On July 5, 2026, JAXA’s Hayabusa2 Extended Mission will perform a high-speed close approach of asteroid (98943) Torifune at approximately 09:30 UTC. During the Torifune encounter, the spacecraft will pass the asteroid at a relative velocity of about 5 km/s, using its onboard instruments to observe a small, undercharacterized body estimated to be roughly 450 m across and possibly elongated. The Torifune flyby is an important test of close-approach guidance technology relevant to planetary defense.
Using the Fink broker, I extracted sparse photometric data points for asteroid (98943) Torifune from January 2025 to January 2026, observed through the Zwicky Transient Facility (ZTF). I then queried NASA Horizons to obtain Torifune’s heliocentric distance, geocentric distance, and phase angle at each observation time, and applied distance and phase-angle corrections to estimate its absolute magnitude. I used a second-order multiband Lomb-Scargle periodogram to fit the corrected lightcurve data, finding a best-fit rotation period of 5.0213 hours. Modeling the asteroid as a triaxial ellipsoid viewed near the equatorial plane and assuming negligible albedo variation across the surface, the 0.635 mag amplitude implies a minimum equatorial axis ratio of a/b ≳ 1.79.
To determine which side of asteroid Torifune was facing us during the Hayabusa2 close approach, I first used a known reference time (T0 = 2460930.949722), where I found the asteroid’s rotational phase to be 0.979419. I then converted the close-approach time, 5 July 2026 at 09:30 UTC, into Julian Date (T = 2461226.8958333). Subtracting (T0) from (T) gave 295.9461113 days, or 7102.7066712 hours. Dividing this by the rotation period of 5.0213 hours showed that 1414.51549822 rotations had passed. Only the fractional part, 0.51549822, affects the final orientation, so I added it to the original phase: (0.979419 + 0.51549822 = 1.49491722). Taking this modulo 1 gives a final rotational phase of approximately 0.49, meaning that at close approach, Torifune was almost exactly half a rotation away from the start of the phase.
Using the 2026-Jul-05 09:30 UTC Horizons rows, Torifune was at RA = 157.899667°, Dec = +10.508333°, Δ = 0.634312859 AU, while Hayabusa2 was at RA = 157.898708°, Dec = +10.508583°, Δ = 0.634308970 AU. I converted both RA/Dec/range positions into Cartesian vectors using r = Δ[cosδ cosα, cosδ sinα, sinδ], then subtracted the Torifune vector from the Hayabusa2 vector to get the apparent asteroid-to-spacecraft vector. Then I compared the asteroid-to-observer vector with the asteroid-to-Hayabusa2 vector using the dot product, yielding θ ≈ 70.18°. Since 70.18° < 90°, Hayabusa2 was viewing the same general hemisphere as Earth. Converting this angle into rotation phase gives Δphase = 70.18/360 = 0.19495 rotations. With the Earth-facing phase 0.494917, the Hayabusa-facing phase is therefore 0.494917 ± 0.19495, so approximately 0.300 or 0.690, depending on the spin direction. In both cases, that means the spacecraft will be facing the smaller side of the asteroid.
Using the 2026-Jul-05 09:30 UTC Horizons rows, Torifune was at RA = 157.899667°, Dec = +10.508333°, Δ = 0.634312859 AU, while Hayabusa2 was at RA = 157.898708°, Dec = +10.508583°, Δ = 0.634308970 AU. I converted both RA/Dec/range positions into Cartesian vectors using r = Δ[cosδ cosα, cosδ sinα, sinδ], then subtracted the Torifune vector from the Hayabusa2 vector to get the apparent asteroid-to-spacecraft vector. Then I compared the asteroid-to-observer vector with the asteroid-to-Hayabusa2 vector using the dot product, yielding θ ≈ 70.18°. Since 70.18° < 90°, Hayabusa2 was viewing the same general hemisphere as Earth. Converting this angle into rotation phase gives Δphase = 70.18/360 = 0.19495 rotations. With the Earth-facing phase 0.494917, the Hayabusa-facing phase is therefore 0.494917 ± 0.19495, so approximately 0.300 or 0.690, depending on the spin direction. In both cases, that means the spacecraft will be facing the smaller side of the asteroid.