As Mars passes behind the Sun, the spacecraft becomes silent

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The space between Earth and Mars is usually buzzing with science data, telemetry and commands for nearly a dozen missions to the Red Planet. But for about a day and a half this November, the interaction between the planets will be quiet as Mars moves behind the Sun.

Tis the season for bonding

During the eclipse season, the Sun’s corona interferes with radio signals used to communicate with missions on Mars.

A solar conjunction with Mars occurs approximately every 25 months. At the time of conjunction, Mars is located on the opposite side of the Sun from Earth.

During conjunction, the radio signals used to send commands from Earth to the spacecraft and receive signals from the spacecraft can be disturbed by the Sun’s active atmosphere – the solar corona.

The period when communications are significantly affected depends on the size and power of the Martian spacecraft’s communications equipment, but typically occurs when the angle on the sky between the Sun and Mars is within 3–4° as seen from Earth.

In 2023, this period will last from early November to early December.

Mars seen from Earth during solar conjunction

As a result of the disturbances, mission controllers cannot send reliable commands or receive data to their spacecraft. Special precautions should be taken.

ESA’s Mars Express and ExoMars Trace Gas Orbiter (known as MEX and TGO, because we enjoy our acronyms), will 'uplink’ all the critical mechanisms the spacecraft need to operate without any contact from Earth for the entire duration. That’s three or four weeks worth of orders when we usually only ship one a week.

Of course, these mergers affect the missions of other spacecraft as well—and this sort of thing isn’t unique to Mars.

Ground stations at full power!

Due to the perturbation of the Sun’s atmosphere during the conjunction season, we need to reduce the amount of data we exchange with MEX and TGO.

We reduce the amount of data we 'uplink’ to MEX, for example from 2000 bits per second to just 250, and reduce the amount of data MEX sends to Earth to 300 bits per second.

We set our Estrac ground stations to maximum transmit power to ensure our spacecraft can be heard loud and clear despite interference.

This limits the information MEX can send to its operators on Earth to 'housekeeping’ data – health status and telemetry – and no scientific data can be sent to MEX.

As with a diver holding his breath, any data collected by MEX’s equipment during a tethered period must be stored in a limited internal memory until the period ends.

ESA’s New Norcia Deep Space Antenna communicates with Mars Express at the start of the 2023 Mars solar conjunction season. At 300 bits per second, this GIF takes over three hours to download.

What’s so special about 2023 Connect?

20 Years and Counting: The Mars Express in Numbers

Mars Express arrived at the Red Planet on 25 December 2003 and is one of Europe’s longest service missions. The team first celebrated 20 years of launch this summer Live webcast from another planet.

This is MEX’s tenth solar conjunction and TGO’s third. However, because the orbits of Mars and Earth have slightly different inclinations, Mars does not usually pass directly behind the Sun.

Orbits of Earth and Mars during the 2023 solar conjunction

The 2023 conjunction is unusual in that it will be the first time Mars has passed behind the Sun’s disk since the arrival of two ESA spacecraft.

When Mars is behind the Sun, for about a day and a half on November 17-18, communication with MEX and TGO will be limited and impossible.

These windows of limited or impossible communication between Earth and Mars will also be a challenge for future human settlers.

Are you worried?

Mars during the 2023 solar conjunction

„At the beginning of the trip, the team was very cautious about docking,” said Mars Express spacecraft operations manager James Godfrey.

“We have stopped all scientific activities. But over the years, we have experienced only minor disruptions.

„In 2019, we discovered that some of MEX’s equipment could continue to be used in a limited way as long as all commands were uploaded before the start of the season, and all scientific data was stored on board until the end of the season.”

„Initially, planning for a merger was a very manual process,” says James Godfrey. „But over the years, it’s mostly become the norm.”

Peter Schmitz, spacecraft operations manager for the Trace Gas Orbiter, adds: „With its large communications antenna and data storage capacity, TGO will be able to continue its data relay operations to Mars’ surface assets for the duration of the link – even when Mars is directly behind the Sun – and transfer all stored data to Earth. Get ready to connect when it’s safe again.”

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