OSIRIS – The Eyes of Rosetta

Introduction

Comets as beautiful phenomena on the night sky have for long fascinated humans and inspired our imagination. Moreover, having witnessed the formation of our Solar System 4.6 billion years ago, comets are also a scientists dream to be studied in all possible aspects. Composed of fluffy dust, several ices and rich organics, it is for long believed that they preserve pristine material from that early time and therefore hold the key to understand the origin of the Solar System with all its planets and ultimately life.

Orbiting the Sun on elliptical orbits, which frequently bring them close to the Sun and as far out as Jupiter and beyond, comets cannot be comprehensively studied with telescopes. In 1986, a fleet of spacecraft from several space agencies was therefore sent to Halley's Comet (1P/Halley). Among them was the European space probe Giotto, which approached Halley to 596 km closest distance and sent the first ever images of a cometary nucleus to Earth. The data from this concerted encounter in deep space not only raised a broad public interest in comets but also put cometary science in Europe and worldwide on a new level.

With the great success of the Giotto mission, the scientific community in Europe advanced the idea of a new cometary mission. The concept of the Rosetta Mission was the next step in cometary science. Referring to the Rosetta Stone, which inscriptions became key in deciphering Egyptian hieroglyphs in the 18th century, the mission’s name reflects its high ambition to decipher the nature of our Solar System’s origin. To keep up with the expectations, the Rosetta spacecraft was equipped with 12 scientific instruments, one of which was the Philae lander, together forming an in-situ laboratory to study the surface, interior, and activity of a comet.

Launched in March 2004 from the European spaceport CSG in French Guiana, Rosetta was pioneering and ambitious in many aspects. Its key advantage over previous missions was that it stayed in the vicinity of a comet for more than two years where missions like Giotto, Stardust, or Deep Impact had performed short and fast fly-bys. The long visit and concerted analysis of an orbiting spacecraft and a lander unit allowed an in-depth study of a comet’s nature and its physical processes to re-write the textbooks about cometary science.

The camera system OSIRIS (Optical, Spectroscopic, and Infrared Remote Imaging System) can certainly be considered the 'Eyes of Rosetta' having covered every step of the spacecraft’s exciting adventure with breath-taking images. It was composed of two cameras with its supporting electronics, a Narrow Angle Camera (NAC) to resolve the cometary nucleus at highest detail, and a Wide Angle Camera (WAC) to keep an eye on the fascinating jets and outbursts of activity at the same time. Equipped with a 2048 x 2048 pixel CCD imaging sensor, OSIRIS was one of the best cameras out in deep space and delivered near to 80.000 scientific images in impressive detail.

This OSIRIS Image Archive presents the entire collection of images taken throughout the journey of Rosetta. It was prepared by the scientists who worked on the development and the operation of the camera system. Join us on an adventure through space! From the launch of the Rosetta spacecraft on-board an Ariane 5 rocket, to a travel through space for more than ten years until its main mission at comet 67P/Churyumov-Gerasimenko. On its way to the comet, Rosetta performed three swing-by manoeuvres around Earth and one at Mars. Moreover, in the spirit of deep space exploration, Rosetta performed fly-bys at two asteroids, Steins and Lutetia, to study these celestial bodies from close by.

On 6 August 2014, the ESA spacecraft finally arrived at comet 67P and found a cometary nucleus with an astounding shape and bizarre landscapes. The landing of Philae on 12 November 2014 marked a milestone in European space history. But at that time, Rosetta’s mission had just begun. The spacecraft remained at the comet for another two years to study its surface and its exotic processes in detail. We witnessed the wake-up of the comet approaching the Sun, the peak of activity in August 2015, followed by the comet’s retreat back into the outer Solar System. The glorious finale of the mission, the landing of the entire Rosetta spacecraft on the surface of the comet, concluded this overly successful endeavour on 30 September 2016 near the pits on the small lobe of 67P.

Building Space Instruments

Scientific instruments for space research are a challenge for engineers and scientists to develop. Going out to the unknown, design requirements are based on expectations and educated guesses. A harsh rocket launch and fast ascent to space, encountering vacuum and cold thermal conditions, deep thermal cycles, and the lack of gravity in flight are demanding environmental conditions contrast the long required lifetime. Hardware maintenance in flight being impossible.

Key instrument parameters for the scientific research such as spatial resolution of the optics, dynamical range of the sensor, focus stability, the choice of colour filters, and throughput with allowed exposure times determine the observational possibilities at the target object. Moreover, the design must allow for the unexpected: For small objects as comet 67P, the knowledge by ground telescopes on the size and shape of the nucleus, surface reflection, composition, its activity and local coma is very limited, derived from a signal that is filling far less than a single pixel of its imaging sensor.

Space instruments are conservative design though driving new technologies. To match the requirements for OSIRIS, a new type, space-qualified sensor had to be developed, allowing the detection of faint activity above the over-exposed limb of the comet’s nucleus by a buried anti-blooming gate, draining out the excess charges by the illumination. A new ceramic material, Silicon Carbide (SiC), allowed thermal floating of the telescope structure and the mirrors of the Narrow Angle Camera at best focus position over a wide range of temperatures; an innovative technological first in space with Rosetta.

The OSIRIS cameras required moving parts: A shutter mechanisms to control the exposure of the imaging sensor (CCD) to a fraction of a percent …

… and filter wheels to select the spectral (colour) band pass of the observation. Re-closable front doors protect the camera optics during launch, accidental Sun exposure, and from the cometary dust when observing in the vicinity of the active comet.

The focal plane with the CCD sensor had to be passively cooled to -120° Celsius to limit the thermal emission of charges into the device, the dark current of the sensor, and to freeze out charges generated through radiation damages by energetic solar and cosmic particles in space.

A digital processing unit, the camera’s computer, had to control the health of the system, monitor and access switches, observe voltages and currents, and thermally stabilise the camera components at the desired target temperatures. It could take decisions on board to safe guard the instrument in case of anomalies. The main task of the unit was the execution of the command sequences that were loaded days or weeks in advance. The transmission of the commands to OSIRIS, and image download to ground, took up to 40 minutes one-way, depending on distance between Earth and the Rosetta spacecraft.

Space missions require a team focused for decades, driven by motivation and enthusiasm for space exploration, and the belief that education, with the Rosetta mission the understanding of formation of our Solar System and life on Earth, will bring people on our planet closer together.

OSIRIS was built by an international collaboration of 9 research institutes from 5 European countries and the European Space Agency with more than 300 engineers and scientists at the peak of development, in a no-exchange-of-funds environment, where motivation and visibility drive the national support, and the mission return balances the efforts.

30 years from mission proposal - 10 for the decision process, 10 to build instruments and spacecraft, and 10 to reach the target comet - made Rosetta a long-term challenge and true generation-bridging endeavour.

Rosetta’s Journey to Comet 67P/Churyumov-Gerasimenko

Rosetta was launched into space on 2 March 2004 on an Ariane 5 rocket from the Guiana Space Centre (CSG) in Kourou. The European space port in French Guiana (South America) is close to the equator, which is optimal to launch rockets by taking advantage of Earth’s rotational (or angular) momentum. After having climbed through cloud layers of the Kourou morning sky into transfer orbit, a 17-minute long burn of the Ariane rocket’s upper stage brought Rosetta onto an Earth escape trajectory, farewell into deep space.

Travel through space never takes the straight way, which would consume more fuel than a spacecraft could carry along. Planet swing-by (also gravity assist or slingshot) manoeuvres serve to accumulate enough velocity to reach far into the outer Solar System. Applying smart celestial mechanics, the spacecraft ‘steals’ a bit of angular momentum from the planet to alter its own path and, most importantly, gain velocity.

A challenging travel like Rosetta’s would take the spacecraft 4.5 times around the Sun before reaching the comet. The spacecraft returned to Earth for its first swing-by 368 days after launch, swung by Mars in February 2007 and Earth again in November 2007.

Of course a spacecraft equipped with this plethora of excellent scientific instrumentation in space would not close its eyes during the swing-bys. The chance was taken to get acquainted with the spacecraft and operate the instrument, which resulted – among other results – in beautiful images of planets Earth and Mars with their moons, providing a glimpse of the performance of the wonderful OSIRIS cameras on board.

Having gained the angular momentum from these swing-bys, Rosetta made it as far out as the asteroid belt and was guided to take the opportunity to fly by asteroid (2867) Šteins in September 2008 at 2.13 astronomical units from the Sun. Visits of these small bodies in deep space are very rare and Rosetta could finally do its first real science! The fly-by at a distance of 800 km from this diamond shaped, 5 kilometre-sized asteroid lasted only minutes but everything was focused to this moment and a great amount of data could be acquired. Every encounter with an unknown world like this helps to shape our understanding of our Solar System neighbourhood and the context of Solar System formation and evolution.

After the exciting fly-by at asteroid Šteins, the path was set to the fourth and last swing-by manoeuvre, again passing by planet Earth in November 2009. After the successful swing-by, Rosetta was finally on a trajectory that should bring the spacecraft as far out as 5.4 au (astronomical units) from the Sun. But on its way there, yet another highlight was awaiting: The fly-by of asteroid (21) Lutetia in July 2010.

With about 100 km in size, this asteroid was much bigger than Šteins and from 3162 km closest distance the high resolution of the OSIRIS Narrow Angle Camera showed a loose regolith covered, cratered surface, resembling our Moon. In spite of its larger size, asteroid Lutetia still shows an irregular shape as only bodies of a sufficient size like asteroid (1) Ceres are heavy enough such that the overburden (lithostatic) pressure is able to round them up.

On its trajectory towards comet 67P, Rosetta went further away from the Sun than any solar-panel powered spacecraft before. The electricity of the huge solar panels (32 m span from tip to tip) would not be enough to keep Rosetta powered. In June 2011, the spacecraft had to be put into hibernation for 31 months. Only minimum thermal conditioning kept the spacecraft from freezing, with a timer set for recovery in 2014. A decision not without risk …

… but it was rewarded with a successful wakeup from hibernation in January 2014. A series of 8 burns of the thrusters brought Rosetta on track to catch up with comet 67P on its orbit around the Sun.

An Encounter in Deep Space

The orbit of comet 67P around the Sun follows an ellipse. Every 6.44 years, it approaches the Sun as close as 1.24 au during the perihelion, only to disappear into deep space as far out as 5.68 au from the Sun. From August 2014 to September 2016, the comet was accompanied by the Rosetta spacecraft, which was performing complex manoeuvres that were specifically designed to accomplish Rosetta’s ambitious mission goals.

Along this timeline selected mission highlights like Rosetta’s arrival at the comet, Philae’s landing …

… the peak of activity or Rosetta’s final landing are iconic, acquired by OSIRIS – The Eyes of Rosetta.

Spacecraft manoeuvres and image acquisitions happened far out in deep space: On its way around the Sun, Rosetta travelled 8 billion kilometres and the distance to Sun and Earth was ever changing with the records of up to 5.3 and 6.3 au furthest distance, respectively.

When ESA’s mission control centre in Darmstadt, Germany, communicated with the spacecraft they used up 70 m diameter antennas and typically sent stacks of commands for a half week, which Rosetta would autonomously follow and people on Earth would eagerly watch.

The last highlight of the mission, landing the Rosetta spacecraft on the comet, was a true challenge. Rosetta had to be navigated while being 720 million kilometres from Earth and the time to transmit a signal took 40 minutes.

But when Rosetta touched down on the comet on 30 Sep 2016 at 10:39:28, this was less than a minute from the time that had been calculated weeks in advance. Unbelievable.

The Science of Rosetta/OSIRIS

When the OSIRIS cameras started to resolve the comet nucleus in June 2014, the prominent bi-lobed shape was a surprise to scientists and engineers. It was exciting and inspired discussions on its formation history and evolution since formation. On the other side, it became a challenge to perform the important measurements of basic parameters that needed to be known immediately after arrival in August 2014: What is the nucleus’ size, mass, and volume? What is the density and its porosity?

OSIRIS images were used to construct a realistic 3D shape model, and from this, size scales and volumes could be determined. The mass was measured from the nucleus’ gravitational pull on the Rosetta spacecraft as this was orbiting around the comet.

From the mass and the volume, the material density was derived: With 0.53 g/cm³ it was lighter than water! The reason for this low density is the nucleus’ porosity, which must be in the order of 70%, meaning that only 30% of the volume is filled with material and 70% is void space. High porosities are well known from theory and experiments on early planet formation in the Solar System, supporting the expectation that 67P is a remnant, thus witness, of this time.

From the nucleus surface topography, for instance on the steepness of cliffs, the material strengths (compression, tension, shear) could be estimated. These are in the order of 10 to 100 Pa, which is comparable to the strength of cigarette ash. The material is thus not only light and porous but also extremely fragile, which is consistent with how we believe small bodies would have formed in the early Solar System.

Soon the comet became active, evaporating ices into space, carrying along dust particles from tiny micrometre grains up to metre-sized agglomerates. Some of this material would fall back onto the nucleus, not reaching escape velocity, and change the surface in appearance: the more active southern hemisphere became rougher while the less active northern hemisphere accumulated back-fall material, appearing like a winter wonderland after fresh snow fall.

With material lost into space, the comet was getting lighter with every passing by the Sun. This could be measured by Rosetta, amounting to 0.1% of its mass by the end of the mission – a layer of 40 cm thickness lost over the entire surface in average, amounting to 10 million tons of removed dust and ice.

The surface would not only change by the material falling back, as many processes were in action. The insolation by the Sun caused ices to sublimate, and induced mechanical stress to the nucleus’ surface layer. Small and large cracks opened and expanded, cliffs and overhangs collapsed, roundish pits opened up, dunes formed and ceased, house-sized blocks on the surface moved from one place to another. Ice patches up to 1600 m² in size appeared on the otherwise dry surface. Some of these lasted for months, others vanished after minutes. Sometimes, changes could be linked to short outbursts of dust and gas activity, which were frequently observed by OSIRIS.

To keep track of all these changes and to locate them on the surface, scientists defined regions based on their distinct morphology, and named these regions after Egyptian deities. The strength and friction properties of the nucleus’ dust determine the appearance of the surface structures, influenced by the local gravity and seasonal effects by the inclination of the rotational axis of the comet.

The data acquired by the Rosetta mission will be studied by scientists for decades to come. The image data from the OSIRIS cameras already led scientists to either support, refine, or even question established theories of our Solar System’s formation. Yet many details of this enormous data set are still to be analysed. The photographs are fascinating as they are, imagine that every image also tells a story about the Solar System when it was very young and Earth in its formation.

Landing on a Comet, End of a Mission!

The end of the Rosetta mission was originally planned for December 2015. The mission was extended until September 2016 due to the scientific success. At that time, 67P and Rosetta would be so far from the Sun such that the power generated by Rosetta’s solar arrays would be insufficient to operate all instruments. One option was to then park the spacecraft at a safe distance from the comet and put it into hibernation – as it was done in 2011 – to continue operation about two years later, seeing the comet waking up again approaching the Sun. The second option was to go close, accepting risks to study the nucleus from distances never observed before, and end the mission with the spacecraft on the surface acquiring measurements and high-resolution images until the very last moment.

Priority was given to science, aiming to understand the physics of the comet close to the nucleus’ surface, looking into the interior structure by the walls of the deep pits in the Ma’at region. Landing had to happen before running out of fuel and solar power. 30 September 2016 was chosen for the ‘Grand Finale’, at the edge of power limitation at 3.8 au of 573 mio kilometer from Sun. Although the velocity during the touchdown would be only about 90 cm per second – very slow walking speed – the high gain antenna needed for communication would certainly lose its remote target and thus contact to Earth. The two scientific instruments that would be operated until the very last second were the ROSINA gas spectrometer and the OSIRIS cameras.

As scientific targets for the landing, the Rosetta Science Working Team picked two roundish pits in the Ma’at region, in particular the pit later named Deir el-Medina. Named after an excavation site in the Valley of Kings in Egypt, the pit on 67P was believed to reveal clues about the comet’s interior and thus formation history. The pit is located on the comet’s small lobe and should be observed during landing. The landing site is right next to it, in a flat area later named Sais.

In the early morning hours of 30 September 2016, the spacecraft was on a straight track towards the comet, which was rotating under Rosetta as the spacecraft was slowly approaching. The observation sequences had been transmitted to Rosetta days in advance, ready to be executed and eagerly followed by the operations team on ground ready to take control should anything go not as planned.

After imaging a large raster of the entire landing area, the OSIRIS cameras were pointed towards the Deir el-Medina pit, which was observed in highest resolution, up to 2 cm per pixel. About 1 kilometre from the surface, the viewing direction was changed by slowly rotating the spacecraft to look straight into the flight direction.

At 10:39:28 UTC, the spacecraft touched down on the nucleus surface and immediately lost communication to Earth, as expected. It took another 40 minutes for the last signals to travel through space and arrive on Earth. For the OSIRIS cameras, these last bits and bytes were a fascinating pattern of small image segments, transmitted in real-time while getting closer and closer.

The last complete image segment was acquired at 10:39:04. During the transmission of the next image the data stream was cut off by the impact of the spacecraft on the surface. This incomplete, very last image was acquired at 10:39:10 at 20 metres from the surface, covering 1 square metre of the nucleus at 2 mm per pixel resolution! This image could be reconstructed to the last message by Rosetta, ending this wonderful and exciting mission.