They call it the Seven Minutes of Terror. Scientists and engineers will be on the edge of their seats as NASA’s Mars 2020 spacecraft screams into the Martian atmosphere, clawing its way down to safe landing speed with aerobraking, rockets, and parachutes. If anything goes wrong, years of work and a seven-month voyage end in a smoking hole on a dusty red plain 100 million km from Earth. And the landing isn’t even the hard part.
If you’re going to spend $2.4 billion sending a lander to Mars, you want something to show for it other than the thrill of the ride. Mars 2020, and its 900kg robotic rover, will look for proof that life once thrived on the Red Planet. This means doing experiments and sending the data safely back to Earth. If the data is lost or corrupted, the whole mission is in vain. That’s the hard part.
Mars 2020 will experience its Seven Minutes of Terror in February 2021. If successful, it will be NASA’s fifth Martian landing in 20 years. It’s hard to exaggerate how dramatically these missions have improved our understanding of the Red Planet. While much attention is on the excitement of the voyages themselves, one of the most extreme challenges is securing the data that makes them worthwhile.
Data is sent back to Earth using radio waves, but there is a limit to the power of the radio transmitter that can be fitted to a lander because it has to be as light as possible – too heavy and current rocket technology can’t launch the vehicle. There’s an engineering trade-off between the lander’s capabilities and its weight.
NASA gets around this by sending two kinds of spacecraft. One has the glamorous job of landing on the planet, driving around, taking photos, and doing experiments. The other sits in orbit around Mars acting as a radio relay station. Because it doesn’t have to carry the equipment needed to land and do experiments, engineers can devote much more of the orbiter’s weight to powerful, high-bandwidth comms.
Most of the time NASA’s Mars landers transmit their data up to their partner in orbit, which then forwards the message to Earth. Radio signals travel between Mars and Earth at light speed. Depending on where the planets are in their orbits, the distance is between 30 million and 400 million km, which is a signal travel time of between 2 and 22 minutes. But the distance is one of the more insignificant comms complications.
A 100-million-km bottleneck
The Curiosity Rover, which landed on Mars in 2012, can store about 8Gb of data from its cameras before it has to upload. The path that data must take back to Earth is complex. First, it’s transmitted to the Mars Reconnaissance Orbiter (MRO) circling the planet at an altitude of about 250 km. But the orbiter is only overhead and able to receive data for a few minutes each orbit – not long enough to send 8Gb. It may be several orbits before the whole data package is sent.
Later – maybe days later depending on other tasks – the MRO turns its comms dish around and squirts Curiosity’s data back at Earth, sending everything twice for error correction. On Earth, the data stream is received by the massive radio dishes of NASA’s Deep Space Network. This task is complicated by the fact that the Earth is rotating – sometimes only part of the data is received before the MRO is out of line of sight and a different ground station takes over.
In real-world conditions, other factors complicate this data path even further. The bit rate varies depending on the constantly changing distance between Mars and Earth. The rover may upload part of its data to a different orbiter (NASA has three around Mars), or even transmit some of it directly to Earth at a much lower bitrate if it happens to be close enough. Different data packets can take different routes to Earth at different rates.
The complex routes back to Earth can mean that 8Gb of neat Curiosity data arrives as scrambled egg, over the course of days or weeks. Putting it back together again is complicated by the fact that time on Earth and Mars doesn’t match up. A day on Mars is 39 minutes longer than a day on Earth. To keep data in the right order – for example to make sure scientists know the correct sequence of photos – Mars times must be converted to Earth time.
And this is only part of the story – the data traffic is two-way. Mission controllers on Earth must send instructions to the rover, about where to drive next, what to photograph, and even what data it should send back in which order. These instructions must follow the same complex path in reverse.
It takes sophisticated application engineering to build a ground station capable of integrating this data to be usable by controllers driving the mission and scientists waiting for results. In the case of Curiosity, the ground station handling data from the French-built instruments on board – including the ones that found the first evidence of flowing water in Mars’ ancient history – was built by Capgemini.
That expertise is being carried forward to the Mars 2020 mission. The French Operations Center for Mars Instruments (FIMOC) has been incorporated into a new, more ambitious data-handling center, with Capgemini continuing to develop its capabilities. If evidence is found that there was once life on Mars, there will be only one chance to get it right.