My paper on asteroid 2025 FA22 has been published in the Minor Planet Bulletin, Volume 53, Number 2 (2026). This is my second publication in the Bulletin, and it encourages me to continue doing high-quality observational and computational astronomy work for publication.
When I measured Chivilikhin's rotation period last summer (see publication), I was revisiting a known object whose last measurement was nearly a decade old. With 2025 FA22, there was no prior data at all. The asteroid was only discovered a year ago and was already flagged as potentially hazardous.
The Observations
All data were collected remotely from my MonitorMyPlanet Observatory (MPC code R60) in Nerpio, Spain. The instrument is a 0.305-m Ritchey-Chrétien telescope operating at f/6 with a 0.75× focal reducer, mounted on a SkyWatcher EQ8-R equatorial mount and equipped with an ASI2600 monochrome camera. The filter wheel contains a full Johnson-Cousins UBVRI set. Images were binned 2×2, giving a pixel scale of 0.848 arcsec pixel⁻¹.
Observations ran across six nights between September 19–26, 2025, covering solar phase angles from approximately 60° down to 25° as the asteroid receded from closest approach. The close approach itself created an immediate technical challenge: at its fastest, 2025 FA22 was crossing the sky at nearly 3 arcseconds per second, fast enough to streak across a fixed exposure. I had to continuously adjust exposure times between 4 and 120 seconds across the night as the apparent angular velocity changed. Standard bias, dark, and flat-field corrections were applied before photometric extraction, and differential photometry was performed using an ensemble of comparison stars within each field, with reference magnitudes drawn from the ATLAS catalog.
Rotation Period and Shape
Period analysis of the combined R-band dataset used a fourth-order Fourier series fit across a periodogram spanning 6–25 hours. The periodogram shows a well-defined minimum at a synodic rotation period of 13.075 ± 0.002 hours. The composite phased lightcurve combining all six nights displays a stable double-peaked morphology with a peak-to-peak amplitude of 0.62 mag — the classic signature of an elongated body in principal-axis rotation. Amplitude error was estimated as 0.06 mag from √2 times the RMS residual scatter.
Modeling the asteroid as a triaxial ellipsoid viewed near the equatorial plane and assuming negligible albedo variation across the surface, the 0.62 mag amplitude implies a minimum equatorial axis ratio of a/b ≳ 1.77. The asteroid is at least 77% longer along its longest axis than its shortest, more like a stretched dumbbell than a sphere. This is a lower bound, as different viewing geometries or surface albedo patches could make the true elongation larger.
Shape matters for planetary defense. From my earlier work on the DART mission, I knew that an elongated body responds differently to a kinetic impactor than a compact sphere - the momentum transfer efficiency, ejecta geometry, and resulting orbital change all depend on how the body is shaped and how fast it is spinning. Characterizing these properties while an asteroid is still in the news cycle, rather than years later, is precisely the gap that independent observers can fill.
Color Photometry and Taxonomy
On September 22, I added a single night of multi-filter BVRI photometry. Because the asteroid was rotating during these observations, I corrected for rotational effects by interpolating R-band magnitudes to the rotational phases of the B, V, and I measurements, then derived weighted mean color indices with uncertainties propagated in quadrature.
The resulting rotation-corrected colors are B−R = 1.22 ± 0.02, V−R = 0.45 ± 0.02, and R−I = 0.36 ± 0.04, yielding derived indices of B−V = 0.77 ± 0.03 and V−I = 0.81 ± 0.05.
Compared with the Sun's colors, 2025 FA22 is consistently redder across all indices. Interpreted within the broadband photometric taxonomy framework of Lin et al. (2018), these colors place the asteroid within the S-complex, overlapping the S-Q region. This is consistent with a moderately space-weathered, silicate-rich surface, likely related to the ordinary chondrite meteorite family.
The full paper is available at the Minor Planet Bulletin archive: https://mpbulletin.org/issues/MPB_53-2.pdf