The source requested anonymity because the deal has not been disclosed publicly.
Northrop Grumman declined to comment and Orbital did not immediately respond to a request for comment. The Wall Street Journal reported the deal earlier on Sunday.
Northrop is valued at $46.5 billion, while Orbital is valued at $6.3 billion. The acquisition price could not be immediately determined, but could exceed $7.5 billion if a typical premium was attached to it, Wall Street Journal said.
The potential transaction comes as rising tensions with North Korea have put additional attention on defense systems.
The Air Force had asked the defense industry last summer for proposals to replace the aging nuclear cruise missiles and intercontinental ballistic missile system as the military moved ahead with a costly modernization of its aging atomic weapons systems.
In August, Northrop received a $328 million contract to continue work on the replacement of the aging Minuteman III intercontinental ballistic missile system from the U.S. Air Force.
The potential deal follows aerospace supplier United Technologies $30 billion agreement earlier this month to buy avionics and interiors maker Rockwell Collins. When completed, that transaction would be the largest in aerospace history.