The explosion last month of an unmanned rocket made by Orbital Sciences Corp. is delaying its planned merger with Alliant Techsystems Inc.

Alliant, better known as ATK, and Orbital announced they are pushing back until Jan. 27 a date for their respective shareholder meetings to approve the merger. ATK wants to combine its aerospace and defense business with Orbital while spinning off its sporting goods business. The original meetings had been scheduled for Dec. 9.

ATK, in Arlington, Va., maintains its defense electronics system division in Northridge. That division performs research and development and manufacturing of weapon systems for the Pentagon and is currently filling a $1 billion missile order. The division has about 250 employees and facilities in Texas and Florida.

Orbital Sciences, in Vienna, Va., builds rockets to launch satellites and missiles. Its Antares unmanned rocket, which was to bring supplies and experiments to the International Space System, exploded Oct. 28 seconds after taking off at the NASA Wallops Island facility in Virginia.

After the incident, ATK and Orbital started a review and analysis of the launch failure and Orbital’s proposed recovery plan. The review will not be concluded prior to the original Dec. 9 meetings date.

“We believe it was responsible and essential to conduct this special due diligence and as a result of our findings, management and our board of directors continue to endorse the previously announced transaction,” said ATK Chief Executive Mark DeYoung in a prepared statement.

The merger will result in a subsidiary known as Orbital ATK Inc. with revenue of more than $4 billion. The transaction is expected to close in February 2015 assuming it is approved by shareholders.