by Duncan Lunan

The loss of the Japanese Moon lander Hakuto-R, the first private lunar landing attempt, has come as a big disappointment.  After a successful launch on December 11th by SpaceX’s Falcon-9  (itself a private vehicle), Hakuto-R ‘went round the long way’ on a low-energy trajectory for capture into lunar orbit on March 20th  (Fig. 1).  After over a month orbiting the Moon, the spacecraft attempted to land  (Fig. 2), carrying a range of experiments including the United Arab Emirates Rashid rover, with a Canadian artificial intelligence system for navigation.  Communications failed at an altitude of 295 feet and a speed of 20.5 miles per hour.  One suggestion was that it might have run out of fuel on approach, resulting in a loss of attitude control;  another is a previously undetected conflict in the programming, like the one which caused ESA’s Schiaparelli Mars lander to crash on the final stage of descent.  For now it remains a mystery.

Before leaving lunar orbit, however, Hakuto-R had been taking some fine images at low level.  An especially striking one was of Earthrise on April 24^th, at an altitude of 62 miles, during that day’s eclipse of the Sun over Indonesia, Australia and the western Pacific  (Fig. 3).  Although the continent shapes are hard to make out  (Australia at left, Asia at right – I think), the umbra and penumbra of the Moon’s shadow are clearly visible, as predicted by the late R.A. Smith in a remarkable painting of the early 50s  (Fig. 4).

The first photographs of the Earth from space were taken in 1946 and 1947  (Fig. 5).  A montage of them was shown by Mike Todd in his introduction to the 1956 version of Around the World in 80 Days, starring David Niven.  He claimed that they had been taken by a Corporal missile like the one I saw at South Uist in 1959  (‘Visitor at Uist’, ON 9^th September 2022), but actually they were taken by V2 rockets from White Sands, New Mexico.  Early orbital experiments by the Vanguard and Explorer satellites led quickly to the TIROS weather satellites in 1960  (Fig.6), followed in 1964 by the Nimbus series, in Sun-synchronous orbit for continuous solar power, which produced continuous images of the Earth below in strips distorted by the spacecraft’s motion  (Fig. 7). 

Experiments in photography of the whole Earth from geosynchronous orbit began with the Applications Technology Satellite series in 1967  (Fig. 8), and they led to the World Weather Watch, with a chain of GEO satellites including ESA’s Meteosat 1 in 1979  (Fig. 9).  The BBC’s weathermen had been complaining for years that they didn’t have a satellite terminal like their US counterparts, and when they finally got one, with matching maps, there were complaints from Scottish Nationalists because the photos from equatorial orbit showed Scotland foreshortened, apparently smaller in relation to England than it really is.  Rectified photographs and maps have been used for many years now, and the Meteosat satellites are now in their 3^rd Generation, with improvements all the way.

Meanwhile, starting with the Gordon Cooper’s final Project Mercury flight in 1963, the astronauts had been continuing to demonstrate the potential of orbital photography for Earth surface studies, and by the time Project Apollo was under way the automated Landsat satellites had begun demonstrating its importance in earnest.  The field of Earth Resources photography, mainly from Low Earth Orbit, is such a large one that I won’t even attempt to cover it – the rest of this article is going to focus on the views from greater distances.

Between the Near Earth Orbit weather and Earth resources satellites 2000 miles up in near-polar orbit, and the Geosynchronous Orbit satellites 22,000 miles up in the plane of the Equator, lies the territory of Medium or Middle Earth Orbit, mainly occupied by navigational and broadband relay satellites like GPS and Starlink.  They don’t carry cameras, and for images at those heights we have to turn to outgoing space probes or Earth slingshot flybys.  The former include the Juno flyby of 9^th October 2013  (Fig. 10), the Osiris-REX flyby of 22^nd September 2017  (Fig. 11), and the Bepi-Colombo flyby of April 10^th, 2020, on its way to Mercury  (Fig. 12)   Departing images were taken by Elon Musk’s Falcon Heavy booster in its first flight on 7^th February 2018, carrying his Tesla Roadster and its Starman occupant to near the orbit of Ceres  (Fig. 13).  The most recent examples are from ESA’s JUICE mission, launched on 14^th April 2023.  Regrettably, the cameras are now busy taking photographs of the RIME penetrating radar array  (Fig. 14), which is stuck, still two-thirds closed, though mission control remain optimistic that they can clear it.

All the Apollo missions photographed the Earth on the way to the Moon and back, at distances well away from the applications satellites’.  The images taken by Apollo 11 (Fig. 15)  and Apollo 17  (Fig. 16)  are particularly significant because they show that the outgoing spacecraft were respectively well north and south of the equator, avoiding the worst zones of the Van Allen radiation belts – which conspiracy theorists continue to insist would have killed them.  It’s often claimed that the Great Wall of China is the only man-made feature visible from the Moon;  the claim is usually attributed to President Nixon on his visit to China, but goes back to 1955 at least.  In reality the Great Wall is built of local materials and doesn’t stand out even in images from Low Earth Orbit, but Fig. 15 shows the feature visible from the Moon which is man-made, according to some experts at least – the Sahara Desert, which is at least partly the product of overgrazing by cattle and goats between the Roman era and the present, though it has earlier incarnations during the ice ages.

The Earthrise photograph taken by Apollo 8 at Christmas 1968 made the biggest impression of all such photographs  (Fig. 17), though Earth had been photographed from the same perspective by Lunar Orbiter 1 in August 1966, and the subsequent image taken by Apollo 11 is perhaps more significant, for the reason stated on it  (Fig. 18).  There are many more images of the Earth from the Moon, taken by humans and by robots, but as they all show the Earth from approximately the same distance, I’m going to keep moving out.

The first photograph of Earth and Moon together was taken by Voyager 1 on 18^th September 1977, from 7.2 million miles  (Fig. 19).  It shows how much darker the Moon is, reflecting only 7% of the sunlight that falls on it.  The corresponding photo at the end of the Galileo flyby, from 3.9 million miles, was significant because the angle of sunlight happened to catch the huge Aitken crater on the lunar Farside, stretching from the south pole to the equator, whose existence was previously unsuspected  (Fig. 20).  Photographs of the Earth and Moon from spacecraft in transit show little more than dots or tiny crescents – the ‘family portrait’ of the Solar System, taken by the Messenger probe from 61 million miles away en route to Mercury, is of interest only for the contrasting brightness’s of the planets  (Fig. 21). 

Many writers have imagined the beauty of Earth and Moon seen from Mars, but due to the constant dust in the Martian atmosphere, they’re actually not conspicuous  (Fig. 22).  The most interesting such photo is the one taken from orbit by Mars Global Surveyor in May 2003, shortly before the end of its mission  (Fig. 23A).  Clearer ones were taken by Mars Reconnaissance Orbiter in 2007, and again in 2016, but the interest of the MGS one lies in the configuration of the planets at the time  (Fig. 23B), showing the size of the gap, occupied by the Asteroid Belt, which prompted Kepler to write, “Between Mars and Jupiter I have put a planet”.  In the 2016 photo the outlines of Asia and Australia are recognisable, recalling the incident in C.S. Lewis’s Out of the Silent Planet  (1938)  when Ransom is shown the Earth through a telescope on Mars and recognises South America – ‘the loneliest moment of his travels’.

No photos seem to have been taken yet of the Earth from the Asteroids or from Jupiter. But in its 14 years orbiting Saturn, the Cassini probe had time to take some unusual photos, including one of Alpha Centauri near the rings, showing it clearly as a double star.  To make Earth easy to find, Cassini photographed it through the rings, just inside the F-ring, in 2006  (Fig. 24), and inside the G-ring in 2013  (Fig. 25)  and 2017.  For the 2013 image people were encouraged to wave to Saturn and send in their photos, which were assembled into an image of the Earth from Cassini’s angle of view.  Critics immediately pointed out that the phase of the Earth was wrong, so the whole montage had to be done again  (Fig. 26).

There was no time to look back at the Earth during the Voyager 2 flybys of Uranus and Neptune, but Carl Sagan persuaded NASA to turn Voyager 1’s cameras back on for one last ‘family portrait’ of the Solar System in 1990  (Fig. 27).  Taken from 4 billion miles, this is likely to be the most distant image of the Earth for a long time.  After passing Pluto and Arrakoth New Horizons is now too far out to capture it, though it is taking long-distance images of Uranus and Neptune, to show the northern hemisphere summer on Uranus from a different angle.  The Voyager montage gave Carl Sagan his Pale Blue Dot title for his book of 1994, and has inspired the ‘Pale Red Dot’ title for the search for planets orbiting nearby red dwarf stars such as Proxima Centauri and Barnard’s Star.  (‘Exoplanets’, ON February 20^th, 2022.) 

A very large space telescope, and a distant occulting disc to cut out the glare from the Sun, would be needed to image the Earth from the distance of Proxima Centauri, though from there the Sun would be a bright star to the left of the ‘W’ of Cassiopeia  (Fig. 28). 

From Tau Ceti, the nearest sunlike star to us, at 11.9 light-years, the Sun is a faint 4^th magnitude star at the foot of the constellation Boötes, and imaging the Earth would be harder still.  For detecting its presence, the two best methods would be astrometric, measuring the effect of Earth’s gravity on the Sun, or by occultation, measuring the dip in the Sun’s brightness as the Earth transited across it.  Like all the planets, Earth doesn’t actually orbit the Sun but they both orbit around the barycentre, their common centre of mass.  But as with all the planets except Jupiter, the Sun-Earth barycentre is actually inside the Sun, and the wobble would be all but undetectable.  Even the Sun-Jupiter barycentre is only just outside the Sun’s surface;  the astrometric method works best with giant planets really close to very small stars.  With the occultation method, the dip in the Sun’s brightness caused by the Earth crossing in front of it would be 0.01%  (Andrew May, Astrobiology, The Search for Life Elsewhere in the Universe, Icon Books, 2019).  And for the occultation method to work, the star in question would have to be very near the Ecliptic  (the plane of Earth’s orbit), as seen from here.  A glance at a star atlas shows that neither Proxima Centauri nor Tau Ceti is anywhere near the Ecliptic.  In fact, because of the Solar System’s tilt to the galactic plane  (Fig. 29), very few other stars are in a position to find Earth by occultation  (Fig. 30), and statistically, it’s thought that there might be only 10 habitable planets elsewhere from which it could be done.  That we’re in such a backwater might be significant, if there are hostile intelligences out there.

It might not, then, be such a good thing that we have made Earth highly visible in microwave, TV and radio wavelengths.  It’s an entertaining thought that the wavefront of the 1964 appearance on the Ed Sullivan show, which launched the Beatles internationally, is now 59 light-years away, beyond 51 Pegasi  (Fig. 31).  But as Sagan and others have pointed out, there’s stuff out there that we might not want others to see, like the Nazi broadcasts of the 1936 Berlin Olympics, or the many reruns of I Love Lucy.  Although the sphere occupied by our transmissions is only 200 light-years across, barely a dot on the map of the Galaxy  (Fig. 32), Patrick Moore was not the only astronomer to suggest that the first message from another civilisation might be ‘Please turn down the volume’ – and that might not be as funny as it first sounds.

G. David Nordley’s novella ‘Empress of Starlight’ was published in Analog, Nov/Dec 2018, and reprinted in his book Around the Stars  (Brief Candle Press, 2019).  It was a finalist for the Hugo Award at the 2019 World Science Fiction Convention, and after lecturing on it to the Astronomers of the Future Club in Troon in August that year, he was kind enough to send me his slide show explaining the concepts in detail  (Fig. 33).  What he had done was to re-examine Freeman Dyson’s 1959 idea that an advanced civilisation could break up all the planets to build a solid shell  (or a sphere of asteroids)  around the Sun, at about the Earth’s orbital distance, to capture and utilise all the Sun’s energy output.  As well as the physical difficulties, there are strong reasons for not doing it, which I discussed in the closing chapter of Man and the Planets  (Ashgrove Press, 1983).   Gerry Nordley suggested that many of the difficulties could be overcome by building a smaller sphere of hexagonal panels with flexible linkages, requiring about the mass of Manhattan Island rather than the destruction of a solar system.  Using such a system as a phased optical array, planets of other stars could be examined in amazing detail.  His worked example involved 11-metres resolution at a distance of 300 light-years  (Fig. 34).  But the downside is that it would make a deadly interstellar weapon, capable of raising a 100-metre spot at a distance of 300 light-years to a temperature of 20 million degrees K, nearly 1000 times hotter than the surface of the Sun.  Prof. R.N. Bracewell pointed out that even at interstellar distance a study of Doppler shifts could reveal the latitude, and timing could reveal the longitude, of a transmitter on a planetary surface, but as well as eliminating it, the senders of a beam to destroy it could just as easily sterilise the entire planet.

In his contributions to my Man and the Stars  (1974)  and his own book Extraterrestrial Encounter  (David & Charles, 1979), the late Chris Boyce argued that ‘Interstellar chequers is not a viable mode of existence’.  If the numbers of civilisations of different types in the Galaxy were roughly equal, he demonstrated that 60% of the trusting would survive, likewise 40% of the suspicious  (like us), but none of the paranoid would make it, even if they set out to eliminate possible competition.  By his untimely death in 1999, he had become more interested in the threats to the percentages who don’t make it, and that would have been a major element in the book ET Presence on which he was working when he died.  Personally, I’m still of the opinion that if we’re not alone, then almost certainly we already have protected status  (‘The Fermi Paradox’, ON April 24^th – May 8^th, 2022).  Shifting all TV to satellite broadcasts from space, and turning off the Ballistic Missile Early Warning radars when we no longer need them, would do a lot to reduce the nuisance value of our present transmissions.  It may take a while for those to come about  (especially the latter), but maybe we should prioritise them if we see stars appearing to go out, as happens at the beginning of Gerry Nordley’s novella.

References

F.J. Dyson, ‘Search for Artificial Stellar Sources of Infrared Radiation’, Science, 131  (1959), p.1667;

R.N. Bracewell, ‘Radio Signals from Other Planets’, Proceedings of the Institute of Radio Engineers, 50  (1962), p.214;

both reprinted in A.G.W. Cameron, ed., Interstellar Communication, Benjamin, New York, 1963.