By Ashley Strickland, CNN<\/p>\n
(CNN) \u2014 The asteroid known as 2024 YR4 is out of sight yet still very much on scientists\u2019 minds.<\/p>\n
The building-sized object, which initially appeared to be on a potential collision course with Earth, is currently zooming beyond the reach of telescopes on its orbit around the sun. But as scientists wait for it to reappear, its revised trajectory is now drawing attention to another possible target: the moon.<\/p>\n
Discovered at the end of 2024, the space rock looked at first as if it might hit our planet by December 22, 2032. The chance of that impact changed with every new observation, peaking at 3.1% in February \u2014 odds that made it the riskiest asteroid ever observed<\/a>.<\/p>\n Ground- and space-based telescope observations were crucial in helping astronomers narrow in on 2024 YR4\u2019s size and orbit. With more precise measurements, researchers were ultimately able to rule out<\/a> an Earth impact.<\/p>\n The latest observations of the asteroid in early June, before YR4 disappeared from view, have improved astronomers\u2019 knowledge of where it will be in seven years by almost 20%, according to NASA<\/a>.<\/p>\n That data shows that even with Earth avoiding direct impact, YR4 could still pose a threat in late 2032 by slamming into the moon. The impact would be a once-in-a-lifetime event for humanity to witness \u2014 but it could also send fine-grained lunar material hurtling toward our planet.<\/p>\n While Earth wouldn\u2019t face any significant physical danger should the asteroid strike the moon, there is a chance that any astronauts or infrastructure on the lunar surface at that time could be at risk \u2014 as could satellites orbiting our planet that we depend on to keep vital aspects of life, including navigation and communications, running smoothly.<\/p>\n Any missions in low-Earth orbit could also be in the pathway of the debris, though the International Space Station<\/a> is scheduled to be deorbited before any potential impact.<\/p>\n Initially, YR4 was seen as a case study in why scientists do the crucial work of planetary defense, discovering and tracking asteroids to determine which ones have a chance of colliding with Earth. Now, astronomers say this one asteroid could redefine the range of risks the field addresses, expanding the purview of the work to include monitoring asteroids that might be headed for the moon as well.<\/p>\n \u201cWe\u2019re starting to realize that maybe we need to extend that shield a little bit further,\u201d said Dr. Paul Wiegert, a professor of astronomy and physics at the Western University in London, Ontario. \u201cWe now have things worth protecting that are a bit further away from Earth, so our vision is hopefully expanding a little bit to encompass that.\u201d<\/p>\n In the meantime, researchers are assessing just how much chaos a potential YR4 lunar impact could create \u2014 and whether anything can be done to mitigate it.<\/p>\n \u2018City killer\u2019 on the moon<\/p>\n The threatening hunk of rock appears as just a speck of light through even the strongest astronomical tools. In reality, YR4 is likely about 60 meters (about 200 feet) in diameter, according to observations in March<\/a> by the James Webb Space Telescope, the most powerful space-based observatory in operation.<\/p>\n \u201cSize equals energy,\u201d said Julien de Wit, associate professor of planetary sciences at the Massachusetts Institute of Technology, who observed YR4 with Webb. \u201cKnowing YR4\u2019s size helped us understand how big of an explosion it could be.\u201d<\/p>\n Astronomers believe they have found most of the near-Earth asteroids the field would classify as \u201cplanet killers<\/a>\u201d \u2014 space rocks that are 1 kilometer (0.6 mile) across or larger and could be civilization-ending, said Dr. Andy Rivkin, planetary astronomer from the Johns Hopkins University\u2019s Applied Physics Laboratory in Maryland. The planet killer that slammed into Earth 66 million years ago and led to the extinction of dinosaurs was estimated to be roughly 6 miles (about 10 kilometers) in diameter.<\/p>\n Smaller asteroids such as YR4, which was colloquially dubbed a \u201ccity killer\u201d after its discovery, could cause regional devastation if they collide with our planet. About 40% of near-Earth space rocks larger than 140 meters (460 feet) but smaller than a kilometer \u2014 capable of more widespread destruction \u2014 have been identified, according to NASA<\/a>.<\/p>\n But astronomers have never really had a chance to watch a collision of that size occur on the moon in real time, Wiegert said. The latest glimpses of YR4 on June 3 before it passed out of view revealed a 4.3% chance<\/a> of a YR4 lunar impact \u2014 small but decent enough odds for scientists to consider how such a scenario might play out.<\/p>\n A striking meteor shower \u2014 and a risk<\/p>\n Initial calculations suggest the impact has the largest chance of occurring on the near side of the moon \u2014 the side we can see from Earth.<\/p>\n \u201cYR4 is so faint and small we were able to measure its position with JWST longer than we were able to do it from the ground,\u201d said Rivkin, who has been leading the Webb study of YR4. \u201cAnd that lets us calculate a much more precise orbit for it, so we now have a much better idea of where it will be and won\u2019t be.\u201d<\/p>\n The collision could create a bright flash that would be visible with the naked eye for several seconds, according to Wiegert, lead author of a recent paper<\/a> submitted to the American Astronomical Society journals analyzing the potential lunar impact.<\/p>\n The collision could create an impact crater on the moon estimated at 1 kilometer wide (0.6 miles wide), Wiegert said \u2014 about the size of Meteor Crater<\/a> in Arizona, Rivkin added. It would be the largest impact on the moon in 5,000 years and could release up to 100 million kilograms (220 million pounds) of lunar rocks and dust, according to the modeling in Wiegert\u2019s study. <\/p>\n Even pieces of debris that are just tens of centimeters in size could present a hazard for any astronauts who may be present on the moon, or any structures they have built for research and habitation, Wiegert said. The moon has no atmosphere, so the debris from the event could be widespread on the lunar surface, he added.<\/p>\n On average, the moon is 238,855 miles (384,400 kilometers) away from Earth, according to NASA<\/a>.<\/p>\n Particles the size of large sand grains, ranging from 0.1 to 10 millimeters in size, of lunar material could reach Earth between a few days and a few months after the asteroid strike because they\u2019ll be traveling incredibly fast, creating an intense, eye-catching meteor shower, Wiegert said.<\/p>\n \u201cThere\u2019s absolutely no danger to anyone on the surface,\u201d Wiegert said. \u201cWe\u2019re not expecting large boulders or anything larger than maybe a sugar cube, and our atmosphere will protect us very nicely from that. But they\u2019re traveling faster than a speeding bullet, so if they were to hit a satellite, that could cause some damage.\u201d<\/p>\n Not all lunar debris that reaches the Earth is so small, and it depends on the angle and type of impact to the moon, according to Washington University in St. Louis<\/a>. Space rocks slamming into the lunar surface over millions of years have resulted in various sizes of lunar meteorites found on Earth.<\/p>\n Preparing for impact<\/p>\n Hundreds to thousands of impacts from millimeter-size debris could affect Earth\u2019s satellite fleet, meaning satellites could experience up to 10 years\u2019 equivalent of meteor debris exposure in a few days, Wiegert said.<\/p>\n Humankind depends on vital space infrastructure, said Dan Oltrogge, chief scientist at COMSPOC, a space situational awareness software company that develops solutions for handling hazards such as space debris.<\/p>\n \u201cSpace touches almost every aspect of our lives today, ranging from commerce, communications, travel, industry, education, and social media, so a loss of access to and effective use of space presents a serious risk to humanity,\u201d Oltrogge said.<\/p>\n The event is unlikely to trigger a Kessler Syndrome scenario<\/a> in which debris from broken satellites would collide with others to create a domino effect or fall to Earth. Instead, it might be more akin to when a piece of gravel strikes a car windshield at high speed, meaning solar panels or other delicate satellite parts might be damaged, but the satellite will remain in one piece, Wiegert said.<\/p>\n While a temporary loss of communication and navigation from satellites would create widespread difficulties on Earth, Wiegert said he believes the potential impact is something for satellite operators, rather than the public, to worry about.<\/p>\n Protecting Earth and the moon<\/p>\n Scientists and astronomers around the world are thinking about the possible scenarios since they could not rule out a lunar impact before YR4 disappeared from view, Wiegert said.<\/p>\n \u201cWe realize that an impact to the moon could be consequential, so what would we do?\u201d de Wit said.<\/p>\n A potential planetary defense plan might be clearer if the asteroid were headed straight for Earth. Rivkin helped test one approach in September 2022 as the principal investigator of NASA\u2019s Double Asteroid Redirection Test, or DART, which intentionally slammed a spacecraft<\/a> into the asteroid Dimorphos in September 2022.<\/p>\n Dimorphos is a moonlet asteroid that orbits a larger parent asteroid known as Didymos. Neither poses a threat to Earth, but the double-asteroid system was a perfect target to test deflection technology because Dimorphos\u2019 size is comparable to asteroids that could harm our planet in the event of an impact.<\/p>\n The DART mission crashed a spacecraft into the asteroid at 13,645 miles per hour (6 kilometers per second) to find out whether such a kinetic impact would be enough to change the motion of a celestial object in space.<\/p>\n It worked. Since the day of the collision, data from ground-based telescopes has revealed that the DART spacecraft did alter Dimorphos\u2019 orbital period<\/a> \u2014 or how long it takes to make a single revolution around Didymos \u2014 by about 32 or 33 minutes. And scientists have continued to observe additional changes to the pair, including how the direct hit likely deformed Dimorphos<\/a> due to the asteroid\u2019s composition.<\/p>\n Similarly, if YR4 strikes the moon and doesn\u2019t result in damaging effects for satellites, it could create a tremendous opportunity for researchers to learn how the lunar surface responds to impacts, Wiegert said.<\/p>\n But whether it would make sense to send a DART-like mission to knock YR4 off a collision course with the moon remains to be seen. It will depend on future risk assessments by planetary defense groups when the asteroid comes back into view around 2028, de Wit said.<\/p>\n Though defense plans for a potential moon impact still aren\u2019t clear, YR4\u2019s journey underscores the importance \u2014 and the challenges \u2014 of tracking objects that are often impossible to see.<\/p>\n Hidden threats<\/p>\n YR4 was detected by the Asteroid Terrestrial-impact Last Alert System, or ATLAS telescope, in R\u00edo Hurtado, Chile, two days after the asteroid had already made its closest pass by Earth, hidden by the bright glare of the sun as it approached our planet.<\/p>\n The same thing occurred when an asteroid measuring roughly 20 meters (about 65 feet) across hit the atmosphere and exploded above Chelyabinsk, Russia, on February 15, 2013, damaging thousands of buildings, according to the European Space Agency<\/a>. While no one died, about 1,500 people were injured when the windows in homes and businesses blew out due to the shock wave.<\/p>\n Trying to observe asteroids is challenging for many reasons, Rivkin said. Asteroids are incredibly faint and hard to see because rather than emitting their own light, they only reflect sunlight. And because of their relatively tiny size, interpreting observations is not a clear-cut process like looking through a telescope at a planet such as Mars or Jupiter.<\/p>\n \u201cFor asteroids, we only see them as a point of light, and so by measuring how bright they are and measuring their temperature, basically we can get a size based on how big do they have to be in order to be this bright,\u201d Rivkin said.<\/p>\n For decades, astronomers have had to search for faint asteroids by night, which means missing any that may be on a path coming from the direction of the sun \u2014 creating the world\u2019s biggest blind spot for ground-based telescopes that can\u2019t block out our star\u2019s luminosity.<\/p>\n But upcoming telescopes \u2014 including NASA\u2019s NEO Surveyor<\/a> expected to launch by the end of 2027 and the European Space Agency\u2019s Near-Earth Object Mission in the InfraRed<\/a>, or NEOMIR satellite, set for liftoff in the early 2030s \u2014 could shrink that blind spot, helping researchers detect asteroids much closer to the sun.<\/p>\n \u201cNEOMIR would have detected asteroid 2024 YR4 about a month earlier than ground-based telescopes did,\u201d said Richard Moissl, head of ESA\u2019s Planetary Defence Office, in a statement<\/a>. \u201cThis would have given astronomers more time to study the asteroid\u2019s trajectory and allowed them to much sooner rule out any chance of Earth impact in 2032.\u201d<\/p>\n NASA and other space agencies are constantly on the lookout for potentially hazardous asteroids<\/a>, defined as such based on their distance from Earth and ability to cause significant damage should an impact occur. Asteroids that can\u2019t get any closer to our planet than one-twentieth of Earth\u2019s distance from the sun are not considered to be potentially hazardous asteroids, according to NASA.<\/p>\n When the new Vera C. Rubin Observatory, located in the Andes in Chile, released its first stunning images of the cosmos in June, researchers revealed the discovery of more than 2,100 previously unknown asteroids<\/a> after seven nights of observations.
Of those newly detected space rocks, seven were near-Earth objects.<\/p>\n