Asteroid 2025 SC79 links two populations near Venus’s orbit

A difficult object to find again

Asteroids deep inside Earth’s orbit are challenging targets for ground-based telescopes. They appear close to the Sun in the sky, where twilight brightness and observations through a thicker layer of atmosphere make faint objects harder to detect.

Scott S. Sheppard discovered 2025 SC79 on September 27, 2025, using the Dark Energy Camera on the Blanco telescope in Chile. The new study reports its recovery with the Keck I telescope on October 7, 2026, followed by observations with the Very Large Telescope, or VLT, on October 24–25.

Here, “recovery” means observing a previously discovered asteroid at a later appearance. Keck recorded four 30-second exposures. The asteroid appeared 19 arcseconds from its predicted position, still within the prediction’s formal uncertainty region. The VLT follow-up extended the observational record to 392 days.

An orbital boundary, not a physical barrier

The study separates two asteroid classes using aphelion: an orbit’s farthest distance from the Sun. In its adopted definitions, Atira asteroids have aphelia between 0.718 and 0.983 astronomical units. Their orbits remain inside Earth’s closest distance to the Sun. The more tightly confined 'Ayló'chaxnim class has aphelia below 0.718 astronomical units, inside Venus’s closest solar distance. One astronomical unit is approximately the Earth–Sun distance.

The adopted orbit of 2025 SC79 has an aphelion of about 0.71887 astronomical units. That makes it an Atira, just outside the 'Ayló'chaxnim boundary. The boundary is a classification rule; an asteroid can change classes as its orbit evolves.

Colours suggest a V-type surface

The VLT team measured brightness through four optical filters. Comparing those measurements reveals how strongly the asteroid reflects different wavelength bands. Calibration against reference stars allowed the researchers to express the results in a standard colour system.

They compared three measured colour differences with templates for 24 asteroid classes. The V-type template gave the closest match, ahead of the R and O templates. The measured colours were approximately g−r = 0.83, r−i = 0.176 and i−z = −0.50 magnitudes, with uncertainties that differed between measurements.

The authors describe the surface as V-like, rather than claiming a confirmed classification. A basaltic fragment from a parent body that had separated into internal layers is consistent with these colours. Spectroscopy—measuring reflected light across many wavelengths—is needed to test that interpretation.

The observations also do not establish a particular parent family. The authors identify the Vesta family as a plausible reservoir, while noting that other differentiated bodies could supply V-type material.

Size depends on assumed reflectivity

Brightness alone does not uniquely determine an asteroid’s diameter: a smaller, more reflective object can resemble a larger, darker one.

Using a typical V-type visible reflectivity, or albedo, of 0.362, the researchers estimate a diameter of about 379 metres. Its formal uncertainty is roughly −25 to +27 metres, holding that albedo fixed. An alternative, population-model-based albedo of 0.222 gives about 483 metres.

Neither value is a direct size measurement. The quoted uncertainties exclude uncertainty in the assumed albedo. The authors also caution that the observations were made at large illumination angles, making the conversion to standardised brightness sensitive to the asteroid’s unknown scattering behaviour. Thermal observations could help constrain both size and reflectivity.

Testing possible orbital histories

The researchers generated 10,000 computational “clones”: slightly different starting orbits consistent with the uncertainties and correlations in the adopted JPL orbital solution. They simulated each orbit for five million years backward and forward, including gravity from the Sun, eight planets and a separate Moon.

Over the 100,000 years preceding the simulation’s May 2026 reference epoch, the ensemble spent an average of 30.2% of its time in the 'Ayló'chaxnim class and 69.8% as Atiras. Over the following 100,000 years, those shares were 3.6% and 96.0%; the remaining 0.4% represented time after clones had been removed following simulated collisions.

Across longer, 500,000-year windows, the average cumulative time in the 'Ayló'chaxnim class was 231,000 years backward and 120,000 years forward. These are ensemble results, not a recovered account of the asteroid’s actual past or a certain forecast of its future.

In the five-million-year forward simulation, 14.56% of clones collided with Venus, 3.77% with Mercury and 0.22% with Earth. These are outcomes within the specified model, not a present-day Earth-impact risk assessment.

A likely source, with important limits

A published population model assigned a combined 95.4% probability to delivery channels associated with the inner Main Belt. Together with the V-like colours, that favours an inner Main Belt origin without identifying an exact birthplace.

The orbital calculations also have limits. They omitted general relativity and nongravitational forces. Their collision-detection method could miss grazing encounters, and backward runs did not remove clones after collisions.

The preprint’s central result is therefore a model-supported connection: an asteroid now classified as an Atira has possible trajectories that include temporary residence inside Venus’s perihelion boundary. The colour observations provide a separate clue to its surface and origin, pending further measurements.

Source: the full supplied manuscript and appendices of the November 2026 preprint, VLT and Keck recovery and characterisation of 2025 SC79, not only its abstract.