A small asteroid, designated 2023 BU, recently completed one of the closest flybys of our planet ever recorded by a near-Earth object. The space rock, roughly the size of a delivery truck, passed just 2,200 miles above the Earth’s surface, soaring over the southern tip of South America. This altitude is significantly lower than the orbits of geosynchronous satellites, which circle the globe at about 22,236 miles, though it remained above the low Earth orbit altitude of the International Space Station.
Astronomers confirmed there was never any danger of the asteroid striking our planet. However, the encounter was so close that it dramatically demonstrated the effectiveness of modern asteroid tracking systems while providing a real-world example of how Earth’s gravity can influence the path of nearby celestial bodies. The flyby, now ranked as the fourth-closest non-impacting approach known, offered scientists a rare opportunity to study a near-Earth object and refine planetary defense protocols.
An Extremely Close Shave
The asteroid made its closest approach at approximately 7:27 p.m. EST, when it passed about 3,600 kilometers, or 2,200 miles, from the planet’s surface. This is a remarkably small distance in astronomical terms, representing less than 3% of the average distance to the Moon. The pass brought the object well inside the ring of geostationary satellites that provide weather and communications services. While many objects pass between Earth and the Moon, very few are observed to come this near.
Scientists at NASA’s Center for Near Earth Object Studies (CNEOS) calculated the precise trajectory, confirming the pass over South America. Traveling at a speed of about 33,300 miles per hour, the asteroid’s journey through the Earth-Moon system was swift. The event was a testament to the precision of orbital mechanics, as even a minor deviation in its path could have had significant consequences, though this particular object was too small to cause damage on the ground.
Discovery and Tracking Systems
A Vigilant Amateur Discovery
The first observation of 2023 BU was made not by a major government-funded observatory, but by an amateur astronomer. Gennadiy Borisov, operating from his observatory in Crimea, discovered the object on Saturday, January 21, just five days before its closest approach. Borisov is a well-regarded figure in the astronomy community, having previously discovered the first known interstellar comet, 2I/Borisov, in 2019. His detection of 2023 BU highlights the crucial role that amateur and smaller observatories play in planetary defense by scanning the skies for new objects.
Rapid Global Confirmation
Following Borisov’s initial report, observatories around the world quickly turned their attention to the newfound object to gather more data. These numerous observations were fed into NASA’s Scout impact hazard assessment system, a program designed to rapidly evaluate the trajectories of newly discovered objects. Scout processed the data and, despite the very limited number of initial observations, correctly predicted the asteroid would make an extraordinarily close pass while definitively ruling out any chance of an impact.
Davide Farnocchia, a navigation engineer at NASA’s Jet Propulsion Laboratory who developed Scout, noted the system’s effectiveness. He stated that this was one of the closest approaches by a known near-Earth object ever recorded and that Scout was able to predict it with high confidence. This rapid response system is a cornerstone of modern planetary defense, providing reassurance that the scientific community can quickly identify and assess potential threats.
An Altered Cosmic Path
An encounter this close with a massive gravitational body like Earth was guaranteed to change the asteroid’s journey through space. Before its flyby, 2023 BU had a roughly circular orbit around the Sun that was very similar to Earth’s, taking 359 days to complete a full revolution. Its low-eccentricity orbit was a key reason for its relatively low encounter speed of 9.3 km/s.
Earth’s gravity acted like a giant gravitational slingshot, accelerating the asteroid and flinging it into a new, more elongated orbit. After the encounter, its trajectory was significantly altered. Its orbital period increased from 359 days to 425 days. This new path will take the asteroid farther away from the Sun, moving it out to a point roughly halfway between the orbits of Earth and Mars at its most distant point, or aphelion. This permanent change to its orbit serves as a clear physical demonstration of gravitational interaction on a cosmic scale.
Profile of a Near-Earth Object
Size and Composition
Asteroid 2023 BU is a relatively small object, with estimates placing its diameter between 3.5 and 8.5 meters (11 to 28 feet). This classifies it as a micrometeoroid, too small to be considered a major threat. Its composition is not known in detail, as its size and the brevity of the encounter made spectroscopic analysis difficult. However, like most near-Earth asteroids, it is presumed to be a rocky body composed of stone, metals, or a combination thereof.
No Threat of Impact
NASA and other agencies emphasized that there was no risk of collision. Even if its trajectory had intersected with Earth, an asteroid of this size would not have reached the surface intact. It would have burned up and disintegrated high in the atmosphere, at an altitude of around 30 kilometers. The result would have been a brilliant fireball, also known as a bolide, but it would have posed minimal threat to life or property on the ground, potentially producing only a few small meteorites. Statistically, impacts from objects around 4 meters in diameter occur about once a year, while impacts from objects 8 meters in diameter happen roughly every five years.
Lessons for Planetary Defense
The flyby of 2023 BU, while harmless, was an important event for the planetary defense community. It served as a successful real-world drill for the entire process of detecting, tracking, and predicting the path of a near-Earth object. The timeline from discovery to close approach was extremely short—only five days—which underscores the need for constant, comprehensive sky surveys.
This event highlights the importance of programs designed to scan the skies and rapidly compute orbits. While 2023 BU was small, a larger object on a similar trajectory would be a serious concern. The ability of the global astronomy community, from amateurs to government agencies, to work together to quickly and accurately assess the situation provides confidence that the systems in place are effective. Every close approach is an opportunity to refine these systems and prepare for the unlikely but high-consequence scenario of a future impact threat.