Why a night launch is the right launch

James Oberg NBC News space analyst

HOUSTON - Doing things "in the dark" has always implied either an activity meant to elude attention, or one that is performed in ignorance. So when NASA launched the space shuttle Discovery in the dark — hours after sunset — what did this imply about its intentions?

Saturday's nighttime launch marks an easing in the safety restrictions that NASA put into effect after the Columbia disaster, almost four years ago. That disaster was caused by damage to the spaceship’s thermal protection system during launch, damage caused by a falling chunk of fuel tank insulation that was actually seen as it happened — but which had consequences that were not understood until it was too late.

Since that disaster, NASA has specifically avoided launching in the dark, so that cameras monitoring the shuttle's ascent could get well-lit views of any damage done to the shuttle as it ascended. Until now. So why is this time different? It's because NASA has finally developed an array of new techniques to look for damage — and to respond if it occurs.

Prior to the Columbia disaster, the only way to notice such damage was to make visual observations of pieces falling off during launch. That's a technique that obviously doesn’t work nearly as well in the dark. Fortunately, NASA’s best new safety techniques don’t depend on seeing what happens during launch — hence NASA’s confidence that a night launch is no added risk.

With the addition of an extension boom for the shuttle’s robot arm, cameras can survey all of the spaceship’s underside, and that’s the focus of the crew's attention on the day after reaching orbit. Then, during the approach to the space station, the shuttle does a slow end-over-end pitch maneuver to show its entire belly to high-resolution cameras in the hands of space station crew members. It is during these two activities — neither available to shuttle crews before the Columbia disaster — that any lethal damage will be detected, in plenty of time to take remedial action.

These procedures, rather than the imagery captured during launch, have now become the shuttle crew’s top line of defense and ultimate alarm system. There are still reasons to be interested in what happens to the shuttle during launch, but the safety of the current flight is no longer a factor in that interest.

Why night flights?
NASA officials say that they need the ability to launch shuttles in the dark in order to fly missions often enough to finish station assembly before the grounding of the shuttle fleet in 2010. The reason for this is based on a particular law of orbital motion — specifically the motion of the shuttle’s destination in space, the international space station.

Any Earth-orbiting satellite traces an ellipse that lies in a plane that also includes the planet's center. That's a consequence of planetary laws of motion first formulated three centuries ago. The plane is usually tipped up at some angle to the equator, and the satellite progresses through its orbital path as Earth is rotating under that path.

But the plane of the orbit is also moving in space, due to gravitational disturbances from Earth's slight equatorial bulge. The plane of the space station’s orbit shifts westward about six degrees of longitude per day (for other orbital tilts and altitudes, the rate varies).

Combined with the eastward rotation of Earth, these opposite motions mean that the station repeats its same path relative to the surface of Earth, somewhat earlier every day. Earth doesn’t have to rotate a full 360 degrees to return to the position at the beginning of the day, since the station’s orbital path is "moving to meet it" through its own westward drift. After Earth has turned 354 degrees, and the station’s orbit has turned 6 degrees, the original alignment is again achieved.

That doesn’t take 24 hours — instead, it takes about 24 minutes less than a full day. So the station flies over the launch site earlier every day by that amount.

If a "launch window" occurs at noon on the first day of a given month, after 10 days it will have shifted four hours earlier. After 15 days, it begins shifting into the darkness before dawn. Only after many additional weeks would the launch window get earlier enough to occur near sunset, and then a bit before sunset, and then earlier every day in the same pattern.

The operational implications of this effect is that for any arbitrary date on the calendar, the launch window may be in daylight or in darkness, based on the position of the station’s orbit. Combined with other scheduling constraints, including Russian launch schedules and the orientation of the station orbital plane to the sun, the result is often an unsolvable scheduling problem — that is, if daylight launches remain a requirement.

Allowing the shuttle to launch at night frees up the schedulers and offers enough launch opportunities that the spaceships would not have to wait weeks or months on the ground for the shifting orbits and day/night cycles to line up again — all the while using up the precious remaining certified lifetime of the space hardware.

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