A comet dubbed Siding Spring will make a close pass by Mars late Sunday in what scientists said is a once-in-a-million-years event.
Siding Spring's nucleus will miss Mars by about 140,000 kilometres and its tail was expected to graze the Red Planet after what scientists estimated was a billions-of-kilometres-long journey to get there.
The US space agency is taking steps to gather valuable data from the event. More than a dozen probes, rovers and telescopes are being deployed to document it.
Siding Spring comes from the Oort Cloud, which is made up of material from the formation of the solar system.
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