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November-December 2010

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Retrospect: November 11, 1935: Explorer II Stratospheric Balloon Flight

Many consider the seminal event that marked the start of the Space Race between the United States and the Soviet Union to be the launch of the Sputnik I satellite by the Soviets on October 4, 1957. While Sputnik was the first artificial object to enter outer space and successfully orbit Earth, it was just one milestone in the human quest to leave the gravitational bonds of Earth and reach the upper limits of the atmosphere—a quest that had actually begun decades earlier.

Another race—what might be called a “near space race”—between the Americans and Soviets began in the 1930s with a series of high-altitude balloon flights into the stratosphere. Although setting world altitude records was never the primary objective, that aspect did provide a certain amount of “bragging rights” between the two nations.

The most ambitious of these upper-air expeditions was the one made in the balloon known as Explorer II, which took off on November 11, 1935, from a naturally bowl-shaped canyon in the Black Hills of South Dakota dubbed the Stratobowl. Explorer II was the brainchild of Capt. Albert Stevens, who petitioned his superiors at the U.S. Army Air Corps in 1933 for permission to lead a high-altitude balloon flight whose mission would be the scientific exploration of the stratosphere. With the approval of the Army Air Corps and the cooperation and funding of the National Geographic Society and several commercial partners, the first Explorer balloon took off from the Stratobowl on July 28, 1934. However, after reaching an altitude of just over 60,000 feet, the bottom of the balloon tore, necessitating a descent. At 3,000 feet, the hydrogen-filled balloon exploded, forcing the occupants, Capt. Stevens, Capt. Orvil Anderson, and Major William Kepner, to bail out of the gondola; all three successfully parachuted to the ground.

Caption: U.S. Weather Bureau Daily Weather Map for 8:00 a.m. EST (6:00 a.m. MST), November 11, 1935. An area of high pressure is centered over Minnesota, southeastern North Dakota, and eastern South Dakota. This area of high pressure helped create ideal conditions for the launch of the balloon Explorer II, which set a world altitude record of 72,395 feet.

After going back to the drawing board, a new-and-improved balloon, Explorer II, which relied on helium instead of hydrogen, was ready to be deployed by the summer of 1935. However, during inflation of the balloon on July 11—a day that was described as “ideal” in terms of the weather—a seam ripped apart, venting helium gas and halting the flight. Over the next several months as repairs were made, organizers scrutinized climatological data from the expected flight region to determine the optimal time for another attempt at a launch. “Studies of the weather conditions at Stratobowl in the Black Hills for the last fifteen years disclose that October usually has periods of good weather long enough to make the flight a possibility,” wrote Science News Letter in its September 14 issue.

A meteorological team, which consisted of both military and civilian personnel and which was augmented by soldiers trained in meteorological tasks, reported for duty at the “Stratocamp” between September 7–11 and set up a full weather station for monitoring conditions. Among the various requirements necessary for a successful launch, “perhaps the most essential weather requirement,” Capt. Randolph Williams, the meteorological officer for the mission, stated in his report, was that there be no precipitation and a wind not exceeding four mph during the inflation of the balloon. Clear skies were necessary both during the flight (so as not to interfere with the photographs that would be taken) and landing, when the surface wind was not to exceed 14 mph. “To forecast such weather a day or more in advance requires the reception of a tremendous volume of weather data,” Capt. Williams wrote.

From 5:45 a.m. until around 10:00 p.m., meteorological personnel decoded large volumes of weather data that came in via teletype and plotted a variety of maps and charts, analyzing everything from surface weather data to winds aloft to upper-air humidity and stability data obtained from aircraft flights over Cheyenne, Wyoming; Billings, Montana; Omaha, Nebraska; Fargo, North Dakota; and Oklahoma City, Oklahoma. In addition, within the Stratobowl, observers took hourly readings of temperature, pressure, and humidity daily from 6:00 a.m. until 4:00 p.m. These, along with hourly reports of weather conditions from airplane flights, provided the weather personnel and flight crew with the information they needed as they waited for ideal conditions to arrive. And wait they did.

“The flight itself was fine, but the waiting for the satisfactory weather was hell,” Capt. Anderson later wrote. As Capt. Williams noted in his report, “the situation which offers the greatest security during inflation and flight was that which occurs when a large, cold, dry, polar air mass, creating a high pressure area, moves into the United States and centers during the inflation period over Rapid City.” After weeks of waiting, these conditions became evident on the weather maps of November 9 and 10, which showed an area of high pressure associated with a cold air mass moving southeastward from British Columbia toward Rapid City, South Dakota.

As the cold front advanced toward the Stratobowl on the evening of November 10, preparations began for the massive balloon's inflation. “Save for the severe cold of about six above zero,” Capt. Williams wrote, “conditions were ideal.” The New York Times wrote that “soldiers and the scientists and spectators wore their heaviest clothing for the job.” Stoves in and around tents and buildings not only kept people from freezing, but ensured that the balloon's 115,845 square feet of fabric would remain warm and pliable. “Camp life, even in heated tents, loses some of its charm in zero weather,” wrote Lyman J. Briggs, Director of the National Bureau of Standards, in the official summary report of the flight.

It took eight hours to inflate the balloon (including time to repair a 20-foot tear in the fabric) with enough helium for it to expand to its maximum size of 3,700,000 cubic feet. The Los Angeles Times reported that a “light covering of snow was cleared from the circle in the center of the bowl before the balloon fabric was spread in the ring to take on its cargo of gas.” By 7:01 a.m. on November 11, final preparations were made, and the balloon took off with Captains Stevens and Anderson safely inside the sealed gondola. Clearing the rim of the Stratobowl by just about 50 feet, the balloon ascended at an initial rate of more than 500 feet per minute. By 9:25 a.m., the balloon had reached an altitude of 28,000 feet, where the outside air temperature was −40°F. About an hour-and-a-half later, Explorer II reached its maximum altitude of 72,395 feet, setting a new world altitude record that would stand for 20 years (and eclipsing an unofficial record of 70,200 feet set by a Soviet balloon the year before, which crashed, killing its three occupants). The balloon finally landed at 3:14 p.m. near White Lake, South Dakota, about 240 miles east of where it had taken off.

During its eight-hour flight, Explorer II collected data on a variety of scientific topics, ranging from atmospheric composition to cosmic rays to the effects of high altitudes on micro-organisms. Data on wind direction and velocity were derived from the balloon's flight. These data “may prove of importance to the science of weather forecasting and to commercial aviation,” wrote The New York Times following the flight.

Although it was the last of the great stratospheric balloons of the 1930s, Explorer II proved to be a success, and it set the stage for the real Space Race between the United States and the Soviet Union that was to come.

Contributing Editor SEAN POTTER is a New York-based Certified Consulting Meteorologist (CCM), Certified Broadcast Meteorologist (CBM), and science writer with an interest in weather history.

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