Ordinary Meeting, 2004 December 18

 

Comets and their Exploration by Spacecraft

Prof Hughes first wished to thank the Association for inviting him to deliver the Christmas lecture. He recalled that he had started his career with an interest in meteors – tiny dust grains originally broken from the surfaces of comets, producing streaks of light across the sky upon impact with the atmosphere. With time his interests had moved from the dust grains to the comets themselves, but they illustrated a central theme of the talk to come: comets were dying objects, always getting smaller. The first part of the talk would be a general introduction to comets, followed by a discussion of space missions which had explored them, winding up with a discussion of unanswered questions. The speaker remarked that there was no shortage of these: he had thought of no less than fourteen embarrassingly simple questions which science had not yet succeeded in answering.

Observers had long been familiar with the fact that comets were surrounded by a coma, which had led to the idea, traceable back to Newtonian origin, that they comprised of a sublimating dirty snowball nucleus, though in modern times it had first come to widespread acceptance through the work of Fred Whipple. Another familiar fact was that comets evolved: as they approached the Sun, the rate at which material was lost changed rapidly. Using the familiar example of Halley's Comet, the speaker quantified this: at a distance of 3AU from the Sun, the nucleus was thought to lose an average of one molecule per square centimetre per second, yet at perihelion, 0.5AU from the Sun, it was thought to lose 11,000 molecules/cm2/s. The speaker remarked that as well as ice, this also included rocky debris, the dust from which gave rise to both the Orionid and δ-Aquarid meteor showers.

Another familiar fact was that comets spent much of their lives in an inactive state: they generally only 'turned on' and acquired any noticeable coma at a distance of around 3AU from the Sun. Halley, presently at a similar distance to Neptune, was essentially indistinguishable from an asteroid, and would remain so until around 2061, when its 76-year periodic orbit would bring it back to perihelion. Between perihelia, it did nothing more interesting than follow an elliptical orbit described by Kepler's Three Laws. In the case of its latest return in 1985-6, the speaker recalled that a significant coma had first developed around 1985 November, as it passed the orbit of Mars. By the time it passed the Earth's orbit in early 1986, a full coma and tail had developed. The speaker had frequently been asked what colour comets were: the answer was much the same as that of the Sun, they merely scattered the Sun's rays. The ion tail itself had a rather bluer colouration as it was composed of ionised plasma, which, just as an electrical spark, radiated preferentially blue light.

It was also known that there were a large number of comets in the solar system, though no one knew quite how many. Furthermore, because they spent time both far from the Earth, and rather closer to it, the risk of one colliding with our planet was one to be monitored. Historically, it seemed probable that much of the water in the oceans had arrived in the impacts of comets.

Perhaps comets were the most beautiful of the naked eye offerings of the sky, and the speaker showed a few of his favourite images. These included an exceptional image of Halley, with, in the foreground, a burning dust grain from another comet in the form of a Quadrantid meteor. The speaker also showed an artistic photograph of Comet Bennett from Switzerland, just above the Moon-illuminated Jungfrau. Prof Hughes remarked that this particular comet appeared to have fragmented in 1974, and that this was not well understood. Little was known about the mass, composition, or mechanical strength of cometary nuclei, and so in turn, little was known about what was needed to break them apart. Presently, this seemed to happen entirely randomly. Finally, the speaker showed an image of a more recent comet, Hale-Bopp, a fine example where both dust and plasma tail clearly visible.

Given their aesthetic appeal, it was perhaps not surprising, then, that comets had attracted wide historical attention. Legend had it that Julius Caesar had been instructed by his wife not to attend Senate on that fateful Ides of March, because the appearance of Halley's Comet was a bad omen. The same comet appeared in the Bayeux Tapestry, prophesising the defeat of King Harold at Hastings. And the return of Halley in 1301 had inspired Giotto's Adoration of the Magi (1304-6), in which the Star of Bethlehem appeared as a comet.

The speaker proceeded to discuss their scientific investigation, starting with the work of Aristotle, who had believed comets to be atmospheric phenomena. His conviction that the heavens were unchanging was so firmly held, that he could not accept comets as a part of them. He suggested that earthquakes ejected gas, which rose up and ignited in the upper atmosphere. It seemed that his idea that comets were caused by natural disasters was later turned around: comets came to be seen as a warning of bad times to come.

Moving forward somewhat to the Newtonian era, it was Halley who had first shown that the elliptical orbits of Newton's Theory of Gravity extended to comets. Newton himself had later devised a mathematical technique for deriving five orbital parameters for a given comet, given three observations at different times and places. This was not quite complete, for comets had six parameters, and so Newton had had to assume all comets to follow parabolic orbits, with an eccentricity of unity. He had later passed this work onto Halley, who used it to undertake the painstaking work of finding the orbits of many comets, each taking several weeks. It was through this work that his suspicions had been aroused upon noticing amongst his catalogue three comets, in 1682, 1607 and 1531, each with a near-identical orbit, and each separated by around 76 years. The comet in question was the one now known as Halley's Comet, and it had been the first time that anyone had proposed that they might return periodically.

The speaker remarked that Hevelius had thought that comets were made of material ejected from the surfaces of planets, spun off rather like the hurling of a discus. The curvature of the subsequent trajectory would be determined by the speed of the ejection. By contrast, the opposite was now understood to be true: comets often added new material to planets. It was widely thought that Jupiter and Saturn both harboured rocky nuclei of around 10-20 Earth masses, each composed mostly of cometary material.

The process of this accretion could be illustrated with reference to another famous comet: Shoemaker-Levy 9, discovered by David Levy and Carolyn Shoemaker. At discovery in 1993, it had been observed to be in around 20 pieces, each orbiting Jupiter. A comet orbiting a planet was a rare find, though not unprecedented, the speaker added. Adding together the bits, it was thought to have originally been of around 1.5km diameter, and tracing the orbit back, it was found to have fragmented in 1992 upon a close encounter with Jupiter. Moreover, tracing the orbit forward, it was realised that it would collide with the surface of the planet upon its next return in 1994.

At this point, the speaker had to confess some embarrassment. Having calculated that 99.5% of the cometary material would be reduced to an atomic state upon impact, he had briefed the media that there would be little to see: it would sink into the planet like a stone into a pond. Thus he had been somewhat surprised, on the day of the impact, to hear reports of the large dark scars which were in fact seen. His mistake had been to overlook the 0.5% of the material which would not vanish!

The speaker went on to discuss space missions to comets, remarking that each had cost around half-a-billion pounds. The first, ESA's Giotto mission, had flown past Halley's Comet during its 1985/6 return. In preparing the mission, he recalled that it had been established that two observation windows were feasible, 1985 November or 1986 March. The latter had been selected simply on the grounds that time was in short supply, and it would give a few additional months for preparation. In essence, the plan had been to put Giotto on a trajectory which was close to the nucleus of Halley at a particular time, though at that time it would be hurtling past the comet at a relative velocity of 65km/s in a head-on collision. This had made photography somewhat challenging – the satellite was also spinning once every four seconds – and as any photographer knew, movement and rotation were best avoided when taking pictures.

The closest approach distance to the nucleus, 600km, had been selected deliberately: at this distance Giotto would collide with many of the dust grains surrounding the comet, and its chance of surviving the encounter was almost exactly 50%. Indeed the speaker recalled that a collision with one large grain had knocked Giotto severely off-course, but that this had been rectifiable.

Prof Hughes expressed some amusement that in their scheduling, some broadcasters had overlooked the 8-minute lag in transmissions from Giotto, and so images from the craft had arrived somewhat later than the media had anticipated. However, when they finally came, they had been the first ever taken of a cometary nucleus, and though some had described them as hideous – the speaker himself confessed the quality left something to be desired – they provided the first decisive evidence that Halley had a single solid nucleus. In addition, the speaker remarked upon the large number of craters apparent on the surface, appearing rather similar to the impact craters on the Moon, except that in this case the surface was far too young for these to have resulted from impacts. Many now believed comets to have a foam-like structure, outbursts being seen whenever the sublimation of snow opened up a new hole in the surface, suddenly revealing a vast area of fresh snow to solar radiation.

In addition to imaging, Giotto had also collected dust grains from around the nucleus, and measured their mass distribution and chemical composition using a mass spectrometer. The speaker remarked that this hardware was very similar to that onboard Rosetta, due to arrive at comet 67P/Churyumov-Gerasimenko in 2014, however the latter would be able to gain samples from much closer to the nucleus as a result of actually being in orbit around it.

Reviewing other past space missions, the speaker remarked that three had successfully returned images of cometary nuclei: as already mentioned, Giotto had returned images of Halley in 1985 to a resolution of ~200 metres per pixel, but in addition, Deep Space I had imaged Comet Borelli in 2001 at a resolution of ~50 metres, and Stardust had visited Comet Wild II in 2004, taking images at a resolution of ~20 metres. The latter had also collected dust samples, to be returned to Earth on 2006 January 13, using a similar mechanism to that employed for the Genesis capsule: the samples would parachute down into the Utah desert, being caught by helicopter to prevent their impacting with the ground. In view of the failure of Genesis' parachutes, the speaker hoped for better luck this time around. Looking at past imaging performance, he remarked that a factor-of-ten increase in resolution had been achieved in a decade, and it was hoped Rosetta would achieve sub-millimetre resolution when its landing craft was deployed in 2014.

Presently, Rosetta was flying through the solar system towards its destination, having been launched on 2004 March 2. Its path would take it through three gravity-assist encounters with the Earth, and one with Mars, before arriving very close to 67P/Churyumov-Gerasimenko in 2014 May. At that time, in contrast with Giotto, it would also be in a very similar orbit to the comet, allowing an orbital insertion burn to be undertaken, putting it into an orbit around it. It would then follow the comet's nucleus as it approached perihelion in 2015 August. The speaker was hopeful that it would remain active for at least one-and-a-half of the comet's six-year orbits. Prof Hughes remarked that Rosetta had been little in the news in recent months: this was, he explained, simply because it was working nominally. All was going to plan, but as there was little data being returned in flight, there was little news to report.

Moving on to discuss unanswered questions, the speaker remarked that perhaps the most obvious was the mass of comets. The physical dimensions of cometary nuclei could be determined from the images taken by the aforementioned three spacecraft, but as little was known of their composition, so their physical density was also unknown. One might guess that since they were thought to contain a lot of ice, they would share a similar density. But ice was known to be a strong material, and by contrast, comets broke apart under the influence of quite weak gravitational encounters. It seemed possible they were more similar to snow, but in that case, the density would be a mere twelfth that of ice, and so already a factor of ten uncertainty could be seen to have crept into mass estimates. This was a question that Rosetta would answer immediately: as soon as it entered orbit around 67P/Churyumov-Gerasimenko, an orbital radius and period would be known, and from these, using Newton's Laws, the comet's mass could be derived. But by the same token, the speaker remarked that without knowing the mass of the body it was entering orbit around, there were big uncertainties as to what orbital insertion manoeuvre would be required: no one knew how easy it would be to enter orbit, without first knowing the mass of the comet.

Other missions, of interest in the nearer future, included NASA's Deep Impact, due to launch in 2005 January, to make a rendez-vous with 9P/Tempel I in 2005 July. It was to impact it with a 370kg projectile, at a collision velocity of 10km/s. The speaker remarked that the illuminated face of the comet was only 4km across, and so this shot required precision aim. NASA had confidently suggested that a crater of dimensions 100m across and 20m deep would result: a claim he viewed sceptically given the lack of data concerning the mechanical strength of the nucleus. The volume of the crater could be accurately estimated by calculating the energy released in the impact, and comparing with the sublimation energy per unit volume of water ice, but he thought it equally possible that the projectile might sink into the nucleus leaving behind it a 4-metre-wide tunnel. In any case, the results were keenly awaited.

Another proposed mission, though sadly scrapped due to budget constraints, was the Comet Rendez-vous Asteroid Flyby (CRAF) mission, which had planned to project a dart-shaped spacecraft into the nucleus of a comet. It had been hoped that an antenna on the back end would be left protruding from the surface of the nucleus to return data. However, the speaker had thought this an exceptionally challenging proposal, given the lack of information about the material into which it would be impacting, since the dart might either disappear completely into the nucleus, or else rebound from the surface. Nonetheless, it would have been an interesting experiment.

A more complete review of Prof Hughes' excellent discussion of the outstanding questions of cometary astronomy can be found in his own paper on the subject, to be published in a future issue of this Journal.

Following the applause for the speaker's excellent lecture, a member of the audience asked whether there had been any attempt to measure the mass of the three comet nuclei which spacecraft had encountered at close range, since in each case there would have been a small trajectory change as a result of the nucleus' gravitational pull, and this might have been measurable. The speaker said that he was not aware of any such work, and thought that if such a technique had successfully obtained a mass estimate, it would have been headline news. The Meeting then broke for tea, after which the President welcomed Mr Martin Mobberley to present his Sky Notes.

Fairfield

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