Just how many threatening asteroids are there? It's complicated. - Space.com
Nov 22, 2021 2 mins, 36 secs

"We're racking up the numbers for these populations, but at the same time, there is no known threat right now to Earth," Kelly Fast, who is a near-Earth object observations program manager at NASA's Planetary Defense Coordination Office, told Space.com.

"There's nothing, there's no asteroid that we know of that poses a significant threat to Earth.".

The art of protecting Earth from an asteroid impact is called planetary defense, and there are two key stages to the process.

NASA's Double Asteroid Redirection Test (DART), launching later this month, is a mission designed to test the second stage of planetary defense, diverting a threatening asteroid from crossing paths with Earth.

"People might think planetary defense is all about deflecting asteroids but it's not," Nancy Chabot, a planetary scientist at the Johns Hopkins University Applied Physics Laboratory in Maryland and the coordination lead for DART, told Space.com.

Fortunately, plenty of those stay far, far from Earth — consider, for example, residents of the main asteroid belt or the Trojan asteroids that flank Jupiter in its orbit.

Related: If an asteroid really threatened the Earth, what would a planetary defense mission look like.

Today, there's a whole host of projects that detect near-Earth asteroids, whether it's their top priority or an opportunity they can make use of.

But surveys on their own aren't enough for planetary defense experts — follow-up observations are crucial to give scientists the data they need to accurately calculate an object's orbit.

"You want to know the asteroid's there, but you really want to know where it's going to be in the future and whether Earth is going to be in the same place at the same time.".

If all those observations find that an asteroid is over a certain brightness (which suggests a certain size, although the two factors don't correlate precisely) and will come within 4.65 million miles (7.48 million kilometers) of Earth, the object is automatically dubbed a "potentially hazardous asteroid." (The distance works out to one-twentieth of the average distance between Earth and the sun.).

But in most cases, despite the ominous terminology, "potentially hazardous asteroids" may as well be called "not currently hazardous asteroids." After all, these are the objects that scientists have already found, and followed, and mapped, and forecast into the future.

So it's the middle size category of asteroids — those more than 460 feet (140 meters) but less than 3,300 feet wide — that most worries planetary defense experts.

While that number is impressive, NASA's planetary defense office estimates that at the current pace, it will take scientists 30 more years to have identified 90% of objects this size, a goal that Congress asked NASA to reach by 2020.

— What would happen if an asteroid were going to hit Earth.

— Planetary defense experts use infamous asteroid Apophis to practice spotting dangerous space rocks.

— Defending Earth against dangerous asteroids: Q&A with NASA's Lindley Johnson.

The quest to map as many nearby asteroids as possible is why the tally of "potentially hazardous asteroids" and near-Earth objects in general is rising so dramatically.

"It's so satisfying to see that number of asteroid discoveries creep up," Nugent said.

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