Internal emails from NASA show that the space agency was unaware of asteroid 2019 OK, described as a "city killer," until the last moment on July 24.

The giant, football field-sized space rock was not detected by researchers until 24 hours before it was set to whiz past Earth at a distance of just 48,000 miles, traveling at 55,000 miles per hour.

“Because there may be media coverage tomorrow, I'm alerting you that in about 30 mins a 57-130 meter sized asteroid will pass Earth at only 0.19 lunar distances (~48,000 miles),” Lindley Johnson, NASA’s planetary defense officer, wrote in a July 24 email, adding the asteroid "was spotted about 24 hrs ago.”

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The internal messages were obtained by BuzzFeed News through a Freedom of Information Act request and have also been verified by Fox News.

Swinburne University astronomy professor Alan Duffy described asteroid 2019 OK as a "city killer" and an asteroid that "would have hit with over 30 times the energy of the atomic blast at Hiroshima" to the Sydney Morning Herald.

NASA officials, including Johnson, were taken aback at the terminology used by Duffy and another Australian astronomer quoted by the news outlet, saying "it might be helpful to ask them to think before they speak.” Johnson also said that Australia is "essentially doing nothing to support Planetary Defense," according to the internal emails.

The emails first obtained by Buzzfeed also indicate that NASA needs better asteroid detection, specifically mentioning that the ATLAS telescope and the PAN- STARRS observatory “need to detect slower objects.”

Following media reports of the close encounter with 2019 OK, CNEOS issued a statement on Aug. 6 that the damage from the space rock could have been troublesome.

"If 2019 OK had entered and disrupted in Earth’s atmosphere over land, the blast wave could have created localized devastation to an area roughly 50 miles across," CNEOS wrote. "If the asteroid had entered over the ocean, it would have been a bad day for any sailing vessels in the vicinity, but the sea would have absorbed the great majority of the impact’s energy and it is doubtful that a tsunami would have been created."

In a statement provided to Fox News, NASA Public Affairs Officer Allard Beutel said that NASA and other U.S. agencies are leading the international efforts to respond to a possible impact from a near-Earth object (NEOs).

"In 2018, the White House released the National Near-Earth Object Preparedness Strategy and Action Plan, which identifies key steps that U.S. agencies need to take to better prepare the United States – and the world – for detecting and responding to a possible impact," Beutel wrote in an email to Fox News.

He added that NASA has been directed to keep track of all NEOs 140 meters and bigger, noting they are 35 percent complete with NEOs that size and "approximately 96 percent complete" for those 1 kilometer and bigger. "Strategic investments in our space-based programs, will lead to the benefit of all of humanity as we continue to catalogue any NEOs that pose a potential threat. One such investment is the planetary defense-driven test, the Double Asteroid Redirect Test (DART), scheduled to launch in 2021, to demonstrate the possibility of using a kinetic impact to change the motion of an asteroid in space.”

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The space agency will launch its first DART mission in 2022. In April, NASA awarded a $69 million contract to SpaceX, the space exploration company led by Elon Musk, to help with DART.

The next time an asteroid of this magnitude will come that close to Earth is a decade from now. Asteroid 99942 Apophis (named for an Egyptian god of chaos) will come within 19,000 miles of Earth on April 13, 2029, Fox News has previously reported.

Asteroids that come within 0.05 astronomical units and measure more than 460 feet in diameter are known as "potentially hazardous" NEOs, according to NASA. They are tracked by the NASA's Center for Near Earth Object Studies (CNEOS). According to a 2018 report put together by Planetary.org, there are more than 18,000 NEOs.

Last month, Musk said on a podcast that Apophis is not something to worry about, but eventually, a "big rock" will hit Earth and as of right now, there's nothing we can do about it.

NASA has been preparing for planetary defense from asteroid strikes for years. A recent survey showed that Americans prefer a space program that focuses on potential asteroid impacts over sending humans back to the Moon or to Mars.

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In 2016, NASA formalized the agency’s prior program for detecting and tracking NEOs and put it inside its Science Mission Directorate. Last June, NASA unveiled a 20-page plan that detailed the steps the U.S. should take to be better prepared for NEOs – such as asteroids and comets – that come within 30 million miles of the planet.

In addition to enhancing NEO detection, tracking and characterizing capabilities and improving modeling prediction, the plan also aims to develop technologies for deflecting NEOs, increasing international cooperation and establishing new NEO impact emergency procedures and action protocols.

Separately in April, NASA Administrator Jim Bridenstine said that an asteroid strike is not something to be taken lightly and is perhaps Earth's biggest threat.

"We have to make sure that people understand that this is not about Hollywood, it's not about movies," Bridenstine said at the International Academy of Astronautics' 2019 Planetary Defense Conference in College Park, Md., according to Space.com. "This is about ultimately protecting the only planet we know right now to host life, and that is the planet Earth."

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Fox News' Brie Stimson contributed to this report.