By Duncan Lunan

To achieve a rendezvous with Halley’s Comet, which is in retrograde orbit around the Sun, a conventional rocket vehicle would have had to launch in 1978 and make a very tight swing around Jupiter.  The alternatives were to use gentler systems such as a solar sail or ion drive, but all three missions were deemed to be too difficult or too reliant on new technology for the US to attempt.  (I was at the Jet Propulsion Laboratory in Pasadena when the news of cancellation came through from Washington, and a wave of disappointment spread through the facility.)  The ion-drive is now established technology, and Deep Space 1, the NASA mission to Borelly’s Comet, was the first mission to use it fully.  

The first probe to encounter a comet was ICE, International Cometary Explorer, which went through the tail of Comet Giacobini-Zinner in August 1985.  It was done very cleverly with International Sun-Earth Explorer 3, whose orbit was ‘cranked up’ by multiple lunar passes to send it through the Earth-Sun Lagrange 2 point, to a rendezvous with Comet Giacobini-Zinner.  The trajectory was a mirror image of one devised by Robert Farquhar in 1972,³⁷ and he ‘just happened’ to be in charge of ISEE-3 when it became ICE.  When asked in an interview if he had ever envisaged that, his reply was “Oh yes”.  Among the most important data obtained were magnetic field vectors within the tail of the comet, relative to the surrounding Solar Wind.

(Robert Farquhar had a reputation for independent action, having landed the NEAR-Shoemaker probe on the asteroid Eros at the end of its mission in defiance of orders from NASA not to do so.  In 2014, when ICE passed by the Earth again, ‘citizen scientists’ regained contact with it, and when Farquhar was asked why it hadn’t been switched off in 1986, he replied, “Oh, I must have forgotten to do that.”)

In 1986 ICE passed up-Sun of Halley’s Comet, as the small fleet of two Japanese and two Russian probes passed closer to it and Europe’s Giotto probe dived through the coma, obtaining close-up pictures of the nucleus before the camera was taken out by a dust particle.  In late 2005 amateur observers had noted with concern that there appeared to be several objects within the head of the comet, as if the nucleus had fragmented, though the Pioneer-11 probe orbiting Venus saw no evidence of that.  The Russian Vega probes eventually showed that there were two bright jets coming off from the nucleus, which was still intact.  

 Although Sir Fred Hoyle and Prof. Chandra Wickramasinghe had predicted that the nucleus would be dark, covered in tar-like organic compounds, the general belief was that the nucleus would be icy and Giotto’s camera had been programmed to track the brightest object in the field of view. As a result it locked on to one of the two jets, and as the real-time pictures were in false colour, all that could be seen was a blur of contour lines.  I was one of the first to realise that the green-coded shape at top left of the screen was actually the nucleus moving out of view, and it’s on record that Mrs. Thatcher, watching in incomprehension, declared that it would be the last time British money was wasted on anything like that.  When the pictures were processed in true colour, the nucleus turned out to be very black indeed and shaped rather like a peanut, with one circular feature which does not appear to be an impact crater.  Similar ones on Comet Wild 2’s nucleus are definitely pits eroded by escaping gases;  and after intensive processing, the Soviet Vega data generated an image of the Halley nucleus showing darker mottling, like that subsequently found on Comet Borelly  (see below).  Giotto went on to Comet Grigg-Skjellerup in 1992, but with no pictures because of the damage to the camera from dust particles in Comet Halley’s coma.  Ironically. although it approached to within 200 km, much closer than its Halley flyby, very few dust hits were recorded, even though the comet generates a meteor shower, the Pi Puppids.  The main data obtained was on magnetic fluctuations around the nucleus, caused by the interactions of hydrogen and oxygen ions with the Solar Wind.

In 1989, three major missions were on the drawing boards of NASA and ESA, each with the active participation of the other.  ESA’s was the Rosetta Mission  (see below);  NASA’s were to be Comet Rendezvous Asteroid Flyby  (CRAF)  and Cassini, planned to be the first of a new generation of Mariner Mark II spacecraft following the highly successful series which had first visited Mars, Venus and Mercury, plus the more advanced and renamed Vikings and Voyagers.  CRAF was to visit the asteroid Hamburga in January 1998 and rendezvous with Comet Knopf in August 2000, embedding penetrator probes in the nucleus in July 2001, and separating from it to explore the tail after perihelion passage in December 2002.  The Cassini mission would orbit Saturn and deliver the Huygens lander to the surface of Titan;  in the event, only that gained the necessary funding within the NASA budget.

After an unsuccessful encounter with the asteroid Braille, in September 2001 the ion-drive mission Deep Space 1 passed Comet Borelly, believed to be from the Kuiper Belt.  Like Halley’s the nucleus proved to be dark, this time shaped more like a bowling pin 8 kilometres in length, with multiple small jets coming mostly from one side of it.  The surface was extremely rugged, but smoother towards the middle of the elongated shape.  Little water or hydrated compounds could be detected at the surface, perhaps suggesting that the crust is old and its volatiles have gone.

In 2004 the Stardust probe visited Comet Wild 2, taking samples for return to Earth.  The nucleus looked quite unlike Borelly’s, with multiple pits or craters, the larger ones flat-bottomed, and at least ten small but active jets.  In January 2006 the samples taken were successfully returned, coming down in Utah after the fastest atmosphere entry ever accomplished.  Although the design was similar to the Genesis mission, which crashed on return to Earth the previous year, the technical error which caused that had fortunately not been repeated.  This was the first successful retrieval of samples from space by an unmanned probe since Luna 20 in 1972.  The samples contained a wide range of organic compounds, including nitrogenous ones, but at first no hydrous silicates or carbonates which might have formed in the presence of water.  But later iron and sulphur compounds were found which has formed in the present of liquid water, while oxygen isotopes revealed that the mixture of rocky material had formed in different parts of the Solar System, some of it close to the Sun.  How it got there, if it’s from our Sun at all, remains mysterious. 

In 2005 on the 4^th of July, appropriately enough, the comet Tempel 1 was struck at 37,000 mph by a copper projectile massing one third of a ton, released from the passing Deep Impact probe.  Unlike any of the other comets imaged hitherto, this one is just a lumpy mass, with no obvious shape – strewn with craters and pits, but with one strangely smooth area, still present when the comet was revisited in 2011.  

The explosion was equivalent to 4.8 tons of TNT and the crater was expected to be seven stories deep and the size of a football pitch.  After the flybys of Comets Halley, Borelly and Wild 2, this would be our first chance to study the interior of a comet.  It should have grown brighter dramatically in binoculars, perhaps even becoming visible to the naked eye and possibly fragmenting into two or more pieces.  From time to time a comet does that – famously, Biela’s Comet in December 1845, and more recently Comet West in spring 1976.   If Tempel 1 broke up the probe might not survive it, but we’d get spectacular close-ups and learn a great deal about the comet’s interior structure.  What actually happened was very different:  a huge plume of water vapour and dust particles, with the consistency of talcum powder, completely concealing the crater.   It now appears that the plume consisted of wet clay, which was nothing like what we expected.   Because the dust cloud had prevented a view into the interior of the comet, the Stardust mission was redirected to pass Tempel 1 in February 2011.  The impact crater was identified but turned out to be insignificant.  Density measurements suggested huge voids inside it, with perhaps 75% of its volume just empty space.  It’s not clear whether they’ve been formed by outgassing, or whether the nucleus is just a loose cluster of icebergs below the dusty crust and clay.

After its encounter with Tempel 1, Deep Impact was retargeted to pass Comet Boethin in December 2008.  Meantime it was given a set of much more distant targets:  starting in January 2006, it was watching five stars already known to have planets, to observe the dips in starlight as the planets transit the faces of the stars, and maybe detect smaller planets as well.  At the time of the Earth flyby in 2010, the comet had disappeared, and another short-period comet, Hartley 2, was selected for an encounter in 2012. 

That proved to be another big surprise:  looking a little like Comet Borelly at first glance, but with a definite dumbbell-shape and a smooth bar in between.  Similar features have been seen on the asteroids Eros and Itokawa, and on smaller moons of Saturn such as Calypso, Methone and Atlas.  The former are thought to be due to dust grains migrating toward the asteroid’s centre of mass, the latter to ice stretching under tidal stresses.  But it’s not clear why Tempel 1 should have one when its shape, though lumpy, is more nearly spherical.  Other exciting news from the comet was that its water content was isotopically similar to Earth’s oceans, suggesting that they might indeed have been delivered by Kuiper Belt objects.   But more recent missions have suggested they’re not all like that.    

‘Rosetta, Are You Better?’

Europe’s sample-return comet mission Rosetta was originally to be a joint US-European mission to Comet Schwassman-Wachmann 3.  Its name expressed the hope that it would provide the key to understanding the early history of the Solar System, as the Rosetta Stone did for ancient Egyptian hieroglyphs.  The US lander was to attach itself to the comet by harpoon, drill for samples and refrigerate them for return to Earth.  

I always had a soft spot for the Rosetta mission.  After the US withdrew from the project, in September 1993 I represented the Glasgow Herald at a ‘Town Meeting’ convened in London to debate whether the next mission selected for development in ESA’s ‘Cornerstone’ programme should be a stripped-down, more conventional Rosetta, with no return to Earth and possibly no lander.  The mission now was to reach Comet Wirtanen, launching in January 2003, to reach the comet in summer 2012, with the reinstated lander later named Philae, after the island on which the Rosetta Stone was found.  The eventual launch was in late February 2004, retargeted to Comet 67P/Churyumov-Gerasimenko in May 2014, making a successful Mars flyby and another in July 2010 of the asteroid Lutetia.  The rendezvous burn in 2015 was a success, and Rosetta became the first spacecraft to orbit a comet, sending down the Philae lander with less success the following year, because the harpoons intended to anchor it to the surface failed to fire.

The nucleus of Comet 67-P proved to be a dumbbell, like many of the asteroids, a ‘contact binary’ formed by the fusion of two objects in the Kuiper Belt.  It showed a spectacular landscape of cliffs, peaks and deep valleys, in one of which Philae was embedded, and an extremely active surface with dust and gas coming off in great quantities, particularly hydrogen sulphide and methane.  The water proved to be rich in organic compounds, but its isotope ratios were very different from Earth’s water, confusing the issue of where we got ours from, once again.  Observations continued throughout the comet’s closet approach to the Sun, when there were major colour changes on the surface and in the coma as red dust was emitted and fresh blue ice was exposed.

After 12 years in space, and with the spacecraft deteriorating as it and Comet 67-P moved away from the Sun, its final task was to photograph a pit called Deir el-Medina in detail.  Deep pits like that were discovered late in the mission and it’s thought that they are the main channels through which the comet releases gas during its passages around the Sun, while nodules visible within them may be the primal material of the Solar System itself.  The spacecraft was not designed to survive landing and was programmed to switch off at the moment of touchdown.

Reporting on the mission for the Herald, Jeff Hawke’s Cosmos and others, I’d often quoted Fame and Price, saying that Rosetta was not only “better” but “well, well, well”.   On September 18^th 2016 Rosetta “turned in for tae tak a lang sleep”, to quote a much older song, and I might have quoted the previous verse and added, “maybe in time I’ll forget her”, but for a shameless piece of sentimentality perpetrated after Loss of Signal by the controllers in the live coverage from the ESOC control centre.

As the mission has gone on, ESA has often illustrated it with videos and stills from a cartoon series, drawn by Carlo Palazzari, with the collective title ‘Once Upon a Time’, anthropomorphosising the spacecraft and Philae.  After the signal was lost, ESOC dramatised the descent with a video sequence by Palazzari which ‘would bring a tear to a glass e’e’, as we say, showing Rosetta exchanging farewell messages with Earth and leafing nostalgically through her photo album during the descent, gazing fondly at images of Philae before briefly spotting the lander asleep in the distance, finally closing the book and going to sleep herself as the solar panels snapped off.  That seemed to be the end of it, but as the applause died down, the screen came back to life with an image captioned “Maybe… someday?”, showing both frost-covered spacecraft on the screen of a future mission control, with the speech balloon, “We’ve found them!”  ‘Someday’ may not be so long, because so much was learned from the mission that there are strong calls for a second one.  NASA turned down the proposal in favour of the Dragonfly helicopter probe to Titan, but ESA may yet take the bait.  Palazzari has drawn Rosetta and Philae having a picnic, using the broken-off solar panels as a windbreak, while they wait for our return.

Comets as Spaceships

If we can use asteroids for interstellar travel, what about comets?   Isaac Asimov suggested that too  (‘Steppingstones to the Stars’, in Asimov on Astronomy, Macdonald & Jane’s, 1974).  The only asteroids which could be used would be chondrites or carbonaceous chondrites, the ones which contain the necessary elements for life-support;  but comets have them in even greater abundance, so much so that the late Sir Fred Hoyle and Prof. Chandra Wickramasinghe argued that life originates in comets and then seeds planets.  Comet nuclei are mostly water ice but contain silicates in the form of dust as well as frozen methane, ammonia, carbon dioxide, trace elements and a wide range of organic compounds.  

Prof. Freeman Dyson advanced the idea that genetically engineered trees could be planted on comets and grow to such size that their foliage could sustain photosynthesis even at the distances from the Sun of the Kuiper Belt or the Oort Cloud.  (Disturbing the Universe, Harper & Row, 1979.   A painting of it by David A. Hardy appears in Ian Ridpath’s Worlds Beyond, Wildwood House, 1975, and when I asked about it, he did two more for me).  Although the incoming energy from starlight would be very low, the collecting area would be as large as a continent and it could sustain plant life over an interstellar journey.  The trees could be bred to extract metals from the comets and concentrate them so that cuttings could be sold on return to the Sun – at which point the trees themselves, adapted to live in starlight, would presumably need sunglasses.  Proper industrial forestry and coppicing of such a treeworld would be a job for a full space habitat workforce. 

I used that idea in a story for the Daily Record’s ‘Lance McLane’ strip called ‘I Talk to the Trees’, in which the trees had developed a communal intelligence – not something that I imagine Clint Eastwood was thinking of when he sang it in the musical, though we’re all familiar with the joke version at the time: ‘I talk to the trees, that’s why they put me away…’  I decided to make my trees sequoias or redwoods and I drew heavily on Redwoods, the World’s Largest Trees by Jeremy Joan Hewes  (Hamlyn, 1981). 

As these trees reach their great height and age they are supported by cores of dead wood, but can remain standing if the core is destroyed by fires, termites etc, so the cores are unnecessary in the low gravity of a comet nucleus and can be removed.  I turned their whole ecology inside out, putting humans, bats and many other creatures inside the hollow trunks and roots, and I suggested to the artist Sydney Jordan that the bats might adapt quickly into the Night Stalkers of Dougal Dixon’s After Man  (Granada, 1982), though still having eyes.  Sydney decided to keep the wings as well, which is less plausible in low gravity and limited flight space.

In other planetary systems, there may well be comets to make use of.  For example, Tau Ceti, one of the nearest Sun-like stars to us, is ten billion years old, twice the age of our Sun, and Britain’s UKIRT infra-red telescope on Hawaii has discovered that it has a huge entourage of comets, at least twenty times as many as we have  (The Sky at Night, 6^th February 2006), subject of another spectacular painting by David A. Hardy.  If Hoyle and Wickramasinghe are right, or if some other civilisation has reached Tau Ceti, who knows what might be going on out there?

(There’s more discussion of trees on comets in Duncan Lunan’s Incoming Asteroid!  What could we do about it?  (Springer, 2013), available from bookshops or through Amazon.  Full details are on Duncan’s website, http://www.duncanlunan.com.)

Also in this series:

• Comets Part 3.   Cometary Impacts
• Comets Part 2
• Comets