Halley’s comet highlighted in Royal Astronomical Society meeting.

Since antiquity, comets have been associated with portents and omens. The most famous comet of all is probably comet Halley or Halley’s comet, implicated in King Harold’s defeat by the Saxons in the 1066 Battle of Hastings and depicted in the 230-foot long Bayeux tapestry. Even as late as the 20th century, comets hadn’t completely shed their reputation as `terrifying apparitions’; In 1910 Halley returned to Earth’s vicinity and astronomers predicted that the comet’s tail will engulf our planet on the 19th May of that year.  This caused widespread panic as people thought they would be poisoned by the compound cyanide recently discovered in the tail using the then brand new method of spectroscopy.

Sir Isaac Newton. Source: Wikipedia Commons

Edmund Halley FRS. Source: Wikipedia Commons

The first major stride in making sense of these celestial visitors came with the publishing of Principia Mathematica by Sir Isaac Newton in 1687. In it, Newton proposed that the motion of objects in space obey certain physical laws such as the inverse-square gravitational law of force and that these laws can be expressed mathematically to explain the orbits of the planets. The significance of the work was picked up by many of Newton’s contemporaries including Edmund Halley, then Astronomer Royal in Britain. Both patron and supporter of Newton’s work, Halley applied Newton’s law of universal gravitation to his observations of a comet that appeared in 1682, showed that the comet was making one full revolution around the Sun every 76 years. He then proposed that the comets recorded in 1531 and 1607 were, in fact, previous apparitions of the same comet and further predicted that the comet will be seen again in 1758. Although Halley did not live to see the comet’s next return, the comet was indeed picked up by an amateur astronomer in Germany on Christmas Day 1758*, confirming Halley’s prediction 70 years ago. The comet’s recovery was heralded as a successful test of Newtonian physics, the comet was named in Halley’s honour and thus comets were welcomed into the Age of Reason.

*The same day as Newton’s birthday!

Halley’s vindication was a momentous occasion in the history of astronomy, yet it didn’t answer the more fundamental question of what a comet actually is. The answer to that question would have to wait until the 20th century and would require actually going out to meet comet Halley in deep space. In 1950, Harvard astrophysicist Fred Whipple proposed that comets are created when an object the size of a small asteroid – the so-called nucleus of the comet – approaches the Sun, causing ice on the nucleus to evaporate with the gas escaping into space taking bits of the comet with it. This so-called “dirty snowball” hypothesis appealed to astronomers because it explained in a simple way the gas and dust observed in comets by ground-based telescopes. But testing it would require sending a spacecraft to look at a nucleus up close, something beyond the capabilities of 1950s technology.

Fred Whipple with a real-life model of his dirty snowball concept. Source: https://www.daviddarling.info/encyclopedia/W/Whipple.html

The year 1957 saw the beginning of the space age as the first artificial satellite of the Earth was launched. Shortly after, humanity began to send robots to investigate the Moon and nearby planets and, by the 1970s, the technology of space travel had finally matured to the point where it was possible to visit a comet. But which one? That question answered itself as astronomers realised that comet Halley would return in 1986; what better target than the object that inaugurated the scientific study of comets 200 yr ago?   

The plan to visit Halley’s comet very quickly became an international affair as several spacefaring nations and agencies decided to join the Halley “bandwagon”. One notable absentee from the international effort was NASA, as budgetary cutbacks in previous years nixed the American plan for a mission to Halley. Instead of building its Halley probe from scratch, the Soviet Union decided to modify two of its Venus probes to fly past the comet after dropping off capsules in the planet’s atmosphere. Meanwhile two newly-minted space-faring entities used this opportunity to stage their first ever mission to deep space: Japan would send two probes to Halley while Europe would send one. The European probe was named Giotto, after the 14th century Italian painter Giotto di Bondone who included Halley’s comet in his work “The Adoration of the Magi”. Its mission was simple: to fly through the comet’s head where the nucleus was supposed to be and get close enough to prove (or disprove) Whipple’s “dirty snowball” theory. 

During 1984 and 1985, the different ships of what became known as the “Halley Armada” set off from Earth one by one, expecting to reach the comet when closer to the Sun in March 1986. As the exact location of the nucleus was not known, the idea was for the Russian probes to scout out the location of the nucleus, allowing Giotto to precisely target its very close pass. This ambitious plan was not without risk, however: because Halley orbits the Sun in the opposite sense than the Earth does, each spacecraft would have to meet the comet nearly head on, flying past it at the breakneck speed of 70 km/s*. Under those circumstances, a chance collision with a particle no larger than a grain of rice would impart enough energy to put the unlucky spacecraft out of action.

*At that speed, one would travel from Armagh to Belfast in 1 sec of time.

Giotto at Halley’s comet. Credit: European Space Agency.

The nucleus of Halley’s comet as seen by the Giotto spacecraft. Credit: European Space Agency.

In the event, every spacecraft in the Armada survived their brush with Halley’s comet. Giotto in particular came to about 600 km of the nucleus, threading a true “eye of the needle” in astronomical terms. The spacecraft did not emerge entirely unscathed from the encounter however, as dust impacts had temporarily upset its attitude in space. Its camera had also been rendered useless, but not before snapping the pictures that everyone was hoping for, showing the 10-mile wide nucleus of the comet spewing off jets of gas and dust into space.

It has now been 37 yr since Halley’s comet graced our skies. The comet nucleus has long returned to the outer reaches of the outer solar system from whence it came, slightly beyond the orbit of Neptune. This week the comet is as far from the Sun as it can be and exactly at the halfway point between the 1986 apparition and the next one, expected in the year 2061. To mark the occasion, the Royal Astronomical Society is hosting a meeting of cometary scientists this Friday 8th December at its Burlington House HQ in Piccadilly, Central London. The meeting will focus on how our understanding of comets has advanced since Halley’s comet was last seen and the potential breakthroughs that might come to pass until 2061. The meeting is open to both fellows and non-fellows of the RAS, for in-person or online participation.


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