What to Know About the Massive 'Planet Killer' Asteroid Passing Near Earth

On June 27, a massive asteroid the likes of which are rarely seen on this side of the solar system will be making a very close approach to Earth. The event will providing a great opportunity for millions to learn about similar space rocks and what could happen in a potentially catastrophic asteroid collision. 

How big is the asteroid?

2011 UL21 orbits the sun once every three years and is estimated to be somewhere between 1.1 and 2.4 miles wide, per SpaceReference. This makes it larger than 99 percent of known near-Earth asteroids according to the European Space Agency. It's been designated as a "potentially hazardous asteroid" due to its size. 

"The term 'Potentially Hazardous Asteroid' (PHA) is a precise formal definition, referring to minor planets larger than approximately 140 meters that can come within 7.5 million kilometers from the Earth," Virtual Telescope Project scientific director Gianluca Masi said in a statement. “In other words, only the largest asteroids capable of approaching close enough to our planet are flagged as PHAs, which does not mean they are going to hit the Earth, but they nonetheless warrant a better monitoring."

How close will the 2011 UL21 asteroid come to Earth?

The space rock is expected to pass our planet at around 58,000 miles per hour. It will come within 4.1 million miles of Earth, according to NASA's Jet Propulsion Laboratory, thereby qualifying it as a potentially hazardous object. Still, it will be approximately 17 times farther away from Earth than the moon. 

Is the 2011 UL21 asteroid the biggest to approach Earth?

2011 UL21 is big, but it pales in comparison to other space rocks that have had an impact on Earth history. It's likely at least 10 times smaller than the Vredefort asteroid, the largest space rock to ever hit Earth, and also around five times smaller than the space rock that wiped out the dinosaurs around 66 million years ago. 

However, while any potential impact from 2011 UL21 would be less extreme than these historic collisions, the asteroid still has the potential to cause damage on a continental scale and eject enough debris into the atmosphere to trigger significant climatic changes, which is why it is considered a "planet killer."

Does the asteroid pose a threat?

While 2011 UL21 isn't projected to collide with Earth anytime soon, its sheer size is enough to permanently alter the course of planetary history if it did so. Asteroids of this size are sometimes referred to as "planet killers" for this reason, as the damage could reach across continents and release enough debris into the atmosphere to cause major environmental changes around the globe. 

How can you watch the asteroid's approach live?

If you're interested in watching the approach, you can tune in to a free livestream from the Virtual Telescope Project that will show the view of the asteroid from the Bellatrix Astronomical Observatory in Ceccano, Italy. The stream begins June 27 at 4 p.m. ET and the close approach is expected to happen around 15 minutes later.  

If you have a good telescope, you might even be able to see the asteroid just by looking up at night. It will be at its brightest on June 28 and 29 and visible across the Northern Hemisphere. 

When will it approach Earth again?

2011 UL21 won't get this close to Earth again until 2089. At that time, the asteroid will get within 1.7 million miles of Earth, more than two and a half times closer than it will get this week.