About the size of a football field: The impact crater left behind
A team of geologists believes it has found the incoming space rock's impact crater, and dating suggests its formation coincided with the celestial vision said to have converted a future Roman emperor to Christianity.
It was just before a decisive battle for control of Rome and the empire that Constantine saw a blazing light cross the sky and attributed his subsequent victory to divine help from a Christian God.
Constantine went on to consolidate his grip on power and ordered that persecution of Christians cease and their religion receive official status.
Civil war
In the fourth century AD, the fragmented Roman Empire was being further torn apart by civil war. Constantine and Maxentius were bitterly fighting to be the sole emperor.
Constantine was the son of the western emperor Constantius Chlorus. When he died in 306, his father's troops proclaimed Constantine emperor.
But in Rome, the favourite was Maxentius, son of Constantius' predecessor, Maximian.
With both men claiming the title, a conference was called in AD 308 that resulted in Maxentius being named as senior emperor along with Galerius, his father-in-law. Constantine was to be a Caesar, or junior emperor.
The situation was not a stable one, however, and by 312 the two men were at war.
Constantine overran Italy and faced Maxentius at the Milvian Bridge over the Tiber a few kilometres from Rome. Both knew it would be a decisive battle with Constantine's forces outnumbered.
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