Between now and early December, radio signals used to send commands from Earth to Mars spacecraft – and to receive signals from the spacecraft – could be disturbed by the solar corona, its outermost atmosphere. Therefore, Earth’s communications with spacecraft, probes, rovers, and future humans on the Red Planet are currently limited. Reprinted from the European Space Agency. EarthSky Editions.
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Mars spacecraft falls silent
The space between Earth and Mars is typically packed with scientific data, telemetry and commands that flow to and from nearly a dozen missions to the Red Planet. But for about a day and a half, communication between the planets will be silent as Mars passes behind the Sun on November 18.
Solar conjunction to Mars occurs approximately once every 25 months. During the conjunction, Mars is located on the opposite side of the Sun from Earth.
At the time of conjunction, radio signals used to send commands from Earth to spacecraft and to receive signals from spacecraft can be disturbed by the Sun’s active atmosphere, the solar corona.
The length of time during which communications are significantly disrupted depends on the size and power of a Mars spacecraft’s communications equipment. But it typically occurs while the angle in the sky between the Sun and Mars as seen from Earth is between 3 and 4 degrees.
In 2023, this period runs from early November to early December.
There are no terrestrial instructions for Mars spacecraft
As a result of the outage, mission controllers cannot reliably send commands or receive data from their spacecraft. Special precautions must be taken.
For Mars Express and ESA’s ExoMars Trace Gas Orbiter (known as MEX and TGO, because we like our acronyms), this means turning on all the critical instructions the spacecraft would need to operate without any contact from Earth for the entire period. That’s three or four weeks of commands, when we normally only send one week at a time.
Of course, these conjunctions also affect the missions of other space agencies, and this kind of thing is not exclusive to Mars.
Ground stations at full power!
Due to the disturbance of the solar atmosphere during the conjunction season, we have to reduce the amount of data we exchange with MEX and TGO.
We have reduced the amount of data that we “uplink” to MEX, for example, from 2,000 bits per second to just 250, and we have reduced the amount of data that MEX sends to Earth to just 300 bits per second.
We also set our Estrack ground stations to maximum transmit power to ensure our spacecraft hears us loud and clear despite the disturbance, meaning we use a louder voice but say fewer words.
This limits the type of information MEX can send to its operators on Earth to “cleaning” data – health status and telemetry – and is too low for MEX to send any scientific data.
Like a diver holding his breath, any data collected by the MEX instruments during the conjunction period must be stored in limited onboard memory until the period ends.
What makes the 2023 conjunction special?
Mars Express arrived at the red planet on December 25, 2003 and is one of Europe’s oldest missions. The team celebrated 20 years since launch this summer with the first live webcast from another planet.
This will be the tenth solar conjunction of MEX and the third of TGO. However, because the orbits of Mars and Earth have slightly different inclinations, Mars does not normally pass directly behind the Sun.
The 2023 conjunction is unusual because it will be the first time that Mars has passed behind the solar disk since the arrival of the two ESA spacecraft.
As long as Mars is behind the Sun for approximately a day and a half on November 17-18, communication with MEX and TGO will not only be limited, it will be impossible.
These limited or impossible communication windows between Earth and Mars will also represent a challenge for future human colonizers.
Are you worried?
James Godfrey, Spacecraft Operations Manager at Mars Express. he said:
At the beginning of the mission, the team was very cautious about conjunctions. If something goes seriously wrong during the conjunction period, it could be difficult to recover the spacecraft until it’s all over.
We used to suspend all scientific operations. But over the years we’ve only had minor disruptions.
In 2019, we discovered that we can continue to use some of MEX’s instruments on a limited basis, as long as all commands are loaded before the start of the season and all science data is stored onboard until the end of the season.
Originally, planning conjunctions was a very manual process. But over the years, it has become largely routine.
Peter Schmitz, Spacecraft Operations Manager at Trace Gas Orbiter, added:
With its largest communications antenna and data storage capacity, TGO is able to continue its data relay activities to Mars surface assets throughout the conjunction period – even when Mars is directly behind the Sun – and prepare to transfer all stored data to Earth when it is once again safe to do so.
In short: the solar conjunction to Mars happened yesterday, November 18, 2023. This was when Mars was most directly behind the Sun in this 25-month period. In 2023, Mars conjunction season takes place between November 5th and December. During this period, data exchange between Mars spacecraft and Earth is limited.