Original mission: Zebulon Pike, a 27-year-old U.S. Army lieutenant, was dispatched to map the western and southern boundary of the Louisiana Purchase, find the headwaters of the Arkansas and Red Rivers, conduct diplomacy with Native people and gather intelligence on Spanish settlements in the Southwest.

Commissioned by: The U.S. government.

Achievements: He returned 51 captured Osage men, women and children to their village and opened discourse with the Osage, Pawnee and Comanche nations. After, Pike and 15 others ascended the Arkansas River to the Colorado Front Range. In November 1806, Pike and three others attempted to climb what is now known known as Pike’s Peak, but were thwarted by snowy, wintry conditions.

Missteps: Pike incorrectly identified Leech Lake—instead of Lake Itasca, 25 miles away—as the source of the Mississippi River. And after making a series of wrong turns, Pike mistook the Rio Grande for the Red River, which he had been seeking. He made camp in a part of southern Colorado that was still claimed by Spain, and during the night of February 26, he and his men were captured by Spanish soldiers. Pike was taken to Chihuahua, Mexico, for questioning, but was released and repatriated.

“The fact that he gets captured is testament to the geopolitical confusion of the time,” says Deverell. On his return, Pike was initially considered culpable in a plot by his mentor General James Wilkinson and former U.S. vice president Aaron Burr to carve out a new nation in the Southwest but was ultimately exonerated.

Original mission: Stephen Harriman Long, a major in the U.S. Army Corps of Engineers, led a handful of expeditions from 1819 to 1823, including his 1820 journey, the first purely scientific survey of the West for the U.S. government. Long was instructed to ascend the Platte River and then explore the headwaters of the Red and Arkansas Rivers.

Commissioned by: The U.S. government.

Achievements: His interactions with the Omaha, Oto and Pawnee nations yielded the first detailed descriptions of their lives and customs.

Mission not quite accomplished: His group failed to suss out the sources of the Platte, Arkansas or Red Rivers.

Impressions: Stunned by the extraordinarily rich wildlife they encountered in the region around Colorado Springs, the expedition’s members were also struck by what they perceived as the barrenness of the Great Plains. Long wrote that the region was “unfit for cultivation and of course uninhabitable by a people depending upon agriculture.”

Long “characterizes those vast basins and open plains areas as the Great American Desert,” explains Deverell. “They are, in a sense, frightened by it. They've never seen anything quite like it. And simultaneously, they're astonished by how magnificently tall and robust the Rocky Mountains are.”

Original mission: Over the course of a decade, John Frémont led five ambitious expeditions to map the western U.S. for the benefit of would-be settlers and also to signal the young country’s intention to expand all the way to the Pacific coast.

Commissioned by: Mostly the U.S. War Department.

Who he was: Frémont’s life was a morass of success, scandal, investigation and dishonor. An abolitionist, he led a genocidal slaughter of Native Americans. One of the first two senators from California, he was the first Republican nominee for president (losing in 1856 to James Buchanan). He later became governor of the Arizona Territory, but was forced to resign. He became rich in the California Gold Rush but died destitute. He was also an expert in wilderness exploration, an officer in the U.S. Army’s topographical corps and a leader of five expeditions to survey the West.

Achievements: During those expeditions, he discovered and named the Great Basin and established the elevation of Great Salt Lake. He explored the Sierra Nevada and, with his wife, wrote what has been described as a “virtual guidebook to the Oregon Trail,” demonstrating a pathway that would be followed by multiple waves of settlers. He also, says Deverell, was mindful of the need to document his findings with pictures as well as words.

Bad luck: His expedition to Oregon and California gathered some 1,400 medical or medicinal plant specimens, but a mule carrying most of them lost its footing and fell “from a precipice into a torrent.” The rest got soaked by rising floodwaters from the Kansas river.

Photo savvy: “It’s fascinating to me that a number of these expeditions were very careful to bring along, at first, artists whose ability to document the landscape and the people is valued, and then, very quickly, photographers,” Deverell explains. “Frémont is one of the first to bring along a daguerreotypist. His career is marked by scandal, and he actually is a scoundrel, but there are things that he did quite well—there’s no doubt about it.”

Commissioned by: The U.S. Congress.

Original mission: A naturalist, geologist and soldier who lost much of his right arm at the Battle of Shiloh in the Civil War, Powell was commissioned to survey the lands along the Colorado River and its tributaries.

Achievements: His three-month expedition included the first recorded passage by white men through the entirety of the Grand Canyon.

Harrowing moments: Rough rapids, leaky boats, moldy food and a campfire whipped out of control by wind that they had to escape via a dangerous rapid at dusk. One of the group’s three boats was wrecked at Disaster Falls, along with a third of their food supply and key equipment. One of the 10 expedition members, Frank Goodman, declared on July 5 that he had had enough adventure and promptly left; three others followed suit the following month and were never seen again.

Legacy: Powell was later director of the U.S. Geological Survey and, says Deverell, he remains a figure of great significance in the history of the Western United States. “We continue to make reference to Powell because of his ability to make bold pronouncements about topography, landscape and water,” he explains. “He was a very important ethnographer of Indigenous people. He made it down the Colorado River with only one arm. He’s larger than life.”

Original mission: Determine the most practical and economical route for a transcontinental railroad from the Mississippi River to the Pacific Ocean. Three routes were surveyed: northern, central and southern.

Commissioned by: The U.S. Congress.

Achievements: Surveyors, scientists and artists compiled cartographic, ethnographic, zoological and climatological information over an area of 400,000 square miles that has been described as “probably the most important single contemporary source of knowledge on Western geography and history.”

These expeditions, says Deverell, “are about territorial acquisition, about modernization and technology, about the rising dispute between the North and the South, as to where you're going to put that railroad line. They look a lot like the early science expeditions, because they’re taking notes, they’re drawing pictures, they’re mapping, they’re meeting with Indigenous people. But the underlying purpose is very different. It’s: ‘We are going to build a railroad across this whole country.’”