UPDATED 1:45 p.m. EDT

WASHINGTON — The U.S. Air Force is shutting down a key part of its network for tracking satellites and orbital debris, possibly as soon as Oct. 1, according to an Aug. 1 memo obtained by SpaceNews.

Gen. William Shelton, commander of Air Force Space Command, “has directed that the Air Force Space Surveillance System be closed and all sites vacated” effective Oct. 1, the memo said.

The letter, signed by Austin Frindt, a contracting officer with Air Force Space Command, was addressed to Five Rivers Services of Colorado Springs, Colo., operator of the current Space Fence tracking system. The Space Fence, a planned replacement for which is on hold pending a Pentagon-wide review, is a line of VHF radars stretching across the southern United States.

Deployed in the 1960s, the VHF Space Fence includes three transmitter sites and six receiving stations. It is responsible for approximately 40 percent of all observations performed by the Air Force-run Space Surveillance Network, which includes other ground-and space-based sensor assets, said Brian Weeden, technical adviser at the Secure World Foundation, a nonprofit organization dedicated to space sustainability.

The memo explained to Five Rivers Services that the Air Force was not exercising its option for a fifth year of a contract to provide management and logistical support for the nine field stations. Lori Thomas, president of Five Rivers Services, declined to comment and referred questions to the Air Force.

‘This is your notice to begin preparing the sites for closure,” the memo said. “A specific list of action items will be provided as soon as it is finalized. A specific date to turn off the mission system has not been established yet, but will be provided to you immediately upon determination.”

Andy Roake, a spokesman for Air Force Space Command, pointed to the automatic budget cuts known as sequestration, in an email Aug. 5.

“In this tough, sequestered budget environment, we’re considering many options, but for FY14, no final decisions have been made,” he said.

Though part of a broader surveillance network, the VHF Space Fence is crucial because it can track objects up to 24,000 kilometers away. Other sensors in the network generally track objects at altitudes lower than a few thousand kilometers, Weeden said.

“The Space Fence is very important as it gives an ‘uncued tracking’ capability,” Weeden said. “Because it’s constantly transmitting, it can detect objects without being tasked to do so. There are some other sensors in the network that can do uncued tracking to some degree, but the Space Fence is rather unique in the sheer size of the detection coverage it has.”

The Space Fence, along with operators at the Joint Space Operations Center, can observe objects down to the size of a basketball and make precise determinations of their characteristics, location and movement. Each month the system is responsible for logging more than 5 million satellite observations, according to an Air Force fact sheet.

In April, Shelton said two of the Space Fence’s receiver sites had been placed in cold storage, one in Glennville, Ga., and one in Hollandale, Miss., reducing the overall accuracy and effectiveness of the system. The change was made as part of the Air Force’s response sequestration.

In July, the Air Force released a request for proposals to operate the aging system beginning in September 2015 — one year after the Five Rivers Services’ contract was set to expire. The request said the Space Fence “has been identified as a critical defense system and, therefore, shall be manned on a 24-hour, 7-days-a-week, 365-days-a-year basis at transmitter sites and 8-hour, 7-days-a-week, 365-days-a-year basis at receiver sites.”

A full-scale development contract for an updated version of the Space Fence had been expected in 2012 or early 2013, but on July 16, Shelton said the multibillion-dollar project is being held up due to a wide-ranging Pentagon review that includes major acquisition programs. The review is examining scenarios under which the Pentagon’s budget is cut by $150 billion, $250 billion and $500 billion during the next decade.

Lockheed Martin Mission Systems and Sensors of Moorestown, N.J., and Raytheon Integrated Defense Systems of Tewksbury, Mass., have developed competing designs for the new network of ground-based radars, which would be capable of tracking greater numbers of smaller objects than the current Space Fence.

Shelton said in July that engineers at Eglin Air Force base in Florida were looking for ways to improve the current Space Fence as a contingency plan should the Pentagon elect not to go forward with the next-generation system.

In April, Shelton said one-third of the Space Fence’s receiver sites had been placed into cold storage, reducing the overall accuracy and effectiveness of the system as part of the Air Force’s response to the automatic budget cuts known as sequestration.