Chapter 1
Light Curves, Orbits, and Reservoirs
1.1 Light Curves
The reason for the ancient fascination with comets was almost certainly because of
their rapid brightening and their apparent motion against the background stars
combined with their distinctly non-star-like appearance. The first aspect to be
understood scientifically was the orbital motion which was explained by Edmund
Halley. He correctly deduced that there were similarities between the comets of
1531, 1607, and 1682 and predicted that it was the same comet. He thus established
that this comet was not on a hyperbolic orbit but on a very eccentric orbit (LancasterBrown 1985) and predicted its return. He also supported the publication of Newton’s
work on celestial mechanics from which it could be determined that such a comet
would be on a (roughly) elliptical orbit with the Sun at one of the foci of the ellipse
and an axis of symmetry along the long axis of the ellipse passing through the Sun
and the pericentre position. However, the concept of why brightening and fading
occurs and its relationship to the sublimation of an ice-dirt mixture was only clearly
understood in the middle of the twentieth century.
It is now known that the cause of the non-stellar appearance is the sublimation of
a gas/dust mixture from the surface of a small (typically 10 km diameter) nucleus. It
should be recalled that the existence of a single solid nucleus at the centre of
cometary activity was still the subject of serious scientific debate in the 1950s
(e.g. Lyttleton 1951) and not conclusively proven until the spacecraft encounters
with comet 1P/Halley in 1986 (Fig. 1.1; Keller et al. 1986). But long before
confirmation of this concept, observers of a bright comet in the sky could identify
three main elements.
The coma (or “head”) of the comet is nebulous in appearance and often roundish
(Fig. 1.2). The brightness arises from a combination of reflected sunlight from dust
particles and resonant fluorescence from neutral gas radicals. The dust tail is seen in
reflected sunlight and its tail-like nature results from solar radiation pressure (SRP)
acting on emitted dust particles. It extends in the anti-sunward direction away from
© Springer Nature Switzerland AG 2020
N. Thomas, An Introduction to Comets, Astronomy and Astrophysics Library,
https://doi.org/10.1007/978-3-030-50574-5_1
1
Light Curves, Orbits, and Reservoirs
1.1 Light Curves
The reason for the ancient fascination with comets was almost certainly because of
their rapid brightening and their apparent motion against the background stars
combined with their distinctly non-star-like appearance. The first aspect to be
understood scientifically was the orbital motion which was explained by Edmund
Halley. He correctly deduced that there were similarities between the comets of
1531, 1607, and 1682 and predicted that it was the same comet. He thus established
that this comet was not on a hyperbolic orbit but on a very eccentric orbit (LancasterBrown 1985) and predicted its return. He also supported the publication of Newton’s
work on celestial mechanics from which it could be determined that such a comet
would be on a (roughly) elliptical orbit with the Sun at one of the foci of the ellipse
and an axis of symmetry along the long axis of the ellipse passing through the Sun
and the pericentre position. However, the concept of why brightening and fading
occurs and its relationship to the sublimation of an ice-dirt mixture was only clearly
understood in the middle of the twentieth century.
It is now known that the cause of the non-stellar appearance is the sublimation of
a gas/dust mixture from the surface of a small (typically 10 km diameter) nucleus. It
should be recalled that the existence of a single solid nucleus at the centre of
cometary activity was still the subject of serious scientific debate in the 1950s
(e.g. Lyttleton 1951) and not conclusively proven until the spacecraft encounters
with comet 1P/Halley in 1986 (Fig. 1.1; Keller et al. 1986). But long before
confirmation of this concept, observers of a bright comet in the sky could identify
three main elements.
The coma (or “head”) of the comet is nebulous in appearance and often roundish
(Fig. 1.2). The brightness arises from a combination of reflected sunlight from dust
particles and resonant fluorescence from neutral gas radicals. The dust tail is seen in
reflected sunlight and its tail-like nature results from solar radiation pressure (SRP)
acting on emitted dust particles. It extends in the anti-sunward direction away from
© Springer Nature Switzerland AG 2020
N. Thomas, An Introduction to Comets, Astronomy and Astrophysics Library,
https://doi.org/10.1007/978-3-030-50574-5_1
1