WASHINGTON — NASA plans to soon start a 45-day effort to restore communication with the Mars rover Opportunity, a timeframe that has elicited criticism from those both within and outside the project.

In an Aug. 30 statement, NASA said it would begin a 45-day campaign of active efforts to restore communications with Opportunity once skies above the rover cleared to a sufficient level. The rover has been out of contact since early June, when a major dust storm deprived the rover of solar power.

That dust storm, which at one point encircled the planet, is fading. “The dust haze produced by the Martian global dust storm of 2018 is one of the most extensive on record, but all indications are it is finally coming to a close,” said Rich Zurek, project scientist for the Mars Reconnaissance Orbiter, which has been monitoring the storm. Zurek said in the statement that there had been no signs of dust storms within 3,000 kilometers of Opportunity “for some time.”

In the statement, NASA said that once the skies above Opportunity clear to a sufficient degree, it will begin a communications campaign to restore contact with the rover by sending commands to it. “Assuming that we hear back from Opportunity, we will begin the process of discerning its status and bringing it back online,” John Callas, Opportunity project manager at the Jet Propulsion Laboratory, said in a statement.

That effort will begin once the optical depth, a measure of the haziness of the skies, drops below 1.5. That optical depth, also known as tau, is typically around 0.5 under normal conditions. At the time contact was lost with Opportunity in early June, tau has risen to 10.8, a record high value that indicated that the sun was almost completely obscured by the dust storm.

In its statement, NASA did not publish an updated value of tau. In a mid-August update, JPL stated that tau had dropped to 2.1, then rose again to 2.5. In subsequent Opportunity mission updates, JPL said that tau was decreasing, but did not give a specific figure.

The announcement, though, attracted criticism because it limited the active part of the recovery to 45 days. “You have to be kidding me. 45 days after a Tau of 1.5. This can’t be based on any real analysis of the situation,” tweeted Mike Seibert, a former flight director and rover driver for Opportunity who is no longer at JPL. He said that JPL attempted “active listening” of Spirit, the twin of Opportunity, for 10 months in 2010 and 2011 when that rover stopped transmitting before giving up.

You have to be kidding me. 45 days after a Tau of 1.5. This can't be based on any real analysis of the situation. Someone in the MER Project, Mars Program or elsewhere has to be trying to kill the mission for non-technical reasons.#SaveOppy #WakeUpOppyhttps://t.co/Zill7w0Gmx — Mike Seibert (@mikeseibert) August 30, 2018

“I’ll be blunt: 45 days is absurdly short, and certainly arbitrary,” Scott Maxwell, another former Opportunity rover driver, said. Starting those efforts while tau was still as high as 1.5 is “not nearly as generous as our trusty, faithful, brave Opportunity deserves,” arguing that the project should wait until tau drops to around 0.7.

I'll be blunt: 45 days is absurdly short, and certainly arbitrary. And starting the clock from when tau (atmospheric opacity) reaches 1.5 is … let's say it's not nearly as generous as our trusty, faithful, brave Opportunity deserves. (Tau ~ 0.7 — half that — more reasonable.) https://t.co/vuZhHD4bcK — Scott Maxwell (@marsroverdriver) August 31, 2018

Those currently working on the Opportunity mission are also disappointed and surprised by the 45-day limit to listening efforts, according to project sources not authorized to speak on the record.

Callas, in the statement, argued that if Opportunity did not respond to communications attempts after that 45-day campaign, it likely meant the spacecraft had suffered a mission-ending malfunction. “If we do not hear back after 45 days, the team will be forced to conclude that the sun-blocking dust and the Martian cold have conspired to cause some type of fault from which the rover will more than likely not recover,” he said.

“The 45-day active listening period was decided based on input from across the team,” JPL spokesman DC Agle said Aug. 31. “The 45-day period, which has not started yet, is meant to span the most likely time to hear from the rover; that is, when the skies clear and there is the most amount of sunlight.”

Another factor, Agle said, is decreasing sunlight because of seasonal changes. After the 45 days, “the likelihood of hearing from the rover starts to decrease as solar insolation starts to decrease (it’s decreasing now) and temperature starts to decrease, albeit slowly.”

Should Opportunity fail to respond to active communications efforts, JPL will continue to listen for signals from the rover, but not attempt to send any commands. That effort addresses “the unlikely chance that there is a large amount of dust sitting on the solar arrays that is blocking the sun’s energy,” Callas said.

Former rover engineers like Maxwell have argued that the active listening phase should run through the end of the year, covering the portion of the Martian year where dust devils could sweep dust off the rover’s solar panels, as has happened in the past.

Agle said that listening campaign will continue at least through January 2019 to cover that potential cleaning phase, for a total of at least eight months of efforts to hear from the rover. “The most likely recovery is for Opportunity to autonomously wake up and talk to us,” he said. “That’s why we are listening all that time.”

“We will keep trying to get our Martian friend back online. We will not give up on #Oppy even after the 45 days of plan we have put in place!” tweeted Thomas Zurbuchen, NASA associate administrator for science. “We will keep trying, and trying… In fact I told the Mars team that I want to get updates on a regular basis!”