The asteroid is classified as a near-Earth asteroid
A massive "planet killer" asteroid is set to make an exceptionally close pass by Earth on Thursday, June 27, speeding past our planet at approximately 58,000 mph (93,000 km/h). This "potentially hazardous" object, comparable in size to a mountain, represents one of the largest space rocks to come this close to Earth in over a century - and you can witness this near encounter live.
Known as 2011 UL21, the asteroid is classified as a near-Earth asteroid, meaning its orbit 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 once every three years. According to SpaceReference.org, estimates based on past observations place its diameter between 1.1 and 2.4 miles (1.7 to 3.9 kilometres), making it larger than 99% of known near-Earth asteroids, as noted by the European Space Agency(ESA).
2011 UL21 is estimated to be at least ten times smaller than the largest space rock to ever impact Earth, such as the Vredefort asteroid, and about five times smaller than the asteroid believed to have caused the extinction of dinosaurs approximately 66 million years ago.
Despite its smaller size compared to these historical impactors, 2011 UL21 is still classified as 'potentially hazardous' due to its potential for causing widespread damage. It has the potential to cause continental-scale damage and release enough debris into the atmosphere to induce significant climatic changes.
On June 27, 2011 UL21 will pass by Earth at a distance of approximately 4.1 million miles (6.6 million km), closer than it has been in at least 110 years according to simulations by NASA's Jet Propulsion Laboratory (JPL). At this proximity, NASA classifies it as a potentially hazardous object, although it remains about 17 times farther away from Earth than the moon.
While 2011 UL21 does not currently pose a threat to Earth, it is noteworthy as one of the largest asteroids to approach within 4.7 million miles (7.5 million km) of our planet since 1900, as highlighted 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 km) of our planet - more than two and a half times closer than its current pass, according to JPL simulations.
There is no foreseeable risk of a known "planet killer" asteroid colliding with Earth within the next 1,000 years. However, close encounters with smaller asteroids are anticipated in the coming years, such as the asteroid Apophis, which is large enough to potentially devastate a city and will pass closer to Earth than some satellites in 2029.