Rosetta mission: Six things we’ve learnt about comets from the super spacecraft and its probe Philae
By James Bullen
If the launch of the Rosetta mission had gone to plan back in 2004, we would never have heard of comet 67P/Churyumov-Gerasimenko. Instead, we’d be talking about a comet called 46P/Wirtanen.
But the launch was delayed by four days and 46P/Wirtanen missed its chance to be the most studied comet of all time.
Luckily, 67P had what was needed: it was a short-period comet that loops around the Sun between the orbits of Jupiter and Earth once every 6.6 years.
And it was going to be in the right place, at the right time to rendezvous with Rosetta.
When the mission began scientists knew very little about 67P, which was first discovered in 1969, other than that it was about four kilometres in diameter and travelled at a speed of about 135,000 kilometres per hour.
Twelve years later, data sent back by Rosetta and its probe Philae from 67P has drastically reshaped the way scientists think about comets.
And the work is just beginning for astronomers — it will take years to get through the huge amounts of data sent back.
Here are just six things we’ve learnt.
Building blocks of life found
Scientists always suspected that comets may have aided in the creation of life on Earth — and Rosetta helped them confirm it.
During the early years of the solar system, when cometary bombardments were more common, astronomers believe comets carrying organic compounds crashed into Earth, seeding future life.
If scientists could find the building blocks of life on a comet out in space, it would lend weight to the notion they helped kick-start life on our planet.
“The discovery of these elements, these complex carbon molecules, was the primary goal of the mission and Rosetta and Philae massively succeeded in confirming that,” said Warwick Holmes, who worked as an avionics engineer on the European Space Agency’s Rosetta project.
Using its onboard instruments, Rosetta made repeated detections of glycine — an amino acid — and phosphorus in the area around comet 67P in May this year, especially when gas jets blasted dust from within the comet out into space.
These are two of the most critical substances necessary to the creation of life. Glycine is associated with the creation of proteins, while phosphorus helps create DNA and cells and is essential to all living organisms.
Oxygen surprise in comet’s coma
While oxygen is a common element in the universe, its simplest molecular form — O2 — is hard to find because it usually binds with other molecules or atoms.
Scientists were extremely surprised to find molecular oxygen in 67P, because it would have had to survive in a pristine condition since the very beginnings of our solar system 4.5 billion years ago.
They believe the discovery adds evidence to the idea comets come from that period, and are now exploring how the presence of oxygen in 67P might provide clues as to how the solar system formed.
Not a potato, more like a duck
When Rosetta launched from Earth, the scientists had no idea of what the comet it was setting out for looked like. Every aspect of the mission, including the landing of Philae, was designed with the team in the dark about the shape of 67P.
“We were hoping something like a potato, or even spherical. But it was anything but — it was this very strange duck-shaped feature,” Mr Holmes said.
Commonly called the “rubber duck” because of its similar appearance to the common bath toy, scientists found 67P is actually made up of two roughly oval bodies joined together.
The comet is about 24 cubic kilometres in size — four kilometres by three-by-two — fitting nicely over Sydney’s CBD if placed on top of it.
Scientists believe 67P was formed in a low-speed collision between two previously separate comets. That gives clues about what the early solar system would have looked like — dense, dusty, and full of rubble.
It’s a dust ball not a ‘dirty snowball’
67P challenged the expectations of the ESA team in other ways when they discovered its surface was covered with smooth dust plains and craggy, rocky cliffs.
Mr Holmes said the scientists expected to find water ice on the surface of the comet — in keeping with the popular “dirty snowball” hypothesis, which describes the idea that a comet is mostly made of ice, with some dust and rock inside of it.
But 67P contradicts that hypothesis — there’s very little ice on its surface. Instead, its appearance is diverse and very dusty. Vast dust dunes and smooth plains cover parts of the comet. Other features on the surface include rocky boulders, cliffs, and pits.
“The popular theory is that comets are big balls of ice frosted or dusted by carbon chemistry,” Mr Holmes said.
“But in our case it seems we’ve got a lot more dust and carbon chemistry than we have ice, different to the ratio that was first thought.”
Analysis of the dust floating above the comet shows the dust is made up of spheroidal sub-micrometre grains, providing clues about how planets in our solar system first formed.
A comet like 67P probably didn’t bring water to Earth
Along with being responsible for bringing substances critical to the creation of life to Earth, scientists have long thought that comets also brought water to our planet, forming the Earth’s oceans, as they crash-landed into its surface after Earth had cooled.
Scientists hoped they’d find water on the comet with a similar “isotopic ratio” to that found on Earth — meaning the ratio of hydrogen and deuterium in the water on Rosetta would be the same as the ratio on Earth.
Eleven comets have previously been measured to find their “isotopic ratio” — and just one has matched the ratio on Earth, Comet 103P, which comes from the Jupiter family of comets — a group of comets that are influenced by the giant planet’s gravity.
Comet 67P is also from the Jupiter family. Yet its isotopic ratio is entirely different to Earth and comet 130P, appearing to scuttle the hypothesis that Jupiter-family comets contained water similar to that found on Earth, and lending weight to the idea asteroids, rather than comets, may have greater responsibility for bringing water to our planet.
“It’s actually made the answer to that question [of whether comets brought water to Earth] far more challenging. People thought there was a nice, easy answer, but there actually isn’t,” Mr Holmes said.
And if that’s not enough … the comet ‘sings’
When Rosetta drew close to 67P two years ago, scientists discovered it seemed to be emitting a strange, unearthly song.
Before long, they’d determined that it was the plasma in the comet’s environment interacting with the magnetic field carried by the solar wind to produce “magneto-acoustic waves”.
The oscillations made by these waves were picked up by the instruments on Rosetta at a frequency far below the range at which humans can hear. But by increasing that frequency by a factor of about 10,000, the comet’s “song” is revealed.
The scientists said the plasma waves detected were so unusual they’d be poring over the observations for years to come.