The Hubble Space Telescope snapped a portrait of Comet C/2013, alias Siding Spring. The comet is projected to whisper past Mars, and astronomers are trying to predict if our Martian explorers can expect a fresh coating of comet-dust in October

The Hubble Space Telescope photographed Siding Springs on March 11th, after the comet had passed within Jupiter's orbit. Edging closer to the sun is ice to sublimate and dust to be released from the nucleus, forming a glowing cloud called a coma. This coma utterly blinds any attempt to peek at finer details of the comet (left), but the glow-glare can be tamed during processing (right).

We still can't see the nucleus — it is but a wee speck in the 20,000 kilometer-wide coma and we are 568 million kilometers away. But, you can spot a pair of jets coming out of the nucleus in opposite directions, spouting more sublimated ice and freed dust into the coma.

When we last looked in on Siding Spring on January 21st, the comet was crossing our orbital plane. From those observations, astronomers calculated the speed of dust coming off the nucleus. The new observations of the dual jets from the nucleus will help astronomers figure out the direction of the nucleus's pole, and the comet's axis of rotation.


Why so much fuss over a comet that will probably never be naked-eye visible? Siding Spring is going to get very, very close to Mars on October 19th, 2014. At just over 135,000 kilometers between the comet and Mars, that's less than half the distance between the Earth and moon, and well close enough to sprinkle some of our Martian explorers in comet dust. By calculating out as much as we can about this comet and it's tail, astronomers will be able to determine if any of this dust will be hitching a ride on our spacecraft, rovers, or the red planet itself.


After its Martian encounter, Siding Spring will make it's closest approach to the sun on October 25th, staying well outside Earth's orbit with a perihelion of 209 million kilometers. It will then start it's journey back through the outer solar system, and out into the distant reaches of Oort Cloud to restart its million year orbit.

Image credit: NASA, where you can also learn more about comets. Yes, the front page covered this a few minutes ago. Bah! An earlier version of this article referred to the comet as "Sliding Spring," not "Siding Spring." Thank you to Benjamin Pope for the correction!