What's the Mars Solar Conjunction, and Why Does It Matter?

Soon, Mars and Earth will dance around the sun for an event formally called the Mars solar conjunction. In simple terms, it's a period when Mars and Earth are on opposite sides of the sun. So what's the big deal?

For roughly two weeks, every two years, the solar conjunction takes place. This year it's happening between Oct. 2 and Oct. 14. During this time, the sun obscures the two planets from each other, essentially making Earth and Mars invisible to each other. That means communication with NASA spacecraft on Mars is reduced to a quiet chatter.

Why No Comms?

Normally, the sun ejects hot, ionized gas from its corona, which then makes its way deep into space. That's usually not a problem, but during solar conjunction, this gas can interfere with radio signals when engineers try to communicate with their spacecraft on and above Mars. Commands can be corrupted and result in unexpected behavior from Mars' mission equipment.

So, a communication moratorium is put in place during which, mission controllers at NASA's Jet Propulsion Lab (JPL) turn off some instruments, collect and store data from the Odyssey and Mars Reconnaissance Orbiters, which in turn, accumulate data from the on-surface Curiosity and Perseverance rovers and InSight lander, although those are stationary during this time.

Only the Odyssey Orbiter will attempt to relay any data to Earth during the solar conjunction, knowing that some info will be lost. However, NASA will stop sending new instructions to Mars during this time to avoid unexpected results from misinterpreted signals.

What Happens During Conjunction?

While NASA stops sending new signals to its spacecraft during the solar conjunction, controllers front-load their communications and send two weeks' worth of messages in advance to avoid the increased risk of radio interference. And it's a rare opportunity for those working on these missions to take time off, assuming there's no other projects needing their attention. Just like you when you have your out-of-office message on, they'll check in after the solar conjunction ends.

When it's over, the spacecraft will send the data they've collected to NASA's Deep Space Network, a system of massive Earth-based radio antennas managed by the JPL. Engineers will spend about a week downloading the information before resuming normal communication operations.

If it's determined that any of the collected data is corrupted, engineers can usually have that data retransmitted, similar to your asking a colleague to resend a lost or unreadable file, just from a lot farther away.