Into the great unknown: Rosetta and the crazy plan to catch a comet
From Fukushima to the darkest corners of the ocean, robots built for extreme environments and an appetite for discovery continue to enlighten our understanding of places too dangerous to tread. Those launched into deep space may be the most daring examples, continually pushing the limits of human ingenuity and expanding our understanding of the universe. In this series New Atlas profiles space probes, both past and present, tasked with pushing the boundaries of science by leading us into the great unknown. This week: Rosetta, an audacious plan to land on a comet for the first time.
Launched: March 2004
Subject of study: 67P/Churyumov–Gerasimenko
Current location: Frozen on the surface of 67P/Churyumov–Gerasimenko
Just like archaeologists turn their tools to materials on Earth for clues about the planet's distant past, studying ancient remnants floating through space can also reveal key secrets of the early solar system. But what happens when that material is a rubber ducky-shaped comet tumbling through space at around 135,000 km/h, and the archaeologist is a fridge-sized robot that needs to somehow be landed on it? This was the dilemma faced by scientists at the European Space Agency (ESA), as they embarked on a first-of-a-kind mission to study comet 67P/Churyumov-Gerasimenko.
While comet flybys had been performed before, at the outset of the ESA’s Rosetta mission to the 67P/Churyumov-Gerasimenko nobody had ever entered orbit around a comet, let alone try and land on one. The reason it was such an alluring prospect for the space agency is that comets are essentially time capsules, containing frozen chunks of ice, rock and dust that have locked away key materials from the formative stages of the solar system, around 4.5 billion years ago.
Digging into a comet to study its subsurface material, in addition to all kinds of other science at the surface, could therefore reveal fascinating insights about how Earth and other planets of the solar system came to be. Even more so as it moves closer to the Sun and begins to outgas its highly secured materials, forming its iconic tails and atmosphere known as a coma.
But how do you go about catching one? The Rosetta probe’s journey from Earth to 67P/Churyumov-Gerasimenko took meticulous planning on part of the ESA scientists, but even then there would be a lot to work out on the fly.
The spacecraft was launched in March of 2004 atop an Ariane 5 G+ rocket, but not with enough thrust to place it into orbit around 67P/Churyumov-Gerasimenko, whose path takes it around the Sun at distances as close as 186 million km and as far as 800 million km.
Instead, Rosetta used the gravity from one flyby of Mars, three of Earth and a detour out beyond Jupiter to slingshot towards the comet over the course of a roundabout 10-year journey. All the while, mission control was making adjustments to its trajectory, with the commands taking up to 50 minutes to reach the spacecraft, until it finally arrived at 67P/Churyumov-Gerasimenko following a 6.4 billion km (3.9 billion mi) trip and 31-month period spent in hibernation.
Mission scientists knew very little about the surface (and therefore a safe spot to try to land) until Rosetta made history and entered orbit around 67P/Churyumov-Gerasimenko in August of 2014. But that would change very quickly. As the spacecraft zeroed in on its 4-km-wide (2.5 mi) target, a picture started to emerge of world far more jagged and spooky than we ever expected.
Through Rosetta's early observations from its triangular orbit, we also learned that the comet was losing water at a rate of 300 milliliters (10 oz) per second, and it has an average surface temperature of -70° C (-94° F), suggestive of a particularly dark and dirty comet that absorbs rather than reflects sunlight.
From here, the Rosetta spacecraft’s imaging instruments began to scope out locations to set down its 100-kg (220 lb) Philae lander. This fridge-sized robo-lab was packed with scientific instruments including cameras, imaging systems, spectrometers, gas analyzers and devices to measure acoustics. Critically, it also included a drilling system to burrow 9 inches below the surface and collect samples for analysis of the comet’s chemical composition.
But lowering this lander to the surface presented a very different challenge to touching down on a planet or moon. As Rosetta orbited 67P/Churyumov-Gerasimenko and built out a map of its surface, mission control weighed up its options looking for the greatest possible scientific return from the landing site, but one that wouldn’t compromise mission safety.
What it needed was square kilometer of clear landing space free of slopes, boulders and other hazards, and one that was exposed to sunlight to power the lander’s batteries. It also needed a clear line of site for radio communications with Rosetta. On top of that, the team needed to avoid landing during the comet’s close approach to the Sun, where its outgassing could provide perhaps the most hazardous obstacle of all.
Site J was what the scientists settled on for the location of the first ever soft-landing on a comet, and on November 12, 2014, the team released the Philae lander from its mothership and sent it towards the surface.
The lander drifted towards 67P/Churyumov-Gerasimenko without power, instead relying on the comet’s incredibly weak gravitational field to pull it in towards the surface. This took place over seven hours, with the lander capturing farewell images and magnetic field measurements along the way.
As it touched down on the surface, a set of drills burrowed into the comet, while a harpoon was deployed to anchor Philae in place. Or so mission control had thought. What it learned through telemetry data soon after was that this anchoring equipment had actually failed, causing the lander to bounce back into space not once, but twice. It eventually wound up in a hole around 1 km (0.6 mi) from its intended home at Site J.
Here, laid on its side in the shadows with only two of its three legs touching the surface and some of its solar panels buried in comet dust, Philae would be unable to draw the solar power it needed to keep its batteries charged and instruments operating.
The charge already in its batteries would allow for around 60 hours of operation, and with the clock ticking the team instructed the lander to activate all of its instruments and return as much data as it could via the Rosetta mothership. In this relatively small window of discovery, Philae was actually able to carry out around 80 percent of its initial scientific studies and relay invaluable data to Earth. It also (kind of) carried out the first ever drilling operation on a comet.
While this command was carried out successfully, later analysis would sadly reveal it had no scientific return as the drilling instrument did not actually reach the ground.
With the batteries drained, Philae then entered hibernation mode. There was a chance that enough sunlight would hit its solar panels to re-establish contact with the Rosetta orbiter, albeit a very small one. Around six months later, the ESA instructed Rosetta to beam a wake up signal to the sleeping lander over an eight-day period, but it was not to be woken.
A few months later, in June of 2015, Philae miraculously showed some signs of life, though was only able to make eight intermittent contacts over a period of almost four weeks. These sporadic signals aside, the lander remained incommunicado until the ESA decided to say goodbye for good in July of 2016.
The Philae part of the mission mightn’t have been the raging success that the ESA hoped, but landing on a comet, albeit a little clumsily, was a monumental achievement in space exploration. Furthermore, while all of this was going on Rosetta continued to orbit 67P/Churyumov-Gerasimenko and gather a wealth of scientific data.
Through Rosetta’s observations we now know there are active sinkholes on 67P/Churyumov–Gerasimenko and that the comet smells like rotten eggs, an aroma created by the ammonia, hydrogen and other chemicals at the surface. We know that it once had a tiny temporary moon and that the comet is blacker than charcoal. We also know that its atmosphere contains key amino acids and molecules, considered the building blocks of life, and that its surface features significant quantities of water ice.
Rosetta gently crashed into 67P/Churyumov–Gerasimenko in September of 2016, gathering data until the very end and bringing its history-making 12-year journey to a dramatic finale. But the data and images it collected throughout will continue to enlighten our understanding of these primitive bodies for some time yet.
In next week's edition of "Into the great unknown" we look at Juno, a mission to understand the origins of Jupiter. For more on pioneering space probes, check out previous installments.