Space Notes:  Solar System Exploration

by Duncan Lunan

In 1973 to 1978 I chaired a discussion project on Solar System exploration and development.  The resultant book had originally been meant for publication in 1975, but was already too big and had to be split into New Worlds for Old, the New Look of the Solar System, and Man and the Planets, the Resources of the Solar System.  With his permission, both books used the terminology and reasoning of a 1968 paper by Prof. Krafft A. Ehricke, ‘A Strategic Approach to the Solar System’, making use of its matter and energy resources for the benefit of humankind.  New Worlds for Old went to press in December 1978 for publication the following year.  Coming at the end of ‘the golden decade of space exploration’, it had been rewritten five times, three more since the split in 1975;  as I said at the time, it was like trying to write a gazetteer of the mediaeval world, with a Columbus or a Marco Polo reporting back every time you thought you had it finished.   

To link the two books, I ended New Worlds for Old with a chapter titled ‘The Case for Continuation’.  In what was to become the format of ‘The Sky Above You’ five years later, and still is, I started with the Sun, then the Moon, then the planets in order outwards.  In each case, I summarised what had been accomplished so far, and outlined what would be needed for the next stage of the Strategic Approach.  After 43 years, it’s interesting to review it, seeing what’s been done since with how few missions  (for the most part), and put what’s happening now into context.

Fig. 1. Pioneer 6, 7, 8 & 9 Nov 1975 (NASA)

In 1978, the biggest investigation of the Sun had been done with the Apollo Telescope Mount on the Skylab space station, five years earlier.  NASA was investigating the environment around the Earth’s orbit with Pioneer 6, 7, 8 and 9  (Fig. 1), and Europe had sent two Helios probes to within the orbit of Mercury  (Fig. 2). 

Fig. 2. Helios-A orbit (Messerschmidt )

Studies continued with International Sun-Earth Explorer (1978), then Ulysses, which flew over the poles of the Sun in 1994-95, and SOHO, stationed at the Earth-Sun L1 point in 1995, and still going strong after a dramatic long-distance rescue in 1998.  Other missions have included STEREO, two spacecraft which observed the Sun from opposite sides in 2011, and the highly successful Solar Dynamics Observer, launched in 2010 with instruments to study the Sun at multiple wavelengths, all still operational.  The Parker Solar Probe, launched in 2018, has penetrated the Solar corona and will make its closest approach to the Sun in December 2024, at less than 10 times the Sun’s radius, after its 7th and final Venus flyby  (Fig. 3).  Less dramatic but still important, Europe’s Solar Orbiter made its closest approach to the Sun at 30 solar radii in 2022  (Fig. 4), and remains at work.

Fig. 3. Parker Solar probe orbit plot, NASA

In 1978 the Moon was in a period of neglect after the last Apollo mission in December 1972 and the sample return by Luna 24 in 1976  (Figs. 5 & 6).  (Luna 25 was scheduled for 2022, but didn’t happen after western experiments were pulled thanks to the invasion of Ukraine.)  Mariner 10 took a few images in 1978 and the Japanese Hiten probe failed in 1990, but otherwise there was nothing until the Galileo flyby of 1990, on the way to Jupiter.  The Clementine probe of 1994 was primarily a military sensor test, but was a great success, so dedicated lunar missions resumed with Lunar Prospector in 1998, followed in 2009 by L-Cross and Lunar Reconnaissance Orbiter, which is still active today.  Probes from other nations such as Europe’s SMART-1  (2003), Japan’s Kayuga-Selene  (2007), India’s Chandrayaan-1  (2008), followed by NASA’s Grail  (2011)  and LADEE (2013), have all contributed to the new build up of knowledge.  China has had two lunar rovers meanwhile, one of which is still operational on the lunar Farside, and brought back lunar samples with Chang’e-5.  Many more satellites and landers are now planned, with crewed missions to resume with Artemis 3, and the Lunar Gateway space station to become a major part of future development.   

Fig. 7. Mariner 10 orbit, Venus flyby and first Mercury pass (NASA)

In 1978 the planet Mercury had been visited by only one spacecraft, Mariner 10, which made three flybys starting in March 1974  (Fig. 7).  Mercury’s resonant rotation makes its day equal to two Hermian years, 188 days, and the dynamics of Mariner 10’s first pass gave it an orbital period of the same length, so it photographed the same side of Mercury on each pass  (Fig. 8).  In the North Lanarkshire Astronomy Project, 2006 to 2009, at first I was still telling school classes that the unphotographed hemisphere of Mercury was the largest unexplored surface in the Solar System, bigger than the whole of Pluto.  By analogy with the Earth, Moon and Mars, I predicted that it might be quite different from the known side, and so it turned out in the first flyby by Messenger in 2008.  I had written in 1978 that we needed landings, rovers and sample return, and while there were rumours of an orbiter, no funding had been allocated.  The orbiter was Messenger, which made orbit around Mercury on its third pass in 2011, after two flybys, and continued until 2015.  Its successor, Europe’s Bepicolombo, will enter orbit in December 2025, after one Earth flyby, two past Venus and six past Mercury itself.  There was talk of a lander, with a soil sampler based on the one developed for Britain’s Beagle 2 on Mars, but in the end it carries a Japanese Magnetospheric Orbiter, which will separate from Bepicolombo upon final arrival.

Fig. 8. Mariner 10 Mercury flyby, by Gavin Roberts for New Worlds for Old, 1975

Flybys of Venus were the first interplanetary missions of the 1960s, followed by Soviet landers and orbiters through the 1970s and 80s.  Going to press in December 1978, New Worlds for Old was just in time to cover the results from the Pioneer 11 landers and the first ones from the Pioneer 12 orbiter.  Pioneer 12 remained in orbit till 1992, mapping the surface in ever greater detail, and was followed by the Venus Orbiting Imaging Radar  (Magellan), which did so even more thoroughly  (Fig. 9).  Europe’s Venus Express orbiter studied the atmosphere until 2014, and that rôle has been continued by Japan’s Akatsuki probe, very successfully, despite being in a higher orbit than intended.  The Rocket Lab company of New Zealand and USA means to send a private entry probe to Venus in 2023 or 2025, and NASA is looking at a variety of proposals for a Venus Flagship mission in the 2030s  (Fig. 10), as well as smaller missions called DaVinci and Veritas, with no commitment as yet.

Fig. 9. Magellan Venus radar map

Mars exploration in the 1970s had climaxed with the nuclear-powered Viking landers, dedicated to the search for life  (see ‘The Book of Mars’, ON 8th January 2023).  Due partly to budget cuts and mission failures  (characterised as ‘The Great Galactic Ghoul’), it would be 20 years before the next Mars landing.  (What differences were noted?  An abandoned supermarket trolley, an Irish theme pub and a Big Issue seller.)  The success of Mars Pathfinder and its Sojourner rover meant that there were multiple missions at each Mars opposition, every 2.5 years from then on.  The accumulation of dust on the solar panels of the Spirit and Opportunity rovers meant that they were expected to shut down after 60 days, but chance cleaning by passing dust devils kept them working for 6 years and 14 years, respectively.  Europe’s Mars Express, which also arrived in 2004, is still at work in Mars orbit, as are Mars Climate Observer and Mars Reconnaissance Orbiter.  The MAVEN orbiter is currently in ‘safe mode’ pending solution of a technical problem.  The stationary landers Phoenix and Insight succumbed to frost and dust respectively, but the Curiosity rover is still climbing the central peak of Gale crater after 11 years, and Perseverance has been roving Jezero crater for two years now, helped by the Ingenuity helicopter  (see ‘Rotorcraft in Space’, ON 5th February 2023).  India’s MOM orbiter has been retired, and China’s Tianwen orbiter has mysteriously shut down along with its Zhurong rover, but most of the fleet up there is still operational.  Europe’s Exomars rover should have been launched last year by Russia, but that one will have to wait till another launcher is found;  meanwhile, Europe is building the return vehicle for the samples being deposited by Perseverance at the foot of the river delta which it is about to explore.

Fig. 11. Asteroid Belt

The Asteroid Belt, between Mars and Jupiter  (Fig. 11)  was passed through without incident by Pioneer 10 and 11 on their way to Jupiter, arriving in 1974 and 1975.  No asteroids were photographed until the Galileo spacecraft imaged Gaspra, Ida and its satellite Dactyl in 1991 and 1993, respectively.  The NEAR-Shoemaker space craft orbited Eros in 2000, finally landing on it without authorisation, but proving it could be done.  Rosetta photographed asteroids Steins And Lutetia on its way to Comet Churyumov-Gerasimenko, and Dawn ion-drive spacecraft orbited first Vesta, the brightest of the Main Belt asteroids, then Ceres, the largest of them, ending its mission in 2018.  Last year the DART mission impacted Didymos, the moon of asteroid Dimorphos.  Japan’s Hyabusa probe returned only a few dust grains from its encounter with asteroid Itokawa, but Hyabusa II did better with asteroid Ryugu, and the returned samples are now under intensive study.  OSIRIS-REX is bringing back samples from the Potentially Hazardous Asteroid Bennu, for delivery to Earth on September 23rd this year.  On November 2nd NASA’s outbound Lucy probe will pass within photographic range of the small, newly discovered Main Belt asteroid 152830, which has been named ‘Dinkinesh’  (‘you are marvellous’, in the Aramaic spoken in Ethiopia).  At 0.4 miles in diameter, the asteroid is slightly larger than Bennu.  One of Lucy’s large solar panels is still only 98% open, and attempts to latch it have ceased now that the spacecraft is so far from the Sun, but it’s thought that it will be able to complete powered flybys of Jupiter’s Trojan families of asteroids  (Fig. 12).

Fig. 12. Lucy mission to Greek and Trojan asteroids in the Sun-Jupiter L4 and L5 points

New Worlds for Old was too early for the Pioneer 11 flyby of Saturn or the Voyager flybys of Jupiter in 1979, and there were some like the late John Braithwaite who believed that to be a mistake.  But the editor Paul Barnett and I agreed that rather than delay for over a year, and possibly have to rewrite the book yet again, it would be better to go ahead, if only because the Voyagers might discover little to add to the Pioneer flybys of the mid-70s.  In the event, their discoveries were the sensation of the decade, including the revelation that Jupiter’s four large moons were worlds in their own right, quite different one from another.  Those discoveries and the ones at Saturn became the fourth draft of Man and the Planets, published in 1983.  By then, what had been the Pioneer Jupiter orbiter had become the Galileo mission, launched by the Space Shuttle in 1989.  It watched the impacts of Comet Shoemaker-Levy on Jupiter in 1994, and released the Galileo Entry Probe into Jupiter’s atmosphere  (Fig. 13 – see ‘Balloons in Space’, ON 12th February 2023). 

Fig. 13. Galileo entry probe, December 7 1995

Although its main antenna failed to open and limited the number of images it could return, Galileo successfully rephotographed all the moons as well as the planet and made numerous major discoveries before its final plunge into the atmosphere in 2003.  These were added to by the Cassini and New Horizons spacecraft in flybys on their way to Saturn and Pluto, respectively.  Currently Jupiter is being orbited by the Juno spacecraft, the first to use solar power at that distance from the Sun.  In 2016, Juno was to enter a close ‘Science Orbit’ from which it would eventually fall into the atmosphere, but that was cancelled due to thruster problems and currently it’s extended to 2025 at least, making repeated passes over the planet’s probes and now passing the Galilean moons, with the Io encounter coming up  (Fig. 14).  Europe’s JUpiter ICy moons Explorer  (JUICE)  is due to launch on April 14th, to arrive in 2031.  NASA’s Europa Clipper mission has now been reallocated from the Space Launch System to launch next year on Elon Musk’s Falcon Heavy, and the ‘Clipper’ name is now a misnomer – to get there will take two years longer than originally planned. 

Fig. 14. Jan 2021, 42 orbit extension through Sep 2025 to study north polar cyclones, ring system, Ganymede (2 flybys), Europa (3 flybys) and Io (11 flybys).

Pioneer 11 successfully passed Saturn in 1979, in a rehearsal for the Voyager encounters of 1980 and 1981.  As well as the planet and the rings, the Voyagers photographed all the major moons except Phoebe, the outermost.  Phoebe was of particular interest as a possible Centaur, a stray from the Kuiper Belt  (see below), so it was first to be photographed by Cassini on its way into the Saturn system in 2004, remaining in orbit till 2017.  In that time it made extensive studies of the rings and moons, particularly Titan, because of its unique atmosphere and surface conditions  (see ‘Balloons in Space’).  In 2015, Astronomy Now published Saturn, Exploring the Ringed Planet, covering all but the last two years of the mission, and I can’t do better than to recommend it  (Fig. 15).  Produced by Pole Star Publications at £9.99, it’s now available to Astronomy Now subscribers at £5, and very well worth it.   The next mission to Saturn will be NASA’s Dragonfly probe to Titan, arriving in 2028  (see ‘Rotorcraft in Space’).

Fig. 15. Saturn, Exploring the Ringed Planet, Astronomy Now, 2015

Uranus has been visited only once, by Voyager 2 on Burns Night in January 1986, provoking some inevitable puns.  Currently inexplicable features were found on its largest moons, all but one of them named after Shakespearian characters, and with the discoveries of liquid water below the surfaces of Europa, Ganymede, Enceladus, Pluto, and possibly Io, Callisto, Mimas and Titan, there’s growing scientific demand for an orbiter mission, possibly a mothercraft powering sub-probes by laser  (Fig. 16).

Fig. 16. Sustained CubeSat Activity Through Transmitted Electromagnetic Radiation (SCATTER), Sigrid Close, Stanford

Neptune too has been visited only by Voyager 2, in 1989.  The images of the planet and its moons, particularly Triton, raised far more questions than answers  (see ‘Uranus and Neptune’, ON November 14th, 2021).  When Voyager 2 was en route from Uranus to Neptune, in 1986 I visited the Jet Propulsion Laboratory in Pasadena, and was briefed on the options by Mike Urban, one of the Voyager 2 controllers, whom I had met in 1984  (Fig. 17).  I wrote an article which ended in block capitals with the words WE WANT TO SEE TRITON, which attracted something of a following, and it may happen again some day.

Fig. 17. Mike Urban, Voyager 2 control room, Jet Propulsion Laboratory, 1986

Pluto has also been visited only once, by New Horizons in 2015.  It lost its status as a planet while the spacecraft was in flight, being relegated to ‘dwarf planet’ and other unflattering new names  (Fig. 18).  For historical reasons  (and nationalistic ones)  there is a US campaign to reinstate it.  As with Mercury in 1978, however, we face a long wait to find out what New Horizon didn’t see on the other hemisphere of Pluto and its large moon Charon, during the one-day flyby, and there are design studies  (no more than that, at present)  for faster ways to get there and do it.

Fig. 18, How I Killed Pluto, Mike Brown, California Institute of Technology

One reason for Pluto’s relegation is that it’s now seen as just the largest member of the Kuiper Belt, a band of icy, rocky objects which Gerard Kuiper predicted would be found beyond the orbit of Neptune.  Indeed, one of the first to be found looked larger than Pluto and was named Eris, after the goddess of discord.  At least three of them have rings, possibly impact debris, but at least one is too far out for that.  After passing Pluto, in January 2019 New Horizons imaged the extraordinary binary object Ultima Thule, now called ‘Arrakoth’, and half of it has a saucer shape which might be due to erosion in interstellar space.  It brings to mind the motto of the Astronomy Section of ASTRA, coined by the late Danny Kane as a curator of Airdrie Public Observatory:  Non modo spectare sed etiam adire, which may be translated as ‘Not only to look at them but also to go there’.

New Horizons is heading out in the direction of the Galactic Centre, still with plenty of fuel and power, possibly capable of lasting till 2040.  Scans are being made for any more Kuiper Belt objects in its path, and there’s time yet in which to find one.  The most extraordinary cases of longevity, however, are Voyagers I and II, launched in 1977 and both still operational.  For the first draft of Man and the Planets, in 1975 Gavin Roberts produced a diagram showing the planets to scale, with the orbits of the planets to a different scale behind them.  It was based on a model of the visible Solar System which a friend and I built in 1960 to get him his Astronomers Badge in the Scouts – its base was the lid of a tea chest, mounted on a sledge runner cut in half, with a central lightbulb, a ping-pong ball painted up as Jupiter, a ball from a baby’s rattle for Saturn, with the rings painted on cellophane, and graded beads for the planets.  It was amazing how many phenomena like eclipses and occultations could be demonstrated on it.  I would probably be using it still, if the planets hadn’t oddly been stolen in a break-in at the ASTRA rooms, leaving the board behind.

But in his 1975 drawing, Gavin had included a line beyond the orbit of Jupiter labelled ‘limit of the Solar Wind’  (Fig. 19).  Readings from Pioneer 10 and 11 suggested that as they approached Jupiter, the Solar Wind was becoming turbulent and breaking up.  As it turned out, though, the Wind smoothed out beyond there – Paul Benson, a curator at Airdrie Observatory, redrew the figure with new data and no Solar Wind boundary, for Man and the Planets.  The Voyagers were still immersed in it when they crossed the orbit of Pluto.  Voyager I didn’t cross the boundary, technically entering interstellar space, until 2012, and Voyager II not till 2018.  Both are still within the Sun’s gravitational field, and indeed they’re far from leaving the Kuiper Belt and entering the Oort Cloud, the vast spherical volume of comets surrounding the Solar System  (Fig. 20).  One by one their experiments are running out of power and being turned off;  but behind them, there is still a great deal of exploration to be done.

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