Our Last Full Day on Orbit
Dispatch #10 (Sunday, December 26)

The time on orbit has gone so fast. It is hard to believe that in the last week we have captured the Hubble Space Telescope, performed three space walks to repair and service it, and deployed it back into space to observe the cosmos. Today was mostly a day of cleaning, stowing all the tools we took out to service Hubble, and putting the space suits away for the last time on this flight.

We also did an orbit adjust burn to bring us down to a slightly lower orbit than Hubble. Orbital mechanics has caused us to advance far above the telescope in only a few hours, so that we can no longer see it. Earlier we could see Hubble at sunrise as a bright star on the horizon, set in front of a sea of stars. I am so pleased that we have returned Hubble to the heavens to again be a bright star in astronomical research.

Once the cabin was put in order we had time to spend doing some of our favorite things in space. Of course, going up to the flight deck with all the lights out and gazing at the Earth and stars at night is high on that list.

From our vantage point some 300 miles above the planet, the Earth at night is all but a dark orb. Flying over the Gulf of Mexico we could see city lights dominating the coastline of California, cities down to Mexico City, up through Houston, St. Louis, even to Chicago, and east to Atlanta and Florida down to Miami and the Florida Keys. Once over Florida, we could see the entire East Coast of the United States up to Washington, D.C. and New York. At night it would seem that the United States is an interconnected system of cities and interstates in a kind of national nervous system.

Photo of the Moon over the airglow of Earth's atmosphere.

Once out over the Atlantic heading south the surface darkens, and with some dark adaptation the horizon fills with a layer of light. This feature of the 'dark' Earth is called the air glow layer. It is caused by the delayed emission of light from nitrogen and oxygen atoms energized by the Sun during the daytime. Most of the light is emitted at altitudes of about 95 kilometers, and even at that height the atmosphere looks perilously thin. Of course most of the habitable atmosphere occurs in the first five kilometers, with the highest flying airplanes only able to achieve about 30 kilometers.

As we circle the Earth at 8 kilometers per second stars slowly rise through this green and red thin veil of light. The Milky Way cuts across the sky like a cloud let loose from the Earth rising across our windows. A pleasure for the astronomers on board was to see the Southern Cross, the Magellenic Clouds, and all the southern hemisphere constellations that we can't see from Houston.

With ethereal constancy stars continued to rise through the mist, piercing the visible horizon, reminding us that the horizon we see is just the top of the air glow. At one point after the moon rose we saw a kind of strange cloud 10 or 15 degrees above the horizon, looking like a higher layer, at perhaps 200 kilometers. We speculated about whether the layer was real or a reflection. It had a relative motion to us and slowly passed out of view. What strange physical phenomena caused it we may likely never know.

There are still many mysteries in the Earth's atmosphere that we are just beginning to observe and study. The Compton Gamma-Ray Observatory has observed high energy x-rays being emitted from upward going lightning in energetic thunderstorms, called sprites.

From space we observe thunderstorms at night as localized flashes of lightning in a sea of grayish cloud tops, illuminated by a half-moon. Lightning seems to go off in one place creating a cascade of bolts that can go on for hundreds of miles before stopping as quickly as it started. Then, from some other spot it starts all over again with the staccato events traveling off in a different direction. Occasionally you can see a precise bolt, going in a random circuit cloud to cloud, and other times a single cloud just lights up in a strange pattern of diffuse light.

After traveling across the Atlantic observing the stars, Venus cast her bright image in our direction. At first we were slightly confused by her presence because she seemed so bright. Almost in Scorpio's pincers, there could be no real ambiguity or confusion, only to be confirmed by the signal of the impending sunrise.

Sunrise occurs over a period of about three or four minutes, with the sky going from a starlit wonderland to a painfully bright sun. In the transition is the most beautiful spectrum of hues from a deep red, changing to fire orange, with many layers and the silhouette of clouds on the horizon, with an edge of bright yellow near the limb. The colors continue to brighten and expand, until the Sun asserts itself over the horizon, and with a brief presence of a red-orange sphere, turns into our familiar bright Sun surrounded by the blackness of space.

Hubble Space Telescope

Tomorrow we leave this magical world of weightlessness, stellar tours, space walks, and earth views. The world we return to is dominated by gravity, which at least for a few days will seem like a weight hung around our necks and our psyche. But for the crew here aboard Discovery, at least we trade our most excellent adventure for the warmth of our families in this holiday season. And of course we bring back the great satisfaction of returning the Hubble Space Telescope to operation, and memories of a great expedition. For myself, I also found that I have a new relationship with the Hubble, and when I go out to view the transit of the observatory over Houston at dusk or dawn, I will smile and think of the first space walk when I reached out for the first time to touch the telescope.