Mountain-Sized Asteroid to Make Close Approach to Earth Tomorrow

On June 27th, a colossal asteroid dubbed a "planet killer" will make a notably close pass by Earth, traveling at an astounding 58,000 mph (93,000 km/h). This "potentially hazardous" asteroid, comparable in size to a mountain, marks one of the closest approaches of such a massive space rock in over a century. This event can be witnessed live.

Known as 2011 UL21, this near-Earth asteroid has an orbit that periodically brings it within 1.3 astronomical units (AU) of the sun, or about 1.3 times the average distance between Earth and the sun. It completes an orbit around our star every three years. Based on past observations, 2011 UL21's diameter is estimated to be between 1.1 and 2.4 miles (1.7 to 3.9 kilometers), making it larger than 99% of known near-Earth asteroids, according to the European Space Agency (ESA).

While 2011 UL21 is significantly smaller than the largest space rocks to have impacted Earth, such as the Vredefort asteroid or the one that caused the extinction of the dinosaurs 66 million years ago, it is still classified as "potentially hazardous." Despite its relatively smaller size, it has the potential to cause widespread damage and could release enough debris into the atmosphere to trigger significant climatic changes.

On June 27, 2011 UL21 will pass by Earth at a distance of about 4.1 million miles (6.6 million kilometers), the closest it has been in at least 110 years according to simulations by NASA's Jet Propulsion Laboratory (JPL). Although it remains about 17 times farther away from Earth than the moon, NASA classifies it as a potentially hazardous object due to its proximity.

Although 2011 UL21 poses no immediate threat to Earth, its approach is significant as it is one of the largest asteroids to come within 4.7 million miles (7.5 million kilometers) of our planet since 1900, as noted by Gianluca Masi, an astrophysicist and director of the Virtual Telescope Project (VTP).

You can observe the closest approach of 2011 UL21 through a free livestream provided by the VTP, featuring views from the Bellatrix Astronomical Observatory in Ceccano, Italy. The livestream begins at 4 p.m. ET on June 27, with the closest approach expected approximately 15 minutes later.

For those with suitable telescopes, the asteroid will be brightest on June 28 and June 29, visible from the Northern Hemisphere under optimal conditions. At its peak brightness, it will shine similarly to Proxima Centauri, the nearest known star to the sun, as noted by VTP.

2011 UL21 will not approach Earth this closely again until 2089, when it will come within 1.7 million miles (2.7 million kilometers) of our planet - more than two and a half times closer than its current pass, according to JPL simulations.

While there is no foreseeable risk of a known "planet killer" asteroid colliding with Earth within the next 1,000 years, close encounters with smaller asteroids are anticipated in the coming years. For instance, the asteroid Apophis, large enough to potentially devastate a city, will pass closer to Earth than some satellites in 2029.