-year crewed mission to Mars will have to be self-sufficient for weeks

If astronauts were already on the surface of Mars while this happened, they might be receiving detailed instructions from NASA for highly delicate procedures, and mistaken or corrupted instructions could, possibly, have deadly ramifications. Even when routine instructions are heard loud and clear, faulty equipment or hardware could create an emergency, as was the case when Apollo 13 had “a problem.” Mars solar conjunctions happen every two years, which means any four-year mission (the maximum time humans can remain exposed to radiation) will have at least one two-week period of little-to-no contact with Earth, and need to be 100% self-sufficient.

Of course, this could be circumvented with a relay satellite positioned between the Earth and Mars, perhaps one-quarter of the way back in the Earth’s orbit (like how satellites can transmit messages around the spherical Earth despite the planet blocking line-of-sight radio contact). But the truly terrifying scenario may lie centuries ahead of us: One day, we may settle on a world beyond our solar system. If an alien world in another solar system moved behind its host star, we would have to wait for their planet to emerge from the other side of its host star. But actually, this entire problem is nothing compared to the vast distances between the stars. The closest star, Alpha Centauri, is 4.5 light-years away. That means any signal to humans on some interstellar mission there would hear no reply until at least 9 years had passed. Luckily, signals to Mars take minutes, not years. But sending signals in space could pose serious challenges to the future of space travel.