An Interplanetary Tour

Imagine being a 10-year-old in 1961, during an era when space exploration was taking giant leaps forward. The world was captivated by the stars and the infinite possibilities they represented. For a child with a fascination for space and planets, it was a time of boundless imagination and wonder. And I was that kid.

The Soviet Union's success with Sputnik and Yuri Gagarin's historic flight had ignited a global interest in space. In the United States, President John F. Kennedy had just announced the ambitious goal of sending an American to the moon before the end of the decade. Space was not just a distant realm; it was the new frontier, a place of adventure and discovery that seemed within reach.

Plus, there were science fiction books written for kids, for example one of my favorites, The Wonderful Flight to the Mushroom Planet and its sequels. They were real science fiction, too: The science was accurate for the day. And the wonder was real.

This was before Pluto was demoted to dwarf planet and excluded from the list of major planets. We had a mnemonic for remembering their names in order: My Very Educated Mother Just Served Us Nine Pizzas, in which the first letter of each word stood for Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. (Today's mnemonic is My Very Educated Mother Just Served Us Nachos.) Even though little was known of them at the time, each planet was a world of its own, just like Earth but different, with its own set of conditions that encouraged a young mind to consider things that required a different perspective than did everyday life.

In today's essay I'd like to briefly examine each of the worlds in our solar system, hopfully recapturing the wonder of 60 years ago as well as the very satisfying additional knowledge that has come our way since.

Mercury

Mercury, the smallest planet in our solar system (it's just a little larger than Earth's moon) and the one closest to the Sun, has been known since ancient times. It is one of the five classical planets visible to the naked eye and was observed by ancient Babylonian astronomers in the 2nd millennium BCE. The planet is named after the Roman messenger god due to its swift movement across the sky.

For many years, astronomers believed that Mercury was tidally locked to the Sun, always keeping the same side facing it, similar to how the Moon is with Earth. However, this was disproven in the mid-1960s when radar observations showed that Mercury actually has a 3:2 spin-orbit resonance, rotating three times for every two orbits around the Sun.

By 1960, astronomers had determined that Mercury had a very thin atmosphere and completed an orbit around the Sun every 88 days. The planet's surface was believed to be similar to that of the Moon, showing extensive mare-like plains and heavy cratering, indicating that it had been geologically inactive for billions of years.

The first spacecraft to visit Mercury was NASA's Mariner 10, which conducted three flybys in 1974 and 1975, providing the first close-up images of the planet's surface. These flybys revealed that Mercury had a magnetic field and a very thin atmosphere known as an exosphere. Following Mariner 10, the MESSENGER spacecraft was launched in 2004 and became the first to orbit Mercury in 2011. MESSENGER's mission provided a wealth of data, including unexpected evidence of water ice at the planet's poles.

Today, we know that Mercury has a dynamic magnetic field about 1% the strength of Earth's, and no natural satellites. The planet's environment is extreme, with surface temperatures ranging from -173 °C at night to 427 °C during the day due to its lack of a significant atmosphere to retain heat. Despite its proximity to the Sun, Mercury is not the hottest planet in our solar system: Venus holds that title due to its dense atmosphere. Current models suggest that Mercury has a solid silicate crust and mantle overlying a solid outer core, a deeper liquid core layer, and a solid inner core. The European Space Agency and JAXA launched a joint mission to Mercury, BepiColombo, in 2018, which is set to arrive in 2025 to continue the exploration of this enigmatic planet.

Mercury has a very eccentric orbit, which means its distance from the Sun varies significantly, ranging from about 29 million to 43 million miles. This contributes to its extreme temperature fluctuations and makes it a planet of extremes.

Venus

Venus, often hailed as Earth's twin due to its similar size and proximity, has been a beacon of wonder since ancient times. The Babylonians documented Venus as early as 1600 BCE, marveling at its brightness and movements across the sky. By 1960, our knowledge of Venus was still shrouded in mystery, largely due to its dense cloud cover. We knew it was similar in size to Earth and speculated about a warmer surface, but the true conditions remained elusive.

The era of space exploration brought us closer to unveiling Venus' secrets. Mariner 2, launched by NASA in 1962, was the first successful mission to fly by Venus, revealing a planet with no magnetic field and surface temperatures high enough to melt lead. This marked the beginning of a series of probes, including the Soviet Union's Venera missions, which provided the first images of the Venusian surface.

Today, we understand that Venus is a world of extremes. Its thick atmosphere, composed predominantly of carbon dioxide, creates a runaway greenhouse effect, resulting in surface temperatures averaging around 462°C. The planet's rotation is peculiar, spinning in the opposite direction to most planets, which means a person on the surface would see the Sun rise in the west and set in the east. Although, with no magnestic field, even the terms east and west would require redefinition.

As for a bit of trivia, Venus is the only planet in the solar system named after a female figure: the Roman goddess of love and beauty. This is fitting, as from our vantage point on Earth, Venus is often the brightest natural object in the night sky, second only to the Moon, and can even be visible during the day under the right conditions.

Earth

Yes, I'm including Earth as one of the planets. The concept of Earth as a planet among others had been suggested as early as 250 BCE by Aristarchus of Samos, but this fact wasn't widely accepted until the end of the 17th century during a period of significant advancements in astronomy and a shift in the understanding of our solar system from an Earth-centric structure to a Sun-centric (heliocentric is the actual term) structure.

The first photo of Earth from space was taken on October 24, 1946. It was captured by a 35mm motion picture camera mounted on a V-2 missile launched from the White Sands Missile Range. The camera snapped a new frame every second and a half as it ascended to an altitude of 65 miles, providing a breathtaking view of Earth against the blackness of space. Although the camera was destroyed upon impact with the ground, the film, protected in a steel cassette, survived. These historic images offered humanity its first glimpse of our planet from a perspective beyond the atmosphere

The iconic image of Earth from the moon is known as Earthrise. It was taken by astronaut William Anders on December 24, 1968, during the Apollo 8 mission, which was the first crewed voyage to orbit the Moon. This photograph captures Earth's beauty as it appears over the lunar horizon, symbolizing the unity and fragility of our home planet. Nature photographer Galen Rowell described Earthrise. as the most influential environmental photograph ever taken.

The photograph has had a profound impact, not only for its stunning visual appeal but also for its representation of Earth's solitary presence in the vastness of space. It has been credited with inspiring the environmental movement and fostering a sense of global stewardship. The Earthrise image continues to be a powerful reminder of our planet's uniqueness in the cosmos and the importance to us of taking care of it.

Mars

Mars, the enigmatic Red Planet, has been a point of fascination since ancient times. Its discovery predates recorded history, as it is one of the five planets known to ancient astronomers, visible to the naked eye. Babylonian astronomers were tracking Mars as early as the 2nd millennium BC.

By 1960, our understanding of Mars had evolved significantly. We knew it had polar ice caps and seasons similar to Earth's, but its reddish hue and fluctuating brightness sparked imaginations with possibilities of alien life. The first successful mission to Mars was NASA's Mariner 4 flyby in 1965, which returned the first close-up images of the Martian surface.

Since then, numerous probes have been sent to Mars. The Viking landers in the mid-1970s were the first to touch down and conduct experiments on the Martian soil. In the decades that followed, orbiters like Mars Global Surveyor and rovers such as Spirit, Opportunity, and Curiosity have revolutionized our understanding of the planet.

Today, we know Mars as a dusty, cold desert world with a thin atmosphere composed mostly of carbon dioxide. It has the largest volcano in the solar system, Olympus Mons, and the deep canyon Valles Marineris. Current missions continue to explore its surface and atmosphere, searching for signs of past life and assessing its potential for future human habitation.

Mars' Olympus Mons (Latin for Mount Olympus) is not only the largest volcano in the solar system; it is also the tallest known mountain at 16 miles (25 kilometers) high. That's nearly three times the height of Mount Everest!

Jupiter

Jupiter, the largest planet in our solar system, has also been known since ancient times. It was observed by Babylonian astronomers as early as the 2nd millennium BCE. In 1960, we understood Jupiter to be a massive gas giant with a thick atmosphere primarily composed of hydrogen and helium. Its Great Red Spot, a giant storm, had been observed for centuries, and Jupiter was known to have at least 16 moons.

The first spacecraft to visit Jupiter was Pioneer 10, which arrived in 1973, followed by Pioneer 11, Voyager 1 and 2, and later Galileo, which entered orbit in 1995. These missions have provided us with invaluable data about Jupiter's magnetic field, its ring system, and its numerous moons.

Today, we know that Jupiter has a very dynamic environment with powerful storms and a strong magnetic field. Its atmosphere is characterized by bands of clouds and the iconic Great Red Spot. Jupiter has 79 known moons, and its largest, Ganymede, even has its own magnetic field!

Jupiter actually emits more energy than it receives from the Sun due to its internal heat source; and its magnetic field is so strong that, if visible, it would appear larger than the full moon in our sky!

Saturn

Saturn, the sixth planet from the Sun, has been known since prehistoric times because it is easily visible to the naked eye. Ancient civilizations, including the Babylonians, were aware of Saturn's presence in the sky. The planet was first recorded by the Assyrians around 700 BC, and they described it as a sparkle in the night.

In 1960, Saturn was understood to be a gas giant with an extensive ring system, although the rings' composition and structure were not yet fully comprehended. It was known to have several moons, and its atmosphere was believed to be primarily composed of hydrogen and helium. The planet's rotation period and the fact that it emitted more heat than it received from the Sun were also known.

The first spacecraft to visit Saturn was Pioneer 11, which provided the first close look in September 1979. This was followed by the twin Voyager 1 and Voyager 2 spacecraft, which conducted flybys in 1980 and 1981, respectively. These missions revealed intricate details about Saturn's rings and moons. The most extensive exploration of Saturn came with the Cassini-Huygens mission, which orbited the planet from 2004 to 2017 and provided a wealth of information about Saturn's environment and its moons.

Today, we know that Saturn's atmosphere is mostly hydrogen and helium, and it has a strong magnetic field. The planet's environment is characterized by extreme temperatures, pressures, and wind speeds, making it inhospitable to life as we know it. However, some of Saturn's moons, such as Enceladus and Titan, have conditions that might support life.

Saturn is the flattest planet in our solar system due to its rapid rotation, which causes it to bulge at the equator and flatten at the poles. Additionally, Saturn's magnetic field is about 578 times more powerful than Earth's!

Uranus

Uranus, the seventh planet from the Sun, has a unique place in the history of astronomy. It was first discovered by Sir William Herschel on March 13, 1781, using a telescope, making it the first planet to be discovered with such an instrument. Herschel initially thought it was a comet or a star, but it was later confirmed to be a new planet, expanding the known boundaries of the Solar System for the first time in modern history.

By 1960, astronomers knew that Uranus was an ice giant made primarily of various ices, such as water, ammonia, and methane, along with a rocky core. The planet's atmosphere was known to be mostly hydrogen and helium, with a trace amount of methane that gives it a pale blue color. However, much about the planet remained a mystery due to its distance and dim visibility.

The only spacecraft to have visited Uranus is NASA's Voyager 2, which made its closest approach on January 24, 1986. During the flyby, Voyager 2 discovered 10 new moons, studied the planet's cold atmosphere, examined its ring system, and observed its five large moons.

Today, we understand that Uranus has an extreme axial tilt of about 98 degrees, probably caused by a collision with an Earth-sized object long ago. This results in extreme seasons, each lasting about 21 years. The planet also has a complex ring system and a magnetosphere. It has 27 known moons, with the largest ones being Titania, Oberon, Umbriel, Ariel, and Miranda.

Uranus is the only planet in our solar system that rotates on its side, and its name, unlike the other planets named after Roman deities, comes from Greek mythology. Uranus is named after the Greek god of the sky. Additionally, all of Uranus's 27 moons are named after characters from William Shakespeare's plays or characters in the works of Alexander Pope.

Neptune

Neptune, the eighth and most distant known major planet from the Sun, has a storied history of discovery and exploration. It was first observed on the night of September 23–24, 1846, at the Berlin Observatory by astronomer Johann Gottfried Galle, working from calculations made by French mathematician Urbain Le Verrier. This discovery was a triumph of mathematics, as Neptune was the first planet found through predictions rather than empirical observation.

By 1960, astronomers had figured out that Neptune was an ice giant, with a blue hue due to the presence of methane in its atmosphere. However, much about the planet remained enigmatic, as its great distance from Earth limited detailed study.

The only spacecraft to have visited Neptune is Voyager 2, which conducted a flyby on August 25, 1989. This historic encounter provided a wealth of data, revealing Neptune's dynamic atmosphere, characterized by supersonic winds, and its system of rings and moons.

Today, we know Neptune's environment is harsh and volatile, with temperatures and pressures too extreme for life as we understand it. The planet's atmosphere is primarily hydrogen and helium, with traces of methane, water, and ammonia. Deep beneath the cloud tops, Neptune may harbor a vast, hot ocean of water surrounding its rocky core.

Neptune's winds are some of the fastest in the solar system, capable of breaking the sound barrier on Earth! Additionally, it seems that Neptune was originally closer to the Sun and migrated to its current position over hundreds of millions of years.

As we stand on the brink of new horizons, the excitement for further exploration of our solar system's major planets is palpable. Each mission brings us closer to unraveling the mysteries of our celestial neighbors, fueling our curiosity and our quest for knowledge. The potential discoveries that await in the vast expanse of space are not just scientific triumphs but also a testament to human ingenuity and the relentless spirit of adventure. The journey ahead promises to deepen our understanding of the cosmos and our place within it, inspiring future generations to continue reaching for the stars.

How exciting it must be, to be a kid today!