(CNN) -- The underwater pulses that an Australian navy ship detected over the weekend have not been heard since -- but authorities are not letting that deter them in the search for missing Malaysia Airlines Flight 370.
"We have at least several days of intense actions ahead of us," Australian Defense Minister David Johnston told reporters Tuesday. "We're throwing everything at this difficult, complex task."
Investigators hope the signals could be locator beacons from the plane's data recorders, but they're not sure yet. But buoyed by the hope that they're closing in, they reduced the size of the search area Tuesday.
Now, they are focusing on a smaller area in the Indian Ocean: 30,000 square miles (77,580 square kilometers) about 1,400 miles (2,270 kilometers) northwest of Perth. That's about a third of the size of the previous search zone.
"Instead of looking at an area the size of Texas, we're now looking in an area the size of Houston," aviation expert Geoffrey Thomas told CNN.
Time, however, is running out.
The batteries powering the beacons, which are designed to start sending signals when a plane crashes into water, last about 30 days after the devices are activated.
Tuesday marks the 32nd day since the plane, carrying 239 people, disappeared en route from Kuala Lumpur, Malaysia to Beijing.
Experts have said it's possible that the batteries could last several days longer if they were at their full strength.
"We need to continue ... for several days right up to when the point at which there's absolutely no doubt that the pinger batteries will have expired," said retired Air Chief Marshal Angus Houston, the chief of the Australian agency coordinating the search.
Retired Royal Air Force Lt. Col. Michael Kay said he sees the hunt for the pingers going another week and a half.
"We know that the batteries can last up to 40 days," Kay told CNN. "If I was Angus Houston, I would be putting the search out to at least 42, 43 (days) to make absolutely sure that the batteries had failed."
Racing the clock
The race against the clock is the "No. 1 challenge" searchers face, U.S. Navy Cmdr. William Marks told CNN. One hundred and thirty-three search missions have been carried out so far, but they have been fruitless.
"We haven't quit since we initially heard these signals," he said. "We've been going continuously around the clock and we haven't been able to reacquire them."
Searchers are still scouring the waters, but their optimism is "more cautious" now, he said.
"As hours pass," he said, "our optimism is fading away, ever so slightly."
Investigators have described the search not as finding a needle in a haystack, but rather trying to find the haystack.
Narrowing down the search area Tuesday was a key step, said David Gallo, director of special projects at the Woods Hole Oceanographic Institution.
"They've cut down the size of that haystack incredibly," he said, "into the size of something manageable."
But the search area ships and planes are scouring still presents major challenges.
The ocean water is deep, and a massive cyclone churned through the area just two weeks ago, packing wind speeds of more than 160 mph. At the time, search crews were looking at a different part of the Indian Ocean.
But now that the focus of the search area has shifted, the possibility that large waves from the storm made it even harder to find debris is a reality investigators must face, according to CNN meteorologists.
"This was an area that looked like a washing machine in the first place, but now we know it was even worse than that," CNN meteorologist Chad Myers said.
Cheers erupted Saturday when the team aboard Australia's Ocean Shield first reported a possible signal had been heard.
The Australian ship is carrying two key pieces of U.S. equipment to help scan the water for signs of the plane: a towed pinger locator and a Bluefin-21 underwater vehicle.
The first detection lasted for more than two hours; a second lasted for about 13 minutes.
The signals, detected about 1,750 kilometers (1,100 miles) northwest of Perth, Australia, were consistent with those sent by a flight data recorder and a cockpit voice recorder, Houston said. They were heard in seawater about 4,500 meters (14,800 feet) deep.
"The audible signal sounds to me just like an emergency locator beacon," Houston said. "We are encouraged that we are very close to where we need to be."
If they are heard again, an underwater drone could be deployed to take photos before officials would know whether Flight 370 has been found. That process could take days, and possibly a week or more.
"Until we have stopped the pinger search we will not deploy the submersible," Houston said. "We will not deploy it unless we get another transmission in which we'll probably have a better idea of what's down there."
Teams are also still investigating pulses detected Friday and Saturday by a Chinese ship about 600 kilometers (375 miles) southwest of the area that the Ocean Shield is searching.
The signals detected by the Chinese weren't as sustained as those picked up by the Ocean Shield, and the Chinese vessel's detection gear isn't thought to be as advanced as the U.S. pinger locator.
Houston said Monday that they were probably separate events, pointing out that the search area where they were detected is over 300 nautical miles long.
Hoping for a miracle
But some friends and relatives of passengers said they're not putting too much stock in Monday's news. Relatives have repeatedly heard from officials that they thought something had been found, only to have their hopes dashed when those finds have not led to the plane.
"Until they physically locate the bulk of the plane with the black box intact and passenger bodies, I won't believe it," said Sarah Bajc, the partner of American passenger Philip Wood.
At a candlelight vigil in Beijing on Monday night, some relatives sobbed and others bowed their heads.
"If the plane is there, it's there. We can't change it," the husband of one passenger said. "But I am still hoping for a miracle to happen."
CNN's Jethro Mullen, Matthew Chance, David Molko, Will Ripley, Judy Kwon, Ed Payne and Mitra Mobasherat and journalist Ivy Sam also contributed to this report.
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