What is the Torino scale, and why do asteroid impact odds rise and then fall?
A newly found asteroid's chance of hitting Earth often goes up for a few days, and then drops to zero. That is how the maths should behave.
Impact odds for a new asteroid often rise at first because of how orbits are measured. Right after a discovery, there are only a few observations, so the object's future path is uncertain, and Earth sits near the centre of that zone of uncertainty. As more observations arrive, the zone shrinks. If Earth is still inside it, Earth covers a larger share of what is left, and the probability goes up. If the asteroid will really miss, the shrinking zone then slides off Earth and the risk falls towards zero.
The Torino scale is how astronomers and the public describe where an object stands. It is an integer scale from 0 to 10 that combines probability and damage potential into one number. A 0 means a negligibly small chance of collision, or an object too small to penetrate the atmosphere. A 10 means a certain collision with an object big enough to cause a global disaster. It is defined only for impacts less than 100 years away.
Objects rated 1 are discovered several times a year, and may last a few weeks until a longer eliminates any possibility of a collision. Only three asteroids have ever ranked higher: Apophis, which reached 4 for four days in late 2004, the highest recorded; 2004 VD17, which had a 2 from February to May 2006; and 2024 YR4, which had a 3 from 27 January to 20 February 2025.
Even a 0 does not mean harmless. The Chelyabinsk meteor of 2013 would have rated 0, whatever its probability, because its energy was only about 0.5 , even though it broke over 3600 windows and injured around 1500 people.
Step 1: Discovery
Few observations, a wide zone of uncertainty around Earth
Step 2: Zone shrinks, odds rise
Earth covers a larger share of what remains
Step 3: More observations
The orbit is pinned down
Step 4: If it misses
The zone slides off Earth and risk falls towards zero
Highest Torino ratings ever recorded
Torino level
| Peak rating | |
|---|---|
| Apophis (late 2004) | 4 Torino level |
| 2004 VD17 (2006) | 2 Torino level |
| 2024 YR4 (2025) | 3 Torino level |
Quiz me
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Recap
A new asteroid's impact odds usually rise at first and then fall to zero, because Earth starts near the middle of a shrinking zone of uncertainty.
💡 A trick to remember it · A shrinking fog: Earth starts in the middle of it, so odds first rise, but when the fog clears past Earth, they drop to zero.
Surprising fact · Apophis scored a 4 for just four days in late 2004, the highest rating ever recorded.
Connects to
- 🔭 How do astronomers find and track asteroids, and what will the Vera Rubin Observatory add?
- 🐍 Will asteroid Apophis hit Earth, and what happens at its 2029 flyby?
- 🔭 What is a near-Earth object, and how many are there?
- ☄️ Did an asteroid or a volcano end the dinosaurs?
- 🎲 How did an unfinished game of chance give birth to probability theory?
- Palermo scale
- Orbit determination
- Impact event
Sources (2)
No source, no claim. Every fact in this lesson (14 claims) cites at least one of these.