Communication with Mars spacecraft lost during solar conjunction
The space between Earth and Mars is usually filled with communication data and commands as multiple missions take place on the Red Planet. However, during the conjunction season, this communication will fall silent for about one and a half days in November as Mars passes behind the Sun.
Solar conjunction for Mars occurs approximately every 25 months when Mars is on the opposite side of the Sun from Earth. During this time, the radio signals used for communication with spacecraft at Mars can be disturbed by the Sun’s active atmosphere, known as the solar corona. The duration of this disturbance depends on the size and power of the spacecraft’s communication equipment but typically occurs when the angle between the Sun and Mars in the sky is within 3-4°. In 2023, this period will last from early November to early December.
The disruption caused by the solar corona means that mission controllers cannot reliably send commands or receive data from their spacecraft. To overcome this challenge, special precautions have to be taken. For ESA’s Mars Express and ExoMars Trace Gas Orbiter (TGO), all critical instructions for spacecraft operation need to be uploaded before the conjunction period begins. Normally, only one week’s worth of commands are sent up at a time, but during conjunction, three or four weeks’ worth of commands are uploaded. This also affects missions from other space agencies, not just those related to Mars.
Due to the disturbance from the Sun’s atmosphere, the amount of data exchanged with Mars Express and TGO has to be reduced. The data uplinked to Mars Express is reduced from 2000 bits per second to 250, and the data sent down to Earth from Mars Express is reduced to as little as 300 bits per second. The Estrack ground stations are set to maximum transmission power to ensure clear communication despite the disturbance. However, this limitation in data exchange means that Mars Express can only send “housekeeping” data, such as health status and telemetry, and cannot send any science data. Any data gathered by Mars Express’s instruments during the conjunction period must be stored in limited onboard memory until the period is over.
The 2023 conjunction is special because it will be the first time Mars passes directly behind the Sun since ESA’s spacecraft arrived. This means that communication with Mars Express and TGO will not only be limited but also impossible for approximately one and a half days on 17-18 November. These windows of limited or no communication pose a challenge for future human settlers on Mars as well.
The concern for mission controllers during conjunction periods is that if something goes seriously wrong with the spacecraft, it could be difficult to recover it until the conjunction period is over. However, over the years, the team has only experienced minor disruptions and has found ways to continue using some instruments in a limited way as long as all commands are uploaded before the season begins and science data are stored onboard until the season ends. Planning for conjunctions has become routine with time.
Peter Schmitz, the Spacecraft Operations Manager for Trace Gas Orbiter, adds that TGO, with its larger communications antenna and data storage capacity, can continue its data relay activities for Mars surface assets throughout the conjunction period, even when Mars is directly behind the Sun. The stored data can then be downlinked to Earth when it is safe to do so.
To wrap it up, the solar conjunction between Earth and Mars poses challenges for communication between spacecraft and mission controllers. However, through careful planning and precautions, these challenges can be overcome, ensuring that critical instructions are uploaded before the conjunction period begins and data can be stored onboard until communication is restored. The 2023 conjunction is particularly significant as it marks the first time Mars passes directly behind the Sun since ESA’s spacecraft arrived. These conjunctions also highlight the potential challenges that future human settlers on Mars will face. Overall, the ability to navigate and adapt to these disruptions showcases the resilience and ingenuity of space exploration missions.
