Solar System in Minutes

by Giles Sparrow

  354 THE KUIPER BELT AND BEYOND

  27.09.92 02:35

  27.09.92 06:42

  28.09.92 06:58

  This sequence of images was used to discover Albion (ringed) in 1992.

  Varuna and Ixion

  The Kuiper Belt Objects 20000 Varuna and 28979 Ixion were two of the first more substantial ice dwarfs to be discovered, in 2000 and 2001 respectively. Varuna proved to be another cubewano, with a roughly circular orbit at around 43 AU from the Sun, while Ixion is a plutino – an object in an inclined, Plutolike orbit, ranging between 30 and 49 AU.

  Steady changes to Varuna’s brightness indicate that it rotates once every 6.34 hours, with an elongated shape that reflects more or less sunlight depending on its orientation. Both worlds are red in colour, but there is a distinct difference in their surface brightnesses – Varuna is extremely dark and reflects back just 4 per cent of the light striking it, while Ixion is lighter, reflecting back 15 per cent. Spectroscopic analysis of Ixion’s light has even revealed some of the materials on its surface. They include pure carbon, and also complex tholin chemicals, formed as solar wind particles alter carbon-based molecules in its surface ices.

  356 THE KUIPER BELT AND BEYOND

  An artist’s impression of the elongated KBO Varuna

  Quaoar

  When it was found in 2002, 50000 Quaoar became the largest known KBO after Pluto. Images from the Hubble Space Telescope resolved the shape of its disc, allowing its diameter to be estimated at 1,260km (783 miles) – it is now recognized as a dwarf planet. Unlike Pluto, however, Quaoar is dark, with a surface that reflects just 9 per cent of light that strikes it. Its overall colour is reddish, although not as red as many smaller KBOs. In 2004, traces of water ice were detected on its surface, followed by methane and ethane ices.

  Quaoar is another cubewano, with a roughly circular, low-inclination orbit ranging between 42.0 and 46.3 AU from the Sun. Cubewano orbits concentrate in this region to avoid orbital resonances with Neptune at around 39.5 and 48 AU. Objects that stray into the danger zones beyond find their orbits altered by repeated interaction with the giant planet; at the inner edge they may stabilize in more eccentric, Pluto-like orbits, while at the outer edge they can be ejected into the distant ‘scattered disc’.

  358 THE KUIPER BELT AND BEYOND

  Large Blackboard

  Haumea

  Discovered in 2004, the Kuiper Belt Object 136108 Haumea has about one-third the mass of Pluto and is classed as a dwarf planet. In this case, however, the designation does not mean that Haumea is spherical – variations in its reflected light indicate that it is an elongated ‘ellipsoid’, with dimensions of around 2,100 × 1,600 × 1,100 km (1,300 × 1,000 × 700 miles), spinning on its axis once every 3.9 hours.

  Haumea’s shape and its speedy rotation are linked – its equator bulges outwards due to rapid motion that weakens the inward pull of gravity. The best explanation for the fast rotation itself is a violent collision in Haumea’s past, which left the rocky core mostly intact while scattering fragments of the icy mantle to create a ‘collisional family’ of objects in related orbits. The collision is also thought to have created Haumea’s two known satellites – Namaka and Hi’iaka. A destructive impact between other small moons probably generated the raw materials for the thin ring that was discovered around Haumea in 2017.

  360 THE KUIPER BELT AND BEYOND

  Hi’iaka

  Haumea

  Namaka

  Eris

  The large Trans-Neptunian Object 136199 Eris is the best-known member of the ‘Scattered Disc’ – a group of objects in highly elongated and inclined orbits with a separate evolutionary history from the plutinos and cubewanos of the ‘classical Kuiper Belt’. First catalogued as 2003 UB313 (and nicknamed ‘Xena’), Eris was named for the Greek goddess of discord following its designation as a dwarf planet and the ‘demotion’ of Pluto.

  Eris was discovered near the outer edge of a long 558-year orbit that carries it between 38 and 98 AU from the Sun, but has, nevertheless, been subject to intense study. Eris is almost the same size as Pluto, but in contrast to most KBOs, its surface is distinctly grey and highly reflective, probably due to surface ice. The 2005 discovery of Dysnomia, a satellite in a

  15.8-day orbit, revealed Eris’s mass to be 27 per cent greater than Pluto’s, suggesting that it contains much more rock than ice. This could generate sufficient heat to power geological activity and perhaps even warm a subterranean ocean.

  362 THE KUIPER BELT AND BEYOND

  An artist’s impression of

  Eris and Dysnomia illuminated by distant sunlight

  Makemake

  The dwarf planet 136472 Makemake is in an oddity among large KBOs. The general shape of its 310-year orbit, taking it between 38.5 and 53.1 AU from the Sun, puts it in the ‘classical Kuiper Belt’, but its inclination of 29 degrees from the plane

  of the solar system means it is considered dynamically ‘hot’. In other words, it did not form in its present orbit, but was instead scattered here by a past interaction with some other object.

  At about three-quarters the size of Pluto, Makemake is one of the brightest KBOs. Variations in its brightness, due to surface markings, reveal that it spins every 7.7 hours. Nitrogen ice is not as widespread as on Pluto, but frozen methane is present in large grains, along with ethane and reddish tholin chemicals. Against expectations, Makemake seems to lack even a low-pressure atmosphere. A small moon, currently saddled with the ungainly moniker S/2015 (136472) 1, was announced in 2016 – evidence

  for a satellite had previously remained elusive because, in stark contrast to Makemake itself, this moon is as black as charcoal.

  364 THE KUIPER BELT AND BEYOND

  Makemake was expected to have a thin atmosphere similar to Pluto’s, but the abrupt disappearance of stars passing behind it, with no prior dimming, reveals that it is effectively airless.

  Sedna

  Touted as the first member of a previously hypothetical ‘Hills Cloud’ of icy objects that venture far beyond the Kuiper Belt (see page 26), 90377 Sedna takes some 11,400 years to orbit

  the Sun. At the time of discovery in 2003, it was a mere 89.6 AU from the Sun (closer, in fact, than Eris – see page 362). But after passing perihelion in 2076 at a distance of 76 AU, it will begin its long retreat to aphelion at an estimated 936 AU.

  Sedna is the reddest object in the solar system – redder even than Mars. Its crust is thought to be dominated by methane, methanol and nitrogen ices, with a thick coating of carbon-based tholin chemicals providing its red hue. In 2014, astronomers announced the discovery of another object in a similar (though smaller) orbit, lending weight to the idea that the Oort Cloud

  has an inward extension close to the plane of the solar system (see page 370). Objects following these orbits must have been pulled into them from closer to the Sun, perhaps during a close encounter with a passing star early in the solar system’s history.

  366 THE KUIPER BELT AND BEYOND

  page 26

  page 362

  page 370

  An artist’s impression shows sunlight reflecting on the icy surface of Sedna at the very edge of the known solar system.

  Planet Nine?

  Does another large planet await discovery in the darkness

  of the outer solar system? The prospect of such a major discovery has tantalized astronomers since the early 20th century, and Pluto’s discovery in 1930 was the serendipitous outcome of a deliberate search for one such ‘Planet X’. However, as improvements to mathematical modelling and computing power refined our understanding of solar system dynamics, earlier concerns about planets not behaving as they should gradually faded away.

  In recent years the quest has reignited, with the suggestion that the orbits of some of the solar system’s most distant objects are aligned by an unseen influence. One possibility is that these distant worlds are affected by the gravitational pull of a ‘Planet Nine’ with ten times Earth’s mass.
Perhaps there is an ice giant ejected from closer to the Sun during the planetary reshuffle produced by the Nice model (see pages 42–45), or even a rogue planet captured from interstellar space.

  368 THE KUIPER BELT AND BEYOND

  pages 42–45

  2010 GB174

  Planet Nine?

  Sedna

  2007 TG422

  The orbits of the solar system’s most remote bodies appear to cluster on one side of the Sun – are they influenced by the presence of an undiscovered ninth planet?

  The Oort Cloud

  The existence of a spherical cloud of comets surrounding the solar system was first proposed in order to explain

  the fact that long-period comets approach the Sun from all directions, and that ‘fresh’ icy comets continue to appear. Today, the so-called Oort Cloud remains a theoretical necessity that eludes direct observation, but its inferred structure has grown more complex. About 80 per cent of its comets are

  now thought to reside in an inner, doughnut-shaped extension called the Hills Cloud (stretching from about 2,000 to 20,000 AU from the Sun). This is the reservoir into which most comets were ejected during encounters with the giant planets in the early days of the solar system. From here, occasional collisions and close encounters eject comets into orbits with aphelia in the outer Oort Cloud, perhaps three times further from the Sun. Here, solar gravity is so weak that their orbits are easily circularized by the influence of other stars, before further encounters either strip them away into interstellar space or send them plunging back towards the inner solar system.

  370 THE KUIPER BELT AND BEYOND

  Outer Oort Cloud

  Orbits of the planets (not to scale)

  Hills Cloud

  Large Blackboard

  Limits of the solar system

  Most astronomers favour one of two definitions for the

  edge of the solar system. The more limited of these is

  the heliopause – the boundary at which the outward stream

  of the solar wind (see page 64) falters and dies. This boundary, encountered by the Voyager 1 space probe at 121 AU, defines

  the heliosphere within which the Sun’s influence is paramount. Somewhat awkwardly, however, it suggests that many objects orbiting the Sun are actually beyond the edge of the solar system.

  A more generous definition, then, is the Hill sphere, the region

  in which the Sun’s gravity is capable of retaining bodies in orbit around it. An accurate calculation of the Hill sphere is complex – the influence of other stars (and even the vast distant mass of our galaxy’s central hub) has to be taken into account. And even if an object moves into the sphere, there is no guarantee that

  it will necessarily be captured into orbit. Most estimates place the Hill sphere’s outer edge about one light year (64,000 AU) from the Sun, but some put it at about twice that distance.

  372 THE KUIPER BELT AND BEYOND

  page 64

  This schematic

  shows major elements

  of the solar system on an exponential scale, where each division is ten times larger than the previous one.

  10 AU

  100 AU

  1,000 AU

  10,000 AU

  100,000 AU

  1 AU

  Termination shock

  Heliopause

  Sun

  Mercury

  Venus

  Earth

  Mars

  Jupiter

  Saturn

  Neptune

  Uranus

  Pluto

  Sedna (aphelion)

  Oort Cloud

  Heliosphere

  Interstellar space

  Exploring the solar system

  For most of human history, astronomers have had to study the planets from afar. At first they simply tracked the planets’

  movements with the naked eye and using ingenious devices such as the astrolabe. Later they puzzled over the features they could see through telescopes. As the technology available for observing the sky improved, so too did the solar system’s complexity, as new moons, asteroids and even entire planets were discovered.

  The launch of the Sputnik 1 satellite in 1957 saw the beginning of a huge leap forward in our knowledge of the solar system. More powerful rockets and the electronic revolution soon meant that automated probes could venture beyond Earth’s immediate orbit, visiting the Moon and our immediate planetary neighbours, before spreading out across the solar system.

  In the decades since, these scientific robots have developed many forms for exploring these alien worlds in our place, from orbiting surveillance platforms and atmospheric probes to sophisticated geological laboratories and intrepid rovers.

  374 EXPLORING THE SOLAR SYSTEM

  Lunar reconnaissance

  In the late 1950s, the Soviet Union took an early lead in the ‘space race’ against the United States with a series of Luna probes that made flybys of the Moon or crashed into its surface. Putting a spacecraft in lunar orbit, however, was a far greater challenge since it required the use of braking rockets to slow the probe on arrival. Although this was first achieved in 1966 by the Luna 10 mission, the US space agency NASA soon had its own series of successful Lunar Orbiter probes mapping the Moon and looking for potential human landing sites. While the US crash-landing Ranger missions of 1964/65 sent back images that revealed the extent of lunar cratering for the first time, both sides struggled with attempts at a more difficult soft landing until February 1966, when Luna 9 sent back the first images from the Moon’s surface. NASA followed Ranger with a series of Surveyor soft-landers that paved the way for the Apollo missions. Although beaten in the race to put humans on the Moon, the Soviets continued to launch increasingly sophisticated Luna missions into the 1970s, including two successful robotic Lunokhod rovers, and three automated ‘sample return’ missions.

  376 EXPLORING THE SOLAR SYSTEM

  The Apollo missions

  Even after many decades, the Apollo programme remains an unsurpassed achievement in human spaceflight. The product of a concentrated national effort throughout the 1960s saw US astronauts Neil Armstrong and Buzz Aldrin land on the Moon on

  20 July 1969. They reached the lunar surface using a sophisticated three-part vehicle launched on the Saturn V, still the most powerful rocket ever built. The Apollo 11 mission was followed by five more successful flights, in which ten more astronauts walked on the Moon between 1969 and 1972. While the driving force behind the Apollo programme was political, it produced a vast scientific bounty that transformed our understanding of the solar system as a whole. The six landing sites deliberately targeted interesting lunar geology, such as the Lunar Apennines, the boundaries between seas and highlands, and the ejecta blankets of major craters. Some 382 kg (842 lb) of carefully labelled rock samples were returned to Earth, allowing scientists to build up a precise timeline of lunar events, such as the Late Heavy Bombardment, which can be translated to the history of other worlds.

  378 EXPLORING THE SOLAR SYSTEM

  The Moon after Apollo

  With the end of the Apollo programme, the focus of space exploration turned elsewhere and the Moon was largely neglected. Interest was reignited in 1994 by the US Clementine mission, a small satellite that returned some 1.8 million images

  of the lunar surface in just two months of operation. Clementine discovered subtle colour differences linked to certain minerals, and confirmed the existence of a huge depression, the South Pole–Aitken Basin, with permanently shadowed craters that might contain ice useful to future colonists. In 1998, NASA launched the Lunar Prospector probe into orbit. Its mission was to compile a comprehensive mineral map of the Moon, and to study lunar gravitational and magnetic fields. In the new millennium, other nations also took a renewed interest in the Moon. In 2003, the European Space Agency launched its own lunar probe, while 2007 saw the launch of lunar orbiters from Japan and China, with India following a year later. More recent NASA missions have fo
cused on answering specific questions about the Moon’s structure and particles in near-lunar space.

  380 EXPLORING THE SOLAR SYSTEM

  A 3D view of the lunar surface from Japan’s Kaguya orbiter

  Venusian landers and orbiters

  After early failed attempts at a Venus flyby with the Venera 1 and 2 probes of the early 1960s, the Soviet Union set out to make the first landing on the surface. Veneras 3 through 6 were intended to parachute directly into the atmosphere upon arrival. Venera 3 lost contact during entry in March 1966 and failed to return any data, while Veneras 4, 5 and 6 returned data from the atmosphere, but lost contact before touching down. The more heavily armoured Veneras 7 and 8 (1970 and 1972, respectively) were the first to return data from the surface. Later missions took a different approach – Veneras 9 through 14 were delivered by an orbiting mothership that also acted as a radio relay, allowing images to be sent back for the first time. Answering fundamental questions about the Venusian landscape and geology, however, would require a different approach, using orbiters with radar-mapping equipment to pierce the clouds. The Pioneer Venus Orbiter and Veneras 15 and 16 made early contributions, but the most detailed maps of Venus originate from NASA’s Magellan probe (1990–94).