---
title: "Why a comet's dust tail does not simply trail behind it"
description: "Dust inherits the comet's velocity, then sunlight changes its orbit. Grain size, release time and viewing angle make the tail curve."
canonical_url: "https://sandboxphysics.com/stories/comet-dust/why-comet-tails-bend/"
language: "en"
page_type: "story"
alternate_languages:
  zh-CN: "https://sandboxphysics.com/zh/stories/comet-dust/why-comet-tails-bend/"
last_modified: "2026-10-28"
---

[Comet dust, from light pressure to real images](https://sandboxphysics.com/stories/comet-dust/)

L1 · Comet dust dynamics

# Why a comet's dust tail does not simply trail behind it

Sandbox Physics · 2026-10-28

A comet can leave the Sun with its tail pointing ahead of it. Nothing has reversed the direction of a wind in space. The dust was already moving around the Sun when it left the nucleus, and sunlight has been changing that motion ever since.

## The dust already has somewhere to go

Imagine opening your hand while riding a fast train. A dropped object keeps the train's forward motion; it does not begin at rest beside the track. A grain released by a comet likewise keeps almost all the nucleus's orbital velocity. Gas escaping from the warm nucleus adds a smaller launch velocity, which helps spread the cloud in three dimensions.

Now let the nucleus and grain continue separately. Solar gravity turns both paths inward. Sunlight also transfers outward momentum to the grain. The result is a new solar orbit, with a different curvature and often a different period. The visible tail collects grains on many such paths. A photograph compresses this history into one shape.

Reading: Finson & Probstein · Burns

## A push matters more to a small grain

The amount of sunlight a grain intercepts grows with its cross-sectional area. Its mass grows with its volume. For spherical grains of the same density and pressure efficiency, the ratio of radiation pressure to solar gravity, called $\beta$, therefore grows as the radius gets smaller. Our teaching assumptions give about 0.57 for a one-micrometre grain and 0.0057 for a hundred-micrometre grain.

The word push can be misleading here. In those two examples solar gravity remains stronger than radiation pressure, so the net central force is still inward. The small grain's path turns inward less sharply than the nucleus's path. That difference is enough to separate them. Some released grains escape altogether because they inherited a speed too large for the weakened attraction.

Reading: Burns · Finson & Probstein

## One photograph mixes size and age

Fine grains released recently can appear near coarser grains released earlier. A broad tail does not label the grains by their age. Astronomers calculate families of possible paths: a synchrone joins grains released at one time, and a syndyne joins grains with the same force ratio but different release times. Their overlap shows which explanations a photograph can distinguish.

The Lab lets you compare those two families directly. A single burst spreads many sizes along a curve. Continuous release of one size makes a different curve. A realistic cloud also has a distribution of launch velocities and grain properties. Recovering those properties takes further image or production information.

Reading: Finson & Probstein · Li et al.

## The Sun and the observer both matter

The broad dust tail often lies roughly away from the Sun, with curvature inherited from the orbital motion. Its apparent direction need not be exactly antisolar or exactly opposite the comet's velocity. Rotate the same three-dimensional cloud and it can look broad, narrow or almost straight. Looking nearly along the orbital plane piles different depths onto similar image positions.

The ISON tutorial makes this comparison with actual HST pixels. At the selected April 2013 exposure, the projected antisolar direction was about ninety-two degrees east of north, and the direction opposite the comet's heliocentric motion was about seventy-nine degrees. Broad outer-image sectors peak near ninety degrees under the tutorial's coarse estimator. This establishes an orientation comparison; several grain histories still agree with it.

Reading: JPL Horizons API and observer/vector conventions · MAST HST program 13198 and public data-use policy · Li et al.

## Two other tails and forces need different physics

An ion tail contains charged material coupled to the solar wind and magnetic field. The neutral dust model on this page cannot predict that tail. Color photographs sometimes show both, which is one reason a single arrow drawn behind the nucleus tells too little.

Poynting–Robertson drag is another radiation effect. It slowly removes orbital energy and angular momentum from bound grains. At one astronomical unit its circular-orbit time scale is hundreds to many thousands of years for the grain sizes here. It is separated from the months-long dust fan in the Lab. Uneven thermal emission can also change an orbit, but predicting its direction requires a rotating body's temperature pattern. The final reading guide develops that additional question.

Reading: Burns · Vokrouhlický et al.

## Try the observation

### [Watch one orbit become many](https://sandboxphysics.com/labs/comet-dust-tails/)

1. Compare Fine grains fan out with Heavy grains follow. Keep your eye on the nucleus and the cyan reference grain.
2. Select One burst, many sizes, then One size, many ages. Look for the different curves before reading the lower diagram.
3. Switch between Above the orbit and Near the orbital plane. The particles keep their positions; their projection changes.

[Next: From photon momentum to a grain's reduced solar gravity](https://sandboxphysics.com/stories/comet-dust/light-pushes-small-grains/)

## Sources and calculation scope

1. [Finson & Probstein, A Theory of Dust Comets I (1968)](https://adsabs.harvard.edu/pdf/1968ApJ...154..327F)

   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.

2. [Burns, Lamy & Soter, Radiation Forces on Small Particles (1979)](https://ntrs.nasa.gov/citations/19790070469)

   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.

3. [Li et al., HST observations of Comet ISON, arXiv:1311.0826v1](https://arxiv.org/abs/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.

4. [MAST HST program 13198 and public data-use policy](https://archive.stsci.edu/proposal_search.php?id=13198&mission=hst)

   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.

5. [JPL Horizons API and observer/vector conventions](https://ssd.jpl.nasa.gov/horizons/manual.html)

   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.

6. [Vokrouhlický et al., The Yarkovsky and YORP Effects, arXiv:1502.01249v1](https://arxiv.org/abs/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.