Talking to Mars

This week, PhD student Diana Hayes takes a look at how we communicate with spacecraft dispersed throughout the solar system. Given the number of assets at Mars, the situation on the red planet is particularly complex and therefore particularly interesting. Like any infrastructure, maintenance is critical to enabling future science if we expect to continue to count on the capable systems we have in place today.

by Diana Hayes

Here on Earth, we’ve built a sophisticated global telecommunications network that allows us to talk to people on the other side of the planet nearly instantaneously with very little difficulty, whether that be through the internet, texts, phone calls, or video chats. It’s easy to take this interconnectivity for granted. For example, if I have data that I want to share with a collaborator in Europe, can upload it to a Microsoft server from my computer, send them a OneDrive link, and they can download it to their computer; unless something goes wrong somewhere along the way, there’s very little thought from either of us about the intricate complexities of the systems that make this possible.

But what if your collaborator is on another planet?

This is a question that every interplanetary mission needs to answer. For many missions, the answer is simple: the Deep Space Network (DSN). The DSN consists of 14 radio telescopes spread across three sites in Spain (Madrid), the United States (Goldstone), and Australia (Canberra). This spacing ensures that at least one telescope has a direct line-of-sight to every location in the Solar System further than about 30,000 kilometres from Earth. The data rates seem laughable compared to what we can achieve on the ground, as they’re limited both by the size and power of the DSN telescopes and the antennas that can be put on a spacecraft, as well as the distance between Earth and a given spacecraft. For example, as I write this, one of the DSN telescopes is talking to Juno (orbiting Jupiter) at 50 kilobits per second, while another is talking to the Mars Reconnaissance Orbiter at 550 kilobits per second. For comparison, the wireless internet connection here at York typically averages around 100 megabits per second.

A screenshot of NASA’s DSN Now page, highlighting a communications session between one of the deep space antennas in Canberra and the Mars Odyssey orbiter.

For orbiters and flyby missions, the DSN is good enough. You can put a reasonably large, powerful antenna on your spacecraft and talk directly to Earth with minimal difficulty. For landed missions, the situation is more complex. When landing on another planet, minimizing the weight and size of your spacecraft is essential, so you can’t really just use a big antenna unless you’re willing to sacrifice science instruments for it. The solution we’ve come up with is relay communications. Rather than talking directly to Earth, a rover or lander first transmits its data to a spacecraft in orbit, which then uses its more powerful antenna to transmit that data to Earth. Because the lander-orbiter distance is much smaller than the lander-Earth distance, you can get away with much smaller antennas on the lander. 

Over time, this strategy has led to the development of the Mars Relay Network (MRN), which consists of a number of Mars orbiters that rovers and landers on the surface can use to talk to Earth. At the moment, there are four orbiters in the MRN: the Mars Reconnaissance Orbiter (MRO), the Trace Gas Orbiter (TGO), Mars Odyssey, and Mars Atmosphere and Volatile Evolution (MAVEN). Relay communications is not the primary mission of any of these orbiters, but they all were specifically outfitted with the instruments needed to serve as part of the MRN.

There is one obvious limitation to using this method to communicate with our landers and rovers on Mars: data can only be transmitted when an orbiter is above the horizon. During rover planning, this means that we need to keep track of when orbiters are available to receive data, and to prioritize which data we want to receive first since we can’t always downlink all of the data on the rover during a single comms pass. Fortunately, because we understand the movements and capabilities of our orbiters pretty well, this is really only a minor problem. 

Top: The elevation of the Mars Reconnaissance Orbiter as seen from Curiosity during the first week of 2025. Most of the time, the orbiter is below the horizon, passing over Gale about twice a day. Bottom: A close-up of the highest-elevation pass of MRO on January 4, 2025, highlighting how the orbiter is only above the horizon  for a very short period of time, about 15 minutes, which limits the amount of data that can be transmitted.

Less avoidable is the motion of the planets themselves. Every few years, Earth and Mars pass on opposite sides of the Sun, an event we call “solar conjunction.” When this happens, interference from the Sun can cause data corruption in the signals sent through the MRN. Corrupted data received from Mars isn’t the end of the world, since the data aren’t deleted from the rovers’ computers until they are verified to have been received intact on Earth, so any missing data can just be re-transmitted. Of more concern is data transmitted to Mars, which usually consists of rover commands. We really don’t want to send corrupted commands to our rovers, so during conjunction, we stand down from operations until the apparent angular separation between the planets is large enough for data to be transmitted reliably.

The apparent angle between Mars and the Sun, as viewed from Earth between the start of 2023 and the end of 2026. About once every two years, this angle is so small that the Sun blocks data transmission between the two planets, a period known as “solar conjunction.”

The MRN has become a critical piece of infrastructure in the exploration of Mars, but it’s facing an imminent crisis: it’s aging. Odyssey, the oldest part of the MRN, was launched in 2001, while the newest (TGO) was launched in 2016. The fragility of this age was highlighted in December of last year when the DSN lost contact with MAVEN. Although recovery efforts are ongoing, NASA has admitted that it’s unlikely that communication with MAVEN will be re-established. MAVEN had the highest data transmission capabilities of any of the four orbiters, so its likely loss leaves a gaping hole in the MRN. The other orbiters can at least partially fill that hole, but it’s obvious that something needs to be done before we lose another orbiter. Some proposals have been made, such as the Mars Telecommunications Orbiter (MTO), which was cancelled in 2005 before being revived in NASA’s 2025 budget. Dedicated telecom orbiters are much less sexy (scientifically speaking) than science orbiters, which makes justifying their cost to national legislatures much more difficult, despite their importance. 

By the way, this isn’t limited to just Mars. For example, the Huygens probe, which landed on Saturn’s largest moon Titan in 2005, relayed its data through the Cassini spacecraft. Notably, Dragonfly, NASA’s upcoming rotorcraft mission to Titan, will not be using a data relay, instead opting for direct-to-Earth communication. To make this possible with any kind of appreciable data volume while not using the much larger antennas found on spacecraft like Juno, Cassini, and Voyager, Dragonfly will be taking advantage of the large amount of energy provided by its nuclear power system (similar to the one used by Curiosity and Perseverance), which will allow it to transmit much stronger signals with a much smaller antenna.

If you’re curious about what the DSN and MRN are up to right now, check out NASA’s DSN Now (https://eyes.nasa.gov/apps/dsn-now/dsn.html) and MRN (https://eyes.nasa.gov/apps/mrn/) websites.