SPACE: ‘Once-in-a-millennium’ asteroid flyby will be visible to much of the world in 2029.

Three years before the skyscraper-size asteroid Apophis makes its very close (but safe) flyby of Earth, scientists have already begun charting exactly when and where billions of people can watch it sweep across the sky.

Speaking at an “Apophis T-3 Years” workshop held earlier this month at the University of Padua in Italy, retired cartographer Michael Zeiler and astronomer Rick Fienberg shared detailed visibility maps charting the asteroid’s passage across Earth’s skies.

According to their calculations, roughly 90% of the world’s population — about 7.6 billion people — lives in regions where Apophis could, in principle, be seen with the naked eye on April 13, 2029. The actual viewing success will depend more on earthly considerations, however, including cloud cover and the extent of light pollution.

Known formally as 99942 Apophis, the space rock will not resemble a blazing meteor tearing through the sky. Instead, scientists say it will appear as a point-like speck of light gliding steadily across, which, at its closest approach, will appear to move by about the apparent width of the full moon every minute.

Maybe we should try taking a once-in-a-millennium opportunity to practice diverting a large asteroid.