The Fate of Comet Halley

Without a doubt, the most widely known comet has got to be Comet Halley. First observed in ancient times perhaps as early as 446 BC, it has returned to the inner solar system approximately every 76 years since then to inspire or frighten our ancestors. Its periodicity was first noted in 1705 by famed English astronomer Edmond Halley who used the then-new theory of gravitation developed by Isaac Newton to predict its return. The reappearance of Comet Halley in 1759 was hailed as a triumphant vindication of Newtonian physics and the predictive power of orbital mechanics. It is also the origin of the comet’s official astronomical designation, 1P/Halley – named after its discoverer, Halley, and the first recognized periodic comet, hence “1P/”.

Since that famously predicted appearance, Comet Halley has returned to the inner solar system three times with the last perihelion pass taking place in 1986 (when many of my readers were either too young to remember or were not even born yet!). During that close pass in 1986 – the first since the start of the Space Age – Comet Halley was studied at close range by a veritable armada of spacecraft from Europe, Japan and the now defunct Soviet Union (see “The Missions to Comet Halley“). All these years later, Comet Halley has completed over a third of its journey around the Sun as it makes its way through the depths of the outer solar system and towards its next close encounter with the Sun in 2061.

Being so well studied, many might expect that we know much about Comet Halley including its orbit and where it came from. Astronomers have determined with great precision that Comet Halley is currently in an elliptical orbit with a semimajor axis of 17.83 AU, an eccentricity of 0.967, an inclination of 162.26° (i.e. it orbits backwards around the Sun compared with the planets) and a period of 75.32 years. Despite the fact that many parameters of its orbit are known to a precision as good as one part per million, past studies of the evolution of the orbit of Comet Halley as it is perturbed by Jupiter and other planets have shown it to be chaotic.  By “chaotic” they do not mean random but in the sense that very slight differences in the assumed initial conditions of the comet’s orbit lead to increasingly divergent predictions of its position as we look further back in time. This seems rather ironic considering that the prediction of Comet Halley’s return three centuries ago was considered proof of Newton’s physics and the appearance of the clock-like regularity of the Solar System implied by his models.

A view of the nucleus of Comet Halley from the ESA Giotto spacecraft acquired near its closest approach on March 14, 1986. (ESA/MPAe/Lindau)

The properties of its orbit and composition derived from measurements by spacecraft and ground-based instruments strongly suggest that Comet Halley originally came from the distant Oort Cloud. It is also known that its orbit has experienced a variety of resonances in previous millennia and that it has been strongly affected by Jupiter as well as Saturn, to a lesser extent. Various orbital calculations performed over the past several decades have shown that it is impossible to make any meaningful predictions of where precisely Comet Halley was more than just a few thousand years ago.  As a result, it simply is not possible to determine exactly when Comet Halley left the Oort cloud and how long it has spent among the outer planet before its first close pass through the inner solar system.

But what about its future? Just like its past, the future of Comet Halley is impossible to predict with any certainty. Studies performed over the last third of a century have confirmed that its orbit is chaotic. But a new study (which has just been submitted for publication) of the orbital dynamics of Comet Halley has been performed by Marco A. Muñoz-Gutiérrez (Instituto de Astronomía, Universidad Nacional Autonoma deMexico) and two colleagues which has allowed them to figure out the likely fate of the comet and estimate how long it will take.

Unlike earlier studies, the analysis of the numerical integrations of the orbit of Comet Halley performed by Muñoz-Gutiérrez et al. has allowed them to define precisely for the first time a quantity known as the Lyapunov exponent. This mathematical quantity characterizes the rate of separation of infinitesimally close trajectories and is vital in predicting the time until the onset of chaotic behavior. By exploring the orbital evolution of 33,300 test particles divided amongst four different series of long numerical integrations that explored variations of orbital parameters in different ways, Muñoz-Gutiérrez et al. were able to determine the chaoticity of the comet’s orbit as a function of phase space and create what they called “survival time maps” as well as accurately calculate the Lyapunov exponent.

Muñoz-Gutiérrez et al. found a value of the Lyapunov exponent that shows that the orbit of Comet Halley is chaotic on times scales of less than a century suggesting its position can not be accurately predicted over periods of time much greater than this. Their maps suggest that the survival time scale for test particles whose orbital parameters differ by less than today’s observational uncertainties for the size of the semimajor axis or eccentricity ranged from 10⁴ to 10⁶ years with an average of 300,000 years. It seems likely that Comet Halley will either collide with another body in the solar system or be ejected outright in as little as 10,000 years. If it somehow escapes either of these fates, Muñoz-Gutiérrez et al. conclude that it seems likely that the orbit of Comet Halley will evolve to have an even higher eccentricity but lower inclination than its current orbit.

But it seems unlikely that the nucleus of Comet Halley will last the tens of thousands of years or longer to meet this last potential fate. During each perihelion passage close to the Sun, the nucleus loses a small amount of mass due to the evaporation of ices and the loss of dust. At its current rate of mass loss, it has been estimated that the nucleus of Comet Halley will be severely diminished or even cease to exist in just 15,000 years. Only time will tell if Comet Halley ends its existence as a stream of dust in solar orbit, impact another body in the solar system or is ejected from the system of its birth and into the cold void of interstellar space. While our current civilization is unlikely to survive that long (historically, few civilizations last more than a handful of millennia), perhaps our distant descendants will know of Comet Halley’s fate.

Related Reading

“The Missions to Comet Halley”, Drew Ex Machina, March 6, 2016 [Post]

“A Personal Adventure in Home Computing: The Origin of Comet Shoemaker-Levy 9”, Electronic Journal of the Astronomical Society of the Atlantic, Volume 5, Number 8, March 1994 [Issue]

General References

M.A. Muñoz-Gutiérrez, M. Reyes-Ruiz and B. Pichardo, “Chaotic Dynamics of Comet 1P/Halley; Lyapunov Exponent and Survival Time Expectancy”, arXiv:1409.7762 (submitted to Monthly Notices of the Royal Astronomical Society), September 27, 2014 [Preprint]