We don’t need the most powerful microprocessor in the world to conquer the universe. The real secret is to make the most of the operating characteristics of a device, even if it is a bit outdated and does not have excellent performance. Let’s see how the Perseverance rover landed on Mars safe and sound.
Humankind closer to Mars
After millions of kilometers, NASA’s Perseverance rover finally landed on the surface of Mars on February 18, 2021. We can consider the event as a historic mission because, despite the distance, the prohibitive conditions of the Universe and the weak hardware, a human device has arrived smoothly on the red planet. Remember that light travels the same distance, on average, in about 12 minutes. The radio signals, images and information collected, therefore, take the same time. For this “impossible” mission, NASA used a processor mounted in Apple’s iMac G3 computer with 1997 architecture with a calculation frequency of only 200 MHz and a RAM memory of 256 MB. It isn’t even among the fastest processors in the same commercial series.
Why such a low technology?
There is always a reason behind a choice. The aircraft’s designers were more concerned with the reliability and robustness of the on-board computer than with the computing power. A simple smartphone is much faster. The mission to Mars needed the CPU to be always working. A bombardment of radiation can seriously and easily damage the electronics of a modern processor. The processor of the G3, on the other hand, is not destroyed by radiation and is able to withstand intense radiation and temperatures. On Earth, the speed of processors can be dramatically increased without any problems. The electrical, thermal, atmospheric and magnetic conditions are very comfortable and the Earth’s atmosphere defends us with a solid protective shield. In the universe, however, the situation is very different. The presence of radiations of enormous power, with an energy between 100 MeV and 1 GeV, could modify the functioning of the circuits, and the impact of a single ionized particle on the processor is enough to create calculation errors or even its destruction.
A slow but reliable processor
The chip used in the rover is slow by modern standards, but meets all reliability tests (see Figure 1). Even prolonged exposure to radiation does not compromise its functioning. Obviously the G3 with its 200 MHz of speed disappears in the presence of today’s microprocessors with more than 3 GHz of clock however, it is appropriate to say, “Slow and steady wins the race”. High reliability is the element that most characterizes the processor, even in critical conditions. This is a purpose built and radiation resistant version. It can operate at temperatures between -55 ° C and + 125 ° C (-67 ° F to + 257 ° F) and the atmosphere of Mars is very cold. It is also very thin, causing all kinds of radiation to penetrate. The RAD750 can safely withstand these conditions. Requests for a spacecraft are highly critical. Saving a few millimeters of space or a few grams of weight is a fundamental factor. Memory was kept very low, with 2 GB of Flash, 256 MB of working RAM and 256 kilobytes of EEPROM. The processor has “only” 10.4 million transistors, 1000 times less than today’s smartphones. The rover has two brains, one of which starts in the event of a problem with the first processor.
The Perseverance rover is equipped with an inertial measurement system (IMU) that provides 3-axis information about its position to perform precise vertical, horizontal and lateral movements. The sensors connected to the computer record many parameters, such as temperature, electrical voltage, energy storage of solar cells and power, in order to keep all operating conditions stable. With this data it is possible to control information exchange operations, snapshots and other commands that are useful for exploring the new environment.
This is not the first time this type of processor has been used. The Earth is surrounded by RAD750 devices in more than a hundred satellites, and none of them have failed so far. Today, we use portable devices (notebooks, smartphones, etc.) with extraordinary powers that feature incredible calculation capacity. A highly critical space mission can be managed can be managed with much less.