Chasing a comet

Following the Rosetta spacecraft on its 6.2 billion kilometre journey past Mars and through asteroid belts


It was “seven hours of terror”: the carefully orchestrated launch of a tiny robot lander that was to anchor itself onto a strange, peanut-shaped rock travelling at 38 kilometres a second. A rock twisting and spinning 550 million kilometres from Earth—so far that communication between the lander and its earthly controllers took 28 minutes each way. Mission Control had detected a problem with its final preparation. As it touched the surface of the comet, harpoons were meant to eject from each of its three feet, safely securing it to the icy surface. To counteract the recoil from the harpoons, which could push the lander back into space, a small thruster was installed on its roof. But the counter-thruster was unresponsive. Control had to go ahead with the launch and hope for the best.

The comet, the romantically named 67P/Churyumov–Gerasimenko, was formed billions of years ago at the beginning of time. It had since been on a long, lonely journey through space, caught in the gravitational pull of the sun, around which it revolves in a great spiralling ellipse once every six years. At its furthest (known as its aphelion) it is 849 million kilometres from the sun, just a bit further away than Jupiter at its own aphelion. At its closest (known as its perihelion) it is 186 million kilometres away, just a little closer to the sun than Mars at its own perihelion.

While the comet careers towards the sun, its ancient ice heats up and evaporates and fractures, reducing its mass. The smaller the comet becomes, the stronger the sun’s gravitational grip on it, until eventually the pull will become too much and the comet will dissolve entirely in the sun’s blistering inferno.

A spacecraft, named the Rosetta orbiter, had been on its own epic journey around the Solar System. Leaving Earth more than ten years before, it had since covered well over six billion kilometres, passing precariously through our main asteroid belt twice and within 250 kilometres of Mars.

Along with twelve scientific instruments, including two cameras, it carried a micro-etched nickel alloy disc inscribed with 13,000 pages of text in 1,200 languages—a physical tribute to its namesake, the Rosetta Stone. It also carried a tiny probe which would be launched onto the comet’s surface: the Philae lander, named after the obelisk which, along with the Rosetta Stone, was used to decipher Egyptian hieroglyphics.

To reach the acceleration needed to meet up with the comet, Control had to use advanced gravity assist manoeuvres. By racing towards the Earth at an angle passing by its gravitational field and slingshotting away obliquely, the Earth’s gravitational energy is transferred to the probe, boosting its speed. The mathematics required to successfully pull off a flyby are mind-bending. Rosetta performed three of these manoeuvres until it was moving fast enough to match the immense speed of the comet.

On the second and closest slingshot, as it raced towards the Earth, the probe was detected by another space agency and misidentified as a rapidly approaching asteroid. For a nervous moment, there were fears a 20 meter asteroid could impact the Earth—a rare and dangerous event. The mistake was soon resolved and the handful of concerned scientists collectively exhaled.

On the third and final flyby, Rosetta was flung far out into our Solar System in a broad arc. To preserve its limited energy reserves, it had to be placed into hibernation mode while it curled out towards the comet. For the next 31 months, the probe would be entirely out of contact with Control, travelling in silence towards its target. A single piece of space debris could destroy Rosetta and end the project, which had now been running for a decade, employing some 2,000 people. For the scientists involved, to be out of touch with the €1.3 billion probe for moments is considered disastrous—two and a half years is a traumatic eternity.

The ambitious project, headed by the European Space Agency (EPA), aimed to be the first mission to orbit and ‘soft land’ on a comet, collecting a vast array of valuable scientific data on the way. Surprisingly little is known about these ancient cosmic objects. There’s evidence comet impacts were the origin of half the Earth’s abundant water. By studying the primitive composition of comets—their gases, dusts and structures—we get an insight into the early evolution of our Solar System. With all our Apple Watches and Hadron Colliders, it’s strange to imagine we’re still asking questions like, What is a comet? Where do they come from and what do they do? And what can they tell us about the birth of the universe?


In January 2014, Rosetta was finally switched back on again. Thousands held their breath while the signal was sent on its billion kilometre round journey to the tiny probe and back. Nerds cried and hugged each other in empirical joy when the monitor lit up with a blip of life after two and a half years of silence. Rosetta was now just months from arriving at the alien object—all they had to do was ensure that the trajectory of the probe aligned with that of the comet, where they could begin their scientific dance through space.

Six months later, images of the comet finally began streaming in—with each approaching image, it grew in size. It was nothing like they imagined. The ancient, icy rock measured kilometres in each dimension, weighing around ten billion tonnes. But rather than the typical semi-round potato shape, it was comprised of two lobes joined by a narrow bridge. It had a ‘head’ and a ‘body’—described as peanut- or duck-shaped. The first images revealed “a world littered with boulders, towering cliffs and daunting precipices and pits, with jets of gas and dust streaming from the surface”, said the EPA in its joyous announcement. It was exciting, yet worrying. It’s difficult to land something on a unpredictable surface—never mind the half a billion kilometre distance and hour long time delay between the probe and its controllers.

It took the EPA six weeks to decide on the best landing spot, during which Rosetta quietly circled the comet, collecting whatever information it could. The spot had to be just right: not just flat and hard (they weren’t sure if the surface would be ice as hard as steel, or dust as soft as baby powder) but also in a position that would capture enough sunlight on its solar panels to power the scientific instruments onboard. They chose a spot on the comet’s formidable head, dubbed Agilkia.

Finally, on November 24, Rosetta was prepared to launch its tiny lander—named Philae—towards the surface of the comet. It was a manoeuvre of immense technical complexity. Travelling at a scorching 1m/s, Philae took seven hours to reach Agilkia, where it was to softly settle and begin its next stage of scientific inquiry, lasting several months. They wouldn’t know if the landing had been a success—if Philae had gently touched down with its tripod legs harpooning the ground and drills securing it to the icy surface—until an agonising hour later.

The signal came in: Philae was sitting snug on the surface. But not where they’d hoped. The problem with the counter-thruster recognised before launch was realised; the lander hadn’t secured to the surface but instead bounced hundreds of metres towards the larger ‘bodily’ lobe of the comet. The timeline of events was recorded: 15:34GMT, Philae made contact with the comet and immediately rebounded. Due the almost complete lack of gravity and slow rotation of the comet, it didn’t make contact again until 17:25GMT, where it bounced a few more times before finally settling at 17:32GMT—just shy of two hours after initial contact. In total, the lander had travelled almost a kilometre from its target landing spot.

It was an historical and technological triumph. We’d landed a tiny craft on an ancient comet hurtling through the inner Solar System at an unfathomable speed. However, there was a major problem. The area Philae had ultimately settled lurked somewhere in the shadow of one of the comet’s major cliffs. The lander’s solar panels would not receive any sunlight on its humble journey. Without any source of backup power, Philae fell asleep. It would not be able to complete its mission and help unlock the secrets of the universe. Instead, over the coming months, it would continue circling the Sun on its silent voyage through the vastness of space, with its orbiting big brother Rosetta keeping doleful watch.