AC11 · Comet dust dynamics
Comet dust, from light pressure to real images
Dust leaves a comet with its orbital velocity. Follow how sunlight changes that motion, derive the release families and the much slower PR clock, then compare a frozen HST image with JPL geometry.
Why a comet's dust tail does not simply trail behind it
Dust inherits the comet's velocity, then sunlight changes its orbit. Grain size, release time and viewing angle make the tail curve.
02 · L1From photon momentum to a grain's reduced solar gravity
Derive the radiation-pressure force ratio, calculate its size dependence and distinguish reduced inward attraction from outward net acceleration.
03 · L2Inherited velocity turns light pressure into a dust tail
Derive the grain's energy and escape threshold, construct synchrones and syndynes, and test a three-dimensional release model.
04 · L3Why Poynting–Robertson drag takes away orbital energy
Derive the first-order drag, its negative orbital power and the circular drift law, then separate it from thermal-lag recoil.
05 · L3Project frozen JPL dust geometry onto a real HST image of ISON
Reproduce a real HST direction measurement with exact UTC–TDB conversion, light time, TAN/CD projection and explicit relative-force checks.
06 · L4Read PR drag and thermal recoil without mixing their clocks
Read the fixed Burns 1979 and Yarkovsky review equations and figures, define a thermal model, and separate orbital fits from force measurements.
Sources and calculation scope
- Finson & Probstein, A Theory of Dust Comets I (1968)
ApJ 154, 327–352, DOI 10.1086/149761. Sections I–III, especially pp. 329 and 334–335 / Figure 4: post-gas-drag dust motion, reduced solar gravity, synchrones and syndynes. We add an explicitly assumed launch law for the teaching scene.
- Burns, Lamy & Soter, Radiation Forces on Small Particles (1979)
Icarus 40, 1–48, DOI 10.1016/0019-1035(79)90050-2. Sections III–IV derive the first-order radiation force; section VI, equation (19) / Figure 7 discuss pressure efficiency; sections VII–VIII, equations (47)–(50) give reduced-gravity orbits and orbit-averaged PR evolution. The paper was read in the author-hosted 48-page PDF.
- Li et al., HST observations of Comet ISON, arXiv:1311.0826v1
Section 2 gives the April 2013 HST observation sequence and original reduction; the sunward inner jet and extended tail motivate excluding the inner two arcseconds in our coarse direction diagnostic. Our 2026 archive single-frame reduction is different from the paper's combined images.
- MAST HST program 13198 and public data-use policy
HST/WFC3 F606W dataset ic7301ooq. The tutorial freezes ic7301ooq_drz.fits, 13,579,200 bytes, SHA-256 bd436ea87d7ab05ba8e10280e03ea04887db7055841260fb8b4bdf0b547e33f7. MAST public-mission data-use policy permits use with attribution: HST/MAST, program 13198, PI J.-Y. Li; STScI operated by AURA under NASA NAS5-26555.
- JPL Horizons API and observer/vector conventions
API response version 1.2; API documentation version 1.3. Frozen comet record 90004196, solution JPL#57 dated 2025-Nov-21_13:42:48, DE441. Read TIME_TYPE, TLIST, ICRF/FRAME vector conventions and observer quantities 1, 19, 20, 21, 24, 27, 30. The saved requests and responses preserve the adopted time scale and geometry.
- JPL Solar System Dynamics astronomical parameters
Exact AU, day and speed of light; planetary-system GM values for the sampled relative-force check. Solar gravitational parameter 132712440041.93938 cubic kilometres per square second and luminosity 3.828 times ten to the twenty-sixth watts are frozen from the Horizons Sun metadata. The teaching density and pressure efficiency are assumptions, not measured ISON grain properties.
- Vokrouhlický et al., The Yarkovsky and YORP Effects, arXiv:1502.01249v1
Fixed 2015 version, 24 pages. Section 2.1, equations (1)–(3), treats diurnal/seasonal semimajor-axis drift; section 2.2 explains thermal/shape refinements; section 3.1 and Figures 2–3 discuss astrometric detections including Golevka and 1992 BF. This is a source-reading guide, not a new thermal simulation or a claim about the latest detections.