Montage of photos showing Comet 67P (left) and the comet's coma as seen with the OSIRIS wide-angle camera (right). (Photo : European Space Agency)
The fiery "tail" emerging from the coma or halo of comets are their most distinctive feature as seen from the Earth. Comet 67P/Churyumov-Gerasimenko's tail is growing and should be at its largest by mid-August.
Four months from today, or on August 13, Comet 67P will reach perihelion, or the moment it's closest to the Sun along its orbit. This will place the comet being orbited by the Rosetta probe of the European Space Agency (ESA) some 185 million kilometers from the Sun between the orbits of Earth and Mars.
Rosetta will watch the gradual evolution of the comet as it forms its coma. It began orbiting the comet on August 2014 and landed a robotic probe named Philae on the comet's surface on Nov. 12. The probe has since gone missing but ESA hopes perihelion will allow the dormant probe to receive enough sunlight to again communicate with the Earth.
The dramatic expansion of the coma over the next few months will not endanger the orbiting Rosetta probe since most of the material comprising the coma will be ice and dust. Water dominates up to 90 percent of the material that outflows from the comet's surface.
The coma is the nebulous envelope around the nucleus of a comet. It forms when the comet passes close to the Sun on its highly elliptical orbit. As the comet warms, its ices sublimate or changes from solid to gas without passing through the liquid phase.
These escaping gases carry streams of dust out into space, and together these slowly expand to create the comet's fuzzy atmosphere, or coma.
As the comet continues to move closer to the Sun, the warming continues and activity rises. Pressure from the solar wind causes some of the materials to stream out into long tails, one made of gas, the other of dust.
The comet's coma will eventually span tens of thousands of kilometers, while the tails may extend hundreds of thousands of kilometers. Both will be visible through large telescopes on Earth, said ESA.
But it is Rosetta's close study of the comet, from just a few tens of kilometers above its surface that enables the source of the comet's activity to be studied in great detail, providing context to the more distant ground-based observations.
ESA also released a spectacular montage of 18 images showing the comet's activity from many different angles as seen between January 31 (top left) and March 25 (bottom right) when the spacecraft was at distances of about 30 to 100 km from the comet.
At the same time, Comet 67P was at distances between 363 million and 300 million km from the Sun.
After perihelion, Rosetta will continue to follow the comet, watching how the activity subsides as it moves away from the Sun and back to the outer solar system again.
