Watch Livestream of Potentially Hazardous 4,000ft Asteroid Passing Earth

An asteroid that could have a diameter as great as 4265 feet is headed towards a close approach to Earth on March 4. Though asteroid (138971) 2001 CB21 will safely miss our planet, that doesn't mean skywatchers will miss out.

The Virtual Telescope Project 2.0 is giving space enthusiasts the opportunity to observe (138971) 2001 CB21 as it passes Earth at a distance of around 3 million miles safely from the comfort of their own home.

Though this may seem like a great distance, it is close enough for NASA to classify it as a Near-Earth Asteroid. It is also large enough to be classified as a Potentially Hazardous Object, even though there is little chance of it hitting Earth in the next 100 years.

The live feed from the telescope will begin at 10 pm ET (03:00 UTC) on March 4, 2022, on Virtual Telescope's WEBTV channel.

For anyone who wants to see the object's approach to Earth, the asteroid can be tracked virtually at NASA Jet Propulsion Laboratory's Small-Body Database Lookup website. Using the simulator it is possible to wind the orbit of (138971) 2001 CB21 to see when it will cross paths with Earth.

(138971) 2001 CB21 on February 7
An image showing where the asteroid (138971) 2001 CB21 is in space in relation to earth on February 7, 2023. Small-Body Database Lookup/NASA

According to NASA's Center for Near-Earth Object Studies (CNEOS), when (138971) 2001 CB21 makes its close approach to Earth it will be traveling at about 7.5 miles per second.

That is about 13 times faster than a bullet fired by an M16 rifle or 18 times faster than the maximum speed of a Lockheed Martin F-16 jet fighter.

CNEOS has calculated the orbit of (138971) 2001 CB21 for the century at least, and while it takes just 384 days (1.05 years) to orbit the sun, it won't come this close to earth again for some time.

The next time(138971) 2001 CB21 makes a close approach to Earth within around 3 million miles will be March 6, 2043, when it will come within 2.99 million miles of our planet.

CNEOS estimates a diameter for (138971) 2001 CB21 of between 1837 and 4265 feet. The reason for this disparity is down to how astronomers estimate the sizes of these space rocks.

How Do NASA Estimate Asteroid Sizes?

Determining Asteroid Sizes
A NASA diagram shows how astronomers calculate the diameters of asteroids. CNEOS say an asteroid with a diameter of between 330 meters and 750 meters will pass Earth on January 7, 2022. NASA

To determine the diameters of asteroids, astronomers measure the visible light from the sun reflected off its surface, a measure known as albedo.

Size isn't the only determining factor in how much light an asteroid reflects, however. It can also depend on the material from which the asteroid is composed and how loose or tightly packed that material is at the object's surface

Albedo also depends on the color of the asteroid, so a lighter "chalky" asteroid that has looser or dusty material at its surface will appear deceptively large to astronomers, while an asteroid with a charcoal-like surface will reflect less light, thus appearing smaller than it actually is.

So, from millions of miles away, a large and dark asteroid could appear to be the same size as a smaller, lighter colored one.

Another way of estimating an asteroid's size involves measuring the amount of heat it gives off as it travels through space by observing it in infrared light.

This could be a better indicator of size than reflected visible light, as a larger object would appear brighter in infrared light, unaffected by the amount of visible light it reflects.

According to NASA's Jet Propulsion Laboratory at the California Institute of Technology, which operates CNEOS, the best way to determine asteroid size is a combination of observing the visual light it reflects and measuring its infrared signature.

Asteroid and Earth
An illustration of an asteroid passing earth. An asteroid with an estimated size of up to 4200 feet will pass earth on March 6, 2022. Ratpack 223/GETTY