Fastest Spinning Asteroid: 2025 MN45 Record-Breaking Rotation Discovery

Time-Domain Astronomy and the Revolution in Solar System Science

The Vera C. Rubin Observatory and LSST Camera: Unprecedented Capabilities

1. The World's Largest Digital Camera

   The Legacy Survey of Space and Time (LSST) Camera mounted on the Vera C. Rubin Observatory's Simonyi Survey Telescope in Cerro Pachón, Chile, is the world's largest digital camera ever constructed for astronomical research. With 3,200 megapixels—an unprecedented resolution for a wide-field instrument—the LSST Camera is coupled to an 8.4-meter primary mirror, creating an optical system with an extraordinarily large light-gathering power and wide field of view. The camera covers 9.6 square degrees of sky in a single exposure, an area roughly equivalent to 40 times the area of the full moon. This exceptional combination of resolution, sensitivity, and field of view enables the rapid, deep survey observations that characterize the LSST. The detector itself consists of 189 CCDs arranged in a hexagonal focal plane, with each CCD consisting of a 4,096 × 4,096 pixel array. The camera is designed to detect objects as faint as magnitude 24.5, reaching depths unprecedented for a wide-field survey. Most importantly for asteroid discovery and characterization, the LSST Camera can capture the sky with remarkable speed, imaging the same field of view multiple times in rapid succession over the course of a single night, enabling precise measurement of how brightness changes minute by minute.

2. The First Look Commissioning Observations

   Before beginning the full Legacy Survey of Space and Time, the Rubin Observatory conducted an intensive commissioning phase to verify the performance of all systems and demonstrate the science capabilities that the instrument would ultimately achieve. During this commissioning phase, which occurred in April and May of 2025, the Simonyi Survey Telescope and LSST Camera obtained seven nights of intensive observations under a dense, rapid cadence optimized for testing rather than following the planned survey strategy. These "First Look" observations were conducted over approximately 10 hours of observing time, compressed into a short observational window. Despite the limited duration and non-standard cadence, the commissioning observations demonstrated the unprecedented discovery power of the LSST system. The First Look dataset included roughly 340,000 individual asteroid detections representing approximately 2,103 previously unknown asteroids—a reminder that the main asteroid belt remains incompletely surveyed, and that powerful new instruments will continue to expand the inventory of small solar system bodies. Within this dataset, astronomers identified 19 asteroids with remarkably rapid rotation rates, and one object—2025 MN45—stood out as having rotation characteristics previously thought impossible for objects of its size.

3. Measuring Rotation Periods via Photometric Variability

   The measurement of asteroid rotation periods from rapid time-series photometry is a technique that has been employed for decades, but the LSST Camera brings unprecedented precision and enabling power to this method. As an asteroid rotates, different surface regions—some darker, some brighter—rotate into and out of view from Earth's perspective. This causes the asteroid's brightness to vary periodically as it spins. By obtaining images at multiple epochs separated by minutes to hours, and measuring the asteroid's brightness at each epoch, astronomers can construct a light curve—a plot of brightness versus time. The periodicity of variations in the light curve directly reveals the rotation period. The LSST Camera's rapid cadence capability—imaging the same field multiple times per night—is ideal for this purpose. For 2025 MN45, observations spanning seven nights and approximately 10 hours of cumulative data provided sufficient temporal coverage to definitively measure the rotation period as 1.88 minutes. The repeated images and high photometric precision of the LSST Camera ensured that this measurement was robust and reliable. For comparison, earlier surveys studying asteroid rotation typically had much longer intervals between observations, making detection of very rapid rotation difficult or impossible. The LSST's capabilities have opened a new discovery space for ultra-fast rotators previously beyond the reach of observational techniques.

Analysis I: The Discovery of 2025 MN45 and Properties of Ultra-Fast Rotators

1. The Record-Breaking Rotation Period

   Among the 76 asteroids in the Rubin First Look dataset for which rotation periods were reliably determined, 2025 MN45 emerged as exceptional. At 710 meters in diameter—larger than any previously known fast-rotating asteroid—this main-belt asteroid completes a full rotation every 1.88 minutes, or 113 seconds. To place this in perspective: a point on the asteroid's equator moves at a velocity of approximately 3.7 kilometers per second, or roughly 13,000 kilometers per hour (8,000 miles per hour). This is comparable to the escape velocity from Earth's surface, and it exceeds the escape velocity from many smaller asteroids. By any measure, the rotation of 2025 MN45 is extreme. Prior to this discovery, the previous record for the largest asteroid capable of such rapid rotation was considerably smaller—typically sub-kilometer objects. The discovery of a 710-meter asteroid spinning faster than nearly all previously known objects demanded explanation. The lead authors, recognizing the exceptional nature of the discovery, undertook detailed analysis to understand how such a massive object could maintain rotational stability at such extreme spin rates without fragmenting or shedding material.

2. The Spin Barrier and Asteroid Composition

   Theoretical models of asteroid stability, developed over decades of observational and computational work, predict a fundamental limit to how fast an asteroid can spin without flying apart. This limit, termed the "spin barrier," arises from the balance between the centrifugal force generated by rotation and the gravitational self-attraction and material strength that hold the asteroid together. For a given asteroid composition and size, there exists a maximum rotation rate beyond which centrifugal forces overcome the asteroid's cohesive strength. For small asteroids, composed primarily of loose material bound mainly by gravity, this spin barrier is relatively low—objects a kilometer or so in size begin to experience structural problems at rotation periods exceeding roughly 2.2 hours. However, for objects in the main asteroid belt, the prevailing model holds that such large asteroids are "rubble piles"—loosely bound collections of rocky fragments and boulders held together primarily by their mutual gravitational attraction. Such objects are expected to have very low material strength, making them particularly vulnerable to disruption if they spin too rapidly. The theoretical expectation is that a 710-meter rubble pile spinning with a period of 1.88 minutes should fragment, its constituent pieces flying off as centrifugal forces overcome gravity. Yet 2025 MN45 clearly does not fragment. This demands a reassessment of asteroid composition and structure. The researchers conclude that 2025 MN45 must be composed of material with exceptional cohesive strength—either solid rock or consolidated clay. Rather than a loose rubble pile, this asteroid appears to be a monolithic or coherent body, fundamentally different from the expected composition of large main-belt asteroids. This discovery suggests that current models of main-belt asteroid composition may be incomplete, and that some large asteroids retain or have reformed into coherent structures far stronger than rubble-pile models predict.

3. Implications for Asteroid Formation and Collisional History

   If 2025 MN45 is indeed a coherent, solid body as the observational evidence suggests, how did it acquire such extreme rotation? Asteroids rotate with angular momentum determined by their formation history and subsequent collisional encounters. An asteroid spinning at 1.88 minutes likely either formed with this rotation or acquired it through a high-impact collision that transferred angular momentum. Theoretically, a solid core remnant could be produced from the disruption of a larger parent body in a catastrophic collision. After the collision, the parent body shatters, but the densest, most coherent core fragments survive and remain in orbit. Such a core would retain the high angular momentum imparted by the collision that fragmented the parent body. Over time, through gravitational reaccumulation, such fragments might reassemble into a coherent body while retaining their initial high spin rate. Alternatively, 2025 MN45 might represent a primordial core or dense central region of a larger asteroid that was excavated by a massive impact. The point is that the extreme rotation of 2025 MN45 appears to encode information about its collisional history. It is likely a fragment or remnant of a larger body, reformed into its present configuration through the violence of asteroid collisions. This connects observations of individual asteroids to the broader processes of collisional cascades and planetary formation that have shaped the asteroid belt over billions of years. As more fast-rotating asteroids are discovered and characterized, they will provide an increasingly detailed fossil record of the impact processes that have sculpted the solar system.

Analysis II: A Population of Fast Rotators and Broader Survey Implications

1. The 19 Fast Rotators from First Look Data

   Although 2025 MN45 is the most extreme example, it is not alone. The Rubin First Look observations identified 19 asteroids with remarkably rapid rotation rates: 16 super-fast rotators with periods between roughly 13 minutes and 2.2 hours, and 3 ultra-fast rotators completing full rotations in less than 5 minutes. All 19 objects are larger than 90 meters in diameter—football-field-sized or larger. This population of fast rotators is itself surprising. Before Rubin, most known rapidly rotating asteroids were small, near-Earth objects accessible to ground-based surveys through their proximity to Earth and correspondingly high brightness. The main-belt asteroids, being more distant and fainter, were undersampled in rapid-rotation surveys. The Rubin First Look data demonstrate that the main asteroid belt harbors a previously unrecognized population of large, rapidly rotating bodies. The existence of this population raises questions: How common are such fast rotators? What fraction of large main-belt asteroids exhibit rotation periods below the theoretical spin barrier? Are they predominantly single monolithic bodies or bound pairs? Do their properties correlate with orbital elements, indicating formation from specific collisional events? These questions will be addressed through the full LSST survey, which is expected to discover millions of previously unknown asteroids over its 10-year operation.

2. Anticipated Discoveries During the Legacy Survey of Space and Time

   The discovery of 2025 MN45 and the 19 fast rotators during a brief, non-optimized commissioning phase provides a tantalizing preview of the discoveries anticipated when the LSST survey formally commences its decade-long operation. Unlike the dense, rapid First Look observations optimized for commissioning, the regular LSST cadence will follow a more systematic pattern designed for long-term survey continuity. The survey will obtain images of the entire Southern Hemisphere sky multiple times over the course of nights, with revisits on timescales of days, weeks, and months. This systematic cadence is less ideally suited for detecting extreme rotation periods than the commissioning observations, but over a decade of accumulated data, even fast rotators will be detected through their periodic photometric variations. Projections suggest that the LSST survey will discover millions of previously unknown asteroids—a revolutionary expansion of the known asteroid population. Among these millions, we anticipate discovering hundreds or thousands of additional fast-rotating objects, including potentially even more extreme spinners than 2025 MN45. The population statistics of fast rotators, constrained by such a comprehensive survey, will enable detailed modeling of asteroid formation, collisional evolution, and the physical properties of main-belt asteroids. Additionally, the LSST's exquisite photometric precision and broad wavelength coverage will enable detailed color analysis of asteroids, constraining their composition and mineralogy. The combination of rotation periods, colors, brightnesses, and orbital elements for millions of asteroids will provide an unprecedented census of the small-body solar system.

3. Connections to Planetary Science and System Stability

   The characterization of asteroids and their properties informs broader questions about planetary system formation and stability. The asteroid belt is understood to be a remnant of the early solar system's primordial planetesimal disk. Planets migrated through this disk, gravitationally scattering planetesimals and ejecting many from the solar system entirely, leaving behind the asteroid belt we observe today. The collision rates, impact velocities, and outcome distributions of collisions among planetesimals and asteroids determine the size distribution, orbital properties, and composition of the surviving asteroid population. By studying the rotation rates, collision signatures, and structural properties of modern asteroids like 2025 MN45, we can constrain models of this early dynamical history. Furthermore, understanding asteroid properties has direct implications for hazard assessment and planetary defense. Asteroids capable of potentially impacting Earth (near-Earth objects) require careful characterization to assess impact probability, energy release, and potential effects. The rotation properties revealed by time-domain surveys inform models of how such objects would fragment during atmospheric entry or impact with Earth's surface. While the asteroids discovered in the Rubin First Look data are main-belt objects posing no known threat to Earth, the techniques and observational capabilities demonstrated will revolutionize our ability to discover, characterize, and assess the threat posed by near-Earth objects, advancing both scientific understanding and planetary protection goals.

Conclusion: A New Era of Solar System Discovery