YR4: everything to know about the potential "city killer" space rock that has a 2.3% chance of striking Earth on Dec. 22, 2032.

Scientists say the threat of a newly discovered asteroid has risen slightly in the past few weeks, as the world's telescopes rush to track its course.

YR4 has a 2% chance of hitting Earth in 2032. Astronomers are tracking it to refine estimates of its size and orbit to see how much of a risk the space rock poses.