Bi-lobed Comet Nucleus Tumbles Along in Its Orbit

When the Rosetta mission planners selected Comet 67P/Churyumov–Gerasimenko (67P) as a rendezvous target, they probably didn’t expect the tumbling, rubber ducky-shaped blob of ice and dust they’re now seeing as the spacecraft gets closer to its August 6th arrival at the comet. The past few weeks have been a parade of discoveries at the comet, and today astronomers announced that they’ve seen the coma really start to thicken up around the nucleus. New images from July 25th show a distinctly blobby coma.

Comet 67P has been slowly getting more “active” as it experiences more heat from the Sun, and in April astronomers began seeing evidence of dust production. This happens as the ices (volatiles) start to sublimate (sort of like watching dry ice in sunlight) and they carry along dust and rocky bits as they move away from the comet.  Eventually, that coma should thicken and stretch out to form a dust tail (and presumably a plasma tail, too).  When that happens, Rosetta, and a small lander called Philae, will be there to measure and report on all the action. The lander will also take samples of the comet to study. What it finds will help astronomers understand the comet’s chemical makeup, which also gives them insight into the conditions in the solar system as the comet formed (some 4.5 billion years ago).

The comet, along with Rosetta and Philae, will make its nearest approach to the Sun in August 2015. That point, called the comet’s perihelion, lies between the orbit of Earth and Mars. The comet was nudged into its current orbit by a close encounter with the planet Jupiter in 1959. Before that, it never came closer than a point somewhere near the Asteroid Belt (around 2.7 astronomical units (2.7 times the distance between Earth and the Sun)).

The Rosetta mission was designed to study all aspects of a comet. This includes using a probe to sample the surface and near subsurface of the nucleus. It’s important to get good images, since the comet’s icy body is tough to “see” as it grows a coma. However, there are other aspects of a comet that also tell scientists quite a bit about its makeup and history. One of the most important is to determine the changes in production rate of water vapor and other gases from a comet as it moves in its orbit, particularly as it begins to “feel” the influence of the Sun.  It turns out that Comet 67P’s nucleus is a sweaty one, losing the equivalent of a couple of glassfuls of water each second!  That rate will likely get higher as the comet nears the Sun, when it could lose many times that much water vapor. Water is a big part of a comet’s nucleus — one of the so-called “volatiles” that outgas as comets experience increased solar heating. The other volatiles are carbon monoxide, methanol, and ammonia. The Rosetta spacecraft has an instrument called the Microwave Instrument for Rosetta Orbiter (MIRO) that measures the volatiles sweating off the comet. The Optical, Spectroscopic and Infrared Remote Imaging System (OSIRIS)  gives astronomers a sophisticated way to take both wide- and narrow-angle views of the comet.  Along with ultraviolet spectrometer, radio-wave instrument, and other subsystems, Rosetta is giving the world a multi-wavelength way to study a comet up-close and personal.

August 6th will be a busy day for the Rosetta team, and I’m sure a LOT of people will be following the progress of the mission. If you want to read more about Rosetta and its cometary target, check out the mission web pages and the Max Planck Institute for Solar System Research’s news updates. Press briefings will be live-streamed, so if you’re interested in real-time information, check ’em out!