NASA Administrator Michael Griffin says his agency has instituted massive safety upgrades since the Columbia disaster.

But he told members of the House Science Committee on Tuesday that the planned mission of the shuttle Discovery -- set for mid- to late July -- is "a test flight."

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Interactive: How Shuttles Work

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In fact, he said each of the next two flights should be considered a "test flight" -- with all the risks that go along with that.

Griffin said there's no way to tell whether the design improvements are effective unless the shuttle returns to space.

He says he's satisfied that the risk involved is "acceptable," but he cautioned members of the committee that the only way to eliminate risk entirely is to pull the plug on the shuttle program.

On the mission, the seven-member crew will test a new thermal-tile replacement system and replace a failed gyroscope on the international space station.