Torino Scale: a classification system for impact risks

The Torino Scale

The Torino Scale is a tool created to communicate to the public the impact risk of an asteroid or comet on Earth. It assigns each object a rating from 0 to 10, calculated from two parameters: the probability of impact (the chances that the object will collide with Earth) and the kinetic energy of the object (the violence of the potential impact).

Five risk categories

To simplify reading, the scale's ratings are grouped into five colored zones:

• White (0): no risk, or risk so low that it can be dismissed.
• Green (1): extremely low probability of impact; the object deserves follow-up purely as a precaution.
• Yellow (2 to 4): impact unlikely, but the object warrants close monitoring, because future observations could change the rating.
• Orange (5 to 7): serious threat; continuous monitoring and action plans are necessary.
• Red (8 to 10): certain or near-certain impact, with devastating consequences at local, regional or global scale.

How should these zones be interpreted?

The zones mainly express a required level of attention, not a certainty: a rating in the yellow does not mean that a collision will occur, but that the object must continue to be observed in order to refine its trajectory. Conversely, a rating in the red requires urgently activating planetary defense measures. Finally, a rating is not fixed: it rises or falls with new observations, and the vast majority of detected objects quickly fall back into the white or green zone.

Case studies: asteroids at the heart of monitoring

Historical and monitored objects

Near-Earth asteroids are systematically observed as soon as they are discovered, and their trajectories are continually recalculated. The history of these temporary alerts shows that initial estimates almost always evolve toward the risk being lifted thanks to additional observations.

• 1950 DA: a large asteroid about 1 km across whose trajectory presented a notable impact probability in 2880. Recent modeling, incorporating the Yarkovsky effect, has considerably reduced this risk.
• Bennu (101955 Bennu): discovered on September 11, 1999 by the LINEAR project, it was studied closely by NASA's OSIRIS-REx mission, which returned samples in 2023. Its impact probability in 2182 remains low but monitored (level 1 on the Torino Scale).
• Apophis (99942 Apophis): discovered in 2004, it briefly reached level 4 on the Torino Scale, a record at the time, due to a risk of impact in 2029. Subsequent observations brought this risk back to 0. Its exceptional close approach on April 13, 2029, at only about 32,000 km from Earth, will be observed by the whole world, without any danger.
• 2011 AG5: a few months after its discovery in January 2011, it was among the rare objects classified above level 0 (risk in 2040). Accumulated observations allowed it to be definitively removed from risk tables in December 2012.
• 2019 OK: this asteroid, about 100 meters across, passed about 70,000 km from Earth on July 25, 2019. It was detected only a few hours before its passage, illustrating the limits of detecting objects arriving from the direction of the Sun.
• 2023 DW: discovered in February 2023, it briefly presented an impact probability in 2046 of about 1 chance in 600, before observations completely removed it from the risk list in March 2023.

Recent cases: the 2024 YR4 alert

Discovered on December 27, 2024 by the ATLAS system, asteroid 2024 YR4 (≈ 60 m) triggered an unprecedented alert: in early 2025, the probability of impact on December 22, 2032 reached nearly 3%, i.e. level 3 on the Torino Scale — a record since Apophis. New observations, including those from the James Webb Telescope, completely ruled out the Earth risk as early as February 2025; only a low probability of impact with the Moon in 2032 remains.

In the same spirit, 2020 XR (≈ 390 m) passed close to Earth on December 4, 2024 at 2.2 million km, without danger. These repeated alerts almost always end the same way: thanks to monitoring programs (ATLAS, Pan-STARRS, Catalina), the accumulation of observations lifts the risk within a few weeks.

Monitoring and prevention of near-Earth objects

Monitoring of near-Earth objects (NEOs) is Earth's first line of defense. Programs such as NASA's Planetary Defense Coordination Office, the Spaceguard project, and the automated surveys Pan-STARRS, Catalina and ATLAS continually detect and track these objects. Each new detection feeds risk tables (Sentry, NEODyS) that are regularly recalculated, as seen with 2024 YR4.

Deflection: from theory to practice

In the event of a serious threat, several deflection scenarios are under study. The most mature is kinetic deflection: striking the asteroid to slightly modify its speed, and therefore its trajectory. Other options, such as the gravitational tractor or a nearby nuclear explosion, remain theoretical scenarios studied by space agencies.

This strategy was validated for the first time in real conditions by NASA's DART mission: on September 26, 2022, the probe struck the asteroid Dimorphos, a small satellite of Didymos, and succeeded in reducing its orbital period by 32 minutes, well beyond the minimum 73 seconds targeted. The European mission Hera, launched in October 2024, came to inspect the impact crater and precisely measure the effectiveness of the maneuver.

The challenge of these programs is simple: the earlier a threatening object is detected, the more feasible a deflection is; a tiny push years before impact is enough, whereas it would be too late to act on an object detected at the last moment, like 2019 OK.