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 , 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.