Final frontier

A Shot in the Dark

More dusty than rocky: Comet Churyumov-Gerasimenko.
  • Why it matters

    Why it matters

    No one has orbited or landed a spacecraft on a comet before, so little is known about these fast-moving and fragile celestial bodies. The Rosetta mission hopes to gather huge amounts of data.

  • Facts


    • The Rosetta probe has taken 10 years to reach its quarry.
    • It is currently hovering 50 kilometers above the comet’s surface.
    • The comet and probe are hurtling towards the Sun at up to 135,000 kilometers per hour.
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The pictures of the landscape, which no person has ever seen before, are razor sharp. Over a jagged hillside lies a placid plain. Further in the distance, on an opposing cliff, is a dusty valley.

“That is my favorite place,” said Holger Sierks of the Max Planck Institute for Solar System Research in Göttingen, who leads a team of 80 researchers. Together, they developed the OSIRIS camera that took the image. It is no Earthly panorama, but instead shows the surface of comet 67P/Churyumov-Gerasimenko.

The celestial body, shaped like a rubber duck, is currently 400 million kilometers (250 million miles) from Earth and racing toward the Sun. Since the beginning of August, it has been accompanied and circled by the European space probe Rosetta, which in November will attempt to dispatch a lander onto its surface.

Osiris is only one of 20 scientific instruments on Rosetta. In the science of comets, pictures speak volumes, so Mr. Sierks received loud applause when he presented his pictures to delegates at the European Planetary Science Conference in Lisbon earlier this month.

But images can also be deceptive. The shots of the comet appear to show boulders, dried-up riverbeds and volcanic craters. Yet the jagged surface is actually more like powder snow than rock. “If one could sit on it, it would feel like a rumpled bedspread,” said Fred Goesmann, manager of another Rosetta instrument. “It would make more of a ‘poof’ than a ‘plunk.’”

“If one could sit on it, it would feel like a rumpled bedspread.”

Fred Goesmann, Rosetta mission scientist

The images remind Mr. Sierks of the steep coastline in his home state of Schleswig-Holstein, in northern Germany, but he said the comet is not really comparable with anything on earth. “I hope the earthly analogies don’t obstruct the path to discovery,” he said.

Comets are remarkable bodies. They have been watched closely for centuries and read as messengers of fate. But it was not until 1986 that close-up images of a comet were captured. These came after the European Giotto probe performed a very fast fly-by of Halley’s comet, showing a pockmarked, potato-shaped lump. Today, scientists are still working with that data.

Comet missions are complicated, expensive and rare. Between 2001 and 2010, only NASA was able to observe comets close up, but like Giotto, these fly-bys were conducted from between 250 and 2,000 kilometers away, and lasted split seconds as the probes hurtled past their targets at tens of thousands of kilometers per hour.

Rosetta is the first mission to specifically target a comet. It is ambitious and expensive. The trip of 6.4 billion kilometers took 10 years. To gain momentum for the voyage, the probe had to circle the Sun four times, Earth three times and Mars once. Costs have exceeded €1 billion.

The probe arrived near Churyumov-Gerasimenko at the beginning of August. Since then, a team at the European Space Operations Center in Darmstadt, Germany, has guided Rosetta through a series of complicated maneuvers around the comet, which measures roughly four kilometers in diameter. These were necessary because its nucleus generates too little gravitational pull for a stable orbit.

Rosetta is now hovering just 50 kilometers above the jagged surface of the comet. Its exterior is unusual. Unlike any comet observed before, Churyumov-Gerasimenko consists of two parts connected by a narrow neck. “The closer we go, the more multifaceted the images become,” said Mr. Sierks.

Comets are actually “dirty snowballs”: Mixtures of dust and ice, loosely held together by microgravity and intermolecular forces.

His pictures have thrown up more questions than answers. On one area of the surface, for example, the Osiris team found 330 meter-high lumps. “How did they get there?” Mr. Sierks asked his audience at the Lisbon conference, “and what are they made of?” None of his listeners had an answer.

The origin of the comet’s many circular depressions is also unclear. They can’t be volcanic craters because the comet is ice cold. Minus 53 degrees Celsius is the highest recorded surface temperature.

Comets are actually “dirty snowballs”: Mixtures of dust and ice, loosely held together by microgravity and intermolecular forces. Little more is known about them but it is hoped Rosetta will greatly expand the knowledge base. The probe has already gathered more data than all previous comet missions combined.

At the Lisbon conference, the teams of researchers exchanged information from the results of their 20 different scientific instruments. While the images made the biggest splash, their meaning will only be fully revealed after comparisons with the much more abstract study of spectral lines, plasma and microwaves.

For example, Fabrizio Capaccioni and his team calculate the temperature of the comet by measuring its infrared radiation. When a spot becomes shaded, the temperature “drops very quickly, at least four degrees per minute,” he said. “The material must be a bad conductor of heat. It is probably porous.” About 70 to 80 percent of the nucleus consists of nothing at all.

Martin Hilchenbach, another comet expert from the Max Planck Institute for Solar System Research, and his team have turned their attention to the comet’s tail. They constructed a spectrograph to analyze the dust inside it, which turned out to be surprisingly large and angular. But they can’t yet say what it consists of because their equipment temporarily stopped working in August.

It has since been repaired, and, like all the other instruments on board Rosetta, is ready for action. Only the control nozzles are causing small problems.

Are we nearly there yet? Rosetta will dispatch the Philae probe to the comet’s surface. Source: DPA


The project to track the comet has been successful, despite being dogged by staffing troubles. Long-time project director Gerhard Schwehm retired at a critical time and the managing scientist, Rita Schulz, the first woman in an ESA leadership position, left after power struggles with other participating scientists. She was replaced by Matt Taylor, a tattooed Briton who, wearing camouflage-print shorts and red sneakers, raved about Rosetta in Lisbon. It is the “sexiest mission ever,” he said.

Mr. Taylor has also had to negotiate a management path. There is continuous disagreement over the orientation of the satellite, about allocations of the short observation times and over the frequency range for the radio transmitting research data back to Earth. “If everyone is equally dissatisfied, then I was successful,” said Mr. Taylor.

The jostling will get especially bad in the middle of November when Philae, a washing machine-sized landing device, will be set down on the comet. Subject to it falling over a cliff or sinking into a hole, it is expected to deliver huge amounts of data.

All the while, the comet is getting more active. Churyumov-Gerasimenko is currently throwing about 25 tons of dust and gas into its tail every day, and in a few weeks it could be a hundred times closer to the Sun. Most of the material is coming from the steep cliff walls, and many will most probably break apart.

In especially active spots, the surface can even sink by dozens of meters, especially on the backside. This region, which makes up 15 of the 40 square-kilometers of the comet’s surface area, has been in the shade so far, and is in total darkness.

“That is the greatest unknown”, said Mr. Sierks.

This article originally appeared in the newspaper Die Zeit. To contact the author:

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