Asteroid 152637, also known as 1997 NC1, swept past Earth on June 27 at 11:15 UTC, passing at a distance of roughly 1.6 million miles. The space rock spans an estimated 0.6 miles across, a diameter comparable to the width of a small town, yet scientists still cannot pin down its exact size. That uncertainty is the real story: competing measurements place the asteroid’s diameter anywhere between 0.75 and 1.65 km, and the June 2026 flyby offered a rare chance to narrow that gap using ground-based radar.
Why a 1.6-million-mile flyby still demands attention
A distance of 1.6 million miles sounds comfortable, and it is. The asteroid posed zero collision risk during this pass. But the encounter matters because of what it reveals about how well planetary defense systems can characterize objects that are already cataloged. The nominal close-approach distance was approximately 0.017 au, or about 6.7 to 6.8 lunar distances, according to the European catalog of near-Earth objects. That is close enough for the Goldstone Solar System Radar in California’s Mojave Desert to bounce signals off the asteroid’s surface and build delay-Doppler images, which yield far more precise size and shape data than optical telescopes alone.
The core tension is straightforward. Infrared observations from the Spitzer Space Telescope produced one set of diameter and albedo values for 1997 NC1, while visible-light data pointed to a different combination. Those two solutions sit at opposite ends of the 0.75 to 1.65 km range. Goldstone radar observations during this close approach were expected to deliver diameter precision tight enough to rule out at least one of those competing albedo solutions, potentially cutting the current size uncertainty by a third or more. If radar confirms the asteroid sits near the upper end of that range, it would rank among the larger near-Earth objects to pass this close in recent decades.
Discovery at Haleakala and three decades of tracking
The asteroid was first spotted on July 5, 1997, by the Near-Earth Asteroid Tracking program, known as NEAT, operating from Haleakala on Maui. NEAT was a joint effort between NASA and the Jet Propulsion Laboratory designed to survey the sky for objects whose orbits bring them near Earth. In the nearly three decades since discovery, ground-based observatories and space-based infrared telescopes have refined the asteroid’s orbit to high precision, even as its physical properties have remained less certain.
That orbital refinement has been driven in part by long-arc astrometric follow-up and by consistent inclusion of 1997 NC1 in official monitoring lists. NASA’s close-approach database tracks the object’s past and future encounters with Earth, listing the June 27, 2026 flyby years in advance. The JPL Horizons system, which provides high-precision ephemerides for solar system bodies, supplied observatories with the pointing coordinates needed to keep the asteroid in their fields of view throughout the encounter, allowing for coordinated campaigns across multiple longitudes.
The diameter range of 0.75 to 1.65 km reflects a fundamental challenge in asteroid science. Size estimates derived from reflected sunlight depend on assumptions about how reflective the surface is. A dark, coal-like surface reflects little light, so the object must be larger to appear as bright as it does. A lighter surface means the object can be smaller. Spitzer measured thermal emission to estimate albedo independently, but the resulting values conflicted with what visible-light photometry suggested. Without radar-derived shape models, neither solution can be confidently preferred, leaving a broad band of possible diameters and masses.
Competing size estimates and what radar can settle
The gap between 0.75 km and 1.65 km is not a minor bookkeeping detail. It represents a factor-of-two difference in diameter, which translates to roughly an eightfold difference in volume and, assuming similar composition, in mass. For planetary defense planning, mass is what determines the energy an impactor would deliver. An asteroid at the upper end of the range would release dramatically more energy in a hypothetical collision than one at the lower end. Resolving the ambiguity therefore has direct consequences for how agencies prioritize monitoring and, eventually, any deflection resources that might one day be needed.
Goldstone radar works by transmitting a coded radio signal at the asteroid and analyzing the echo. The time delay gives precise distance, and the Doppler shift across the echo reveals the object’s rotation rate and rough shape. By mapping echo power as a function of delay and frequency, scientists reconstruct two-dimensional “delay-Doppler” images that can be inverted into three-dimensional shape models. Together, these measurements constrain diameter to a level that optical and infrared telescopes cannot match for objects at this distance. A precision of roughly 15 percent on diameter from a single radar apparition would be sufficient to distinguish whether the asteroid’s albedo is consistent with a dark carbonaceous surface or a brighter silicate one, eliminating one of the two solutions that have coexisted since the Spitzer observations.
Radar also offers a check on the orbit itself. While 1997 NC1’s trajectory is already well determined, each radar ranging measurement pins down the asteroid’s distance to within tens of meters, tightening the uncertainty on its future path. This is particularly valuable for objects of this size, where even small non-gravitational forces, such as the Yarkovsky effect from uneven heating, can subtly shift the orbit over decades. Incorporating radar data into orbital solutions allows dynamicists to test whether such forces are detectable and to update long-term impact probability assessments accordingly.
Why 1997 NC1 is a useful test case
From a planetary defense perspective, 1997 NC1 is large enough to be consequential but not so large as to be vanishingly rare. An object around a kilometer across could cause regional to continental devastation in the unlikely event of an impact, making it a key scale for risk modeling. Yet the asteroid’s current orbit keeps it at safe distances for the foreseeable future, turning it into a natural laboratory rather than a cause for alarm.
That makes the June 2026 flyby an ideal opportunity to test how well observation networks perform when an object of known orbital parameters but uncertain size comes into radar range. Optical telescopes can measure light curves to infer rotation period and search for evidence of a binary companion. Infrared instruments can refine thermal properties. Radar can tie those threads together by delivering a direct constraint on size and shape. Comparing the pre-encounter predictions to the post-encounter radar results will show where current models succeed and where they need revision.
The flyby also underscores the importance of maintaining and upgrading infrastructure. The loss of the Arecibo Observatory in Puerto Rico removed one of the world’s most powerful planetary radars, placing a greater burden on facilities like Goldstone. Each successful campaign demonstrates what remains possible with the current toolkit-and highlights the scientific return that could come from future dedicated planetary radar systems.
From one asteroid to the broader population
While the focus of this encounter is 1997 NC1, the implications ripple outward to the wider near-Earth object population. Every time an asteroid’s size, shape, and spin state are pinned down, researchers gain another data point for understanding how these bodies form, evolve, and respond to forces such as sunlight, thermal re-radiation, and tidal encounters. Those insights feed into population-level models that estimate how many objects of various sizes remain undiscovered and how they are distributed in orbital space.
In that sense, the June 27 flyby was not merely a distant pass of a harmless rock but part of a long-running experiment in how humanity maps and monitors its small celestial neighbors. By combining precise orbits from tools like the Horizons ephemerides with close-approach records and targeted radar campaigns, scientists are steadily turning once-fuzzy points of light into well-characterized worlds. For 1997 NC1, the payoff is a sharper answer to a basic question-how big is it?-and, with it, a clearer picture of the risks and realities of the near-Earth environment.
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*This article was researched with the help of AI, with human editors creating the final content.
