When a new asteroid makes headlines for a possible impact, the news usually quotes a number from one of two scales. They sound similar, but they were built for different readers and answer different questions. This article looks at how each one works in practice, using real cases. For the wider picture of how asteroids are tracked, see How NASA tracks asteroids.
The Torino Scale is a communication tool. It gives journalists, officials and the public a single whole number from 0 to 10. The Palermo Scale is a working tool for specialists. It gives a continuous number that lets astronomers rank many objects against each other and decide which one deserves the next hour of telescope time.
A useful rule of thumb: Torino tells you how worried to be, Palermo tells an astronomer where to look first.
The Torino Scale was proposed in the late 1990s and presented at a 1999 conference in Turin, Italy. It combines two ingredients: the probability of an impact and the energy such an impact would release. A rare impact by a huge object and a likely impact by a small one can end up with the same rating, because the scale weighs both.
The levels are grouped by colour:
The scale was revised in 2005. According to reports at the time, the numbering and the way the hazard is calculated stayed the same, but the wording changed. Levels 2-4 were once described as "meriting concern" and are now described as meriting attention by astronomers, a change meant to avoid frightening people over objects that nearly always turn out to be harmless.
The Palermo Technical Impact Hazard Scale was described by Steven Chesley and colleagues in a 2002 paper in the journal Icarus. Its core idea is to compare a specific risk with the background risk: the average chance that an object of that size or larger hits Earth during the same period of time anyway.
In simple terms, the Palermo value is the base-10 logarithm of the impact probability divided by the background probability over the time left before the possible impact. Because it is a logarithm, each whole step is a factor of ten:
Values between -2 and 0 are generally described as deserving careful monitoring. Almost every object on NASA's public risk list sits well below that range, at strongly negative values.
Apophis, December 2004. Early orbit calculations, based on only a few days of data, briefly gave Apophis a Torino rating of 4, the highest ever assigned to an asteroid. The rating fell as new observations, including images from earlier in 2004, tightened the orbit. The full story is in Is Apophis dangerous?
2024 YR4, early 2025. This asteroid, roughly 40 to 90 metres across by the estimates published at the time, rose to level 3 on the Torino Scale when its estimated chance of hitting Earth in December 2032 peaked at about 3%. That made it the second-highest rating ever given. By late February 2025, after about two months of observations, the Earth impact chance had dropped close to zero and the rating returned to 0. ESA noted in a newsletter that on the Palermo Scale the object ranked lower than some earlier cases, mostly because of its smaller size.
This pair of cases shows why both scales exist. For 2024 YR4, the Torino number captured public attention. The Palermo number told specialists that, even at its worst moment, the risk was modest next to the natural background for objects of that size.
When an asteroid is newly found, its orbit is a cloud of possible paths, and Earth may sit inside that cloud. Each extra observation narrows it. In most cases the cloud shrinks until it no longer includes Earth, and both ratings drop to zero. A rise in rating therefore signals uncertainty more than danger. It is also why astronomers rush to follow up a worrying object quickly, as described in our guide to measuring asteroid size and speed.
To learn what happens if a real threat is ever found, read about planetary defense, and for the object that will pass close in 2029, see the Apophis 2029 guide.
Facts in this article are drawn from public scientific sources. For primary information, see: