The giant inflatable doughnut-shaped object floating inside the Rotunda of the U.S. Capitol might seem like a gentle bit of architectural whimsy intended to simply catch falling dust and reduce noise from the major repairs currently underway.

It will, in fact, do both of those things. But it is also a serious piece of safety equipment capable of catching objects weighing as much as 500 pounds that could be dislodged and plummet toward the Rotunda floor as work crews begin a massive renovation and paint job inside the "interstitial space" between the inner and outer Capitol domes.

Workers have begun to strip away at least 12 coats of lead-based paint from the dome in order to reveal the hundreds of hairline cracks that have caused serious leakage in the Rotunda in recent years.

The stripping and repainting is expected to cost some $5.5 million and last through next spring. During the next phase of repairs, which could cost as much as $28 million and take another three years, the Architect of the Capitol's project team will repaint the dome's exterior, repair at least 200 damaged cast-iron plates, repair the lighting inside the Rotunda, improve "life safety" equipment, test for original colors inside the Rotunda and repaint where necessary.

It is a massive undertaking. "Once we disturb an area such as this, and disturb the Congress," project director Jim Ellison says, "we want to be sure we do it all in one fell swoop."

Doughnut in the Dome: The doughnut-shaped safety device made of plastic sheeting in the Capitol Rotunda is capable of catching falling objects weighing as much as 500 pounds.

Dome facts

* Architect: Thomas U. Walter

* Architectural inspiration: St. Pauls, London; St. Peter's, Rome; Pantheon, Paris

* Year construction began: 1856

* Year construction ended: 1866

* Original cost: $1,047,291

* Weight: 8.9 million pounds