As comet 67P/Churyumov-Gerasimenko reaches the closest position to the Sun in its orbit, Ezzy Pearson looks back at what the Rosetta mission taught us about our Solar System’s icy wanderers
On 3 November, the Sun will once again be graced by a visit from one of the Solar System’s frozen travellers – Comet 67P/ Churyumov-Gerasimenko. Like all comets, 67P is a fragment left over from the creation of the planets; a time capsule containing a hint of what the Solar System was like during its formation 4.5 billion years ago.
By understanding comets, astronomers can piece together the history of our planetary system. So in 2004, the European Space Agency (ESA) launched a mission to 67P and began the most detailed exploration of a comet ever undertaken. The comet would act as a Rosetta Stone –a key that would allow them to unlock the secrets of all other comets – so the mission was called Rosetta.
Rosetta arrived at Comet 67P on 6 August 2014 and spent more than two years circling the comet as it drew closer to the Sun. The spacecraft watched the surface sublimate, turning straight from ice to gas, creating its beautiful tail. Its spectrometers examined the gas and dust coming off 67P to sniff out what chemicals were being released as the comet thawed. ESA even managed to set the washing machinesized Philae lander down on the surface, making history as the first ever soft landing on a comet.
From the first images Rosetta sent back, it was making revelations. During approach it discovered that 67P is double-lobed comet, with two round sections stuck together in a formation many liken to a rubber duck. It was when Rosetta got into orbit around 67P that the real investigation could begin however, as the spacecraft was able to begin looking at every facet of the comet.
Water carrier
Many of Rosetta’s instruments were focused on the swirling cloud of gas and dust surrounding the comet – its coma. One of the first measurements taken was the ratio of normal water to ‘heavy’ water, which contains an extra neutron. This ratio – known as the deuterium-to-hydrogen ratio or D/H – remains unchanged over time. As such, planetary scientists can use it to trace the movement of water over our Solar System’s history. Planetary scientists have wondered for decades if comets brought water to the early Earth. If they did, then the D/H for comets should be the same as it is on Earth.
“The D/H was higher than any comet measured so far, which answered [the question of] whether our terrestrial water came from comets. And that answer is a no,” says Professor Kathrin Altwegg, project manager of the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) instrument that made the measurement. “The old idea was that all water on Earth came from comets. Now we know it’s probably less than one per cent.”
That’s not to say comets didn’t bring anything important to Earth, as Rosetta revealed they could have been instrumental in transporting the ingredients of life to our planet.
“We found glycine. It’s the simplest amino acid, and they’re essential for life. To find that in a very cold, lifeless, primitive body was quite a surprise,” says Altwegg. “We also found phosphorus, which is another essential ingredient for life. It’s probable that most of today’s biomass was delivered by comets.”
Meanwhile, other instruments focused on the icy nucleus of the comet. The Comet Nucleus Sounding Experiment by Radio wave Transmission (CONSERT), used radar to look inside Rosetta. It found the body wasn’t a solid lump of ice, but actually a loosely packed collection of pebbles. Around 75 to 85 per cent of the comet’s volume is empty space, suggesting it formed from many small objects bumping together at slow speeds, rather than a high-velocity collision between two large objects.
Hard landing
The surface, however, turned out to be remarkably solid – something the Rosetta team discovered the hard way when they attempted to land Philae. The spacecraft was meant to set down in what appeared to be a region covered in dust, known as Agilika. On 12 November 2014, the landing team rejoiced as the signal came back saying it had touched down. But their joy was short-lived – it appeared the dust was only a thin layer, covering a hard surface. Several of the mechanisms meant to latch Philae to the surface had broken in transit, so the lander had rebounded off the unexpectedly solid comet.
Philae spent two hours drifting above 67P’s surface, eventually coming back down 1km from its intended landing site, in a region named Abydos. It had landed on its side, meaning the drill couldn’t deploy correctly and collect an ice sample. To make matters worse, Philae had ended up in the shadow of a cliff, where its solar panels couldn’t recharge the battery, so it only functioned for the 64 hours its battery allowed.
Though Philae could only achieve a few of its goals, the lander did manage to get a good look at the surface – in two locations. They showed a highly varied terrain: while Agilkia was a field of smooth dust with wind-blown features carved across the surface, Abydos was a dark and rugged terrain of hard ice and jagged cliffs. The vibrations made by Philae revealed that the hard layer seems to be a universal feature across the comet. It seems the constant cycle of freezing and thawing as the comet nears the Sun before passing back into deep space has created a hard shell surrounding the nucleus’s crumbly centre.
An increase in gas
As 67P drew closer to the Sun, Rosetta recorded the comet’s temperature rising from –70ºC to just below zero and the amount of gas given off dramatically increasing. When the spacecraft first arrived, the comet only gave off around 300g of water a second – about a mugful. By the time it reached perihelion on 13 August 2015 that number had increased 1,000-fold and 67P was throwing out two bathtubs-worth of vapour a second.
This huge amount of material coming from the comet made staying in orbit very difficult for Rosetta. The strong breeze constantly pushed against the 32m2 surface of the spacecraft’s solar panels as it struggled to stay close, while dust also hammered away at its entire structure.
But Rosetta survived, remaining in orbit around the comet for another year. As 67P travelled further into deep space and away from the warming Sun, it slowly returned to a slumber. With the comet dormant once again, Rosetta’s job was done. On 30 September 2016, Rosetta began a slow descent towards a smooth spot on the comet’s smaller lobe before setting down on the surface in what would be its eternal resting place.
RIGHT: Dust and high winds made conditions more challenging for Rosetta the closer the comet got to the Sun
Back on Earth, however, the rest would be considerably shorter. The Rosetta team was now faced with the complex task of consolidating all the data gathered during the spacecraft’s long mission, organising the information to make it as useful and easy to interpret as possible.
“That consolidation has been critical,” says Professor Marina Galand from Imperial College London, one of the scientists working on Rosetta data today. “Now we have this amazing data set available to the whole community where you can look at changes over a season, over heliocentric distance, over latitude.”
Importantly, the data set is now properly calibrated. Previously, researchers had only been able to interpret a single experiment at a time, but this step means they can now compare data across all of Rosetta’s 11 instruments.
“With one instrument you can learn up to a certain point. When you start to have different instruments or different types of observation, you can start to fit all the pieces of the puzzle together. That enhances the science return,” says Galand.
Further discoveries
This refined data was released to the public in 2019 and researchers have been combing through it ever since, leading to a slew of discoveries. Galand and her team, for instance, looked at far-ultraviolet (UV) emissions in the comet’s coma. Though the radiation was detected by Rosetta’s UV spectrograph, ALICE, the instrument couldn’t pin-point where the emissions came from on its own.
“We used data sets from different instruments. We used ALICE, which looked at the brightness of specific emissions; another instrument looked at energetic electron fluxes. And we measured the density of the gas from the infrared,” says Galand.
With all this information, the team was able to confirm that the emissions came from a dancing aurora, encompassing the comet as it passed around the Sun. While this ultraviolet emission is invisible to the human eye, there were hints of the 630nm emission produced by oxygen, which is responsible for the red tones seen in Earth’s aurora. Fortunately, the comet’s return to perihelion offers the perfect chance to have another look for this emission.
“We couldn’t observe the comet before at perihelion, because it was on the other side of the Sun to Earth,” says Galand. But that’s not the case this time and many teams are planning on observing the comet this November. “That will give context because we’ve seen in close and now we can see from a different perspective,” she says.
Herein lies the real legacy of the Rosetta mission. Before the orbital mission, no one would have thought to look for aurora around a comet, but now they know it’s there, astronomers can use groundbased telescopes to look for it around 67P – or any other comet that comes by. And this is just one study, which had looked at the data for just a few months. More revelations are waiting to be made as astronomers dig further into the data.
“Today, people are still publishing data from the comet flyby mission Giotto, which was just a few hours of data acquisition in 1986,” says Galand. “Here we have two years of data and we’ve not yet looked at its whole wealth.”
So while the Rosetta mission may be over, but its scientific legacy has only just begun to emerge.
Comet 67P’s journey around the Sun
The comet returns to the inner Solar System on 3 November for the first time in 6.5 years
Time to orbit Sun 6.45 years
Aphelion distance 5.7 AU (850 million km)
Perihelion distance 1.2 AU (186 million km)
Discovered 1969
Rotational period approximately 12 hours
Inclination 7º
Place of origin Kuiper Belt
Next perihelion 11 April 2028
Mass 10 trillion kg
Size of nucleus 4.3km x 2.6km x 2.1km
The future of comet exploration
Comet Interceptor will wait in orbit for a fresh comet to be found
Comet 67P has taught us a lot about the origins of the Solar System, but that’s not to stay it’s remained unchanged since its creation 4.5 billion years ago. It’s passed through perihelion many times, and the cycle of freezing and thawing has changed its composition. If planetary scientists want a comet made of material that’s as it was during the formation of the planets, they’ll need to find a comet on its first trip to the inner Solar System.
Comet orbits regularly get knocked inwards (67P used to orbit much further out until an interaction with Jupiter in 1959 brought its orbit closer to the Sun). But astronomers get little warning and only find out about these new comets when they’re already on their way. So, rather than waiting for one to appear and then hurriedly launching a mission, ESA and the Japanese space agency (JAXA) are working on a joint mission that will wait for a comet in orbit.
Comet Interceptor will launch in 2029, but without a specific target in mind. Instead, it will travel to the second Lagrange point (L2) 1.5 million km behind Earth. And when astronomers spot a suitable comet coming towards us, Comet Interceptor will speed off to investigate. The mission could even become our first ‘interstellar’ explorer if an interesting object from another solar system passes through ours, such as ‘Oumuamua (pictured) or Comet Borisov, which have both visited us in the last few years.
See ‘The Sky Guide – Comets and Asteroids’, for details of how to observe 67P in November
Dr Ezzy Pearson is BBC Sky at Night Magazine’s news editor. She has a PhD in extragalactic astronomy