Traffic in space is getting so congested that flight controllers in the past few weeks have had to nudge three spacecraft out of harm’s way, in one case to prevent the craft from colliding with its own trash. . . .

Officials and private space experts say episodes like these illustrate the danger of a drastic rise in satellites and space debris in Earth’s orbit. Early this year, after decades of growth, the federal catalog of detectable objects (four inches wide or larger) orbiting Earth reached 10,000, including dead satellites, old rocket engines and junkyards of whirling debris left over from chance explosions and weapon tests.

Now, that number has jumped to 12,000. China’s test of an antisatellite weapon in January and four spacecraft breakups in February, one of them mysterious, have contributed to the buildup of debris. Space officials worry that a speeding bit of space junk could shatter an object into dozens or hundreds of fragments, starting a chain reaction of destruction.

Experts said that moving spacecraft out of the way to avoid collisions, once a rare way of dealing with potential threats, is becoming increasingly common.

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