If there happened to be a killer asteroid hurtling on a path to collide with Earth, we now know, thanks to the successful nudge it gave an asteroid last month, that NASA has the ability to deflect it — possibly saving the planet from catastrophic damage, and the human race from extinction. Thankfully, there are no known large asteroids expected to hit the earth over the next 100 years.

That's the good news.

The bad news is NASA estimates that it tracks only about 40 percent of the asteroids large enough that they could cause calamity if they were to hit Earth. To save us, the space agency needs fair warning — years, not months or weeks — to muster the defenses in space needed to safeguard the planet.

"As we say, we can't do anything about them unless we know about them, and when they might be a concern for us," Lindley Johnson, NASA's Planetary Defense Officer, said in an interview.

To truly become a serious defender of Earth, NASA has been working to drastically improve its ability to spot potentially dangerous asteroids, track them over time and calculate well into the future whether it would hit Earth — a series of highly complicated tasks requiring an array of sophisticated telescopes and tracking stations all over the world.

One of the main tools NASA plans to use in the hunt for killer asteroids is the NEO Surveyor, a telescope that would operate in space and be able to see objects in infrared wavelengths, which is critical in the search for asteroids because they are often hard to spot against the darkness of space.

The telescope is expected to launch as early as 2026, and, NASA says, allow it to finally meet a long-overdue congressional mandate that in 2005 directed NASA to find 90 percent of asteroids at least 140-meters in size within a decade. But funding cuts have threatened to delay the telescope program, which has led to an outcry from some corners of the space community.

In a letter to Congress earlier this year, the leaders of two space advocacy groups, the Planetary Society and the National Space Society, wrote that without the Surveyor telescope, "NASA will not achieve the congressional detection mandate for another 30 years."

Johnson said now that NASA successfully smashed a spacecraft into an asteroid and altered its trajectory, a mission known as the Double Asteroid Redirection Test, or DART, "our emphasis now will turn to getting the NEO surveyor mission developed and launched."

The telescope is "extremely important to planetary defense," Johnson said. "It is our best chance of getting a comprehensive survey of the hazardous population. That being said, an asteroid impact, a significant one, is a very rare event. So time is probably on our side. But we really don't know that until we complete the survey. So every year that we delay is a year, possibly, that we don't know about objects coming our way."

In that way, the threat of an asteroid strike is like the coronavirus pandemic, said Casey Dreier, the Planetary Society's chief advocate and senior space policy adviser. "DART is like our vaccine development program. And NEO Surveyor is having a testing regime, so we know where the hotspots are. We know where to focus our efforts, and so we have some kind of understanding of what the threat situation is."

The asteroid NASA hit last month in its DART mission posed no threat to Earth. It was merely a test to see if NASA could hit it and if so, what would happen. The target was an asteroid 7 million miles from Earth called Dimorphos, which is about the size of a football stadium.

NASA chose Dimorphos because it orbits a much larger asteroid, known as Didymos, giving scientists an easy way to measure the effect of the impact. Before the crash, it took Dimorphos 11 hours and 55 minutes to complete an orbit around its larger twin. Afterward, it was 11 hours and 23 minutes — a significant change that made even the most stoic astronomers at NASA gleeful.

"We showed the world NASA is serious as a defender of this planet," NASA Administrator Bill Nelson said after the mission.

Still, space is vast but crowded with the detritus of the violent formation of the universe that whiz around at tremendous speed. NASA cannot see a lot of what's out there, even the rocks hurtling about the Earth's corner of the solar system. Earth gets hit all the time, mostly with small particles, no bigger than a grain of sand. But they "bombard the Earth at the rate of more than 100 tons a day and quickly disintegrate," NASA says.

The vast majority of asteroids that enter Earth's atmosphere also burn up once they hit the atmosphere. But NASA warns that "larger asteroids could explode in the atmosphere or reach Earth's surface intact and cause damage in and around their impact sites."

That is what happened in 2013, when a nearly 60-feet-tall meteor exploded with the force of 30 atomic bombs more than 14 miles above Chelyabinsk, Russia. More than 1,000 people were injured after windows were blown out and fragments fell from the sky. NASA says such events could occur every 30 to 40 years, though it is far more likely to occur over water than populated areas.

Also that year, a large asteroid, 1.6 miles in diameter, flew by Earth at an unsettlingly close distance: 3.6 million miles.

"Had an object this size struck the Earth, the resulting debris would likely have contaminated the Earth's atmosphere, causing partial obstruction of sunlight, acid rain, and firestorms," NASA's Inspector General said in a report.

Given the threat, Congress had grown concerned that NASA was not doing enough to protect the planet from the large rocks whizzing about in space. In 2005, the NASA Authorization Act required the space agency to find, track and characterize the physical dimensions of near-Earth objects — defined as those within 30 million miles — that are 140 meters in diameter or larger. The goal was to have 90 percent of the objects catalogued by 2020.

By 2014, when the Inspector General issued its report, the number stood at just 10 percent, the agency watchdog found, despite a 10-fold increase in the budget for near-Earth object detection, from $4 million to $40 million. That report prompted NASA to create the Planetary Defense Coordination Office, which was stood up in 2016 with the goal of tracking near-Earth objects, issuing warnings, developing ways to defend Earth and coordinating how the United States would respond to an actual threat of impact.

NASA does have some sense of the threats lurking out deep in space. One is an asteroid called Bennu, which is wider than the height of Empire State Building. The Center for NEO Studies at NASA's Jet Propulsion Laboratory predicts that next time it buzzes Earth's tower — flying inside the moon's orbit — will be in 2135. Coming so close to earth will change its orbit by a small amount, which, NASA believes, "may lead to a potential impact on Earth sometime between 2175 and 2199."

The chance of it crashing into Earth during that time frame is tiny, NASA says, just 0.037 percent. Still, the consequence of a crash would be severe, and so the agency is watching it closely.