“The most dangerous asteroids are the most difficult to detect.” Carnegie Science’s Scott S. Sheppard’s caution is no exaggeration a clear summary of the danger that 2025 SC79, a newly found Atira-class asteroid with its orbit contained entirely within that of Earth, presents. Detected September 27 with the Dark Energy Camera (DECam) on the National Science Foundation’s Blanco 4-meter telescope in Chile, this 700-meter diameter object is just the second known asteroid to orbit entirely within Venus’s orbit, at times crossing that of Mercury. Its speedy 128-day orbit about the Sun has it as the record-third fastest asteroid, after 2021 PH27 and Mercury itself.
To find such bodies, twilight surveys are needed. Traditional asteroid hunts are done at night, with a dark sky, under easier visibility conditions. But Atiras, let alone extremely rare Vatiras, hide in the brightness of the Sun, with feeble reflected light drowned out by solar luminosity. Observations at twilight, shortly after sunset or before dawn, provide that slender opening to find them. The deep-field power of DECam, going beyond typical magnitude limits of most wide-area telescopes, has unveiled this formerly unreachable corner of our inner Solar System. Verification came with follow-up measurements with NSF’s Gemini telescope, as well as Carnegie Science’s Magellan telescopes.
From an orbital mechanic standpoint, 2025 SC79’s orbit is remarkably restricted. Atira asteroids do not ever cross Earth’s orbit, but it would require gravitational perturbations, most notably from Venus, to change its path over time. Such changes, simulations indicate, might eventually move an asteroid into a near-Earth orbit, which would raise its risk of impacting a planet. For 2025 SC79, though it now poses no immediate danger, capable of unleashing energies far exceeding those of nuclear weapons, with resultant casualties in the billions depending upon where it impacts.
Its proximity to the Sun leads to harsh thermal conditions for 2025 SC79. Surface temperatures can reach as high as 500°C, ductile enough to vaporize lead, as was seen with 2021 PH27. Its survival under such conditions implies a composition resistant to thermal stress, possibly a metal or of low volatile content. Near-Sun asteroids also suffer from the Yarkovsky–O’Keefe–Radzievskii–Paddack (YORP) effect, a torque generated by differential thermal emission that can alter rotation speeds and, over several hundred thousand years, cause structural collapse. For a body this large, YORP will reduce its lifetime to below a million years, fragmenting it into smaller pieces.
The engineering problem of protecting a planet from an object like 2025 SC79 is daunting. Preemptive measures described in studies of NEO hazards vary from civil defense installations for smaller impactors to active deflection at orbit for larger ones. For a 700-meter asteroid, kinetic impactorspacecraft that collide at high speed could transfer enough momentum to change its orbit if a few years of warning time were provided. A 10-ton spacecraft impacting at 50 km/s with an ejecta efficiency factor (β) close to 10 might produce a 1 cm/s change in velocity enough to avoid Earth by 15,000 kilometers after a decade. Lower β or lower impact velocities would demand multiple mission tries. Slow-push techniques, including gravity tractors, are more questionable for this mass unless decades of warning time are provided, but they provide high-precision control with independence from composition of a body’s surface.
The finding also illuminates a necessity for developing detection technology. Space-mounted infrared observatories such as NASA’s future Near-Earth Object Surveyor will play a central role in discovering asteroid populations hiding in solar brightness, as a supplement to twilight surveys from the ground. Such a telescope will look closer to the Sun with no atmospheres to get in the way, improving detection out to where Atiras and Vatiras live. As Sheppard describes, “We have to keep covering as much sky as possible in twilight, waiting for these near-Sun asteroids to poke their heads out.”
For the time being, 2025 SC79 has passed behind the Sun, out of observational view for a few months. When it comes back, astronomers will aim it for spectroscopic, thermal, and investigations to learn its mineralogy and origins. It is unknown whether it was expelled from the main asteroid belt or was formed in situ in the inner Solar System. With each data point for such a rare discover, models of asteroid population behavior, orbit evolution, and collision probability are improved information critical for protecting Earth from threats that we cannot yet see.
