The Encounter

On July 5, 2026, at approximately 09:30 UTC (18:30 JST), JAXA’s Hayabusa2 Extended Mission will perform a high-speed close approach of the near-Earth asteroid (98943) Torifune. The spacecraft is planned to pass within 1 km of the asteroid at a relative velocity of roughly 5 km/s to observe a small, undercharacterised body estimated to be 450 m across and probably elongated.

There are thousands of asteroids of sizes similar to Torifune, and an asteroid of that size is projected to collide with Earth once every 100 to 1,000 years and is large enough to cause significant damage. Currently, ground-based observations are unable to reveal extensive details of these asteroids. Beyond its scientific value, the Torifune flyby is a demonstration of the high-precision close-approach guidance relevant to planetary defence, which may be needed in a kinetic-impactor deflection scenario against a hazardous asteroid.

Photometry and Rotation Period

Using the Fink broker, I extracted sparse photometric data of Torifune observed through the Zwicky Transient Facility (ZTF) from January 2025 to January 2026. For each observation, I queried NASA JPL Horizons to obtain Torifune’s heliocentric distance, geocentric distance, and phase angle, and applied corresponding distance and phase-angle corrections to estimate its absolute magnitude. Fitting the reduced light curve with a second-order multiband Lomb-Scargle periodogram yields a best-fit rotation period of 5.0213 hours, in agreement with the published value of 5.021516 hours. The folded light curve has an amplitude of 0.635 mag. Modeling Torifune as a triaxial ellipsoid viewed near its equatorial plane and assuming negligible albedo variation across the surface, this amplitude implies a minimum equatorial axis ratio of a/b ≳ 1.79. The true elongation is therefore at least this large.

Torifune’s Spin Phase at Closest Approach

The Face Presented to an Earth-Based Observer

To find which side of asteroid Torifune was facing us during the Hayabusa2 flyby, I used a known reference time (T0 = JD 2460930.95), where the asteroid’s fitted rotational phase was 0.979. I then converted the close-approach time, 5 July 2026 at 09:30 UTC, into Julian Date (T = JD 2461226.90). Subtracting T0 from T gives 295.95 days, or 7102.7 hours. Dividing by the rotation period of 5.0213 hours gives about 1414.5 rotations; only the fractional part, ~0.515, affects the final orientation. Adding it to the anchor phase (0.979 + 0.515 = 1.494) and taking mod 1 gives an Earth-facing rotational phase of about 0.49 as there will be uncertainties coming from the rotation-period error propagated over roughly 1,400 rotations. At the flyby instant, Torifune is therefore almost exactly half a rotation from the phase-zero reference.

The Face Presented to the Hayabusa2 Spacecraft

The phase previously calculated is the orientation as viewed from Earth; the spacecraft, however, viewed Torifune from a slightly different direction. 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. Comparing the asteroid-to-observer and asteroid-to-spacecraft vectors via their dot product yields θ ≈ 70.2°, so Δphase = 70.2/360 ≈ 0.195 rotations. With the Earth-facing phase ≈ 0.49, the Hayabusa2-facing phase is 0.49 ± 0.195, i.e. about 0.30 or 0.69 depending on the spin direction. In both cases, the spacecraft sees the smaller side of the asteroid.

Because the ZTF baseline extends to within months of the encounter, this propagation spans a short interval and the accumulated phase uncertainties likely stay smaller. As the phase is anchored close to the event rather than extrapolated across years, it makes it more reliable for this prediction.

Limitations

The "broader side vs. smaller side" interpretation depends entirely on the phase-zero assumption fitted against the model's actual maxima. Second, the light curve symmetry under a 180° longitude shift, means which of the two "smaller side" ends the spacecraft would encounter cannot be distinguished by these calculations. Furthermore, the spacecraft trajectory and encounter time themselves carry operational uncertainty. The definitive check of ground-based light curve and the geometric reconstruction will come when the flyby images are released by the Japan Aerospace Exploration Agency (JAXA) and comparing the face(s) Hayabusa2 actually recorded against this prediction.