Asteroid PHA 2022 RM4 will make its closest approach to Earth either tomorrow or the morning after, depending on your time zone. As asteroid encounters go it’s not particularly close, passing us at 2.3 million kilometers (1.43 million miles) away, or six times the distance to the Moon. There are, however, aspects to this event that make it stand out besides its timing.
The first is that PHA 2022 RM4’s orbit crosses our own, so one day it could actually hit the Earth, thus the PHA (Potentially Hazardous Asteroid) designation. Moreover, it’s large enough to do serious damage if it hit the Earth, although it’s definitely no dinosaur killer. At somewhere between 330 and 740 meters (1,080-2,430 feet) in diameter, this is not an object to be measured in the size of African wildlife. Once we get a more precise estimate of its size everyone will be able to check if it's longer than the tallest building in their hometown. At the moment, only a handful of cities have buildings so high they can be confident of exceeding it.
Perhaps the most alarming thing about PHA 2022 RM4, however, is in the middle part of its name. We only discovered this object in September this year. All hopes that we had discovered all the short-period asteroids with Earth-crossing orbits larger than 300 meters (980 feet) or so have now been crushed. Instead, we need to accept that the possibility of a large space rock unexpectedly going bump in the night on our planet remains a distinct possibility.
Projects like DART to deflect objects capable of unleashing immense tsunamis or blackening the skies for months are all very well if we have plenty of warning, but little use with less than two months’ notice.
With a 1,400-day (3.8-year) orbit, PHA 2022 RM4 spends most of its time further from the Sun than Mars, but its closest approach is just inside the Earth’s orbit. It’s unusual in how far out of the planetary plane its orbit lies.
No closer approaches to Earth have been calculated, so PAH 2022 RM4 is not a threat for the foreseeable future. However, with an orbit like this it’s likely it will eventually become a problem that needs to be dealt with.
Gianluca Masi of the Virtual Telescope Project succeeded in capturing the below image of PHA 2022 RM4 as a tiny dot two days before its closest approach. The telescope tracked the asteroid as it moved across the sky, making the background stars appear as lines. At the time, the object was almost exactly double its closest distance from Earth.
The Virtual Telescope Project will be back in action tracking the asteroid during its closest approach on November 1, starting at 17:00 UTC.
Under dark skies, skilled observers may be able to pick it up for themselves using medium-sized backyard telescopes, although it will be too far south for many Europeans and North Americans.