While stood on planet Earth, it’s easy to forget that we are sat on a rock hurtling through the Solar System on a path which could feasibly collide with other (smaller) space rocks, largely because we can’t feel that motion.
If you ever have the urge to feel completely at the mercy of random space rocks, we highly recommend heading to NASA’s Eyes on Asteroids website and selecting their closest approach.
NASA and other observatories track the orbits of objects discovered in the Solar System, keeping a particular eye on “near Earth objects” (NEOs) 140 meters (460 feet) and larger in size that could cause devastation if they were to hit Earth. By observing their orbits, astrophysicists are able to estimate the future orbits of the objects, and predict whether they potentially put them in our part of the Solar System.
These objects are given a score on the Palmero scale.
“The scale compares the likelihood of the detected potential impact with the average risk posed by objects of the same size or larger over the years until the date of the potential impact,” NASA’s Center for Near Earth Object studies explains. “This average risk from random impacts is known as the background risk. For convenience the scale is logarithmic, so, for examples, a Palermo Scale value of -2 indicates that the detected potential impact event is only 1 percent as likely as a random background event occurring in the intervening years, a value of zero indicates that the single event is just as threatening as the background hazard, and a value of +2 indicates an event that is 100 times more likely than a background impact by an object at least as large before the date of the potential impact in question.”
The objects are also given a friendlier “Torino” score of 0-10, with a score of 0 meaning the likelihood of impact is zero or thereabouts, and 10 meaning “a collision is certain, capable of causing global climatic catastrophe that may threaten the future of civilization as we know it, whether impacting land or ocean.” These are helpfully color-coded in green, yellow, and red, to clarify the situation to the public.
Over the years of monitoring space objects, astronomers have discovered objects which have strayed from the green zone. However, there have been a couple that made it to level 4; the highest level of the yellow zone.
“A close encounter, meriting attention by astronomers,” NASA explains of the level. “Current calculations give a 1 percent or greater chance of collision capable of regional devastation. Most likely, new telescopic observations will lead to re-assignment to Level 0. Attention by public and by public officials is merited if the encounter is less than a decade away.”
One of these objects was 99942 Apophis. After it was first discovered in 2004, observations placed it at level 2 on the Torino scale. However, further observations in December of that year placed it up to level 4 due to a 1.6 percent chance that the asteroid would hit us in 2029. Further observations ruled out a collision in 2029, as well as in 2036 and 2068, though they will still be close encounters.
Currently, there are no known objects with a Torino score above 0. However, there are objects that require further observations, scoring -0.93 and -1.59 on the Palmero scale, which have not been given a Torino score as the possible collisions take place further than 100 years in the future.
(29075) 1950 DA, an object 1.3 kilometers (0.81 miles) across, scores -0.93 because of a “potentially very close” approach to the Earth on March 16, 2880. This will likely change in the coming years, as more observations are made.
101955 Bennu (1999 RQ36) – more simply known as “Bennu” – currently sits at -1.59 on the Palmero scale. As far as we can tell, it will make several close approaches to the Moon and Earth. In September 2135, it is expected to pass within 0.00143 Astronomical Units (AU) of the Earth, with 1 AU being the distance between the Earth and the Sun. That’s 213,925 kilometers (132,927 miles), which is pretty close in space terms.
As these asteroids orbit, more observations are made, and trajectories refined, meaning that these scores can go up or down. The further in time, the more chance that objects’ orbits can be disturbed from close encounters with other objects (like the Earth).
So far, astronomers have been able to predict the orbits of known objects up to about 100 years in the future. The good news is that “no known asteroid larger than 140 meters in size has a significant chance to hit Earth for the next 100 years”, according to Dr Kelly Fast, manager of NASA’s NEO Observations Program at NASA Headquarters in 2018.
In better news, a team led by Oscar Fuentes-Muñoz from the University of Colorado Boulder was able to go further, predicting the paths of larger asteroids 1,000 years into the future.
“Assessing the impact risk over longer time scales is a challenge since orbital uncertainties grow. To overcome this limitation we analyze the evolution of the Minimum Orbit Intersection Distance (MOID), which bounds the closest possible encounters between the asteroid and the Earth,” the team explain in their paper. “The evolution of the MOID highlights NEOs that are in the vicinity of the Earth for longer periods of time, and we propose a method to estimate the probability of a deep Earth encounter during these periods.”
Using this method, the team were able to rule out the majority of NEOs from hitting our planet within the next thousand years, and could estimate the probability of others hitting us like a bunch of dinosaurs. The probability of being hit before the year 3000 is looking pretty low according to the team, with the most-likely object to hit us – 7482 (1994 PC1) – having only a 0.00151 percent chance of a close encounter, approaching the Earth closer than the orbit of the Moon.
