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Infrasound is usually defined as sound waves below 20 Hz, the nominal limit of human hearing. Infrasound waves propagate over vast distances the Earth's atmosphere: the CTBTO (Comprehensive Nuclear-Test-Ban Treaty Organization) has 48 installed infrasound-sensing stations around the world to detect nuclear detonations and other disturbances. In February 2013, several CTBTO infrasound stations detected infrasound signals from a large bolide that exploded over Chelyabinsk, Russia. Some stations recorded signals that had circumnavigated the Earth, over a day after the original event. A balloon-borne infrasound sensor is expected to have two advantages over ground-based stations: a lack of wind noise and a concentration of infrasound energy in the “stratospheric duct” between roughly 5–50 km altitude. To test these advantages, we have built a small balloon payload with five calibrated microphones. This paper discusses the design, testing and expected sensitivity of the payload. Our current plans call for a launch in August 2016, coordinated with a ground-based explosion and a network of ground-based infrasound sensors.

In September, 2013 the BRRISON mission suffered an anomaly resulting in a total loss of science data for the duration of flight. The Balloon Observation Platform for Planetary Science (BOPPS) mission is in essence a re-flight of the BRRISON mission. The BOPPS mission was designed to study multiple comets and other planetary bodies as well as demonstrate a fine pointing system. The performance of this fine pointing system (FPS), designed for science pointing on the 50 milliarscecond level, will be discussed along with the mission as a whole. Due to a telescope focusing issue that manifested in flight during the allocated FPS demonstration window, thorough FPS characterization could not be performed. However, a calibration dataset demonstrated stable pointing of 33 and 58 milliarcsecond (mas) RMS in instrument Azimuth and Elevation respectively. This performance is considered marginal given the conservative FPS settings it was acquired with.

A balloon-borne Acousto-Optic Tunable Filter (AOTF) hyperspectral imager is ideally suited to address numerous outstanding questions in planetary science. The spectral agility, narrowband wavelength selection, tolerance to the near-space environment, and spectral coverage afforded by AOTFs would enable investigations not feasible from ground-based facilities. A notional AOTF imager design includes both visible and near-infrared channels to take full advantage of the spectral coverage of an AOTF.We explore an example use case of synoptic observations of clouds on the giant planets using the visible channel of such an instrument. Although technical challenges such as detector cooling would require further performance modeling, an AOTF hyperspectral imager is a logical choice for giant planet imaging investigations from a balloon platform. The ability to rapidly acquire hyperspectral image cubes, thereby obtaining spectra of all locations on the planet that could elucidate atmospheric structure and dynamical processes, offers a unique advantage over traditional imaging techniques.

NASA's Balloon Program Office (BPO) typically operates zero-pressure balloons at pressures near 3 torr and super-pressure balloons around 8 torr (around 125,000 ft and 110,000 ft altitude, respectively). These pressures are problematic for managing the temperature of a balloon-borne payload: the pressures are too low to support substantial convective cooling, but high enough to increase the conductivity of MLI (Multi-Layer Insulation). Nevertheless, thermal control of balloon payloads is often critical. Temperature gradients across a mirror or telescope can degrade their optical performance, and thermal emission from telescope mirrors can be a significant background source. We show that a combination of sunshades, earthshades and an enclosure around the OTA (Optical Tube Assembly) can reduce the mirror temperatures by 50 – 90 K, reduce gradients across the mirror to less than 0.5 K and reduce day/night temperature excursions to less than 3 K. We describe the relative importance of convection vs. radiation in the super-pressure and zero-pressure environments, and we compare simulation results and physical test results measured in a environmental chamber.

By virtue of operating at altitudes of 35 km, above 99.5% of the Earth's atmosphere, balloon-borne observatories have several advantages over ground-based and airborne observatories, including space-like image quality, access to the IR spectrum from 1 to 30 microns, very stable photometry and the potential for 100-day missions. We present plans for modifying inexpensive COTS telescopes for use in the stratosphere. The stratospheric environment is a problem for thermal control of a telescope and optical tube assembly (OTA): at 4 torr (the pressure near 35 km), convection is ineffective as a cooling mechanism, but the ambient pressure is easily high enough to thermally short-circuit layers of MLI that would otherwise form an adequate blanket in vacuum. A consumer telescope on a balloon would fail to form reasonable images due to distortion of the OTA and thermal gradients across the primary mirror. We discuss simple modeling of thermal mitigation strategies and the potential to fly modified COTS telescopes (costing less than a few $10Ks) to achieve 0.25″ imaging in UV and visible wavelengths, scintillation-free photometry and IR spectroscopy from 1 to 30 microns.

Balloon-borne telescopes have the potential to provide space-quality imaging. However, long exposures require pointing stability on the order of the optical point spread function (PSF). Consequently, one of the main goals of the BRRISON (Balloon Rapid Response for ISON) payload is the characterization of a fine pointing system. The BRRISON telescope is stabilized at the 5 arcsecond (RMS) level by the gondola's coarse control system. The job of the fine pointing system is to improve the pointing by another two orders of magnitude, from ±5″ to better than ±0.05″. Due to a flight anomaly, this paper discusses the design and ground test performance of the two main components of the fine pointing system: the fine guidance system (FGS) and the fast steering mirror (FSM). The FGS was designed with the goal of providing a 20 Hz optical reference signal with 0.05″ positional acuity. The FSM is specified to provide 100 nRad control of the mirror, translating to 0.0024″ on-sky. We report on measured performance and observed FWHMs during preflight ground testing. During a successful re-flight we expect ∼100X better performance than was achieved in ground testing.

Open source tools and readily available consumer hardware offer many advantages over the typical, custom designs used for control and data handling (CDH) and electronic power systems (EPS), especially in applications where the development time is measured in months, not years. This paper highlights the development of the CDH and EPS systems for the UV-Visible optics bench onboard the Balloon Rapid Response to Comet ISON (BRRISON) balloon gondola, a 5 month effort from conception to flight. The system was designed to power, operate, and collect data from an optical bench which included a fine-steering mirror motion compensation system, one high speed scientific CMOS camera, a science-grade CCD, a fold mirror mechanism, a filter wheel, and an active thermal heating element. To enable rapid development of a system with such complex and diverse requirements, a strategy was employed to use open source and consumer products. SwRI constructed a flight-ready power system designed around an Arduino microcontroller board, and a data collection system which includes a desktop motherboard running Ubuntu Linux onboard a recent Intel processor. The result is a highly portable, modular system which is efficient and inexpensive to reproduce for future high altitude balloon missions. This paper discusses the benefits and challenges SwRI faced using COTS (commercial off-the-shelf) hardware and open source software for rapid development.

Balloon platforms operating in Earth's upper stratosphere offer a unique platform to conduct new, high value planetary science observations of our solar system and exoplanets. There are compelling science drivers for conducting observations from such a balloon platform, with several potential high value science measurements that can be accomplished with one of several instrument concepts. Observations from 100,000 to 120,000 feet, which can last from hours to months, night and day, offer significant advantages over observations from ground and aircraft platforms. The stability of the airmass at float altitude is indistinguishable from space so that diffraction-limited performance can be obtained without adaptive optics, resulting in performance at visible wavelengths better than many ground based assets with larger apertures. With >99% of the atmosphere, and almost all the telluric water and CO 2 , beneath the platform, previously obscured spectral windows are also now open (e.g. water, CO 2 , and the organic fingerprint region of 5–8 µm), others are now fully free from telluric contributions, and observations in the mid through thermal infrared (IR), as well as shortward into the near ultraviolet (NUV), experience more than an order of magnitude less downwelling radiance than do ground based measurements enabling longer integration times and higher contrast observations. Instrument types that would support high value science include broadband and multispectral high spatial resolution NUV-NIR imagers, multispectral and hyper spectral imagers in the 2.5–5 µm range, as well as in the 5–8 µm range.

Recent developments in high altitude balloon platform capabilities, specifically long duration flights in excess of 50 days at over 100,000 ft and precision pointing with performance at the arc sec level or better have raised the question whether this platform can be utilized for high-value planetary science observations. In January of 2012 a workshop was held at NASA Glenn Research Center in Cleveland, Ohio to explore what planetary science can be achieved utilizing such a platform. Over 40 science concepts were identified by the scientists and engineers attending the workshop. Those ideas were captured and then posted to a public website for all interested planetary scientists to review and give their comments. The results of the workshop, and subsequent community review, have demonstrated that this platform appears to have potential for high-value science at very competitive costs. Given these positive results, the assessment process was extended to include 1) examining, in more detail, the requirements for the gondola platform and the mission scenarios 2) identifying technical challenges and 3) developing one or more platform concepts in enough fidelity to enable accurate estimating of development and mission costs. This paper provides a review of the assessment, a summary of the achievable science and the challenges to make that science a reality with this platform.

High altitude balloons are capable of supporting astronomical observations with virtually no image degradation due to atmospheric turbulence. To take advantage of this space-like seeing, a telescope must be pointed and stabilized with sub-arcsecond precision. This problem consists of two parts: providing an error signal, and using it to correct the pointing. This paper addresses error signal acquisition, specifically focusing on modeling and flight testing of the DayStar star tracker.

The ST5000 is a star tracker developed at the University of Wisconsin and used in NASA's sounding rocket payloads. In order to demonstrate the ST5000's suitability for pointed balloon-borne telescopes, we flew an ST5000 on a stratospheric balloon on May 6, 2011. This flight addressed our four basic questions: will the ST5000 work from 120,000 ft (it does), what was the rms performance (about 0.6"), what angular rates of motion would cause the ST5000 to fail (greater than 0.5 degrees/sec) and could the ST5000 serve as a daytime star tracker (not without modifications). We will briefly present the results from the flight and describe the ST5000's quantitative performance. We will also describe the problems with background light as a function of wavelength, altitude and angle from the Sun. We will discuss approaches to improve the daytime performance using infrared detectors and longer focal lengths/reduced platescales. Solutions that have finer platescales have the potential to improve the star tracker's error signal to the 0.1" level, which is better than the diffraction limit of a one meter telescope at 5000 A.