What happens when the pinger dies?

• The batteries powering the pinger on MH370 are expected to die in the coming days


• Pingers emit sounds that can direct searchers to flight data and cockpit voice recorders


• A pinger locator is searching underwater, but can only move about 2 to 3 knots


• Searchers also have a robot that can search the ocean bed for wreckage

(CNN) -- The actual sound is mundane, like the ticking of a second hand on a loud wall clock.

But it's the sound that searchers from around the would are desperately hoping to hear -- the noise of the pinger from missing Malaysia Airlines Flight 370.

The problem is, time's running out. The pinger's batteries will likely die after 30 to 45 days, and Friday marks the 28th day of the search.

So what happens if the pulse of the plane sputters out? Is there any hope of still finding the jet carrying 239 people? Here's what to know about the future of the search:

Let's catch up -- what exactly are the pings, and how do you find them?

Every commercial airplane is required to have pingers -- technically called underwater locator beacons -- to help locate lost planes. One is attached to the flight data recorder; another to the cockpit voice recorder.

The pings sound about once every second. The noise is inaudible to the human ear, but devices such as towed pinger locators (TPLs) can hear the sound from 2 nautical miles away.

The pinger locator has a sensor that resembles a 35-inch, 70-pound yellow stingray and can recognize the flight recorder's chirps up to 20,000 feet below the surface of the water.

The U.S. Navy has sent a TPL to drag behind an Australian ship to hunt for the plane in the Indian Ocean.

What are the challenges of hunting for the plane by pings?

Not only will the batteries powering the ping die after about 30 to 45 days, the sound can be drowned out by weather, noise or silt.

Also, the pinger locator has to be towed slowly. It could take days to cover the 240-kilometer (150-mile) track identified by officials as the latest best guess for where plane could be.

"It is a very slow proceeding search, 2 to 3 knots depending on the depth that you want the hydraphone, that tow pinger locator trailed at," said Capt. Mark M. Matthews, the U.S. Navy's head of TPL operations. "It's going to take time. ... Again, we're searching on what information we do have, our best guess at where it would have been lost. It's the best we can do at this time."

Did the pings help in finding Air France Flight 447 several years ago?

A Phoenix TPL-25 searched for a ping from Air France Flight 447, which crashed hundreds of miles off the coast of Brazil in 2009. That search didn't find the plane, but two years later, searchers found the flight data recorder and the bulk of the wreckage using an autonomous underwater vehicle, or AUV.

What other kinds of high-tech gadgets can searchers use to find Malaysia Airlines Flight 370?

One of the search ships has an underwater robot called the Bluefin-21, which can scour the ocean bed looking for signs of wreckage. But the robot hast not been deployed yet.

Searchers could also use autonomous underwater vehicles, which are typically used in the oil and gas industry to conduct deepwater oilfield surveys.

"The smaller ones are only going to go down to about 5,000 feet," analyst David Soucie said. "The next class is a much more expensive, much larger device. It's 15 by 25 feet because it adds a lot of battery capability and a lot of hydraulic capability."

One of the most sophisticated AUVs owned by Phoenix International was activated and flown to Perth, Australia, to help with the search for Flight 370. The device is yellow, 17.2 feet long and has an in-air weight of 1,600 pounds.

It can be lowered 20,000 feet below the water surface and travels 2 to 4.5 knots, using side-scan sonar to create a map of the seafloor. The rapidly moving probe is also equipped with a still camera.

"They have their own control system so they talk to it with an acoustic modem. It's hard to get sound through the water," said Jami Cheramie of C&C Technology, whose AUV has been called in to search for plane debris in the past. "We will see waterfalls. A picture will scroll and you will see the seafloor be painted in front of you."

Have these underwater vehicles found plane wrecks in the past?

Yes. AUVs played an instrumental role in finding the downed Air France flight, the plane wreckage of Italian fashion designer Vittorio Missoni off the coast of Venezuela, and the HMS Ark Royal, a ship sunk by a German U-81 submarine in World War II. The AUV provided black and white images of the wreckage site.

Let's say they find the flight data recorder and the cockpit voice recorder. Will the mystery be solved?

Not necessarily. The voice recorders have only two hours of recording capacity. And since officials believe Flight 370 flew almost seven hours beyond the point where something went terribly wrong, some crucial cockpit sounds have almost certainly been erased.

On the positive side, the depletion of the battery will not wipe out data. Data has been known to survive years in harsh sea water conditions on modern recorders.

CNN's Rose Arce, Rosa Flores, Mike M. Ahlers, Jethro Mullen and Paula Hancocks contributed to this report.