10 Lessons From ESA’s Comet Mission

In November 2014, the European Space Agency’s washing-machine-sized investigation and analysis lander lab Philae, did something that no other automated probe had ever done before, it successfully landed on a comet. But apart from this monumental achievement what has the lander probe and its Rosetta mothership so far learned about comets? Here are 10 things that we now know…

1)     2 in 1

:Comet CG: 18th February 2015, 198km away from Rosetta.
(Image credit: ESA/Rosetta/NAVCAM, CC BY-SA IGO 3.0 (https://creativecommons.org/licenses/by-sa/3.0/igo/)

After a ten year chase across space, the Rosetta spacecraft discovered that up close its large cosmic fugitive had a somewhat bizarre appearance, looking more like two heavenly bodies fused together rather than a single, round, comet nucleus. With one ‘lobe’ apparently smaller than the other and the two connected by a narrow ‘neck’ of material, scientists were quick to identify a ‘duck-like’ form in the dark body. Whether comet CG’s shape is the result of a low-key, slow-motion impact of two comet nuclei into one lumpy hybrid, a vast amount of ice has simply escaped from the ‘neck’ region, or whether an enormous heavenly body with a strong gravitational pull (such as Jupiter) could have yanked the comet’s mass out of shape in passing, no-one currently knows; but scientists are hopeful that further study will answer such questions.

2)     Tougher than we thought

Diving duck: Viewed from approximately 102.6km away, light room processed version of Comet 67P as seen by the Rosetta satellite’s NAVCAM, revealing the coma’s activity as a glowing cloud of outflowing material around the comet.
(image credit: ESA/Rosetta/NAVCAM – CC BY-SA 3.0 IGO (https://creativecommons.org/licenses/by-sa/3.0/igo/)

As smaller comets are known to have a weak gravitational pull, the European Space Agency expected their probe to prove that 67P would have a soft, fluffy surface. And while the analogy of a Ferrero-Rocher chocolate has already been used to describe its bumpy exterior, at least for the patch on which Philae inadvertently landed, the German Aerospace Centre (DLR) instead said that the comet’s hard surface made it “a tough nut to crack”. Granted the lander did not secure itself to Churyumov-Gerasimenko with all harpoons deployed and all feet resting on the ground as was intended, but armed with a drill 100 times better than the one you or I might have in our homes, it was hoped Philae’s SD2 system would be able to pierce CG’s crust and collect samples down to a depth of 23 centimetres. Yet although the drill started to operate no such material however was collected. While this may suggest that the surface was so hard Philae’s drill could not pierce it, this is thought not to be the case. Rather, it is believed that in its unstable ‘handstand’ position Philae’s drill could not reach the ground. None-the-less thanks to a hammer which was able to smack the comet’s surface Philae’s science team think that the lander is standing in a 10-20cm carpet of dust, beneath which lies a very hard table if ice.

3)     Comets can sing

With the contrast exaggerated to show the faint ice jets streaming off its back, comet 67P perhaps look as much like a ‘dirty snowball’ as it ever will.
CREDIT: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

The concept of an alien spacecraft ‘singing’, a notion made forever iconic in 1977 in the film Close Encounters of the Third Kind was a novel one, not likely to be easily forgotten. However in 2014 people were probably just as unprepared for the real news that ‘very low singing’ was discovered to be coming from an inanimate object inside our Solar System! That object was Comet Churyumov-Gerasimenko and the super low-frequency burbling was detected and recorded by the Rosetta spacecraft. The interpretation of the phenomenon currently thought to be the most plausible is that fluctuations in the magnetic field around the comet may be oscillating particles trailing off comet CG that have been charged by the solar wind, thus creating the mysterious ‘sounds’.

4)     Nothing to drink here

Although ‘water ice’ is generally believed to be one of the key ingredients of a comet, 67P has in some ways challenged the comet template. Any thirsty space travellers wishing to stop off on a comet and easily gather some water ice on the surface for refreshment may be disappointed, as the Philae probe discovered that a on top of a comet nucleus this substance may be in short supply. Radiation from the Sun is known to sublimate (change directly from solid to gas) the water ice on a comet, yet scientists on Earth interpreting Philae’s findings were surprised at just how dehydrated the surface of comet CG already appeared to be, even though still some considerable distance from the Sun.

5)     Hot or cold?

Yikes! – Its watching you: Still some 30km from the comet and with nothing in the vacuum of space to obscure the Rosetta satellite’s view of its target, this remarkable image reveals Comet 67P looking eerily like some monstrous monochromatic duck that has raised its great head out of the darkness to give the automated probe the full benefit of its chilly cosmic stare.
CREDIT: ESA/Rosetta/NAVCAM, CC BY-SA IGO 3.0 (https://creativecommons.org/licenses/by-sa/3.0/igo/)

Yikes! – Its watching you: Still some 30km from the comet and with nothing in the vacuum of space to obscure the Rosetta satellite’s view of its target, this remarkable image reveals Comet 67P looking eerily like some monstrous monochromatic duck that has raised its great head out of the darkness to give the automated probe the full benefit of its chilly cosmic stare.
(Image credit: ESA/Rosetta/NAVCAM, CC BY-SA IGO 3.0 (https://creativecommons.org/licenses/by-sa/3.0/igo/)

As most comets are believed to be formed in the spherical-shaped ‘refrigerator’ of the outer Solar System or Oort Cloud, the stereotypical expectation is that they will be uniformly cold. However Philae proved that comets may be more complex than first thought. This complexity was revealed in the dual temperature detected between 67P’s northern hemisphere and its southern hemisphere. Implying seasonal changes may be at work, Philae discovered that the uppermost section of the comet can reach a relatively warm -45 degree Celsius compared to the southern portion which can plummet to a markedly cooler -70 degrees Centigrade.

6)     Potentially soft-centred

Fitness apparatus?: From a distance of 28.7m and from this angle, magnificent comet Churyumov-Gerasimenko perhaps looks less like a duck and more like some enormous dumbbell of the gods!
(image credit: ESA/Rosetta/NAVCAM, CC BY-SA IGO 3.0 (https://creativecommons.org/licenses/by-sa/3.0/igo/)

Holger Sierk’s science team used Philae to investigate CG’s solid centre, the part of a comet referred to as the nucleus. Their findings were somewhat of a surprise however, as they discovered that the nucleus was made up of rock, dust, and frozen gas. This means that instead of a hard solid centre comet nuclei may in fact be ‘fluffy’ and porous a little like the centre of an Aero egg! By contrast, surface explorations of Comet CG revealed a surface with ‘Ferrero-Rocher style’ bumps and clumps of small bright spots which may be loaded full of ice.

7)     A complex shroud

Comet nuclei are encapsulated in a gassy, dusty envelope that accompanies them as they glide through the Solar System. This murky cloud is called the coma and slight diurnal (day-night) changes in the dust, carbon dioxide, water, and carbon monoxide mixture hint at a complex relationship between the coma and the nucleus that is not yet fully understood, but when it is, may ultimately assist research into how comets evolve.

8)     Pooh- you stink!

A good look at the comet: Approximately 28km from Comet 67P, Rosetta’s NAVCAM snaps this image on the 31st January 2015.
(Image credit: ESA/Rosetta/NAVCAM, CC BY-SA IGO 3.0 (https://creativecommons.org/licenses/by-sa/3.0/igo/)

Philae and Rosetta travelled the gargantuan 4 billion mile distance across space armed with a whole range of instruments and sensors so that as much as could feasibly be ascertained about comets would be gleaned during their long-awaited exploratory visit to comet Churyumov-Gerasimenko. Rosetta rather resourcefully applied its two mass spectrometers to ‘smell’ the shroud of gases in CG’s coma. These instruments detected sulphur dioxide, methane, ammonia, hydrogen sulphide, and formaldehyde. So what does this tell us about comets? Well it means that although a comet with a tail whipping around the Sun can make one of the most magnificent celestial spectacles to people on Earth, up close, they would be less appealing; as these gases surrounding the nucleus would smell just like vinegar, horse dung, cat urine, rotten eggs, and literally ‘death’ all rolled into one!

9)     A striking topography

Gigantic cosmic cliff: Composed from four separate photos taken by Rosetta’s NAVCAM from approximately 12 miles away, amateur astronomer Stuart Atkinson’s crop work focuses our attention on the rugged cliff face, the boulders at the foot of which are approximately the length of an articulated (60ft) bus.
(Image credit: ESA/Rosetta/NAVCAM – CC BY-SA 3.0 IGO (https://creativecommons.org/licenses/by-sa/3.0/igo/) Additional Processing: Stuart Atkinson)

The nucleus of Comet 67P is in total, approximately 4.3km long and 4.1km wide, so a kilometre-tall cliff on this scale, cannot be missed, and Churyumov-Gerasimenko has been found to have many large cliffs. Apart from the fact that a photograph taken by a manmade robot parked on the surface of a 84 000mph moving comet is a mind-blowing and historic achievement, the images taken by both Philae and Rosetta are dramatic in the extreme and in themselves surely provide sufficient reward for all the painstaking-effort and planning that was required to orbit and land a probe on this distant cosmic traveller. Another cool thing mentioned when the researchers discuss this cosmic cliff is that in the strange low-gravity environment of a relatively small celestial object such as 67P, it is thought that a human being would be able to leap of the top of the cliff to land on the flat terrain 1km below without incurring any serious injury, so gentle would be his/her descent!

10) Surprising colour

Move over Hollywood blockbuster special effects! – real surface footage of Comet 67P captured in magnificent detail by ESA’s Rosetta orbiter. (Cliffs of the comet’s larger lobe rise dramatically in the background as Rosetta stares across the ‘neck’ adjoining the two lobes.)
(Image credit: ESA/Rosetta/NAVCAM, CC BY-SA IGO 3.0 (https://creativecommons.org/licenses/by-sa/3.0/igo/)

Often referred to with their ice-dust/dirt composition as “dirty snowballs”, comets are generally much darker and filthier than the average snowball, thus making these opaque objects extremely hard to spot when still far out from the light source at the centre of our Solar System. Not only does this make the date of 67P’s discovery (from its accordingly distant location) remarkable, as it was spotted by its namesake astronomers on a photographic plate back in 1969, but its finding is made all the more impressive (as are the images that have been produced of it) when we learn that it is in reality nearly as black as coal! So while on the one hand we must bear in mind that photos we have seen of Churyumov-Gerasimenko have been greatly enhanced to enable us to see its overall form and surface topography, researchers have now revealed that in pure white light (for example right in front of the Sun), the comet’s true colour would include a distinctive coppery tint!

Viewed from afar: At an apparent magnitude of about 20 here we see the dark and dim Comet Churyumov-Gerasimenko as photographed on a 50-second exposure (with the aid of 40 separate superimposed images) on the 11th August 2014 by one of the ground-based 8m telescopes of the VLT. The background stars were blocked out for maximum exposure of the comet and the Rosetta satellite had at this stage, already plunged deep into the comet’s coma. Even in this dim, grainy image the comet’s coma can clearly be seen starting to develop a tail.
CREDIT: Colin Snodgrass/ESO/ESA

Although on the 13^th August 2015 at perihelion (its closest pass to the Sun) this darker-than-average comet is only expected to reach a too-dim-for-naked-eye apparent magnitude of 11, thanks to Rosetta/Philae’s decade-long chase and unprecedented comet photography Churyumov-Gerasimenko’s place in the history books will undoubtedly remain secure as the best viewed comet ever.

(Article by Nick Parke, Education Support Officer)