DULLES, Va. ( TheStreet) -- Orbital Sciences ( ORB) plans to acquire the spacecraft development and manufacturing business of General Dynamics' ( GD) GD Advanced Information Systems unit. The transaction is expected to close in about four weeks. Financial terms weren't made public. Orbital said the acquisition would enhance its position in the market for national security space systems, including spacecraft used in intelligence and surveillance and missile tracking. The deal also would strengthen Orbital's capabilities to design and make certain satellite systems, the company said in a statement. GD Advanced Information Systems has built 15 small- and medium-class satellites for the U.S. Air Force, NASA, and other government customers over the last 20 years. General Dynamics shares rose 60 cents, or 0.8%, to $73.54 in after-hours trading Thursday. The announcement of the deal was made after the stock market closed. Orbital shares declined 3 cents to $18.61 in the regular trading session. -- Reported by Joseph Woelfel in New York. Follow TheStreet.com on Twitter and become a fan on Facebook.