Scientists should get a grandstand view of a comet on Sunday when it makes a dramatic flyby of 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 icy object, known as Siding Spring, will miss the Red Planet by what is - in astronomical terms - just a hair's breadth, or 139,500km (87,000 miles).
The comet, known as Siding Spring, raced past Mars at 56km per second (125,000mph), missing it by 139,500 km.
Satellites round Mars and rovers on its surface hope to capture the event on their cameras and instruments.
Rovers on the Martian surface and satellites were primed to catch the event on their cameras and instruments.
But the orbiters in particular have been commanded to do so from a safe vantage point.
Siding Spring comes from the Oort Cloud - a spherical region of space far beyond the planets.
Fast-moving dust ejected from the comet poses a small but tangible risk to the probes.
Researchers believe the comet is very little altered from the time of its formation more than 4.5 billion years ago.
The space agencies of America, Europe and India have therefore shifted the satellites so that they will be on the far side of the planet when moving past Siding Spring's debris field.
"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.
This is likely to occur about 90 minutes after the comet has made its closest approach to Mars at 18:27 GMT (19:27 BST; 14:27 EDT).
"So think about a comet that started to travel probably at the dawn of man and it's just now coming in.
Long journey
"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."
Scientists are excited because it is really their first opportunity to study up-close one of the true outlier comets of the Solar System.
The icy core, or nucleus, of the comet, also known as C/2013 A1, is only about 1,000m (3,300ft) wide.
Siding Spring, also known as C/2013 A1, comes from what researchers call the Oort Cloud - a huge spherical region of space far beyond the planets and part-way to the nearest stars.
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.
As such, the comet is experiencing the warmth of the Sun for the first time, and that means it is very little altered from the time of its formation more than 4.5 billion years ago.
Other satellites studied its gas and dust shroud, known as the coma, and the material trailing away from it - its tail.
Its pristine condition should provide insights on the materials that went into building the Solar System.
Specifically, they wanted to examine any interactions with the Martian atmosphere.
"Siding Spring probably got knocked into the inner Solar System by the passage of a star near the Oort Cloud," explained Carey Lisse, from the Johns Hopkins University Applied Physics Laboratory, US.
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.
"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 Curiosity and Opportunity rovers were trying to picture Siding Spring in the sky from their surface locations.
The icy core, or nucleus, of the comet is only about one km across, but the American space agency's Mars Reconnaissance Orbiter will try to picture it and resolve its shape - something that has never been done before for an Oort Cloud visitor.
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.
The other satellites will study its gas and dust shroud, known as the coma, and the material trailing away from it - its tail.
Scientists are expected to release their initial findings in the coming days.
Specifically, they will examine any interactions with the Martian atmosphere.
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.
This is likely to heat up very slightly as material from the comet falls on it. Instruments should also detect some transient chemical changes, perhaps even some circulation changes.
The Curiosity and Opportunity rovers will try to picture Siding Spring in the sky from their surface locations.
This will be challenging, but they may well pick up shooting stars as cometary dust grains shoot through the upper atmosphere and burn up.
"The Mars flyby is exciting since we have so many different instruments on so many different spacecraft working together," said Dr Dan Brown, an astronomer at Nottingham Trent University, UK.
"Not one of these was initially designed for such a project, yet the science teams behind them were able to adapt and set up observations that should give us an in depth view into comets, the matter they are made out of and ultimately what made our Solar System.
"In my opinion this shows how science is sometimes driven by chance encounters and relies on flexibility and creativity of the teams supporting them."
Survey improvements
Siding Spring is approaching Mars on a steep angle from south of the ecliptic - the plane on which the planets and other inner Solar System objects sit.
This is one reason scientists know it to be an Oort visitor. Another telltale is its velocity at more than 50km per second.
And that velocity, says University of Maryland's Jessica Sunshine, is a reminder of why we should be vigilant for these types of comet, which tend to announce themselves with only a few months' warning (C/2013 A1 was first seen in January 2013).
The energy they would impart were they to strike our planet would be immense.
"People remember the Chelyabinsk asteroid that hit at maybe 15km per second. This comet is going almost four times that and would be altogether more impressive," Prof Sunshine told the BBC.
"We're getting better at finding these Oort Cloud visitors further out because our telescopes and automatic surveys are getting very good at picking up objects moving across the sky.
"But if this thing were coming to Earth, we'd have a problem because with only a few months' warning, you couldn't really do anything about it."
Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos
