The BYORP Effect: How Sunlight Torques Reshape Binary Asteroid Orbits

What BYORP Is and Its Physical Basis

The binary YORP (BYORP) effect is a secular radiation torque that reshapes the mutual orbit of a binary asteroid. It was first proposed by Matija Ćuk and Joseph Burns in 2005, and given a rigorous analytic treatment by Jay McMahon and Daniel Scheeres in 2010.

The ordinary YORP effect (Yarkovsky–O'Keefe–Radzievskii–Paddack) torques the spin of a single irregular body, because sunlight re-emitted as thermal photons from its uneven surface carries away momentum asymmetrically. BYORP takes the same physics and applies it to the orbital motion of a two-body system. The crucial precondition is that the smaller body — the secondary — must be tidally locked, keeping one hemisphere permanently facing the primary, exactly as our Moon does toward Earth.

• Because the secondary always presents the same orientation to its orbital motion, its shape asymmetries do not average out.
• The net photon recoil therefore has a persistent component along the orbit.
• That along-track force is what secularly changes the orbit's size and shape.

The Mechanism: From Photon Recoil to Orbital Torque

Every surface element on the sunlit and warm side of the secondary emits thermal photons, each carrying momentum p = E/c. For a perfectly symmetric body these recoils cancel. A real asteroid moon is a lumpy, faceted object, so the emitted photons produce a small net force.

Because the secondary is synchronous, McMahon & Scheeres showed you can average this force over three cycles — the mutual orbit, the heliocentric orbit, and the slow precession of the orbital node. What survives is a constant tangential acceleration parameterized by a single dimensionless BYORP coefficient B, which encodes purely the secondary's shape:

• B measures the effective asymmetric area as a fraction of the body's total cross-section.
• A positive B pushes the secondary outward (growing semimajor axis); a negative B pulls it inward.

The averaged along-track force F ≈ (B · Φ · A) / c, where Φ is the solar flux at the asteroid's heliocentric distance and A is the secondary's cross-sectional area. Feeding this through the orbit-averaged equations yields the semimajor-axis drift. Notably, statistical studies find the effect preferentially shrinks orbits.

Key Quantities and a Worked Example

The drift rate of the mutual semimajor axis scales as

da/dt ∝ B · L☉ / (c · a_hel² · √a_mutual),

where L☉ = 3.83×10²⁶ W is the solar luminosity, a_hel is the heliocentric distance, and a_mutual is the binary separation. The 1/a_hel² captures the falling sunlight; the 1/√a_mutual comes from the orbital dynamics.

• BYORP coefficient B: typically a few ×10⁻² to ×10⁻¹. For the secondary of 1999 KW4, a shape model gives B_nom = 2.082×10⁻².
• Characteristic timescale: ~10⁴–10⁵ years for a D_p = 2 km primary with a D_s = 0.4 km secondary at 1 AU, separated by roughly 1.5 primary diameters.
• Drift rate: the same KW4 model predicts a semimajor-axis change of order 7 cm/yr — meters per century.

These rates sound trivial, but a binary separation of only a few kilometers means BYORP can double or destroy the orbit in a few hundred thousand years, far shorter than the ~10⁷-year dynamical lifetime of a near-Earth binary.

How BYORP Is Detected and Where It Appears

BYORP acts too slowly to watch directly, so it is inferred from tiny changes in the mutual orbital period measured across many years. The workhorse technique is mutual-event photometry: as the secondary eclipses and transits the primary, the combined lightcurve dips. Timing those dips over a decade or more constrains any drift in semimajor axis.

• (66391) 1999 KW4: radar-modeled by Steven Ostro's team; period analysis indicates its mutual semimajor axis grows at about +1.2 ± 0.3 cm/yr, an expanding orbit consistent with a BYORP-driven trend.
• (175706) 1996 FG3: 17 years of lightcurves show a drift of only −0.07 ± 0.34 cm/yr — essentially zero, versus a shape-model prediction of ~2–7 cm/yr, hinting at a tide–BYORP balance.
• Radar systems (Arecibo, Goldstone) supply the shape models that let B be computed independently of the timing.

Roughly one in six near-Earth asteroids larger than ~200 m is binary, so BYORP is a population-wide sculptor, not a curiosity.

BYORP Versus Its Cousins — Tides, Yarkovsky, and YORP

BYORP is easily confused with three neighboring radiation and gravitational effects, but each acts on a different quantity.

• Standard YORP torques a single body's spin, spinning it up or down over 10⁵–10⁶ years — and it is YORP-driven rotational fission that likely creates many of these binaries in the first place.
• The Yarkovsky effect drifts a body's heliocentric orbit (its distance from the Sun) over 10⁶–10⁸ years; BYORP instead changes the internal binary orbit.
• Tidal evolution, driven by dissipation inside the bodies, always expands the mutual orbit and slows any spin toward synchrony.

The pivotal insight is that BYORP and tides can oppose each other. When an inward-pushing BYORP torque exactly balances outward tidal expansion, |da/dt|_BYORP = |da/dt|_tide, the orbit stalls in a stable equilibrium. The near-zero drift of 1996 FG3 is the leading observational candidate for a system caught in exactly this frozen state — a natural laboratory linking two otherwise-hidden dissipation mechanisms.

Significance, DART, and Open Questions

BYORP matters because it governs the fate and demographics of binary asteroids. It can drive a secondary outward until external gravitational perturbations unbind the pair, or inward until the components merge or the secondary is tidally disrupted — recycling the system through a possible YORP-fission → BYORP-destruction loop.

The concept gained fresh urgency with NASA's DART mission, which slammed into Dimorphos, the moon of (65803) Didymos, in September 2022. DART shortened Dimorphos's orbital period, but it may also have altered the moon's shape, spin state, and libration — the very ingredients that set B. The follow-up ESA Hera mission is en route to characterize the post-impact system and pin down its BYORP-relevant properties.

• Open question: Dimorphos's oblate shape means its long axis may circulate rather than librate, which can strongly weaken or even nullify BYORP.
• Debated: the true sign and magnitude of B, whether tidal quality factors Q are large enough for equilibrium, and how non-principal-axis (tumbling) rotation suppresses the torque.

BYORP versus its close relatives — all are sunlight-driven but act on different quantities

Effect	What it acts on	Requires	Typical timescale (NEA scale)
YORP	Spin rate & obliquity of one body	Irregular shape, any spin	10⁵–10⁶ yr
BYORP	Mutual orbit (semimajor axis, eccentricity)	Tidally locked, asymmetric secondary	10⁴–10⁵ yr
Yarkovsky	Heliocentric orbit (drift in/out from Sun)	Nonzero spin, thermal inertia	10⁶–10⁸ yr
Tidal evolution	Mutual orbit (expands it)	Dissipation inside the bodies	10⁵–10⁷ yr
Radiation pressure	Instantaneous push away from Sun	Any illuminated surface	Continuous (non-secular)