A 1.6-kilometer-wide (1-mile) “potentially hazardous” asteroid system passed by Earth over the weekend.

1999 KW4 is a binary system – two space objects close enough in proximity to orbit each other – that was first discovered on May 20, 1999, by the Lincoln Near-Earth Asteroid Research (LINEAR) project in New Mexico. Observations over the last two decades have marked the near-Earth object (NEO) as “potentially hazardous” given its small chance of colliding with Earth. Fortunately for us, this time it came within a relatively safe distance of around 5 million kilometers (3 million miles) from Earth. It's travelling at some 77,200 kilometers (48,000 miles) per hour, making its closest approach at 7:05 pm EST on May 25, and won’t be that close again until 2036. Eventually, its path could collide with Earth, but radar measurements indicate no significant chance of such an event anytime in the next millennium.

The asteroid-pair is unique in its characteristics. The main object spins like a top with a ridge around its equator, according to NASA. Its small moon, roughly one-third the size of Earth, orbits 2.6 kilometers (1.6 miles) every 16 hours. Las Cumbres Observatory says the main object’s shape is “slightly squashed at the poles and with a mountain ridge around the equator, which runs all the way around the asteroid. This ridge gives the primary an appearance similar to a walnut or a spinning top.”

Astronomers know of more than 20,000 NEOs, over 1,800 of which are considered potentially hazardous, but most are small. Space experts don’t typically study NEOs unless it’s important to the safety of Earth. As such,  experts earlier this year launched the International Asteroid Warning Network (IAWN) campaign to test our capability of characterizing and preparing for a hazardous asteroid. NASA and several federal agencies, as well as a handful of international organizations, executed a simulation that mimicked what would happen if an asteroid was set to collide with Earth (hint: it did not end well for New York City).