All set for our first comet landing

For an event intended to celebrate a scientific milestone, the January gathering inside the European Space Agency’s control room in Darmstadt, Germany, was unexpectedly tense and muted. Officials and scientists had assembled to listen out for the world’s most expensive morning wake-up call. A computer on ESA’s Rosetta spaceship was supposed to rouse the probe, then several hundred million kilometres from Earth, from its two and a half-year hibernation and prepare it for a rendezvous with Comet 67P/Churyumov-Gerasimenko.

The alarm on Rosetta had been set for 10am. But engineers warned it would take several hours to warm up the craft’s electronic systems so it could phone home. 

The wait was proving stressful. At 17.30, a one-hour telecommunications window opened. Now was Rosetta’s chance. But three-quarters of an hour later, nothing. “It was a spectacular few minutes of torture,” admitted ESA scientist Martin Kessler. Senior staff paced up and down. Others fiddled with their mobile phones. 

And then, at last, a single spike appeared on the control room monitors. The room erupted as jubilant scientists cheered the awakening of their sleeping beauty. Control staff hugged each other and administrators waved their fists in the air.

“That was the longest hour of my life,” Andrea Accomazzo, Rosetta’s operations manager, admitted. He was not alone. “The spacecraft decided to make us suffer,” said Paulo Ferri, head of ESA’s solar and planetary operations. 

Such anxiety is easy to appreciate. The one billion euro project is one of the most audacious robot space missions ever attempted and has taken two decades to plan, build and launch. Had Rosetta not woken up, all would have been lost and with it an unprecedented chance to demystify something of the beautiful ethereal visitors to our night sky, and also to answer profound questions on the formation of our solar system and the origins of life. 

In the Middle Ages comets were seen as harbingers of doom and destruction. But today’s astronomers take a very different view. They are thought to have terraformed our planet in two fundamental ways. They may have provided it with the first essential ingredient for life: water. They may have also seeded Earth with the organic compounds that are the building blocks of life itself.

A recent series of space missions has transformed our understanding of these astonishing objects. Several probes have intersected a passing comet’s path, whizzing past their targets at colossal speeds to record a few fleeting moments in the lives of these great balls of dust and snow. From these flybys, scientists have glimpsed evidence that comets carry amino acids and other organic chemicals. Might comets have seeded our world with the essential ingredients for life? It is the very question that Rosetta has been designed to answer.

Launched in March 2004, Rosetta will allow scientists to make a long, extremely detailed study of a comet. Rosetta initially launched into a circular orbit round the Sun, but the controllers at Darmstadt have gradually tweaked its path to track the comet (see page 43). As Rosetta swung back toward the Sun on its new elliptical orbit, it was hot on the trail of its quarry, Comet 67P. At present, Rosetta is several million kilometres from its target. However, in May, the spacecraft will begin firing its thrusters and start to close in. By September, the distance between comet and probe will have been cut to 10 km.

For the following 16 months, Rosetta will shadow the comet as closely as any basketball defence while 67P draws closer and closer to the Sun. As the comet warms, water vapour, gases and dust clouds will boil off its surface to create a mighty, luminous tail. And all the time Rosetta will observe the unfolding drama beneath it. Then in November, Rosetta will finally tackle its quarry. It will drop its lander, Philae, right onto the comet’s surface to analyse the composition of its dust and snow, and study the huge geysers of vapour and gas venting into space.

Michael A’Hearn has a long-standing passion for comets, not just because of their beauty and mystery but because they promise to teach us about the origins of the solar system. “These objects are the leftovers from its birth,” says the University of Maryland astronomer and mission member. 

The Sun and planets formed 4.567 billion years ago out of the dust and ice created by exploding stars. From such unpromising material, a wet, dirty cloud slowly coalesced, eventually evolving into the hot disc of material from which the solar system emerged. But at the periphery of the disc, the leftover bits of the cloud mixed together to form comets, creating a flotilla of interplanetary time capsules that scientists have only in the last few decades been able to think of studying up close and personal.

A’Hearn has been along for the entirety of that ride, often in the driver’s seat. Now 73 years old, the white-haired, bushy-bearded astronomer fell in love with comets early on after a career switch from theoretical physics. “I wanted something more practical, more applicable,” he says. Comet chasing fit the bill.

But the astronomer admits that his early involvement with comet missions was not auspicious. In the late 1980s, he worked on the ill-fated US Comet Rendezvous Asteroid Fly-by (Craf) project.

It was meant to follow on the coattails of the success of the world’s first comet catcher, Giotto. Named after the Renaissance painter who depicted a comet in “The Adoration of the Magi”, Europe’s Giotto caught up with the world’s most famous comet, Halley, sweeping past its nucleus in March 1986. Halley was revealed to be a 15 km long peanut-shaped object that possessed three sites on its sunlit side from which jets spewed out several tonnes of water vapour mixed with carbon monoxide and organic material every second. Giotto also revealed that Halley’s body was black rather than white. Until then, scientists had assumed comets were mostly made of a lot of snow and only a little dust.

Craf was a much more ambitious proposal. A’Hearn and his colleagues planned on sending their spacecraft to an asteroid and a comet, flying beside the latter for nearly three years. Too ambitious, it turned out. The mission ran seriously over budget and was cancelled.  Resuming the comet chase took almost two decades. Giotto’s success was finally followed in January 2004 with NASA’s Stardust probe, which flew through the coma – the halo of dust and vapour surrounding the core – of Comet Wild 2. A device containing a foam-like substance called aerogel sprang open to trap dust from the comet’s tail. Dozens of particles, each much smaller than a grain of sand but flying six times faster than a rifle bullet, were caught in the gel. Following this close encounter, Stardust launched the precious dust homeward in a Sample Return Capsule, which entered Earth’s atmosphere on January 16, 2006 and made a safe parachute landing in Utah. 

Scientists anxiously waiting to analyse the capsule’s contents were in for some surprises. At the time, astronomers thought comets were made purely from the frozen leftovers of the material from which the solar system formed. “It was assumed the dust came from other stars and supernovae,” says Stardust’s principal investigator Don Brownlee, a professor of astronomy at the University of Washington, Seattle.

But some of the minerals of aluminium and titanium found in the dust of Comet Wild 2 were not interstellar in origin. They had formed under the white-hot temperatures at the centre of the disc from which our own Sun and planets formed. “I remember presenting our data at the annual Lunar and Planetary Science conference three months after Stardust landed,” says Brownlee. “There were 600 scientists there and I could see their jaws drop round the room.”

Stardust had revealed a strange and unexpected detail about our solar system’s birth. Somehow a huge interstellar conveyor belt had transported dust from its centre back to its edge where it mixed with the primordial ice and dust that already existed there – to form comets such as Wild 2 says Brownlee. Far from being simple balls of ice and dirt, comets were beginning to look like very complex bodies.

The tiny specks of comet suspended in the gel brought back by Stardust produced a second scientific gem: the presence of glycine, an amino acid used by living organisms to make proteins. It was the first time an amino acid had been detected in a comet and it provided tantalising evidence that some of life’s essential ingredients formed not on Earth but in space, and perhaps had been delivered here long ago by comet impacts. 

But a comet’s coma can tell only part of the tale. For it’s next probe, NASA wanted to get inside the heart of a comet. A’Hearn was tasked with leading the mission. Launched in January 2005, Deep Impact reached Comet Tempel that July and fired a 370 kg copper projectile into it. The celestial fireworks blew several thousand tonnes of water, ice and dust into space and created a crater more than 300 feet wide. “One of the best moments of my career,” recalls A’Hearn.

Just as Giotto discovered with Halley’s Comet, the heart of Comet Tempel contained a lot more dust than ice. Evidently comets are not so much dirty snowballs as snowy dirtballs. In addition the dust was finer than anticipated: more like talcum powder than sand. 

“The more we study comets up close, the more variety we find,” says A’Hearn. “Certainly, the notion that there is a single type of comet – as we once thought – is outdated.”

What kind of comet will 67P prove to be? A’Hearn is involved in two experiments on Rosetta to find out: Alice, an ultra-violet imaging spectrometer that will measure gas levels on Comet 67P; and Osiris, a camera that will be used to take high-resolution images of it. 

Other instruments on Rosetta will include a device that will use radio waves to probe the comet’s interior – astronomers still don’t know whether comets are solid or riddled with holes. Another will measure its halo of dust, while a different one will study the comet’s interaction with the solar wind. 

But the most ambitious part of the project is Philae, the little lander that will touch down on the surface of the comet itself. Astronomers have only a narrow window in which to extract as much data as they can – the lander will operate for only two or three weeks before its power runs out.

One of Philae’s most important jobs will be to look for traces of ancient water. It will do it by searching for isotopes of hydrogen. In its early days, our planet was so hot its water would have evaporated into space. Most researchers believe the water we now have on Earth was delivered later on in the planet’s evolution, most probably by comets. Comet hunting has made it possible to test that hypothesis. Earthly water carries a precise signature in its hydrogen isotopes: the ratio of deuterium to hydrogen is precisely 1.56 parts to 10,000. So far measurements made in six comets have produced ratios that are more than twice this figure, casting doubt on comet water as the source of our oceans.

Philae’s first job is to drill into Comet 67P’s surface to collect samples for a mass spectrometer which will measure hydrogen isotopes.

If Comet 67P also carries the wrong ratio of hydrogen isotope, that would be a dampener on the cometary origins of earth’s water. But there is another contender.

What kind of comet will 67P prove to be? A’Hearn is involved in two experiments on Rosetta to find out: Alice, an ultra-violet imaging spectrometer that will measure gas levels on Comet 67P; and Osiris, a camera that will be used to take high-resolution images of it. 

Other instruments on Rosetta will include a device that will use radio waves to probe the comet’s interior – astronomers still don’t know whether comets are solid or riddled with holes. Another will measure its halo of dust, while a different one will study the comet’s interaction with the solar wind. 

But the most ambitious part of the project is Philae, the little lander that will touch down on the surface of the comet itself. Astronomers have only a narrow window in which to extract as much data as they can – the lander will operate for only two or three weeks before its power runs out.

One of Philae’s most important jobs will be to look for traces of ancient water. It will do it by searching for isotopes of hydrogen. In its early days, our planet was so hot its water would have evaporated into space. Most researchers believe the water we now have on Earth was delivered later on in the planet’s evolution, most probably by comets. Comet hunting has made it possible to test that hypothesis. Earthly water carries a precise signature in its hydrogen isotopes: the ratio of deuterium to hydrogen is precisely 1.56 parts to 10,000. So far measurements made in six comets have produced ratios that are more than twice this figure, casting doubt on comet water as the source of our oceans.

Philae’s first job is to drill into Comet 67P’s surface to collect samples for a mass spectrometer which will measure hydrogen isotopes.

If Comet 67P also carries the wrong ratio of hydrogen isotope, that would be a dampener on the cometary origins of earth’s water. But there is another contender.

Anatomy of the landing craft

1. SESAME
Surface Electric Sounding and Acoustic Monitoring Experiment (probing the mechanical and electrical parameters of the comet).

2. CIVA
Comet Nucleus Infrared and Visible Analyser (six cameras to take panoramic pictures of the comet surface).

3. COSAC
The Cometary Sampling and Composition (detecting and identifying complex organic molecules).

4. PTOLEMY
Using MODULUS protocol (Methods of Determining and Understanding Light elements from Unequivocal Stable isotope compositions) to understand the geochemistry of light elements, such as hydrogen, carbon, nitrogen and oxygen.

5. CONSERT
Comet Nucleus Sounding Experiment by Radiowave Transmission (studying the internal structure of the comet nucleus with Rosetta orbiter).

6. SD2
Sampling, drilling and distribution subsystem (drilling up to 23 cm depth and delivering material to onboard instruments for analysis).

7.ROMAP
Rosetta Lander Magnetometer and Plasma Monitor (studying the magnetic field and plasma environment of the comet).

8. MUPUS
Multi-purpose Sensors for Surface and Sub-surface Science (studying the properties of the comet surface and immediate sub-surface).

9. APXS
Alpha Proton X-ray Spectrometer (studying the chemical composition of the landing site and its potential alteration during the comet’s approach to the Sun).

ROLIS
Rosetta Lander Imaging System, providing the first close-up images of the landing site (not visible in image)

This article appeared in Cosmos 56 – Apr-May 2014 under the headline “Comet Landing”