A 60-metre asteroid known as 2018 BY6 routinely cuts across Earth’s orbit and in future flybys could pass our planet only a few lunar distances away. Now a team of astronomers is tracking its long-term trajectory to determine whether it might eventually pose a genuine impact risk.
Thousands of small asteroids, most too faint to detect without telescopes, regularly pass Earth. Almost all will never hit the planet, but researchers must first detect them and then calculate their trajectories with high precision over timescales of hundreds of thousands of years.
This is the task undertaken by Ireneusz Włodarczyk of the Chorzów Astronomical Observatory and colleagues in Lithuania and the United States.
The group has now published an analysis of 2018 BY6 in the journal Icarus. The asteroid, roughly the length of a football field, follows an Apollo-type orbit that intersects Earth’s. It was discovered in 2014 with the Vatican Advanced Technology Telescope on Mount Graham in Arizona, a facility used by Polish astronomers that found 54 new asteroids between 2013 and 2018.
Among these, 2018 BY6 is considered the most dynamically interesting because it makes repeated close approaches to Earth. Its minimum orbit intersection distance is about 0.012 astronomical units — just over 1.5 million kilometres, or a few times farther than the Moon.
The researchers set two goals: to show how much information can be extracted from only a handful of faint observations, and to examine how the asteroid’s motion might evolve over extremely long timescales, including the possibility of an impact. They aimed to demonstrate techniques that could become standard in assessing potentially hazardous objects.
Because the orbit of 2018 BY6 is known with some uncertainty, the team generated 101 “virtual” versions of the asteroid by slightly varying its orbital parameters around the best-fit trajectory.
They then integrated the motion of all these variants one million years into the future, accounting not only for planetary gravity but also for subtle non-gravitational forces: the Yarkovsky effect, a thermal recoil force caused by heat radiating from the asteroid’s sunlit surface, and the YORP effect, a torque arising from uneven reflection and emission of sunlight. These effects act like tiny thrusters that can gradually shift an asteroid’s orbit or spin rate over millions of years.
The results are largely reassuring but scientifically striking. In most scenarios, 2018 BY6 avoids Earth for the next million years, though it passes close to Earth, Venus and Mars. In three simulations, however, the virtual asteroid struck Earth after roughly 270,000, 570,000 and 920,000 years. In these cases, the closest approach fell within Earth’s radius, an impact in simulation terms.
The authors say that these outcomes should not be interpreted as a measurable impact probability, because the long-term motion of small bodies is chaotic and the initial uncertainties are large. The scenarios represent possible evolutionary paths, not predictions.
The physical evolution of the asteroid is just as notable. At the start of the simulation, 2018 BY6 rotates once every 18 hours. After 100 million years, most virtual copies spin in just minutes. This extreme acceleration is driven by the YORP effect, which acts like a constant but very weak wind, steadily increasing the spin of irregular bodies.
Such rapid rotation can cause small asteroids to fracture, shed material or even split into binary systems. The researchers note that the Sun, over immense timescales, not only illuminates and heats these bodies but also gradually reshapes them.
A more immediate chapter arrives in 2029, when 2018 BY6 will brighten to magnitude 16–17 and be accessible to many telescopes, including advanced amateur instruments. Its predicted position is known to within a few arcminutes, enabling precise tracking. Each favourable apparition refines the orbit and improves estimates of the non-gravitational forces acting on it.
The authors encourage professional and experienced amateur observers to use the 2029 opportunity as a coordinated test case.
The study highlights the growing effectiveness of the planetary-defence system. Protecting Earth depends not only on large survey telescopes and high-profile space missions but also on the painstaking work of calculating trajectories, work that even accounts for how quickly a 60-metre rock absorbs and releases heat.
The researchers say that the future is not fixed and instead of a single unavoidable outcome, asteroids like 2018 BY6 follow a spectrum of possible paths, which scientists are increasingly able to map and, in some cases, slightly influence. (PAP)
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