Comet C/2013 A1, also called Siding Spring, will buzz by Mars on October 19th, at a distance of just 86,000 miles. This is just one-third of the distance between the Earth and the Moon.
Now, the Hubble Space Telescope has observed at least two tails emanating from the comet. These tails appear to sprout from opposite sides of Siding Spring.
Comets are large, dirty snowballs, composed of frozen carbon dioxide and water ice, mixed with dust and rock. Billions of these objects exist in the outer solar system, making up the Oort cloud. Occasionally, two comets will collide, sending one or both members plunging toward the Sun. Some collide with our companion star and others fall apart on their first trip around the Sun, as Comet ISON did at the end of 2013. A few fall into steady orbits and make regular trips around the solar system.
As comets approach the Sun, they warm and ice begins to turn into gas which leaves the nucleus, or main body of the comet. This "atmosphere" around the nucleus is known as the coma. Pressure from particles streaming from the Sun pushes the gas back, forming the distinct tails that are a hallmark of comets.
The comet was 353 million miles just inside the orbit of Jupiter when the Hubble Space Telescope turned its gaze toward the visitor from the distant Oort Cloud.
The twin tails of Siding Spring were revealed in image processing. Pictures of the comet taken by the Hubble Space Telescope did not immediately show the features. After processing to remove glow from the nucleus, a pair of tails composed of dust, became apparent. When C/2013 A1 passes Mars, spacecraft orbiting the red planet may be enveloped in this material.
Hubble photographed the comet just as the Earth was perfectly aligned to measure the speed of gas coming off the nucleus.
"This is critical information that we need to determine whether, and to what degree, dust grains in the coma of the comet will impact Mars and spacecraft in the vicinity of Mars," Jian-Yang Li of the Planetary Science Institute said.
Comet Siding Spring was named after the observatory in Australia where it was first discovered by astronomer Robert McNaught, in July 2013.
The object takes one million years to travel once around the Sun. Amateur astronomers will get to see Siding Spring this autumn, using binoculars. The comet will come within 130 million miles of our home planet.
At the time of closest approach between the comet and Mars, cameras aboard spacecraft on the red planet will be focused on the celestial visitor.