StratoCruiser Flight System

Scientific Motivation

Long duration observing platforms operating in the lower stratosphere at altitudes around 65 kft (20 km), would provide critically needed scientific breakthroughs in both the Space Sciences and the Earth Sciences. The StratoCruiser Propulsion Module offers unprecedented mobility and measurement capability combining the persistence of balloon platforms for greatly extended observing times, station keeping over the central U.S. in July and August, and recovery of the experiment following the system deployment.

The “StratoCruiser” long-duration observing system presented here engages the union of four separate advances in engineering, technology, and science: (1) the use of long-duration super-pressure balloon technology for flight durations of six weeks in the stratosphere at altitudes of 20 km capable of sustaining a payload mass of 1500 - 2000 kg on a modest 1 million (1M) ft³ balloon, or larger payloads with increased balloon volume, (2) a stratospheric solar-electric propulsion system for horizontal navigation and station keeping in the stratosphere that provides drive velocities up to 8 m/s for a 1M ft³ balloon volume, (3) a “reeldown” system providing in situ observations over an altitude range of 10 km below the balloon float altitude of 20 km, and finally (4) recognition and engagement of the unique anti-cyclonic flow in the lower stratosphere over the US in summer that provides to-this-point unrecognized opportunities for long duration flights of balloon platforms over the US.

In the arena of Astronomy, Astrophysics and Planetary research:

Critical frontiers in current research rely on being above 95% of the atmosphere for the achievement of observations testing key hypotheses and making new discoveries. The Hubble Space Telescope is one of the most successful Space Science endeavors in human history, and the astronomy community will suffer significantly when this observatory reaches the end of its life. Currently there is no open-space based observatory class facility planned that is capable of acquiring wavelengths short-ward of the red part of the electromagnetic spectrum. Yet it is clear major advances in the understanding of the births of stars, exosolar planetary systems, exoplanet atmospheres, and signatures of life beyond Earth will emerge in the unexplored regions of the electromagnetic spectrum available at altitudes of 20 km. Astrophysicists require more flexible technology development in space-like conditions without space-like difficulty. Clearly underlined in the National Research Council Astronomy and Astrophysics Decadal Study (NRC Astro2010) is the need for a hierarchy of project sizes, both to train our next generation as well as to allow for more immediate and flexible investigations to reignite vitality in the nation’s PhD programs.

At stratospheric altitudes both atmospheric opacity and atmospheric turbulence are sufficiently reduced such that diffraction-limited imagery can be achieved with a large aperture telescope — apertures on the order of 1 - 2 meters.  A telescope of this size has a resolution < 0.1” in the visible. The large aperture translates into greater sensitivity with which to obtain critically important measurements of the faint universe as displayed in Figure 2. Survey experiments become both possible and practical with the combination of long observing times from the StratoCruiser combined with diffraction-limited observations over a wide field of view. These are critical attributes for a new class of survey experiments.

Explicit examples of key areas of astrophysics that would benefit significantly using optical telescopes in the 1-2 meter class from the vantage point of the stratosphere include:

• discovering the first galaxies, stars and primordial structure during the period of re-ionization with highly red-shifted emission that is absorbed by the atmosphere below the tropopause.
• systematic imaging of the low surface brightness sky, wide field surveying for major scientific problems in “near-field cosmology” such as finding Milky Way’s satellites.
• wide field imaging for dark sector cosmology that includes the mapping of clusters, periodic acoustic oscillations and weak lensing.

In the field of planetary studies, the delivery of  water to young terrestrial planets is a key step in developing an understanding of habitable planets as displayed in Figure 3. Thus observations of the condensation line (aka the snowline) in a protoplanetary disk represents a critical measurement in exoplanetary science.

Observations above the water vapor in the Earth’s atmosphere by single telescopes that can observe the universe at infrared through terahertz wavelengths, will provide major advances in the detection of water vapor admission lines from the proto-planetary disks. The strongest and most diagnostically powerful water vapor rotational emission lines from discs fall in the 30 to 300 µm range. In combination with the ordered Keplerian velocity field in disks that provides precise radial information on the molecular emission observed – given sufficient accuracy in the line shapes - critically important observations can be made from the Stratocruiser system. The coherent nature of even the single baseline terahertz interferometer provides just such a platform.

In the domain of Earth Sciences:

We now know, through the union of high altitude aircraft, satellite and NEXRAD weather radar observations, that the stratosphere over the US in summer is vulnerable to ozone loss as a result of a series of factors unique worldwide to the central US in summer.

Several factors, as well as their coupling, are directly involved: (a) convection, as a result of severe storms over the Great Plains, delivers both markedly enhanced water vapor, as well as source species from the lower troposphere, deep into the stratosphere over the US in summer, (b) the depth of convective injection is sufficient to reach altitudes of rapidly increasing available inorganic chlorine that is then catalytically converted on simple, ubiquitous, binary water-sulfate aerosols, to free radical form, ClO, (c) anti-cyclonic flow in the lower stratosphere over the US in summer, as a result of the North American monsoon, contains that convective injection in a gyre that provides time for photochemical catalysis to exert influence, (d) it is the chlorine radical, ClO, that couples to the available BrO radical to form a catalytic cycle rate limited by ClO + BrO → Cl + Br + O₂ that constitutes the mechanism capable of removing ozone; and it is the same mechanism that contributes to ozone loss over the polar regions in winter, (e) the remarkable sensitivity of increases in the skin cancer incidence in the U.S. to fractional decreases in ozone column concentration places strict demands on the quantitative understanding required to accurately forecast ozone losses in a changing climate, (f) the frequency and intensity of severe storms over the Great Plains of the US is increasingly tied in the scientific literature to increased forcing of the climate by increasing levels of CO₂, CH₄ and N₂O in the Earth’s atmosphere. As a result, climate forcing is mechanistically linked to forecasts of ozone reduction in the stratosphere over the US in summer, and (g) with increasing forcing of the climate, linked to increasing frequency and intensity of convective storms over the Great Plains, the convective injection into the stratosphere is inherently irreversible and thus so to is ozone column concentration reduction irreversible. This has critically important human health considerations because the medical community has established that a 5% reduction in ozone column concentration in summer translates to a 15% increase in skin cancer incidence against what is currently a rate of new cases equal to 3.5 million annually in the U.S. alone.

Therefore, this deep stratospheric convective injection over the U.S. in summer links human health, NEXRAD radar observations, solar radiation management and ozone loss from volcanic injection as displayed in Figure 4.

This coupled problem summarized in Figure 4 requires a new observing strategy displayed in Figure 5 in renewed efforts to understand the stratosphere-troposphere response to increased climate forcing by CO₂, CH₄, N₂O, and other greenhouse gases. Critically important quantitative information is needed on:

• the structure of the 3-D velocity fields within the storm structure in the stratosphere,
• the structure of the wave-breaking events associated with the convective intrusion,
• the concentration of species co-injected with water by the convective event,
• both the vapor phase and condensed phase H₂O and HDO concentrations injected by the storm,
• the concentration of (a) the reactive chemical precursors that are normally removed by photolysis and oxidation in the troposphere, (b) chemical tracers injected by the convection,
• subsequent chemical kinetics of the injected chemical cocktail that tracks the time evolution of the source species, reactive intermediates, isotopes and catalytic free radicals that attack ozone.

Efforts to engineer the climate by solar radiation management (SRM) are under serious consideration because reduction in incoming shortwave radiation is one of the only methods that can reverse the rapid irreversible loss of the Earth’s cryosystems. Cryosystems that exert an inordinate control over the global climate structure of the entire planet. However, the addition of sulfate to the lower stratosphere directly engages the same heterogeneous and homogeneous catalytic removal of ozone in the lower stratosphere that is treated throughout this document. This results because an increase in the reactive surface area of sulfate aerosols in the stratosphere shifts the critical temperature for conversion of inorganic chlorine to free radical form to higher temperatures. In order to directly test the impact of sulfate addition to the lower stratosphere, it is necessary to first seed a cylindrical volume of the lower stratosphere with a variety of sulfate-water vapor mixtures as shown in Figure 6, panel A, using the StratoCruiser as the seeding platform. Then the seeding platform must withdraw from the test domain, tracking that volume with a lidar system that can continuously track the position of the seeded region. This is displayed in Figure 6, panel B. Then the reeldown system is used to provide multiple vertical soundings of the seeded region to follow the time evolution of the photochemistry that results from the sulfate-water perturbation as displayed in Figure 6, panel C. The key rate limiting steps involved in the use of stratospheric sulfate aerosols to reduce solar forcing of the climate involve both the UV radiation field as well as the presence of heterogeneous surfaces unique to the lower stratosphere. Thus observations within the lower stratosphere itself are required for the controversial public policy questions associated with solar radiation management.