Rosetta Update: The Scent of a Comet

Days from now, the Philae lander will drop onto Comet 67P/Churyumov-Gerasimenko. In the meantime, the "mother ship" Rosetta is watching the icy nucleus — and smelling the mix of gases escaping from it.

Philae landing art on Comet Comet 67P/Churyumov-Gerasimenko

An artist's portrayal of the moment when Rosetta's Philae lander touches down on Comet 67P/Churyumov-Gerasimenko.
ESA / ATG medialab

In less than a week, teams of comet scientists in Europe and elsewhere will witness the culmination of an effort begun by the European Space Agency nearly two decades ago. A small instrumented probe, named Philae, will descend toward the icy nucleus of Comet 67P/Churyumov-Gerasimenko and gently drop onto its surface — becoming the first spacecraft to land on a comet. (Here's the timetable for the landing on November 12th.) ESA engineers are hoping Philae will operate there for several weeks.

Amid all this drama, however, a second mission is taking place. Rosetta, the washing-machine-size "mother ship", has been slowly orbiting the comet's irregularly shaped nucleus since mid-August. It's been busy for the past 2½ months, not only assessing the terrain for an optimum landing site but also taking stock of the nucleus and its activity.

Rosetta's scientific payload consists of 10 instruments — 11 if you count the craft's radio transmitter, which plays a crucial role in determining the shape and mass of the nucleus. Public interest to date has focused on OSIRIS, the German-built Optical, Spectroscopic, and Infrared Remote Imaging System. The camera's two lenses, 140 and 700 mm in focal length, have provided all of the dramatic views seen to date.

Rosetta instrument schematic

ESA's Rosetta spacecraft bristles with scientific instruments to study the physical and chemical characteristics of Comet 67P/Churyumov-Gerasimenko.
European Space Agency

Other instruments include an ultraviolet imaging spectrograph (called Alice, one of NASA's three contributions), a visible/infrared spectrometer (VIRTIS), a microwave spectrometer (MIRO), and a radar sounder (CONSERT). Four more devices are sampling gas and particles escaping from the nucleus: a mass spectrometer (ROSINA), a microscope (MIDAS) and mass spectrometer (COSIMA) for analyzing captured dust particles, and a detector (GIADA) that assesses the momentum, velocity, and mass of dust particles. A cluster of five sensors (collectively called RPC) will track the comet's interaction with the solar wind.

Most of these investigations won't ramp up until the 2½-mile-long (4-km) comet gets much more active as it nears the perihelion of its orbit next August.

But even now faint jets of gas and dust are escaping into space, and recently the ROSINA team released some its early findings. Led by Hans Balsiger and Kathrin Altwegg (University of Bern), the researchers find that the comet's "perfume" is already quite strong.

Jets from Comet 67P seen by Rosetta

On September 10, 2014, Rosetta's wide-angle camera captured jets of gas escaping from the nucleus of Comet 67P/Churyumov-Gerasimenko (overexposed). The comet should become much more active in the months ahead.
ESA / OSIRIS Team

So what exactly does this comet smell like? Most of the gas escaping into the cloud-like coma from the nucleus consists of water and carbon monoxide, both of which are odorless. But there's also a noxious mixture of trace compounds: whiffs of rotten eggs (from hydrogen sulphide), horse manure (ammonia), and brain-pickling formaldehyde. ROSINA's discerning "nostrils" have also detected hints of almond (hydrogen cyanide) and alcohol (methanol, the toxic variety). A vinegar-like scent from sulfur dioxide is balanced by the sweeter aroma of carbon disulphide.

That's quite a brew — and getting all these whiffs while the comet is still so far from the Sun (3.0 astronomical units, well outside the orbit of Mars) comes as a surprise. "This all makes a scientifically enormously interesting mixture in order to study the origin of our solar-system material, the formation of our Earth, and the origin of life," Altwegg notes.

ROSINA isn't the first attempt to deduce a comet's composition at close range. On March 13, 1986, ESA's Giotto spacecraft flew deep into the coma of Comet 1P/Halley. Three spectrometers on board deduced a gas composition of water (80% by volume), carbon monoxide (10%), carbon dioxide (2.5%), methane, ammonia, and traces of other hydrocarbons. Analysis of Halley's coal-dark dust yielded an intriguing surprise: an abundance of "CHON" particles that consisted entirely of the elements carbon, hydrogen, oxygen, and nitrogen.

No doubt Rosetta and Philae will find Comet 67P/Churyumov-Gerasimenko equally surprising in the days and weeks ahead.

By the way, if you've grown tired of boring animations showing where various spacecraft are headed and what'll happen once they get there, then jump over to YouTube to view Ambition, the Film — a futuristic, 5-minute-long teaser for the Rosetta mission. Watch it in HD, and crank up the volume!


Read all about the Rosetta mission in Sky & Telescope's August issue.