An international team of astronomers has discovered a massive asteroid, roughly 700 meters in diameter, with an orbit that keeps it hidden within the glare of the sun. The object, designated 2025 SC79, follows a path entirely inside the orbit of Venus, making it exceptionally difficult to detect using conventional survey methods that typically scan the night sky away from the sun.
This discovery highlights a critical blind spot in planetary defense efforts. While most near-Earth asteroids are found during nighttime observations, a population of potentially hazardous objects with orbits interior to Earth’s remains largely uncatalogued. These “twilight asteroids” are only observable during a brief window just before sunrise or after sunset. The newfound asteroid is one of the largest of its kind found in recent years and belongs to a small, elusive group of space rocks that could pose a future threat.
Characterizing a New Celestial Object
The newly discovered asteroid, 2025 SC79, is a significant find due to its size and unusual orbit. Measuring approximately 700 meters (2,300 feet), it is large enough to cause catastrophic damage on a continental scale if it were to impact Earth. For comparison, this is considerably smaller than the Chicxulub impactor that led to the extinction of the dinosaurs, but an impact from an object of this size could still have devastating global consequences. The discovery was made on September 27 by astronomer Scott S. Sheppard of the Carnegie Institution for Science.
2025 SC79 is classified as an Atira asteroid, meaning its orbit is entirely confined within Earth’s. It is the 39th member of this rare group. Even more exceptionally, it is only the second known asteroid with an orbit entirely inside that of Venus. It completes a full circuit around the sun in just 128 days, making its orbital period the third shortest of any known asteroid. During its journey, it also crosses the orbit of Mercury.
The Hunt in Solar Glare
Finding asteroids like 2025 SC79 is a formidable challenge for astronomers. The vast majority of asteroid surveys are conducted at night, pointing telescopes away from the sun to avoid its overwhelming glare. Objects that orbit close to the sun are lost in its brightness and are only visible in the brief twilight periods. As Sheppard explained, the most dangerous asteroids are often the most difficult to detect.
Tools of Twilight Detection
The discovery was made using the powerful Dark Energy Camera (DECam) mounted on the Víctor M. Blanco 4-meter Telescope at the Cerro Tololo Inter-American Observatory in Chile, a facility of the National Science Foundation’s NOIRLab. DECam’s ability to capture deep images over wide swaths of the sky makes it uniquely suited for these challenging twilight surveys. By scanning large areas near the horizon just as the sun is rising or setting, astronomers can hunt for faint objects that would otherwise be impossible to spot. Follow-up observations with Carnegie Science’s Magellan telescopes helped confirm the discovery.
Assessing the Long-Term Risk
Currently, 2025 SC79 does not have an orbit that intersects with Earth’s. However, the orbits of such objects can change over thousands of years due to gravitational interactions with other planets. This means that while it poses no immediate danger, it could become a concern for future generations. The discovery of 2025 SC79 follows the announcement of another large, so-called “planet-killer” asteroid named 2022 AP7, which is nearly a mile wide and has an orbit that might one day cross Earth’s path. An impact from an object like 2022 AP7 would be a mass extinction event.
These discoveries underscore the importance of identifying and tracking all near-Earth objects, including those hidden by the sun. Astronomers estimate that most near-Earth asteroids over one kilometer in diameter—the size that could trigger a global catastrophe—have been found, but some may still be lurking in the inner solar system. Only about 25 asteroids with orbits entirely within Earth’s have been discovered to date because of the observational difficulties.
Future Observations and Planetary Defense
After its discovery, 2025 SC79 is now moving behind the sun from our perspective and will be unobservable for several months. Once it reemerges, astronomers will conduct further studies to learn more about its composition and refine its trajectory. This data is crucial for accurately predicting its path far into the future.
The ongoing search for near-Earth objects is a critical component of planetary defense. While the probability of a major impact in the near future is extremely low, the potential consequences are enormous. In recent years, scientists have begun testing methods to defend against a potential asteroid threat. In a landmark 2022 mission, NASA’s Double Asteroid Redirection Test (DART) successfully altered the orbit of a small asteroid by crashing a spacecraft into it, demonstrating a viable technique for planetary defense. Continued vigilance and technological development are essential to safeguard Earth from potential impacts.
