Houston, we have a solution

The challenge: Securing the data

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 900 kg robotic rover, was to 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. 

Transmitting data at light speed

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 cannot 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. 

Introducing the Curiosity Rover

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 sends the Curiosity 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.

Coping with two-way data traffic

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.

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.