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Sunrise and Sunset from space

Sunrise and Sunset from space

For a spacecraft such as the Space Shuttle, the Mir, a Soyuz Capsule or the International Space Station to maintain an orbit around the Earth at relatively low altitudes (anywhere from approximately 175 to 575 kilometers [~95 to 310 nautical miles]) it must travel at approximately 32,500 km/hour (~17,500 nm/hr). At these altitudes and at this velocity it takes about 90 minutes to circle the Earth once, so every 45 minutes the astronauts and cosmonauts onboard see a sunrise and a sunset, a total of 16 each every 24 hours.

To view these orbital sunrises and sunsets is quite spectacular. Their beauty is breathtaking and the colors are extremely vivid. They are one of those events on orbit when more often than not, the work onboard stops and everyone gathers at the window for a brief moment to watch the display before returning to work. Reactions to the sight of the atmospheric limb as its colors dynamically change with the stage of the sunrise or sunset have ranged from rapt silence to exclamatory outbursts. At its most vivid and spectacular moment the lower atmosphere is a deep red which transitions upward with altitude to a bright orange and then to a beautiful sky blue then to a deep blue and finally to a thin layer of violet. Often the tops of cumulonimbus (thunderstorm) clouds are visible within the lower red band of the limb.

This thin band which can be easily obscured from view just by holding a pencil horizontally at arms length in front of one’s eye is all that shields the earth from the harsh environment of space. It is not remarkable then that the few human space travelers who have had the privilege of being able to witness this wonderfully breathtaking sight have become keenly aware of just how frail and fragile the atmosphere really is. From this perspective it is no longer viewed as a vast, unlimited resource but rather a finite and delicately balanced one.

This photograph taken by the crew of Endeavour during the STS-77 mission shows another spectacular view of an orbital sunset taken just before the sun dipped completely below the horizon.

The spectacular eruption of Mount Pinatubo in June of 1991 in the Philippines ejected a cloud of dust and ash as high as 30 kilometers into the stratosphere. A significant portion of this dust and ash cloud was made up of sulfuric acid droplets. It has since been estimated that the volcano ultimately spewed approximately 15 to 20 million tons of sulfur dioxide into the atmosphere over a period of four weeks. Mount Pinatubo is located at about 15 degrees north latitude and by August of that year the eruption plume had spread to about 30 degrees North. This single event dramatically changed what an orbital sunset looked like for months afterwards. Taken by the crew of Atlantis during the STS-43 mission which launched shortly after the Pinatubo eruption the dust and ash from the volcano can be plainly seen as the dark brown band between the now totally orange and light blue color bands of the atmospheric limb. This band is situated well into the stratosphere while the more diffuse dust and ash not confined to the brown band itself thins out away from the band both above and below it causing the change in character of the sunset’s colors. The deep red and blue are no longer present and any hint of violet has been completely obliterated. There were a few other factors which contributed to the change in character of the lower tropospheric portion of the limb as seen in this photograph. The hazy atmosphere included dust from the Arabian Peninsula and North Africa blown westward across the Atlantic to Mexico by strong wind currents, a huge smog bank from eastern North America had spread eastward across the Atlantic and black smoke from the Kuwaiti oil fires burning after the Gulf War spread eastward across the Indian Ocean. This time of year was also the start of the burning season in the southern hemisphere.

Also taken by the crew of STS-43 just a few days later in the mission the band of dust and ash from the eruption is more well defined in this view and there is a greater amount of diffuse dust and ash spread throughout the atmosphere below the main band.

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