Human space travel is tough. It takes a lot of tech to keep people alive up there, and a lot more to get them safely home. But we have few qualms about sending robots on one-way suicide missions to the stars. And it’s not just that we have no intention of bringing them home; we’ve sent robotic probes into environments that we knew full well would destroy them. And in some cases, the very destruction of these probes is part of the scientific experiment. Or in the case of the recent destruction of Cassini, to protect the solar system for future experiments. Today we’re going to memorialize the robots that have given their lives for in the name of exploration.
Venus, with its dense, searing-hot atmosphere is the most ravenous consumer of Earth-made landers. And the Soviet Union few it the most. From 1966 to 1982, 12 probes from the Soviet Venera program we devoured by Venus’ atmosphere. After a few probes were destroyed on descent, Venera 7 became the first human-made probe to land on another world in 1970. It fought against the blistering 445 Celsius heat and crushing 90 atmosphere pressure for 23 minutes before going quiet. This was just long enough to make the first-ever transmission from the surface of another planet: accurate temperature and pressure readings of the atmosphere that killed it. These measurements would have been impossible from outside Venus’ thick atmosphere. Seven more generations of Venera probe landed on Venus, ultimately sending back our first images from the surface of another world. Venus devoured all of them, with Venera 13 surviving the longest at 127 minutes.
Mars is also a favorite destination for doomed robots. In some ways it’s harder to land things on Mars due to its extremely thin atmosphere, which makes parachute-assisted soft landings difficult. There’s a rich history of robot death on Mars, from the ill-fated series Soviet Mars landers - more like crash-landers, to the notorious Mars Climate Orbiter, which accidentally entered the Martian atmosphere due to a unit conversion error.
However let’s focus on the positive - the probes that, despite no prospect of ever returning home, delivered far more science than anyone thought possible. I’m talking about NASA’s Martian rovers Spirit and Opportunity. These identical probes touched down in early 2004 on what was planned to be a 90 day mission exploring the geology of the Martian surface. Their primary goal was to search for signs of past water activity. Their doom was sealed before they launched. Their designers knew that the solar panels would soon be covered by Martian dust, drastically limiting their lifespan. But this time the alien atmosphere proved the savior rather than the destroyer of robots. Right after landing, the power output of the rovers’ panels dropped as expected. But then, inexplicably, sporadic recoveries in power were observed. NASA scientists soon realized that dust was being cleaned from the solar panels by the Martian wind.
With help from these “cleaning events” the rovers rolled on, ultimately returning a wealth of evidence of Mars’ watery past. Spirit finally succumbed to the dust beneath it, becoming stuck in a sand trap. It continued to transmit as a “stationary science platform” for over a year before we finally lost contact in 2010, at which point the little rover had lasted 25 times the original mission plan. And Opportunity? Some say that it still roves to this very day. Actually, NASA says it. Crazily, Opportunity is still fully operational and is still doing science. It’s now traveled 45 km across the Martian surface, more than any other interplanetary surface vehicle. Keep rolling little buddy. One day we’ll come for you.
Spirit and Opportunity may have fought hard and long, but it was Cassini that made the ultimate sacrifice. This orbiter studied Saturn and its moons and rings for 13 years.
Cassini’s most incredible discovery is arguably that of hydrothermal vents on Enceladus. This moon is a world covered in ice, but beneath that ice is world-spanning ocean 30km deep, apparent from the geysers that occasionally erupt from the ice. Cassini found the presence of sand, ammonia, and organic molecules in the spray of these geysers, which tells us that they are powered by hydrothermal vents on a solid ocean floor. Such vents on Earth’s ocean floors host rich living ecosystems, and may have even been the origin of life on Earth.
This discovery of a potentially habitable environment on Enceladus only further sealed Cassini’s fate. One day we’ll land probes on the moon and even drill into its ocean. If those missions find signs of microbial life, we want to be 100% sure that it was not brought there by US. On the remote chance that Cassini is carrying microbial contaminants from Earth, and that it crashes into Enceladus after decommissioning, a sacrifice had to be made, by decree of NASA’s awesomely named Office of Planetary Protection. On September 15th, 2017 Cassini was plunged into Saturn’s atmosphere, where it ended spectacularly in cleansing fire.
The Planetary Protection Agency has issued death sentences before. The Galileo probe was deorbited into Jupiter to protect its moons, in particular Europa, which also boasts a vast ocean under its own icy crust. Juno, a probe sent to Jupiter in 2011, will meet the same fate in 2021.
Where some spacecraft are destroyed to protect future science, in other cases their destruction IS the scientific experiment. Some of the most mysterious entities in our solar system are the comets that dwell in the Kuiper belt, far outside our planetary system. These small, icy bodies are thought to carry many secrets of our solar system’s formation. To unlock the secrets of the comet we have resorted to a time-honored approach to scientific inquiry – throw something at it and see what happens. This was the Deep Impact mission: a probe launched by NASA in 2005 to impact comet Tempel 1.
A large fraction of the spacecraft’s mass was a 370 kg guided impactor – really a robotic probe all on its own. The impactor guided itself headlong into Tempel 1 at 10 km/s, delivering nearly 5 tons of TNT in kinetic impact energy and forming a crater 30 meters deep. 10s of millions of kilograms of comet was ejected in a debris cloud to be analyzed by the surviving spacecraft component of Deep Impact.
Deep Impact’s sacrifice significantly furthered our understanding of comets and paved the way for future ill-fated landers like Philae. This lander for the Rosetta comet mission failed to deploy its harpoons during its 2014 landing attempt. This caused it to bounce into a region too shaded for its solar-powered operation. Even so, the stubborn little robot did manage to send back the first images from the surface of a comet and to detect some organic compounds that had never before been seen in comet material. It’s now silent, riding its eternal cometary home into the distant Kuiper belt.
The robotic explorers I’ve mentioned all found their final resting places. The same can’t be said of the probes we sent to the outer reaches of our solar system. Pioneer 10 and 11, Voyager 1 and 2, and New Horizons are all on trajectories that will fling them into interstellar space. In fact Voyager 1 is already there. After 35 years exploring our solar stem, in 2012 it passed the heliopause – the boundary where the Sun’s magnetic field and solar wind give way to the ambient environment of the Milky Way. Unlike the long-silent Pioneer spacecraft, Voyager 1 still sends faint radio signals, bringing us our first and only direct measurements from beyond our home system. But its signal is fading; its decaying plutonium power source will only support operations until 2025. After that Voyager 1 will meet its very lonely doom; perhaps floating forever in the coldness between the stars.
As much as we like to anthropomorphize, these robots are just robots. But perhaps it’s right to feel pride in their exploits and sadness at their demise. It's our was of honoring the many brilliant scientists and engineers poured their own lives into these ingenious devices. And eventually real live human explorers will follow these “brave” machines. Hopefully with far fewer fatalities. And then we’ll find inspiration from the sacrifices of the robots who gave their little silicon lives to blaze the very first paths into outer space time.