Spacecraft grant us a deep insight into our cosmic neighbourhood and, hence, our origins. Europe set itself an ambitious goal with the Rosetta mission: to observe a comet up close for almost two years – and land on it.
A bold idea was born during the Giotto mission to Halley's Comet in 1985/86 – scientists wanted to investigate a comet, but not just during a rapid fly-by. They wanted an orbiter that would accompany the comet for a longer period as it travelled through the inner Solar System. The plan was to reach the comet before it became active and accompany it to its closest approach to the Sun, where it would eject the most gas and dust. Thereafter, the orbiter would be able to analyse the decreasing gas and dust ejections.
The original plan was to, in collaboration with NASA, collect samples from the comet's nucleus and bring them back to Earth for subsequent laboratory analysis. But this bold vision was discarded.
The new concept was no less ambitious – it detailed the use of a landing module to deposit a laboratory on the surface of the comet and conduct on site analyses over a period of weeks. The main scientific objectives were to conduct research on cometary activity and to acquire new insights into the development of the Solar System. Such a mission would also hold a key position within the scientific programme of the European Space Agency (ESA).
In 1994, ESA decided to plan a several-month long rendezvous with a comet, with the possibility of carrying one or two small landing devices. The Rosetta mission was born.
A journey through the Solar System
Rosetta did not only fly past a comet (like Giotto, for instance); instead, it spent a considerable amount of time accompanying such a celestial body on its journey around the Sun. This meant adapting its route to match the comet's trajectory and substantial changes in speed. In practical terms, the orbiter accelerated by building momentum as it passed by Earth or Mars on what is known as a swing-by manoeuvre: here, the massive planet transfers a negligently small portion of its kinetic energy to the comparatively tiny spacecraft.
But this meant an extremely long flight time of approximately 10 years, during which Rosetta performed several full orbits around the Sun along planet-like trajectories. It repeatedly came close to the inner planets, travelling at everincreasing speeds. On three occasions, Rosetta used these manoeuvres to build momentum during precisely calculated and extremely tight swing-bys around Earth and, on one occasion, Mars.
The orbiter's trajectory became distinctly eccentric, ever further into the Solar System. Its multiple passages across the Main Asteroid Belt were used to analyse two asteroids – Šteins and Lutetia – during brief fly-bys. In addition, the orbiter managed to assist in NASA's Deep Impact mission by acquiring images of the Tempel 1 comet in 2005. By the time it arrived at 67P/Churyumov-Gerasimenko, Rosetta had clocked up around 6.4 billion kilometres, and will complete another 1.6 billion kilometres tagging along with the comet until September 2016.
Flying past Šteins and Lutetia
Rosetta flew past the asteroid (21) Lutetia at a distance of 3169 kilometres on 10 July 2010. Measuring around 106 kilometres in diameter, Lutetia is one of the largest asteroids, big enough to survive the collisions in the Main Asteroid Belt. Its high density (3.4 grams per cubic centimetre) is a strong indication that it possesses a heavy, metal-rich core and a slightly lighter rocky crust and surface. This small celestial body presents a connecting link between larger asteroids like Vesta and the rocky planets in the inner Solar System.
Before that, on 5 September 2008 Rosetta flew past asteroid (2867) Šteins at a distance of around 800 kilometres. Šteins is one of the millions of asteroids that can be found in the Solar System between Mars and Jupiter. It has a diameter of 4.5 kilometres, making it one of the largest of these small asteroids. The shape it has acquired through rapid rotation resembles that of a cut diamond. A large impact crater is clearly visible close to the rotation axis. Consisting of rubble – so-called regolith – the surface is strewn with impact craters. To the human eye, Šteins is rather unspectacular – grey with a slightly reddish hue.
Hibernation and 'Wake-up' call
Rosetta was switched to standby mode in July 2011 to complete the arduous journey to the comet. The reason for this was that the spacecraft's trajectory took it beyond Jupiter's orbit, a faraway spot almost 800 million kilometres from the Sun, where the solar arrays would have been unable to generate sufficient electricity for important functions. After 31 months the craft 'woke up' again on 20 January 2014. At this time, Rosetta was 9 million kilometres from 67P/Churyumov-Gerasimenko, drawing ever nearer to it at a speed of 800 metres per second.
The 'alarm clock' woke Rosetta as expected. The spacecraft autonomously put its systems into operation. The thrusters fired to stop the slow rotation to ensure that the solar arrays faced directly towards the Sun, before the startrackers were switched on to determine the spacecraft's attitude. Once established, Rosetta turned directly towards Earth, switched on its transmitter and pointed its high-gain antenna to send its signal to announce that it woke-up. The signal was confirmed by ESA's Operations Centre (ESOC) in Darmstadt, Germany, at 19:18 CET (18:18 GMT).
Approach and arrival
Rosetta was almost at its destination. Yet the OSIRIS camera had already begun observing 67P/ChuryumovGerasimenko millions of kilometres away, long before the spacecraft's rendezvous date on 6 August 2014. The structure of the comet's nucleus was first revealed in images acquired from a distance of 570,000 kilometres. The comet's complex shape astounded everyone.
An increasingly detailed view of the nucleus emerged as the spacecraft drew closer. It rotates once every 12.4 hours, which means that the comet's day is only half as long as one here on Earth. The rotational axis has a 52 degree inclination with respect to the orbit, which causes pronounced seasonal variations. Only the north was visible before and after arrival, as the south initially lay cloaked in the darkness of a polar night.
Rosetta had maintained its course with staggering precision. Later, the spacecraft took a last swing to enter the comet's orbit with as little difference in velocity as possible. Once it was 'captured' by the comet's tiny gravitational field, Rosetta began circling the body like a satellite. Nevertheless, the thrusters needed to be fired repeatedly to correct its course. ESA mastered all of these complex manoeuvres magnificently. The instruments on board delivered huge amounts of valuable data. Detailed cartographic assessments of the comet commenced with high priority. After all, the comet landing was scheduled for just over three months later.
Beginning of November 2014: 'Green light' from the control centre to begin preparations for Philae's landing.
An image of a comet
Scientists and the general public were fascinated by the images that the Rosetta spacecraft transmitted to Earth. 67P/Churyumov–Gerasimenko turned out to be an exotic, bizarre comet world. Two lobes separated by a kind of 'neck', which are known as the 'head' and 'body', immediately catch the eye. 67P is so distinctive that it cannot be mistaken for another comet or asteroid. Its entire surface shows a surprising variety of landscape – in addition to smooth, dust-covered plains, rugged, rocky cliffs and ridges rise upwards. Numerous pits and sinks are the result of the comet's activity every time it approaches the Sun. Dust that is released with the evaporating ice is obviously not completely swept away into space, but partly falls back on the comet’s surface and creates gently undulating areas, somewhat reminiscent of dunes.
In many places one finds large rocks as large as small houses scattered like boulders across the landscape. Fields of boulders have formed at the base of some slopes. Bare ice is, however, only found in few places. In some areas, cracks several hundred metres long – probably the result of the large temperature differences that led to stress in the comet's nucleus – cut through the landscape. Impact craters caused by meteorites are not visible, as the comet's surface is ever-changing as a result of its high activity.
The search for a landing site
The unexpectedly rugged surface soon raised concerns about whether it would be possible to land safely on the comet's nucleus. Discussions to find a suitable landing site began in August 2014, following Rosetta's first surveys of the comet from orbit. First, the Philae lander team, headed by DLR and CNES, the French Space Agency, selected 10 candidates for potential landing sites, which were also considered technically feasible by ESA. These were presented to the Philae science team in Toulouse at the end of August 2014. The team was asked to shortlist five candidate sites that would be examined in detail before the final selection was made three weeks later.
he engineers' main concern was whether or not a safe landing would be possible. This meant that the morphology of the landing site had to be free of obstacles. In addition, the illumination and the orientation of the landing site with respect to the Sun were important for the landing craft's power supply. The scientists had slightly different priorities, as they were keen to choose a landing site that was scientifically exciting. For example, they wanted to analyse the comet's activity at close quarters and provide all instruments with the best opportunities to observe their surroundings. The final landing site Agilkia and a backup site were selected in mid-September. Eight weeks later, it was time to land on a comet.
Although the landing on 12 November did not go quite as planned, Agilkia turned out to be an excellent choice.
Separation and descent
Wednesday, 12 November 2014 – the big day. The teams worked toward this moment for years – the day that the landing module Philae would touch down on the comet.
Rosetta was moved out of its orbit 30 kilometres above the comet at 06:06 CET (Central European Time) and brought into a near-collision course with 67P. This was the only way to ensure that Philae could be deployed safely towards the landing site Agilkia. Philae was separated from the orbiter at 09:35 CET, 22.5 kilometres from the comet, and drifted towards 67P at an initial speed of 19 centimetres per second. The descent was ballistic – meaning in free fall – and lasted seven hours. There was no way to influence the manoeuvres at this point, especially as the signal travel time was 28 minutes from Earth to the lander.
In the meantime, the OSIRIS camera on board Rosetta transmitted images back to Earth, confirming that all three landing legs were fully extended. Caught in the comet's slight gravitational field, Philae accelerated to one metre per second, or almost walking speed. The lander touched down at Agilkia, just 100 metres from the intended landing site, at precisely the calculated time of 16:34 CET. Loud cheering initially greeted the incoming 'touchdown signal' at 17:03 CET at the control centre, confirming contact with the comet's surface. Even the harpoons designed to anchor Philae in the ice encrusting the comet appeared to have worked. Radio contact also remained stable.
But something was not quite right… Philae continued to move!
What happened to Philae?
Just moments after Philae had touched down on the comet it was clear that not everything had gone entirely according to plan. Telemetry data from the solar generator and measurements by the magnetometer and the MUPUS sensor confirmed this: Philae continued to move!
What had happened?
The harpoons intended to hold Philae in place at the Agilkia landing site had not fired. The ice screws on its feet had also failed to drill into the ground. The 100-kilogram heavy probe on Earth weighed a mere two grams on 67P due to the comet’s low gravity. So Philae bounced off the surface - albeit almost in slow motion. The landing craft ascended, touched down again, bounced a second and a third time, before finally coming to rest some two hours later approximately one kilometre further south on the edge of the large Hatmehit depression. The site was later named 'Abydos'.
Although the Philae team managed to locate the final landing site precisely to within a few tens of metres, the lander did not show up on the photos taken by OSIRIS initially. At least it seemed Philae was not damaged, and radio contact with Earth via Rosetta also proceeded smoothly. The first image that Philae sent from Abydos showed structures indicative of a dark crevasse.
Abydos was a very dark place, exposed to the Sun for just approximately 80 minutes over the course of a 12-hour comet day: that was insufficient to recharge the batteries, but nevertheless enough to operate all of the instruments at least once.
A spectacular mission finale
Philae managed to work for just 64 hours using the electricity supplied by the primary battery. This enabled important measurements, making this first landing on a comet a resounding success.
Afterwards, Philae fell into a kind of hibernation. But the comet was now approaching the Sun, so the lander could generate more electricity. Indeed, Philae beamed a sign of life on 13 June 2015! From then on, though, contact remained sporadic, and the final contact was made on 9 July 2015. Additional experiments were not possible.
Rosetta remained in operation long after the scheduled end of its mission. The spacecraft recorded the increase in cometary activity as 67P approached its perihelion, and its decrease as it moved further away.
By the summer of 2016, Rosetta and the comet had travelled to far beyond the orbit of Mars. Soon, the solar panels would no longer produce sufficient electricity. So ESA planned yet another spectacular, unrivalled manoeuvre to bring the mission to its conclusion – Rosetta would land on the comet, just like Philae. The orbiter was taken to within two kilometres of Churyumov-Gerasimenko in preparation for its final manoeuvre, as well as to observe the comet from close quarters.
This enabled it to acquire extremely high-resolution images using the OSIRIS camera. Thereby, Philae finally was found on 2 September 2016, 'standing' upright in a dark crevasse in Abydos.
Rosetta was set into collision course on 30 September 2016. It touched down gently on the surface of the comet it had so closely observed 786 days. The final communication reached the control centre in Darmstadt at 13:19 CEST. Rosetta continued to transmit measurement data and photos until the very end