Near-Earth Asteroid

2024 YN4 -
the risk that resolved

Discovered in December 2024, asteroid 2024 YN4 briefly appeared on NASA's Sentry risk table with a non-zero impact probability. Follow-up observations eliminated the risk entirely. Its story is a clear illustration of how the planetary defence system is supposed to work.

Will an asteroid hit Earth? →

Discovered

December 2024

By automated survey systems

Estimated size

~50–100 m

Below the 140 m PHA threshold

Peak Torino rating

1 (briefly)

Assigned during peak orbital uncertainty

Current status

No impact risk

Removed from Sentry risk table

Discovery

Automated asteroid surveys photograph the sky continuously. When software detects a point of light that moves between exposures, it flags the object for follow-up. 2024 YN4 was detected this way in December 2024 and assigned a provisional designation by the International Astronomical Union's Minor Planet Center - the body that catalogues all solar system small bodies.

Provisional designations follow a standard format: the year of discovery, a letter indicating the half-month, and a second letter plus optional number indicating the order within that period. "2024 YN4" tells you it was the 117th object discovered in the second half of December 2024. Like most newly found near-Earth objects (NEOs, meaning asteroids with orbits within 1.3 AU of the Sun, where 1 AU is an astronomical unit, approximately 150 million km), it entered NASA's Sentry automated risk assessment system shortly after discovery.

The initial risk assessment

A newly discovered asteroid comes with a fundamental limitation: the orbital solution is only as good as the data behind it. With a few nights of observations, the computed orbit has a large uncertainty region - a cloud of possible trajectories, all consistent with the measurements, but diverging significantly over years and decades.

For 2024 YN4, that uncertainty region included trajectories that intersected Earth's orbit in the early 2030s. The Sentry system, which runs automated impact probability calculations for all newly catalogued NEOs, assigned a non-zero probability to those trajectories. The object briefly reached a Torino Scale rating of 1 - meaning a close encounter meriting attention, though with no cause for public concern.

Survey networks around the world were alerted. Additional observations began almost immediately, extending the arc of data used to compute the orbit. This is normal procedure whenever Sentry flags a newly discovered object.

How the risk resolved

As additional observations came in over days and weeks, the orbital uncertainty region shrank. The range of possible trajectories consistent with the data narrowed to a point where none of them passed through Earth's location at any future date within the relevant window. Sentry removed 2024 YN4 from the risk table.

The timeline - from initial flagging to resolution - is characteristic of how most Sentry entries are resolved. A short data arc generates uncertainty. More data eliminates it. The system is designed to catch every plausible risk early, accept that most will resolve to zero, and escalate the ones that do not. 2024 YN4 was a routine case of the first outcome.

Why this matters for understanding planetary defence

2024 YN4's story is worth understanding because it is representative. The Sentry risk table is not a roster of confirmed threats. It is a working list of objects whose orbital uncertainties have not yet been reduced to the point where Earth can be excluded from all plausible future paths. Most entries on the table at any given time carry cumulative impact probabilities below 1 in 10,000, spread across many possible dates decades hence.

Entries appear and disappear constantly. That is the system working. An object only remains on the table long-term - and warrants serious attention - when follow-up observations fail to eliminate the risk, or when the probability actually increases with more data. Neither happened with 2024 YN4.

The Sentry risk table

Currently, no object on NASA's Sentry risk table carries a meaningful probability of impact in the next century. Objects appear and disappear from the table routinely as observations refine orbital solutions. The table is publicly accessible at cneos.jpl.nasa.gov/sentry - checking it is straightforward if you want to see what is currently listed.

Size context

At 50 to 100 metres across, 2024 YN4 sits below the 140-metre threshold that defines a potentially hazardous asteroid (PHA). The PHA classification requires an object to be both large enough to cause significant damage and to have an orbit that passes within 0.05 AU of Earth's orbit. 2024 YN4 is smaller than the size boundary.

An object of this size impacting in a populated area would release energy comparable to a large nuclear weapon - destructive over tens of kilometres, not hundreds. Significant local damage, but not the regional or global effects associated with objects above 140 metres. The Chelyabinsk meteor of 2013, which injured over 1,400 people, was approximately 20 metres across. 2024 YN4 would be considerably more energetic, but still orders of magnitude below a civilisation-scale threat.

Will an asteroid hit Earth?

The current state of the risk table and what the science says.

Impact probability explained

How Sentry calculates odds and what the numbers actually mean.

The Torino Scale

The 0-10 rating system that turns probability into plain language.

CNEOS explained

NASA's centre for near-Earth object studies and what it publishes.

Common questions

What is 2024 YN4?

2024 YN4 is a near-Earth asteroid discovered in December 2024. Following discovery, it entered NASA's Sentry automated risk assessment system because early orbital solutions - based on limited observations - could not exclude trajectories that intersected Earth's orbit in the early 2030s. Additional observations resolved the uncertainty, and 2024 YN4 was removed from the risk table. It poses no known impact risk.

Was 2024 YN4 going to hit Earth?

No. The initial non-zero probability was a product of orbital uncertainty, not a genuine threat forecast. When an asteroid is newly discovered, astronomers have only a short arc of observations from which to compute its orbit. That limited data allows for a range of possible trajectories - some of which may pass through Earth's location at a future date. More observations narrow the uncertainty region until, in almost all cases, Earth is excluded entirely. That is what happened with 2024 YN4.

How big is 2024 YN4?

Estimated at roughly 50 to 100 metres across, 2024 YN4 falls below the 140-metre threshold for a potentially hazardous asteroid (PHA, meaning 140 metres or wider with an orbit that passes within 0.05 AU of Earth's orbit). An object of this size would release energy comparable to a large nuclear weapon if it reached the surface, causing significant local damage - but nothing on the regional or global scale associated with larger objects.

Why did 2024 YN4 make the news?

Automated risk tables are publicly accessible, and any non-zero entry attracts attention - especially when initial probability figures are quoted without context. For 2024 YN4, the combination of a recently discovered object, a near-term potential impact window, and a measurable (if small) probability created a news cycle. The resolution - more observations eliminating the risk - received less coverage than the initial alert, which is a common pattern in asteroid news reporting.

How does an asteroid get removed from the risk table?

Additional observations extend the data arc and tighten the orbital solution. As the uncertainty region shrinks, the range of possible trajectories narrows. When no trajectory within the remaining uncertainty passes through Earth at the relevant date, the object is removed from the risk table. This process can take days, weeks, or occasionally months depending on how quickly follow-up observations can be obtained.

How often do asteroids appear on the Sentry risk table?

Dozens of objects appear on the Sentry risk table each year. The vast majority are removed within weeks as follow-up observations refine their orbits. At any given time, the table typically lists around 1,000 to 1,500 objects, almost all with very low cumulative impact probabilities spanning decades or centuries. The table is not a list of confirmed threats - it is a list of objects whose orbital uncertainty has not yet been reduced enough to exclude Earth from all future trajectories.