Tethered to the warhead were two seven-foot decoy balloons, which the Eris avoided.

Dormant machines would have flickered to life when first light struck the decoy balloons, electric circuits closing.

All the decoy balloons were gone-no surprise there.

The test included three decoy balloons.

But when it actually performed the tests last October and this January, each flight had but one decoy balloon.

But it later acknowledged that the kill vehicle had initially drifted off course and picked out a decoy balloon rather than the warhead.

But analysis showed that the kill vehicle had drifted off course and had initially homed in on a decoy balloon instead of the warhead.

It suffered initial navigational mistakes and found the target only after luckily homing in on a nearby decoy balloon.

To date, targets in the interception tests have consisted of a single mock warhead and a single decoy balloon.

By contrast, the spherical decoy balloons have more uniform signatures.