Emerging from the elation of a first-rank performance in the Space Shuttle Discovery mission to the International Space Station (ISS), NASA now faces a sobering series of challenges extending over four years.

The Discovery mission, STS-121, suffered little of the foam insulation loss from its external fuel tank that was evident in earlier missions. The loss of a suitcase-sized piece of foam from the tank on the Space Shuttle Columbia during its launch punched a hole in a leading edge of the orbiter wing, which later led to the spacecraft disintegrating during reentry.

NASA will discover next month whether the minor, and safe, foam loss during the July 4 Discovery launch was just a fluke, or was the payoff for safety design changes such as removal of the protuberance air load ramp and reduction of foam applied to various areas.

Aug. 28 to Sept. 7 is when Space Shuttle Atlantis will be launched, permitting NASA to discover whether once again there are but tiny bits of foam breaking off, and whether they break loose late in the ascent, safely above most of the atmosphere, as occurred during the Discovery launch.

While in retrospect the decision to launch Discovery seems well taken, it was not so obvious before the Independence Day ascent of the spacecraft from a pad at Kennedy Space Center, Fla. Top officers in the shuttle program recommended against permitting Discovery to fly, fearing further loss of foam insulation might damage the craft, even though the damage this time would be detected and thus avoid any danger that a crippled craft would be destroyed during reentry.

While those fears were not borne out in the Discovery launch, another safe mission with a clean, undamaged Atlantis returning to land with hardly a scratch will be required to demonstrate that the foam-shedding problem has been resolved, according to our sister publication Space & Missile Defense Report. Assuming that’s the case, then NASA will have to buckle down to a grueling schedule of shuttle launches, where all must go well, mission after mission, if the ISS construction job is to be completed by 2010.

One part of the Discovery mission helped to move the ISS construction program forward. Repeated spacewalks saw repairs to a railway, a tram on the outside of the space station that will permit a construction crane to move to needed locations. The space station components that will be moved into position and mated with the artificial moon in some cases must be brought to the ISS by space shuttles, because the components are too large to be lifted into orbit by transport spacecraft of other nations.

NASA must succeed in each of its planned shuttle flights throughout the next four years, needing a home-run performance in each mission. An ambitious average of more than four missions per year is planned.

As the missions proceed, the ISS will increase in size, and power. When complete, the space station will have more solar array panels, greatly increasing the electrical power to run the larger craft. At some point, the ISS will become so large it will be possible to spot it from Earth with the naked eye during daylight.

The ISS thus far has provided a laboratory moving at 17,500 mph where scientists can conduct experiments that can be performed only in the almost weightless, near-zero-gravity conditions of space.