Solar System in Minutes
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Vikings on Mars
Following early investigations of Mars by Mariner probes, NASA launched a pair of ambitious missions to Mars in the mid-1970s. Each consisted of a two-part spacecraft – an orbiter to carry out a colour photographic survey of the red planet, and a lander that would send back pictures and environmental data from the surface, as well as collecting and analysing samples of the Martian soil. Vikings 1 and 2 arrived at Mars in summer 1976, with their landers touching down on the surface of Mars on 20 July and 3 September, respectively. Viking Lander 1 touched down on Chryse Planitia (see page 146) in what is now known to be a flood channel, while Lander 2 set down further north in a region known as Utopia Planitia, on a desert plain strewn with volcanic debris. Both landers monitored weather conditions, reported the composition of the atmosphere and analysed the surrounding terrain. Their most intriguing experiment searched for signs of microbial activity in the Martian soil – initial results from both landers seemed encouraging, but later attempts to repeat the tests met with negative results.
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Pioneers to the outer solar system
While the inner solar system is relatively compact, the space beyond the asteroid belt is a different matter, with vast distances and long journey times between the giant planets. In 1964, however, NASA engineers realized that a rare planetary alignment in the late 1970s presented a unique opportunity for a ‘Grand Tour’ of all four outer planets, picking up speed at each through a technique called a ‘gravitational slingshot’.
Planning began for the missions that would attempt the tour, but an understanding of conditions around Jupiter and Saturn, and a test of the slingshot principle, were key to its success. To this end, NASA launched the Pioneer 10 and 11 missions. In December 1973, Pioneer 10 became the first spacecraft to fly past Jupiter, revealing the planet’s swirling cloudscapes in detail for the first time. Pioneer 11 followed a year later, executed its course change, and went on to return the first close-up pictures of Saturn, in September 1979. Both spacecraft are now on trajectories that will take them out of the solar system entirely.
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The Voyager missions
The spacecraft that eventually accomplished NASA’s ‘Grand Tour’ of the outer solar system started life as modifications of the trusty Mariner template, after earlier and more ambitious schemes were scrapped in the early 1970s. They were ready for launch by mid-1977, with Voyager 2 launching first, and Voyager 1 setting off on a faster trajectory some 15 days later. By 1979, Voyager 1 had overtaken its sibling, and the two probes swung past Jupiter in March and July respectively, discovering the planet’s rings, volcanic plumes rising from Io and the icy shell of Europa. Voyager 1’s primary mission ended at Saturn in November 1980 – the trajectory required for its close Titan flyby could not also be used to fly on to Uranus. However, Voyager 2 continued onwards, executing a gravitational slingshot at Saturn, in August 1981. This put it on course to encounter Uranus in January 1986 and Neptune in August 1989, revealing them in detail for the first time. Both Voyagers are now leaving the solar system; in August 2012, Voyager 1 crossed the heliopause to became the first probe to send back signals from interstellar space.
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Neptune
25.08.89
V1 launch
05.09.77
V2 launch
20.08.77
Jupiter
05.03.79
25.08.81
Saturn
12.11.80
09.07.79
Uranus
24.01.86 Voyager 2 Voyager 1
Trajectories of the Voyager probes
Missions to comets and asteroids
In the mid-1980s, an international armada of five probes was launched to greet the return of Comet Halley, perhaps the most famous of the solar system’s smaller bodies. Two Japanese missions made long-range observations, while a pair of Russian craft flew closer to locate the nucleus. Finally, ESA’s Giotto probe plunged into the coma, flying within 596 km
(370 miles) of Halley’s solid heart.
Since then, a wide range of missions have expanded our knowledge of both asteroids and comets. NASA’s Galileo probe encountered main-belt asteroids en route to Jupiter in the early 1990s, while NASA’s Stardust and Japan’s Hayabusa returned samples of material from a comet and an asteroid to Earth laboratories (see pages 324 and 178). Longer-term studies include the NEAR-Shoemaker probe, which orbited asteroid 433 Eros for a year, and ESA’s Rosetta mission, which accompanied comet 67P through its perihelion passage (see pages 190 and 326).
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pages 324 and 178
pages 190 and 326
Return to Mars
ollowing the success of the Viking project, there was a
long gap in the exploration of Mars – partly because space agencies’ priorities were elsewhere and partly due to a string of mission failures that became known as the ‘curse of Mars’. This began to change in 1997, when NASA successfully placed its Mars Global Surveyor (MGS) satellite in Martian orbit, and parachuted a lander called Pathfinder (equipped with a small robot rover called Sojourner) onto the surface. MGS delivered images at a far higher resolution than those delivered by Viking, revealing some of the first hints that liquid water might still be present on Mars. Subsequently, the pace of Mars exploration has accelerated, with successes such as the 2001 Mars Odyssey, which revealed vast amounts of ice beneath the northern plains, and Phoenix, which landed near the Martian north pole and monitored its descent into winter. Mars Reconnaissance Orbiter and ESA’s Mars Express have continued MGS’s work, photographing the planet
in stunning detail, while the ExoMars Trace Gas Orbiter hopes to solve the mystery of Martian methane (see page 154).
F
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Mars rovers
Some of the most important insights into Martian conditions past and present have come from NASA’s robotic rovers. Following the success of Sojourner in 1997, a pair of much larger Mars Exploration Rovers – Spirit and Opportunity – landed on Mars in early 2004. Spirit’s landing site was in Gusev, a crater whose floor was thought to be an ancient lake bed, but was subsequently found to be volcanic. In six years of exploration across 7.7 km (4.8 miles) of terrain, Spirit’s discoveries included Martian dust devils and silica sand just beneath the surface, potentially formed by ancient hot springs. Opportunity landed
in Meridiani Planum, an ancient shoreline, where it discovered rocks and minerals that probably formed under water. It has since travelled some 45 km (28 miles) during more than 15 years of operation. In August 2012, another even larger and more complex rover, Curiosity, touched down in Gale, an ancient crater filled with sediment deposits that were laid down underwater. Curiosity continues to investigate whether conditions in this region could once have supported life.
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Galileo and Juno
The brief Voyager flybys of Jupiter provided our first detailed views of the giant planet and its complex moons, but they inevitably left many questions unanswered, and plans were being laid for a follow-up orbiter mission even before the Voyagers launched. Ultimately, this mission – Galileo – did not reach Jupiter until 1995 (taking a six-year route to the planet, in order to enter orbit around it). Galileo deployed a smaller probe into Jupiter’s atmosphere before beginning a series of two-month loops around the planet that would bring it close to each of the major moons on several occasions. During eight years in orbit, it made a wealth of discoveries, such as the buried oceans on Ganymede and Callisto and the true extent of volcanism on Io. Its final act was a death plunge into Jupiter’s atmosphere, in September 2003.
In July 2016, a new spacecraft arrived at Jupiter, aiming to reveal the secrets of the giant planet’s atmosphere, magnetosp
here, polar lights and internal structure. Juno occupies a highly elliptical, near-polar orbit that brings it to within 4,200 km (2,600 miles) of the cloud tops at close approach, providing unique views from hitherto unseen angles.
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Cassini/Huygens
Following the Voyager flybys of Saturn, it was clear that the planet, its rings and varied system of moons held more than enough interest to justify a follow-up orbiter mission in the vein of Galileo. The bus-sized Cassini was to be the most ambitious space probe ever assembled, and became a collaboration between NASA and the ESA, with the latter primarily responsible for the Huygens lander that would parachute to the surface of Titan (see page 272).
Cassini arrived at Saturn in 2004, after a six-year journey involving multiple gravitational slingshots. It remained in orbit for some 13 years, making a series of discoveries, including at least eight new inner moons, intricate structures in Saturn’s rings, the equatorial ridge on Iapetus and, most importantly of
all, the enormous water plumes rising from Enceladus (see pages 246, 280 and 258). With its fuel supply running low, Cassini was deliberately deorbited into Saturn’s atmosphere in September 2017, in order to protect the moons from possible contamination.
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page 272
pages
246, 280 and 258
MESSENGER to Mercury
Entering orbit around Mercury presents a unique challenge owing to the planet’s high orbital speed (see page 70).
Mariner 10 achieved three initial flybys in the 1970s, using a slower-moving orbit that crossed over that of Mercury. However, matching orbit with the planet required NASA’s MESSENGER (Mercury Surface, Space Environment, Geochemistry and Ranging) mission to follow a complex, seven-year flightpath involving one flyby of Earth, two of Venus and three of Mercury itself. By the time MESSENGER entered orbit in March 2011, it had already delivered a wealth of new data about the planet, including images of its previously unseen hemisphere.
MESSENGER remained in orbit until 2015, transforming our view of the solar system’s smallest planet with evidence for surface water ice, carbon-based organic chemicals and a complex volcanic past (see page 78). BepiColombo, a joint European–Japanese mission, should continue to improve our understanding of Mercury when it arrives in the mid-2020s.
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page 78
New Horizons
Launched in January 2006, NASA’s New Horizons probe was already en route to its primary target of Pluto before the International Astronomical Union made its decision to reclassify the largest Kuiper Belt Object (KBO) as a mere dwarf planet. Nevertheless, the mission has been a stunning success, turning many previous ideas about KBOs on their heads.
Racing to catch Pluto on the inner edge of its orbit, and before its thin atmosphere had a chance to condense back into surface ice, New Horizons was the fastest spacecraft ever launched.
It departed Earth at a speed of 16.26 km/s (36,373 mph) and boosted its speed still further with a gravitational slingshot at Jupiter, allowing it to reach Pluto in less than a decade. Over a few short days around its closest approach, it sent back images that revealed Pluto and its giant moon Charon as complex worlds full of unexpected geological activity (see pages 346–51). Following this encounter, it was set on course for a flyby of a much smaller KBO, designated (486958) 2014MU69, in January 2019.
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pages 346–51
An artist’s impression depicts New Horizons during its flyby of (486958) 2014MU69.
The future of manned exploration
Aside from the Apollo missions to the Moon some half a century ago, human exploration of our solar system has mostly been confined to Earth orbit. After decades of false starts, however, this is finally about to change. The growth of commercially operated spaceflight into Earth orbit has allowed the US space agency NASA to refocus on the exploration
of other worlds. A new spacecraft, called Orion, will enter operation in the early 2020s – its advanced design can sustain a crew of four astronauts on missions of up to three weeks, and potentially much longer with additional modules attached. Orion’s proposed missions include a return to the Moon, the establishment of a space station in lunar orbit, investigation of near-Earth asteroids and, ultimately, a manned expedition to Mars orbit in the 2030s. Meanwhile, the China National Space Administration is developing its own plans to send ‘taikonauts’ to the Moon and beyond, and commercial companies, such as SpaceX, are pioneering a new generation of giant rockets that, they claim, could put the first humans on Mars within a decade.
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NASA’s new spacecraft, the Orion Multi-Purpose Crew Vehicle, is designed with interplanetary exploration in mind.
Large Blackboard
Our future in the solar system
While many space advocates argue that colonization of the solar system is humanity’s best hope of avoiding future extinction, the settlement of other worlds presents a huge challenge. Colonists would need to sustain themselves without a constant supply of resources from Earth, and water would play a key role; not just for drinking and growing crops, but also for processing to provide fuel and oxygen for breathing. Bases on our Moon might harvest ice from shaded craters at the poles – the airless environment would limit the scope of settlement, but a lunar base with lower gravity than Earth would make an ideal ‘jumping off point’ for exploration further afield. Mars offers more hospitable conditions, with plentiful ice in permafrost across much of the planet. In the long term, some argue, the planet could even be ‘terraformed’ to create a warmer, wetter world with a thicker atmosphere. Taming the hostile environment of Venus or settling on the moons of the outer planets would present even greater challenges, but if we wish to survive in the truly long term, humanity must go into the solar system.
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Large Blackboard
This artist’s impression depicts stages in the ‘terraforming’ of Mars to provide a habitable environment for human settlers.
Glossary
Asteroid
One of the countless rocky worlds of the inner solar system, mostly found in the main Asteroid Belt between the orbits of Mars and Jupiter.
Astronomical unit (AU)
A unit of astronomical measurement equivalent to Earth’s average distance from the Sun – about 150 million km (93 million miles).
Centaur
A small icy world that orbits between the giant planets of the outer solar system.
Comet
A small chunk of rock and ice that typically orbits as a deep-frozen nucleus in the Oort Cloud or Kuiper Belt. When comets are disturbed, they can fall into elliptical orbits that bring them closer to the Sun, when their surfaces become active and produce streams of gas and dust.
Dwarf planet
A solar system object that, though large enough to otherwise qualify as a true planet, does not possess enough mass and gravity to clear its orbit of other bodies.
Eccentricity
A measure of how stretched or ‘elliptical’ an orbit is – perfectly circular orbits have
an eccentricity of 0, while paths with
an eccentricity of 1
or greater are open trajectories on which a body can escape orbit entirely.
Gas giant
A huge planet, far larger than Earth and dominated by a deep atmosphere of lightweight elements.
Gravity
A force of attraction that acts between all objects with mass.
408 GLOSSARY
Heliosphere
The region of space where the solar wind of particles blown
out from the Sun is consistently streaming outwards.
Hills cloud
An doughnut-shaped inner extension of the Oort Cloud, between about 2,000 and 20,000 AU from the Sun.
Hill sphere
Ice dwarf
An icy world, larger than a comet and including some dwarf planets, orbiting in or around the Kuiper Belt.
Ice giant
A planet larger than Earth whose interior
is dominated by slushy ‘ices’ of chemicals such as water, ammonia and methane.
Inclination
The angle at which an orbit is tilted compared to the plane of the solar system or (in the case of satellites) the equator of the parent planet. Objects with inclinations greater than 90 degrees follow retrograde orbits.
Irregular satellite
A natural satellite in an eccentric, highly inclined or retrograde orbit that shows it did not form in orbit around a planet, but was captured later by its gravity.
Kuiper Belt
A doughnut-shaped ring of ice dwarfs and comets orbiting beyond Neptune.
Lagrangian point
One of several ‘sweet spots’ in a system involving two bodies with substantial gravity, where the influence
of the object is neutralized and a third object can sustain a stable orbit around the more massive body.
Light year
A unit of astronomical measurement equal to the distance travelled by light in one year – roughly 9.5 million million km (5.9 trillion miles), or 64,000 AU.