WASHINGTON — A Rocket Lab Electron successfully launched a radar imaging satellite for Synspective Dec. 21, days after that Japanese company lifted off on the stock market. The Electron rocket lifted off from Pad B of Rocket Lab’s Launch Complex 1 in New Zealand at 9:17 a.m. Eastern. The launch was postponed a few days because of weather and a previous launch attempt was scrubbed Dec. 20 when the company reported an out-of-family reading with an unspecified sensor. The Electron’s payload, a StriX synthetic aperture radar (SAR) imaging satellite developed…