The passage of the comet may affect spacecraft in orbit around Mars, including ESA's Mars Express.
The image on the left above, captured 11 March by the NASA/ESA Hubble Space Telescope, shows comet C/2013 A1, also called Siding Spring, at a distance of 568 million km from Earth. Hubble can't see Siding Spring's icy nucleus because of its diminutive size. The nucleus is surrounded by a glowing dust cloud, or "coma," that measures roughly 19 000 km across.
The right image shows the comet after image processing techniques were applied to remove the hazy glow of the coma revealing what appear to be two jets of dust coming off the location of the nucleus in opposite directions. This observation should allow astronomers to measure the direction of the nucleus’s pole, and axis of rotation.
Hubble also observed Siding Spring on 21 January as Earth was crossing its orbital plane, which is the path the comet takes as it orbits the Sun. This positioning of the two bodies allowed astronomers to determine the speed of the dust 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," said Jian-Yang Li of the Planetary Science Institute in Tucson, Arizona.
The image above shows a series of HST pictures of comet C/2013 A1 Siding Spring as observed on 29 October 2013, 21 January 2014 and 11 March 2014. The distances from Earth were, respectively, 605 million km, 552 million km, and 568 million km. The solid icy nucleus is too small to be resolved by Hubble, but it lies at the center of a dusty coma that is roughly 19 000 km across in these images.
When the glow of the coma is subtracted through image processing, which incorporates a smooth model of the coma's light distribution, Hubble resolves what appear to be two jets of dust coming off the nucleus in opposite directions. The jets have persisted through the three Hubble visits, with their directions in the sky nearly unchanged. These visible-light images were taken with Hubble's Wide Field Camera 3.
Discovered in January 2013 by Robert H. McNaught at Siding Spring Observatory, Australia, the comet is falling toward the Sun along a roughly 1-million-year orbit and is now within the radius of Jupiter's orbit. The comet will make its closest approach to our Sun on 25 October at a distance of 209 million km – well outside of Earth's orbit. The comet is not expected to become bright enough to be seen by the naked eye.
The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency. NASA's Goddard Space Flight Center in Greenbelt, Md., manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy, Inc., in Washington.