PHOENIX — The desperate effort this week to find two hikers who disappeared at the bottom of the Grand Canyon represented the National Park Service's most extensive use yet of drones in a search-and-rescue mission.
The Grand Canyon is the only national park with its own fleet of unmanned aircraft for reaching people who have gotten lost, stranded, injured or killed. Under a program that began last fall, it has five drones and four certified operators.
While the aerial search for the two hikers came up empty, it threw a spotlight on technology that can enter crevices and other rugged spots unreachable by foot while sparing searchers the dangers of going up in a helicopter.
With its steep cliffs, nearly 2,000 square miles and mesmerizing views, the Grand Canyon can be as dangerous as it is captivating.
Rangers were confronted with 1,200 medical emergencies, 293 search-and-rescue missions and 17 deaths in 2016, a year in which the park had nearly 6 million visitors. Last summer, a 35-year-old Yelp executive tripped while hiking, fell backward and was found dead 400 feet below.
"Our historic model was to take the helicopter to look and see," said Grand Canyon chief ranger Matt Vandzura. But now, drones can offer "that same close look but without putting any people at risk. It has dramatically increased our ability to keep our people safe."
The drones are about 18 inches across and 10 inches high, with a battery life of about 20 minutes. Drone operators watch the video in real time and then analyze it again at the end of the day.
The aircraft have been used a few times since the fall.
In November, after a visitor drove off a cliff and died, drones were sent in to examine the trees and brush and make sure it was safe for a helicopter to fly in and lift the car out.
The next month, rangers used a drone to locate a woman who had jumped to her death. Then they rappelled down to retrieve the body.
The aircraft were used Monday through Wednesday in the search for LouAnn Merrell, 62, of Vernal, Utah, and her step grandson, Jackson Standefer, 14. The park also sent out three ground search teams of about 20 people in all, an inflatable motor boat and a helicopter.
Merrell and Standefer vanished last weekend after losing their footing while crossing a creek near the North Rim. They were on a hike with Merrell's husband, Merrell Boot Co. co-founder Randy Merrell, and the boy's mother.
The park scaled back the operation and stopped using the drones but continued to search for the pair. In a statement, the hikers' families backed the decision and said they were "still praying for a miracle."
Other national parks use drones, but for wildlife research. The use of private drones is prohibited in national parks.
James Doyle, a spokesman for the park service's Intermountain region, said other national parks will probably seek their own drone fleets, too. He said the Grand Canyon's extreme topography — it is a mile deep — makes it a perfect candidate.
"It's a wonderful tool for the unfortunate situation we just found ourselves in at Grand Canyon," Doyle said.