Comet Siding Spring skims past Mars
A recently discovered comet has whizzed past Mars, giving scientists a unique chance to study an object from the farthest reaches of the Solar System.
The comet, known as Siding Spring, raced past Mars at 56km per second (125,000mph), missing it by 139,500 km.
Rovers on the Martian surface and satellites were primed to catch the event on their cameras and instruments.
Siding Spring comes from the Oort Cloud - a spherical region of space far beyond the planets.
Researchers believe the comet is very little altered from the time of its formation more than 4.5 billion years ago.
"Siding Spring probably got knocked into the inner Solar System by the passage of a star near the Oort Cloud," said Carey Lisse, from the Johns Hopkins University Applied Physics Laboratory, US.
"So think about a comet that started to travel probably at the dawn of man and it's just now coming in.
"And the reason we can actually observe it is because we've built satellites and rovers and we've now got these outposts at Mars. That's pretty exciting."
The icy core, or nucleus, of the comet, also known as C/2013 A1, is only about 1,000m (3,300ft) wide.
Nasa's Mars Reconnaissance Orbiter was primed try to picture it and resolve its shape - something that has never been done before for an Oort Cloud visitor.
Other satellites studied its gas and dust shroud, known as the coma, and the material trailing away from it - its tail.
Specifically, they wanted to examine any interactions with the Martian atmosphere.
This is likely to have heated up ever so slightly as material from the comet fell on it. Instruments should also have detected some transient chemical changes, perhaps even some circulation changes.
The Curiosity and Opportunity rovers were trying to picture Siding Spring in the sky from their surface locations.
This would have been challenging, but they may well have spotted shooting stars as cometary dust grains sped through the upper atmosphere and burned up.
Scientists are expected to release their initial findings in the coming days.
The comet was first identified by Robert McNaught in January 2013 while using the Uppsala Southern Schmidt Telescope at the Siding Spring Observatory near Coonabarabran, New South Wales, Australia. It is this association that gives C/2013 A1 its common name.