Every day, software at NASA’s Jet Propulsion Laboratory scans the orbits of thousands of known near-Earth objects and flags any that will pass within 4.6 million miles of our planet. That distance, equal to about 7.5 million kilometers, is the formal threshold for what the agency calls a “close approach.” The system does not just track what is coming next week or next month; it catalogs predicted encounters both in the past and into the future, building a running ledger of every object that crosses into that zone.
Why the 4.6-million-mile threshold drives planetary defense decisions
The number is not arbitrary. An Earth Minimum Orbit Intersection Distance of 0.05 astronomical units or less, roughly 7,480,000 kilometers or 4,650,000 miles, is one of two criteria that define a “potentially hazardous asteroid,” according to the Center for Near-Earth Object Studies. The second criterion is size: an absolute magnitude of H 22 or brighter, which corresponds to roughly 140 meters across when a standard reflectivity is assumed. Any object that meets both tests earns the “potentially hazardous” label and receives closer scrutiny from planetary-defense teams.
The public-facing piece of this system is the Asteroid Watch dashboard operated by JPL. According to the dashboard’s own description, it covers asteroids more than about 460 feet (140 meters) in size whose orbits bring them within 4.6 million miles (7.5 million kilometers) of Earth’s orbit. Visitors can explore this information through the main Asteroid Watch page, which summarizes current close-approach activity and links to more technical tools.
A companion module on the same site displays the next five approaches to within that distance, giving any visitor a quick snapshot of what is headed our way. This rolling list is designed to be approachable: it highlights each object’s estimated size, closest-approach distance, and relative speed, translating orbital mechanics into numbers that non-specialists can understand at a glance.
Behind the dashboard sits the Close-Approach Data API maintained by JPL’s Solar System Dynamics group. Its default query parameters are set to return NEO Earth close approaches less than 0.05 au, the same 7.5-million-kilometer boundary, within a defined forward-looking window. Researchers, journalists, and amateur astronomers can pull machine-readable records from this feed, which means the same orbital calculations that inform internal NASA reviews are available to anyone with an internet connection.
One open question is whether the public release of the next-five-approaches module has measurably increased follow-up observations by amateur astronomers. The hypothesis is plausible: a visible, regularly updated list of incoming objects could motivate backyard telescope operators to submit fresh astrometry to the Minor Planet Center within 48 hours of each listed event. No data in the available NASA or JPL documentation confirms or denies that effect, however, so the link between dashboard visibility and observer behavior remains untested in any published study accessible through these sources.
How CNEOS software builds the close-approach record
The CNEOS close-approach overview explains that its system detects predicted Earth close approaches for all known NEOs and tabulates the data organized by time. That phrasing, drawn directly from JPL’s own technical introduction, is precise: “all known” means every cataloged object with a computed orbit, not a curated shortlist. The software runs continuously, updating as new observations refine each object’s trajectory and as newly discovered asteroids are added to the catalog.
For each object, the close-approach computation propagates its orbit forward and backward in time, searching for passages within the chosen distance threshold. When the geometry of an orbit brings it near Earth, the system records the date and time of closest approach, the nominal miss distance, and the relative velocity. These entries are then made accessible through tables and APIs that can be filtered by date range, distance, and other parameters, allowing specialists to focus on the subset of events most relevant to their work.
A separate layer of the same database infrastructure, the Small-Body Database, stores ancillary information including close-approach records and virtual-impactor data. Virtual impactors are hypothetical future impact scenarios that have not yet been ruled out by additional observations. Their presence in the database does not mean an impact is expected; it means the orbital uncertainty still permits one, and more telescope time is needed to close the gap. As new astrometric measurements come in and refine an orbit, most virtual impactors are systematically eliminated.
The size threshold for “potentially hazardous” status introduces a small but real source of confusion. JPL’s Asteroid Watch page phrases it as “more than about 460 feet (140 meters).” The next-five-approaches module rounds differently, stating “about 150 meters.” And a separate NASA Planetary Defense explainer describes the cutoff as “larger than approximately 500 feet (approximately 140 meters).” All three descriptions point to the same underlying criterion, the H 22 absolute magnitude limit, but the translated diameter varies because it depends on assumed surface reflectivity. Readers comparing NASA pages side by side may notice these discrepancies; they reflect rounding and albedo assumptions, not a policy disagreement.
Another subtlety is that the “close approach” boundary and the “potentially hazardous” definition are related but not identical. Many objects that come within 4.6 million miles are far smaller than 140 meters and therefore do not qualify as potentially hazardous, even though they appear on public close-approach lists. Conversely, a large asteroid with an orbit that never dips inside 0.05 au would not be tagged as potentially hazardous under the current criteria, because its geometric relationship to Earth’s orbit keeps it at a safe distance.
Gaps in the public record and what to watch next
Several pieces of the tracking picture are absent from the publicly documented system. No specific object names, flyby dates, or miss distances from the CNEOS close-approach tables appear in the source documentation reviewed here. The dashboard promises the next five approaches, but the underlying data rotates as objects come and go, and no archived snapshots of past dashboard states are publicly linked. For anyone trying to reconstruct how public messaging evolved around a particular object, this lack of historical captures can be a real limitation.
Direct statements from CNEOS scientists or program managers about current detection cadence, false-positive rates, or the fraction of the near-Earth population already cataloged are also missing from these primary pages. NASA has published such figures in congressional testimony and mission reports, but the dashboard and API documentation do not surface them in a way that a casual visitor would encounter. As a result, someone arriving through a search engine may leave with a good sense of upcoming flybys but little context about how complete the survey really is.
There are also practical limits to what close-approach tables can say about risk. A predicted pass inside 4.6 million miles can sound alarming without the surrounding detail: the object’s size estimate, the uncertainties in its orbit, and whether any virtual impactors remain. Those nuances are available in technical databases but are not always distilled into the public-facing summaries. That gap can feed misunderstandings when social media posts highlight a single distance figure without explaining how conservative the threshold is compared with Earth’s actual radius and the vastness of space.
The practical consequence for anyone tracking this topic is straightforward. The 4.6-million-mile boundary is best understood as an early-warning filter, not a line between safety and danger. It defines which objects receive closer attention from planetary-defense analysts and which encounters are highlighted for the public, but it does not by itself indicate that an impact is likely or even plausible. To interpret any individual flyby, readers need the additional layers that CNEOS already computes: object size, orbital uncertainty, and the presence or absence of virtual impactors. As NASA refines its tools and communication strategies, one development to watch will be whether more of that deeper context migrates into the dashboards that most people see first.
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*This article was researched with the help of AI, with human editors creating the final content.
