About comets

Comets are serendipitous celestial visitors. There are numerous ancient records of the occurrence of fuzzy, tailed objects in the sky. Then there are legends that relate these objects to the fall of empires, disasters and epidemics on the Earth. It was Edmund Halley who first successfully used Newton’s gravitation laws in the 18th Century to show that the fascinating celestial objects are actually members of the Solar System. In a year about 20-25 comets may be seen, some new while others are encore. Periodic comets follow highly elliptical orbits while numerous others are parabolic, even hyperbolic. There are supposed to be hundreds of billions of comets as members of the Solar System populating its outermost regions. For an average mass of 4×1016gm, the combined mass of the comets would be just 6-7 Earths only. There are comets that have stayed in human memory for long, the foremost among them being the Halley’s Comet that returns every 75-76 years.

Far away from the Sun, a comet is just a tiny irregularly shaped body, its nucleus alone that is barely a few kilometres in dimensions and rotating about some axis. It is composed of rocky material and icy mixture of water, carbon dioxide, ammonia and methane, sulfur, silicate minerals and complex organic compounds etc. As the tiny form approaches near the Sun, water-ice in the heated surface starts to sublimate. This also loosens up the dirty parts of the nucleus, the meteoroids. The gas and dust create a head (coma, about 10,000 – 1,00,000 km) and a tail, directed away from the Sun. As the comet gets nearer the tail increases in extent that can get millions of kilometres long, and vice versa. Spectroscopic studies of comets from ground based astronomical observatories, with the Hubble Space Telescope as also a number of fly-by space missions to comets have revealed the nature and composition of the comets and their tails in an unprecedented detail. The studies of comets are important for in them lie the clues to the past and present of the Solar System and even to origin of life on the Earth, and of their own. Some astronomers believe building blocks of life dumped on the Earth by comets that smashed into it some 3.8 billion years ago.

A close fly-by in August 2011

A comet shall pass by the Earth from sufficiently close quarters in August 2011. It has already come quite near us. The comet is named 45P/Honda–Mrkos–Pajdušáková. It was discovered on Dec 3, 1948 by Minoru Honda from Kurashiki in Japan – and has been named after Minoru Honda, Antonín Mrkos, and Ľudmila Pajdušáková. It is a short period, Jupiter-Family comet taking 5.25 years to complete one orbit round the Sun, perihelion q = 0.52965 AU, aphelion Q = 5.5115 AU (1 AU= the mean distance of the Earth from the Sun = 149597871 km). It has had many encounters with the planet Jupiter in the past that have influenced its orbit and the period. In 2011, it is the eleventh observed visit of the comet since discovery; in the visits in 1959 and 1985, the comet was missed out. Astronomer P L Lamy and team put to good advantage the Wide Field Planetary Camera 2 of the Hubble Space Telescope on Feb 4 and Feb 5, 1996 to get high resolution pictures of its nucleus despite an active coma around it when the comet came very close to the Earth, just 0.17 AU. It had already passed its perihelion on Dec 26, 1995. An analysis of the pictures suggested its nucleus to be elongated and having a mean radius of 300 metres only. They also found that 11% of the surface was active. The size of its nucleus is variously quoted, its being as large 900 metres to 1.6 kilometres.  The spacecraft Rosetta launched by the European Space Agency on Mar 2, 2004 towards a comet known as 67P/Churyumov-Gerasimenko for a rendezvous in 2014 and then follow it towards the Sun, approached in Jul 2006 to 0.06 AU of the ion tail of the comet 45P/Honda–Mrkos–Pajdušáková also and secured useful information. The picture here is by Tim Puckett of USA taken on 1995 December 28.99, with a 30 cm f/7 Meade LX-200 and a SBIG ST-6 CCD camera in an exposure of 300 seconds.

A passage as close as 0.060085 AU or 23.56 LD (1Lunar Distance = 3,81,584 km; mean value) on Aug 15, 2011, 13:30 IST would be its first ever since discovery. In the last century at least, it has not passed by the Earth as close. That provides an excellent opportunity even for the ground based observers. During this passage, the comet should brighten up to a visual magnitude + 8. In a very rough way, that is about 10 times fainter than the faintest star naked eye can see in a dark sky. It should then be accessible through powerful binoculars and small telescopes, as an early morning object. Around the time, it is a very low declination object, therefore relatively more easily accessible from low latitudes. It would also be moving through the sky rather fast, headed north-east. Only by around Aug 20, 2011, it moves up in declination, from – 26° to – 21° through that day itself, and to be found easily from northern latitudes. For instance, on Aug 15, the day of its closest passage, the comet moves north, from a declination of about  – 70° to about – 61° within a span of 24 hours. Yet for an observer at Delhi it stays below horizon even at the sunrise time (05:49 IST). In contrast, for those at Bangalore, the comet rises on Aug 15 around 03:00 IST. However, it reaches an altitude of only about 7° above horizon until the sunrise, at 06:09 IST. As the comet nears its perihelion that it shall pass on Sept 28.7815 next, it is expected to brighten up to 6.1 mag whereafter it begins to fade.

In the course of its motion round the Sun, there will always be occasions in the future when during a certain return the comet 45P/Honda–Mrkos–Pajdušáková passes by the Earth from sufficiently close. A close encounter is due in its next visit itself, from a distance of 0.09 AU on Feb 11, 2017.

Of a few close encounters with other comets

A comet’s passing by the Earth this close is rare. The comet C/2002 V1 (NEAT), discovered on Nov 6, 2002 passed by the Earth on Feb 18, 2003 from 0.1 AU. A comet known to have passed by the Earth closest in history is the one designated D/1770 L1 (Lexell), discovered on Jun 14.95, 1770 by Charles Messier from Paris. It passed by the Earth on Jul 1, 1770 from a mere 0.0146 AU. That would be 5.724 LD. The comet is named after Anders Lexell who had computed its orbit. The comet IRAS-Araki-Alcock (C/1983 H1) passed by the Earth in 1983 from 5 million km, or about 13 LD.

Near Earth Objects

There is sufficient evidence to suggest that the Earth has been impacted by asteroids and comets ever since the Solar System was formed. These objects are called Near Earth Objects (NEO). An NEO is one with a perihelion distance equal to or < 1.3 AU. Some of these are on or nearly Earth crossing orbits. The NEO population consists of bodies varying in size from the dust sized fragments to those tens of kilometers across; we have herein asteroids that are rocky (the NEAs), and comets that are ice and dust (NECs). The latter are mostly defunct ones. NEAs above 1 km in diameter are estimated to be about 1100 in number. The orbital speeds are of the order 10 km/sec or so. In comparison, the orbital speed of the Earth is 30 km/sec. The NEAs originally belonged to the asteroid belt but were deflected into new orbits due to gravitational interaction with one or more planets. The largest Earth crossing asteroid is 1627 Ivar, about 8 km across and 1015 kg in weight. There were 84 NECs known until May 2010. While NEOs can be potentially hazardous, they offer an opportunity for physical exploitation in the future.  

What if a comet, or an asteroid passes so close as to be deflected enough to hit the Earth? Probability of the occurrence of such an event is extremely small but not zero. For an asteroid of size 250 m to 1 km, that would be an event in several thousand years.  Objects in this size range are not easy to trace as at present. Estimates have been made of the disastrous consequences of such an impact. The NEOs receive constant attention of scientists and the relevant agencies.

In their 1977 science fiction, Lucifer’s Hammer, Larry Niven and Jerry Pournelle developed story of a comet impacting the Earth and the post apocalyptic scenario. Similarly, in the Hammer of God, a 1993 science fiction, Arthur C Clarke wove a plot about an asteroid named Kali headed Earthwards. Both the books have science assimilated into their plots. Then we have the well known 1998 disaster films, Deep Impact about a comet and Armageddon about a rogue asteroid on collision course with the Earth.