ESA Rosetta Image: Coma surrounds comet 67P/Churyumov-Gerasimenko

The coma of comet 67P/Churyumov-Gerasimenko as seen by OSIRIS camera on board ESA Rosetta. 

The coma covers an area of 150 kilometers across. 

This image was taken on July 25th, 2014 with an exposure time of 330 seconds. 

 The hazy circular structure on the right and the center of the coma are artifacts due to overexposure of the nucleus. 

Credit: ESA/Rosetta/MPS for OSIRIS Team MPS /UPD /LAM /IAA /SSO /INTA /UPM /DASP /IDA

Less than a week before ESA Rosetta's rendezvous with comet 67P/Churyumov-Gerasimenko, images obtained by OSIRIS, the spacecraft's onboard scientific imaging system, show clear signs of a coma surrounding the comet's nucleus.

While the OSIRIS' view of the coma covers an area of 150 kilometers across, its outskirts might reach much farther.

"Even though it sounds like a contradiction, imaging the comet's coma from nearby is more difficult than from far away", says OSIRIS Principal Investigator Holger Sierks from the Max Planck Institute for Solar System Research (MPS) in Germany.

At the end of April, for example, OSIRIS had witnessed a distinct rise of cometary dust production from a distance of more than two million kilometers.

At that time, one pixel in OSIRIS' images corresponded to a region in space covering 2500 square kilometers at the nucleus.

ESA Rosetta's OSIRIS camera

The reflected light from all dust particles seen in this column worked together to create a signal.

Now, as the resolution of OSIRIS images increases, a much smaller region and thus far less dust particles contributes to one pixel.

Nevertheless, a new image dating from July 25th clearly reveals an extended coma shrouding 67P's nucleus.

"Our coma images cover an area of 150 by 150 square kilometers", says Luisa Lara from the Instituto de Astrofísica de Andalucía.

However, most likely these images show only the inner part of the coma, where particle densities are highest. Scientists expect 67P's full coma to actually reach much farther.

Another challenge for OSIRIS is the bright nucleus that outshines the surrounding coma.

While OSIRIS is designed to deal with a controlled overexposure in the region of the nucleus, the stray light from this strong source causes artifacts by the optical system.

In the current image, the hazy bright circular structure to the right of the comet's nucleus is such an artifact. The center of the image located around the position of the nucleus is obscured by overexposure.

The nucleus of comet 67P/Churyumov-Gerasimenko as seen from a distance of 1950 kilometers on July 29th, 2014. 

One pixel corresponds to approximately 37 meters. 

The bright neck region between the comet’s head and body is becoming more and more distinct. 

Credit: ESA/Rosetta/MPS for OSIRIS Team MPS /UPD /LAM /IAA /SSO /INTA /UPM /DASP /IDA

In the next weeks, the OSIRIS team will study how the comet's activity has developed so far in approach.

To this end, data obtained from different distances and with different exposure times need to be correlated.

Meanwhile, new images of the comet's nucleus confirm the collar-like appearance of the neck region which presents itself brighter than most parts of the comet's body and head.

The reason for this feature is still subject to discussion. Possible explanations range from differences in material or grain size to topological effects.

Rosetta is an ESA mission with contributions from its member states and NASA.

Rosetta's Philae lander is provided by a consortium led by DLR, MPS, CNES and ASI.

Rosetta will be the first mission in history to rendezvous with a comet, escort it as it orbits the Sun, and deploy a lander to its surface.

Further Mystery Surrounds Plague Death of Scientist

A follow up Story to the Death of a Chicago Scientist caused by Plague - Previous Story

Before Malcolm Casadaban died on September 13, the 60-year-old microbiologist at the University of Chicago had been suffering from flu-like symptoms.

Another victim of swine flu? Perhaps - but an autopsy revealed he had plague bacteria in his blood.

Casadaban was reported to have been working with a weakened vaccine strain of Yersinia pestis, intended for the development of vaccines against plague.

However, doctors are not sure the bacilli in Casadaban's blood were the cause of death, or how they got there. So what's going on?

Y. pestis has been linked to the medieval Black Death, though proof has been elusive. We do know that it kills several thousand a year by causing bubonic plague, and it is considered a potential biological weapon.

The University of Chicago has not revealed which strain the Yersinia was, but says it should have been safe:

This strain is not known to cause illness in healthy adults and has been used in some countries as a live-attenuated vaccine to protect against plague. It has been approved by the Centers for Disease Control and Prevention (CDC) for routine laboratory studies. The weakened strain does not require the special safety precautions required for work with virulent strains.

But the case reminds me of misgivings that have been voiced about weakened strains, which might not be as safe as they seem.

The strain seems likely to have been EV76, which is used as a vaccine in Russia and Madagascar. It is considered unlikely to revert to the virulent strain - but even without reverting, it kills some mice immunised with it.

Worse, up to a fifth of people vaccinated with EV76 develop flu symptoms such as fever, headache, weakness and malaise, according to Rick Titball of the University of Exeter, UK, a leading Yersinia expert. Some even need hospitalisation, he says, though no deaths have ever been reported that we know of.

Of course, we don't yet know whether these findings have any bearing on Casadaban's tragic death. For that we'll need to which strain was in his blood and whether it was the bacteria that killed him.