On this Day 30 Years Ago, Voyager 2 Cruises By Uranus

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In this now famous parting view, a crescent Uranus is majestically set against the backdrop of space as Voyager 2 speeds on to its next destination, Neptune

At a stunning velocity of 57,890 km/h (35,970 m/h) this day 30 years ago, Voyager 2 cruised past the largest of what is known as the Ice Giants, Uranus!

As a subset of the Jovian planets, a set that includes all the outer planets (Jupiter, Saturn, Uranus and Neptune), the Ice Giants consist of Uranus and Neptune, both uniquely distinct in their Blue, Blue/Green hues. These colors are indicative of a high methane concentration in their respective atmospheres with Neptune presenting as the bluer of the two. With red being largely scattered and absorbed by Methane in its upper atmospheres, Uranus presents decidedly blue/green as the blue light passes through the upper atmosphere unabated and reflects strongly from the Methane at a greater depth. Since Neptune’s atmosphere has a higher concentration of methane, this effect is more strongly pronounced and thus, it presents strikingly much bluer.

Launched on August 20, 1977, Voyager 2 was the second and final deep-space probe launched in the Voyager mission series or what has been colloquially referred to as “The Grand Tour”, humanity’s [first] exploration of the outer solar system. In a series of 2 “gravity assists” at Jupiter and Saturn, where a tiny amount of a planet’s orbital angular momentum is transferred to the spacecraft in the form of forward kinetic energy (the energy of motion), Voyager 2 was ultimately accelerated to an astounding 16 km/sec or about twice the earth’s escape velocity, outbound from Saturn to Uranus.

Voyager_2_velocity_vs_distance_from_sunAs can be seen from this chart, the outbound velocity decreases dramatically with distance. Without these gravity assists, the mission beyond Jupiter would have failed since the vehicles would have lacked sufficient kinetic energy, and thus forward linear momentum, to continue  beyond Jupiter, no less explore the realms of Uranus, Neptune and beyond. In short, they would have lacked the necessary velocity to escape the solar system and would have gone into perpetual orbit around the sun with a 5 AU (Jupiter’s distance from the sun) aphelion (the furthers point from the sun in any orbit).

Standard color image of Uranus flanked by a false-color image obtained in UV and violet light by Voyager 2 illustrates the stratified atmosphere of the planet

In addition to a stunning set of images from Uranus that include existing and newly discovered moons, the probe returned volumes of data about the planet’s environment and atmosphere that confirmed and enhanced our understanding of it. Although discovered by William Herschel in 1781 and presenting as a plain, aqua-colored featureless globe, it has a unique set of peculiarities. Its polar orientation is 98 degrees to the vertical meaning that its equatorial region is within 8 degrees of where the polar region should be! In a sense, it could be understood to “roll” around the sun during its 84 earth-year period. Additionally, and like its sister world Neptune, its physical core is not axi-symmetric with its geometric core; said differently, its core is off center, an anomaly thought to be the result of a collision, possibly with Neptune.

Even though the “Grand Tour” ended with the Voyager 2 flyby of Neptune on 25 August, 1989, 12 years after its launch, the mission continues today as an Interstellar Mission, exploring the vast region of space well beyond the planets where the sun’s influence wanes and yields to the combined stellar wind of the galaxy and the environment of interstellar space. As a testament to engineering prowess and mission planning, both probes are still functioning today, almost 39 years after their launch! Launched 16 days after its twin, Voyager 1 explored “only” Jupiter and Saturn. With a round-trip command travel time now of 37 hours, a distance of 134 AU from the sun, it not only has traveled further than anything else ever made in history by human hands, it has now entered interstellar space, a transition point where the solar wind (the continuous stream of changed particles from the sun) has abated and yielded to the stellar wind, the combined wind of the stars in our galaxy! With Voyager 2 soon to follow, Voyager 1 has now become humanity’s first emissary to the stars! Its twin, Voyager 2, explored all four outer planets with each craft carrying a 12-inch gold-plated copper phonograph record containing sounds and images selected to portray the diversity of life and culture on Earth should either craft be intercepted by extra terrestrials. NASA produced a more ambitious message for the Voyagers than they did for the Pioneer mission, a kind of time capsule, intended to communicate a story of our world to who ever finds it! In 40,000 years, when Voyager 1 passes within 1.7 light years of the star AC +79 3888 at a distance of 17.6 light-years, it will then assume the title, quite literately, of “Star Ship” (a light year is the distance light travels in one year). At that rate (2,300 yrs of travel/Ly), it would take approximately 9,800 years to reach Proxima Centauri, the closest star, if it were heading in that direction.

Not only is this mission a testament to endurance, engineering and technical excellence, it is a testament to the indefatigable human spirit and what can be achieved with teamwork, cooperation, some luck and a little bit of genius.

Vaya con Dios, Voyagers!

Imagination is more important than knowledge 585px-Albert_Einstein_signature_1934(invert)
An index of all articles in this blog can be found here.

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