“The biggest problem with success is that it looks easy, especially for those of us who have nothing to do.” Thus spoke Jean-Jacques Dordain on Wednesday, November 12th, just moments after it had been confirmed that a tiny robot vehicle called Philae had safely landed on the surface of a comet half a billion kilometres away from Earth.

That simple statement offers a subtle message on the huge achievement this landing represents. The Rosetta / Philae mission is the story of a 6 billion kilometre journey across space which has taken a decade to achieve, and which has involved some 20 countries. Yet the adventure is in many ways only now starting.

The Rosetta mission actually started 21 years ago, in 1993 when it was approved as the European Space Agency’s first long-term science programme. The aim of the mission being to reach back in time to the very foundations of the solar system by rendezvousing with, and landing on, a comet as it travel through the solar system.

Comets hold enormous scientific interest because they are, as far as can be determined, the oldest, most primitive bodies in the Solar System, preserving the earliest record of material from the nebula out of which our Sun and planets were formed. While the planets have gone through chemical and (in the cases of places like Earth), environmental and geological change, comets have remained almost unchanged through the millennia. What’s more, they likely played an important role in the evolution of at least some of the planets. There is already substantial evidence that comets probably brought much of the water in today’s oceans – and they may even have provided the complex organic molecules that may have played a crucial role in the evolution of life here.

The target for ESA’s attention is comet 67P/Churyumov–Gerasimenko (aka 67P/C-G), an odd-shaped body comprising two “lobes” joined together one in what some in the media have at times referred to as the “rubber duck”. The larger of the two lobes measures some 4.1×3.2×1.3 kilometres in size (2.55×1.99×0.8 miles) and the smaller some 2.5×2.5×2 kilometres (1.6×1.6×1.2 miles). It is a “short period” comet, orbiting the Sun once every 6.4 years and most likely originating in the Kuiper belt, a disk of material from the early history of the solar system, orbiting the Sun at a distance of around 30-50 AU

The primary spacecraft in the mission, Rosetta, arrived in the vicinity of 67P/C-G on August 6th, 2014 becoming the first vehicle in history to successfully enter orbit around a comet. The major reason the mission took so long to reach the comet, having been launched in 2004, is that despite having a relatively short orbital period, 67P/C-G is travelling very fast and accelerating as is falls deeper into the Sun’s gravity well heading for perihelion (it is currently travelling at 18 kilometres (11.25 miles) a second and can reach velocities of 34 kilometres a second as it swings around the Sun). As it is impossible to launch a space vehicle is these velocities, Rosetta was launched on a trajectory which allowed it to fly by Earth twice (2005 and the end of 2007) and Mars once (early 2007), using the gravity of both planets to accelerate it and (in the case of the 2nd Earth fly by), swinging it onto an orbit where it would “chase” and eventually catch the comet.

Following its safe arrival, Rosetta settled into an orbit of some 30 kilometres around the comet in September, and began looking for a suitable place where Philae might land – because until the craft actually arrived in orbit around 67P/C-G, no-one had any idea of what it’s surface might look like. On 15 September 2014, ESA announced a region on the “head” of the “duck” had been selected for the landing, christening it Agilkia in keeping with a contest to name the landing site.

Further observations of the comet were carried out throughout September and October as an overall part of Rosetta’s mission and to gain as much information on the landing site itself. At the same time the spacecraft started manoeuvring itself in closer to the comet, dropping its orbit to just 10 km, ready for Philae’s delivery.

The landing operations commenced around 09:05 UT on Wednesday, November 12th, when Philae detached from Rosetta and started on its long gentle descent. Immediately following the separation, and due to Rosetta’s orbit around the comet, contact was almost immediately lost with the lander, leading to a tense 2 hour wait before communications could be re-established. This happened on cue, with the lander reporting all was OK.

Landing on a comet is no easy task. The gravity is almost non-existent, and there was a very real risk that Philae could, if it struck the surface of 67P/C-G too fast, simply bounce off. Hence the lander’s long, slow drop from the Rosetta spacecraft which the ESA mission scientists dubbed “the seven hours of terror” in recognition of the famous “seven minutes of terror” which marked the arrival of NASA’s Mars Science Laboratory Curiosity on Mars.

To prevent Philae bouncing off the surface of 67P/C-G, ESA engineers came up with two ingenious systems to try to ensure the craft can anchor itself in place. Each of the three landing legs has a small ice drill built into its “foot”, and designed to activate as soon as a leg makes contact with the comet, effectively “screwing” the landing pad against the rock. Little harpoons would also be fired from under the vehicle, tethering it to the comet.

The actual touch-down was expected to take place around 15:35 UT, or shortly thereafter. However, because of the distance involved, it would take the signal confirming touchdown a further 30 minutes to reach Earth. At 16:11 UT,the first telemetry showing Philae had landed was received by mission control in Darmstadt. It promoted a Tweet from the Rosetta media team:

It had initially been thought that the landing systems had performed as anticipated; however, as further telemetry came in, it back apparent that the harpoon system had not actually fired; indeed, the lander may have actually bounced very slightly, leading to mission manager Stephan Ulamec to comment, “today we landed on a comet – twice!” While the failure of the harpoons, and a slight issue with the communications signal between the lander and orbiting Rosetta are not serious enough to compromise the mission, engineers are examining the potential to attempt a re-fire of the harpoons.

However, assuming all goes well, Philae should quickly start into an intensive 60-hour study of the comet. After this time, the primary battery system will be discharged, and the lander will switch to a rechargeable battery system which uses solar cells mounted on the side of the vehicle.

Both systems will allow Philae to conduct an on-the-spot analysis of the composition and structure of the comet’s surface and subsurface material. A drilling system will obtain samples down to 23 centimetres (9 inches) below the surface. These samples will be subject to spectrographic analysis to determine their chemical composition, including the presence of including amino acid enantiomers. Other instruments will measure properties such as near-surface strength, density, texture, porosity, ice phases and thermal properties. In addition, instruments on the lander will study how the comet changes during the day-night cycle, and while it approaches the Sun.

How long the lander survives depends on several factors. The comet is currently “falling” towards the Sun and will grow increasingly active as it does so. already the comet is close enough for dust and gas to be given off as it is warmed by the Sun, forming a faint coma around the rocky nucleus. As this increases, there is a risk that this dust could coat the lander’s solar cells, preventing the batteries recharging. Even if this doesn’t happen, it is likely that by mid-March 2015, the comet will be so close to the Sun, heat will overcome the lander.

Meanwhile, Rosetta will also be carrying out an extensive study of the comet from its orbit. using a suite of instruments including cameras, spectrometers, and experiments that work at different wavelengths – infrared, ultraviolet, microwave, and radio. These will allow the vehicle to gather more high-resolution images of the comet and information about its shape, density, temperature, and chemical composition. In addition, Rosetta will analyse the gases and dust grains in the comet’s coma, which will become more and more active as it approached perihelion and swings around the Sun.

One thing that has come as something of a surprise is that not only is Rosetta able to look at, sniff and even – via Philae – scratch and tickle 67P/C-G, the mission has also been able to hear the comet, which appears to be singing to itself! In August 2014 Rosetta detected oscillations in the comet’s magnetic field, which give rise to an unusual “song” in the 40-50 millihertz range. Far too low for the human ear to discern, ESA issued a recording on the sound on November 11th, 2014, with the frequency increased factor of 10,000 to make it audible.

Via Wayne Kraus

It is hoped that Rosetta will continue to function right the way through perihelion in August 2015, and onwards to around December 2015. However, while 67P/C-G appears to be a lot less active than other comets visited by probes in the past – such as the famous Halley’s Comet “chase” and fly by of 1986 – there is still a risk that significant outgassing from the comet might damage the vehicle or compromise this solar panels.

The Rosetta mission isn’t the first to study a comet; as noted above, Halley’s Comet, which orbits the Sun every 76 years, was visited by no fewer than 5 space vehicles – Europe’s Giotto, and two each from Russia and Japan, all of which flew by that comet in 1986, with Giotto coming to within 600 kilometres of the comets very active nucleus, having been “steered onto” the comet with data obtained by the Russian and Japanese missions. While it had not been expected to survive the encounter, Giotto did continue onwards after passing Halley’s Comet and flew by comet Comet Grigg-Skjellerup in 1992, prior to being switched off. NASA has also carried out three cometary missions, including the Stardust mission of 2004, which gathered samples from the tail of comet Wild 2 and returned them to Earth for analysis in 2006.

However, and as noted, Rosetta marks the first time a vehicle has been successfully placed in orbit around a comet, and Philae the very first craft to land on a comet.

About the Project Names

The Rosetta project uses three interconnected names. The overall name, which is also that of the primary space craft, is taken from the Rosetta Stone, the granodiorite stele inscribed with a decree issued on behalf of King Ptolemy V inscribed in three scripts: Ancient Egyptian hieroglyphs, Demotic script and Ancient Greek. It offered an important key to understanding Egyptian hieroglyphs. Thus the name reflects the overall objectives of the mission, with comets perhaps being the key to our understanding the ancient history of the solar system

Philae refers to a pair of islands just above the First Cataract of the Nile River near Aswan. Said to be the buried place of Osiris, the islands were the home of the The temple of Isis. It was here that the Philae Obelisk was found, on which was engraved a petition in Ancient Greek and Egyptian hieroglyphs, and which further assisted in the decipherment of hieroglyphs alongside the work with the Rosetta stone.

Agilkia is the name of the island to which the Temple of Isis on Philae was relocated stone by stone in the 1960s to remove it from further risk of flooding due to continuing development of the Aswan dams.

There will be more to come from Rosetta and Philae. In the meantime, I’ll leave you with a film on Philae’s landing, featuring music composed especially for the event by Vangelis.

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all images courtesy of the European Space Agency unless otherwise indicated.