This Sunday, something historical will happen: An ancient rare comet will arrive to Mars after millions of years travelling at 53km/s from the Oort cloud. It will look like you can see above, passing just within a third of the distance from the Earth to the moon, engulfing the Red Planet in its large tail.
This is a simulation of how the comet will look from the Victoria crater when it is on its closest point to Mars:
NASA is ready to record this historic event with all the weapons in its arsenal:
Comets from the Oort cloud are both ancient and rare. Since this is Comet Siding Spring's first trip through the inner solar system, scientists are excited to learn more about its composition and the effects of its gas and dust on the Mars upper atmosphere. NASA will be watching closely before, during, and after the flyby with its entire fleet of Mars orbiters and rovers, along with the Hubble Space Telescope and dozens of instruments on Earth. The encounter is certain to teach us more about Oort cloud comets, the Martian atmosphere, and the solar system's earliest ingredients.
There's no danger to Curiosity on the surface of Mars, since it will be protected by the planet's thin atmosphere, but NASA says that there's a risk for the fleet spacecraft now orbiting around the Red Planet — the particles from the tail may damage some of the instruments on board these ships. One of those spacecraft is Maven, which you can see in this simulation of the flyby from orbit:
Watch NASA's narrated video with the complete explanation: