Adapting the Ghost Balloons

Abstract

It has been four years since a balloon with the ethereal acronym GHOST set off from Christchurch, New Zealand, on the first round-the-world flight by a constant-level balloon. That particular balloon stayed in the stratosphere for 102 days, circling the Southern Hemisphere eight times. The flight was one of the early beginnings of a program to test the feasibility of a network of thousands of such balloons drifting slowly around the earth. Posted at various latitudes and altitudes, they would continually monitor atmospheric conditions and relay the information to the ground via orbiting satellites. One projection made at the time envisioned 6,000 balloons airborne simultaneously. As a result of the four years of balloon tests, plus faster-than-expected advances in space technology, the original GHOST concept has undergone major modification. The balloons still have a role to play in the future global observing But the role, as now seen by atmospheric scientists, is no longer as grandiose as it once was. Far fewer balloons will be necessary, and their tasks will be more limited and specialized. The proper generic name for a GHOST (for Global Horizontal Sounding Technique) balloon is superpressure, or constant-level, balloon. Their semirigid Mylar skins keep them at a constant volume, so they rise to the level in the atmosphere at which the mass of the displaced air is equal to their mass, and stay there. Such balloons were first proposed in the late 1950's when the tough Mylar film had just been developed. Lifetime is the key to whether the balloons are economically feasible as tracers of global winds and platforms for simple measurements in the worldwide observing system envisioned by GARP, the Global Atmospheric Research Program (SN: 4/4, p. 342). The tests carried out during the last four years by Dr. Vincent Lally of the National Center for Atmospheric Research have been only half encouraging. In the stratosphere, which begins about 35,000 feet up, the balloons have been a pronounced success. One balloon, launched in September 1967, stayed up 441 days, making 35 trips around the world at an altitude of about 60,000 feet. The average lifetime for balloons at that altitude is six months, but Dr. Lally feels that with more care in manufacture, testing and handling the average can be stretched to more than a year even though they suffer deterioration from ultraviolet light, and lose gas both by pinholes and slow diffusion through the membrane. But in the troposphere, the lower six miles or so of the atmosphere, the balloons have been disappointingly shortlived. Life expectancy has varied from a week to a month, far too short to be feasible. With a two-week life expectancy, world-wide coverage by 600 constant-level balloons at just one altitude would require 15,600 balloons a year. At $2,300 per instrumented balloon, the cost would be more than $36 million. Most of the failures in the troposphere are due to accumulation of ice crystals or supercooled water droplets on the balloon's surface. These increase the balloon's mass so much it falls. No suitable solution has been found. We see no clear way to increase life in this region, haunted by supercooled clouds, without introducing hazardous hardware, says Dr. Lally. While the difficulties with balloons in the troposphere were being encountered, the technology for space-based measurements of the atmosphere was advancing rapidly. The infrared instruments aboard the Nimbus 3 satellite last year (SN: 11/29, p. 509) demonstrated the ability to obtain a temperature profile of the atmosphere. eroded much of the GHOST balloons' mission. Future satellite-borne infrared sensors will be able to obtain enough temperature soundings to provide a global description of the mass field, which permits computation of the winds in the middle latitudes. But such a calculation is dependent on knowledge of the pressures, temperatures and winds around the earth at one specific altitude-a so-called reference level. determination of a reference surface in the Southern Hemisphere is one of two special tasks now assigned to constant-level balloons for the GARP Global Experiment, sometime in 1975 or 1976. As stated in the preliminary report of the GARP planning conference in Brussels March 16-20: This reference surface could best be determined by a network of approximately 300 constant-level balloons measuring ambient air pressure and the temperature and altitude, with an appropriate satellite location and data collection system. The United States and France will share responsibility in establishing the reference-surface balloon The French EOLE constant-level balloon program is similar to the United States' GHOST program. Discussions in the next few months will determine how the sharing will be done. The reference level is also needed in the Northern Hemisphere, but a combination of land-based radiosondes, ships and buoys can probably do the NSF