Finding a Needle in an Infinite Haystack
The night sky contains billions of points of light. Among them, a handful are moving — asteroids and comets tracing their paths through the inner solar system. Finding the ones that could threaten Earth (or the Moon) requires automated systems scanning the entire sky, night after night.
Asteroid 2024 YR4 was discovered by one such system: ATLAS, the Asteroid Terrestrial-impact Last Alert System.
ATLAS: The Early Warning System
ATLAS is a network of four telescopes — two in Hawaii, one in Chile, and one in South Africa — funded by NASA and operated by the University of Hawaii. Together, they survey the entire visible sky every 24 hours.
Each night, ATLAS captures images of the sky and compares them to previous images. Anything that has moved is flagged as a potential asteroid. Software filters out known objects (planets, satellites, known asteroids) and highlights new discoveries.
It was this automated comparison that caught 2024 YR4 on December 27, 2024 — a faint dot that had moved between exposures.
From Detection to Orbit
Once a new object is detected, the clock starts ticking. Astronomers need multiple observations — ideally over several nights — to calculate an orbit. Here is the process:
- Initial detection: ATLAS flags a moving object
- Confirmation: Other observatories are alerted to observe the same object
- Astrometry: Precise position measurements are made at multiple times
- Orbit determination: Using the position data, software calculates the most likely orbit
- Impact assessment: The orbit is projected forward to identify any close approaches to Earth or the Moon
For 2024 YR4, this process went from detection to a preliminary impact assessment in under a week.
The Minor Planet Center
All asteroid observations worldwide flow to the Minor Planet Center (MPC), operated by the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts. The MPC is the global clearinghouse for small body data — every confirmed asteroid gets a designation and its observations are catalogued.
When you see a designation like "2024 YR4", the format tells you it was discovered in the second half of December 2024. The letter code follows a specific system maintained by the MPC.
NASA's Sentry System
Once an orbit is determined, NASA's Sentry system takes over for impact assessment. Sentry is a highly automated collision monitoring system operated by the Center for Near-Earth Object Studies (CNEOS) at the Jet Propulsion Laboratory.
Sentry works by:
- Taking the best-fit orbit and its uncertainty
- Generating thousands of slightly different possible orbits (called "virtual asteroids")
- Propagating each virtual asteroid forward in time
- Counting how many virtual asteroids pass close to Earth or the Moon
If, say, 43 out of 1,000 virtual asteroids hit the Moon, that gives a roughly 4.3% impact probability — which is essentially how the 2024 YR4 probability was calculated.
ESA's NEO Coordination Centre
On the European side, the Near-Earth Object Coordination Centre (NEOCC) in Frascati, Italy, performs an independent assessment. Having two major agencies calculating impact probabilities provides a valuable cross-check.
ESA also operates its own detection systems and maintains the Risk List — a public catalogue of objects with non-zero impact probabilities.
Ground-Based Observatories
Dozens of ground-based observatories worldwide contribute to asteroid tracking. Some of the key facilities include:
- Catalina Sky Survey (Arizona, USA) — one of the most prolific asteroid discovery programs
- Pan-STARRS (Hawaii, USA) — a wide-field survey telescope
- Spacewatch (Arizona, USA) — specialising in faint, small objects
- La Sagra Observatory (Spain) — a European discovery station
Each observation of 2024 YR4 from any of these facilities gets sent to the Minor Planet Center, refining the orbit further.
Radar Observations
When an asteroid passes close enough to Earth, radar telescopes can bounce signals off it. Radar observations are extraordinarily precise — they can determine an asteroid's distance to within metres and its velocity to within millimetres per second.
The Goldstone Solar System Radar in California and the now-collapsed Arecibo Observatory (which operated in Puerto Rico until 2020) have been the primary facilities for radar asteroid observation.
If 2024 YR4 passes close enough to Earth in a future close approach, radar observations could dramatically reduce the orbital uncertainty and settle the impact question definitively.
The Torino and Palermo Scales
Scientists communicate asteroid risk using two standardised scales:
- The Torino Scale (0-10): A simple public-facing scale. Most asteroids are Torino 0 (no risk). 2024 YR4 is notable because it has been assessed at Torino 1 or above — meaning it merits attention.
- The Palermo Scale: A more technical logarithmic scale comparing the impact probability of a specific asteroid to the "background" probability of an equivalent random impact. A Palermo value above 0 means the risk exceeds the background level.
What Comes Next
The future of asteroid tracking is getting brighter:
- NASA's NEO Surveyor, a space-based infrared telescope scheduled for launch, will find asteroids that are difficult to spot from the ground — particularly those approaching from the Sun's direction.
- The Vera C. Rubin Observatory in Chile will begin operations with the most powerful survey camera ever built, expected to discover millions of new asteroids.
- Planetary defence missions like NASA's DART (which successfully deflected asteroid Dimorphos in 2022) prove that we can change an asteroid's orbit if needed.
For 2024 YR4 specifically, continued observations over the next several years will narrow the orbit uncertainty. By the late 2020s, we should know with high confidence whether the December 2032 close approach will result in a lunar impact or a clean miss.
Either way, the systems that found and tracked this asteroid are working exactly as designed — giving humanity years of advance notice for a potential cosmic event.
Data sourced from NASA/JPL, ESA NEOCC, and the Minor Planet Center. Last updated February 2025.