EXECUTIVE SUMMARY:A ROADMAP FOR SCIENTIFIC BALLOONING 2020-2030

Report of the NASA Balloon Program Analysis Group

The NASA Program Analysis Group Executive Committee:Peter Gorham, (chair, Univ. of Hawaii at Manoa), Christopher Walker (Univ. of Arizona),

William Jones (Princeton Univ.), Carolyn Kierans (NASA Goddard), Abigail Vieregg (Univ. ofChicago), James Anderson (Harvard Univ.), Eliot Young (Southwest Research Inst.), Supriya

Chakrabarti (Univ. Mass. Lowell), Robyn Millan (Dartmouth), Pietro Bernasconi (Johns HopkinsAPL), T. Gregory Guzik (LSU)

Contact Information for Balloon Roadmap Chair:Peter Gorham

Department of Physics and Astronomy,University of Hawaii at Manoa

2505 Correa RoadHonolulu, HI, 96822, USA

gorham@hawaii.edu, 808-956-9157

1 INTRODUCTIONThe exploration of the Earth, the stratosphere,

the solar system, and the cosmos beyond it fromballoon-borne scientific instruments has a longand lustrous history. Scientific ballooning hasprogressed in a continuous arc from early be-ginnings in the late 18th century when the firstmeasurements of air temperature vs. altitudewere made from a balloon payload, to our cur-rent era where almost every branch of astro-physics, space science, Earth science, and plan-etary science garner contributions from balloonmissions. There is every reason to believe thatthis rate of scientific productivity will continueits remarkable trajectory into and beyond thenext decade.

Our goals in this report are to distill the sci-entific context and directions from a diverse andcreative community of investigators, educators,and technologists, in a way that will help toprepare us to meet the scientific needs of boththe next decade, and a new generation of in-vestigators who are even now engendering newavenues of research. Scientific ballooning hasover the years acquired another important rolewithin NASA’s broad portfolio of missions: thatof a critical training ground for students andearly-career investigators at many different lev-els. This is due both to the relatively short du-ration of the projects, which is well-matched tothe several-year tenure of both students and re-searchers, and to the low relative costs of balloonpayload and the associated missions, which en-ables greater participation of students and earlycareer scientists in project roles that would oth-erwise be reserved for far more senior personnel.In all of the PAG assessments presented here, wemaintain a goal of preserving such access.

In the following sections, we briefly iden-tify high priority scientific drivers in all of themajor disciplines relevant to the NASA balloonprogram, including Astrophysics, Earth Science,Planetary Science, Solar & Heliophysics, Ed-ucation & Public Outreach, and Balloon pro-gram technologies. The PAG does not here ad-vocate for specific payloads, but rather has se-lected those science drivers across many disci-plines that rely on scientific balloons, and that

we find are of the most compelling significancefor the next decade. After describing these, weconclude with a set of findings and recommenda-tions to the NASA Balloon Program in responseto these scientific priorities. We refer the As-tro 2020 panel also to an online version of bothour draft report, along with a collection of doc-uments from community input that were used inthe process, referenced here.1

2 ASTROPHYSICSWithin the Astrophysics discipline, balloon-

borne payloads are used in a diverse set offields, involving diffraction-limited telescopes,particle detectors, and even compact radio in-terferometers. In this section we summarizethese fields and highlight our perceived priori-ties within them.2.1 Astroparticle Physics. The study ofatomic and subatomic-scale particles of cosmicorigin is a discipline requiring large collect-ing areas to overcome the very low fluxes, viaboth direct measurements using particle trackingdetector, and indirect measurements using sec-ondary emission from ultra-high energy (UHE)particle cascades in large natural targets such asEarth’s atmosphere or Antarctic ice, observedfrom the radio to the visible spectrum. Balloonpayloads have and will continue to play a criticalrole in this area through the next decade.

Cosmic Ray composition and spectrum belowthe knee. The composition of cosmic rays in theTeV to PeV energy range provides unique vis-ibility into important high-energy astrophysicsprocesses in the galaxy and interstellar medium.Balloon payloads still play a critical role in theexploration of particles in this energy regime,since they are able to deliver large mass, largearea to altitudes high enough for primary com-position measurements at low cost.

Cosmic Antiparticles. At tens to hundreds ofGeV, cosmic antiparticles are observed by bothspacecraft and balloon-borne payloads. Thespectral energy distribution of positrons and an-tiprotons may indicate the presence of nearbypulsars in the solar neighborhood, but may alsoarise from dark matter decay scenarios. De-tection of certain particle species, such as anti-helium nuclei or anti-deuterons, could provide

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compelling evidence for particle dark matter.Ultra-high energy cosmic rays & neutrinos.

The mystery of the sources of the highest en-ergy cosmic rays, at Exavolt (1018 eV) to Ze-tavolt (1020 eV) energies, remains a compellingquestion in the field of particle astrophysics. De-tectors using Antarctic ice have now begun toopen a new observing window for neutrinos inthis area,2 and balloon payload sensitivities toEeV neutrinos have grown steadily, providingimportant and unique astrophysical constraints,3and possible evidence of unexpected signals.4Balloon-borne payloads have a unique physicsreach in the ultra-high energy range, with floataltitudes above Antarctica providing a very largeobservable target volume, but at a proximity notavailable from space, thus giving an importantadvantage in a low particle energy threshold toevents arising from particle interactions near theEarth’s surface.2.2 Exoplanets & Stellar Astrophysics. Highaltitude balloons, especially long-duration bal-loons, are real alternatives for large satellite mis-sions. They demonstrate cutting-edge technolo-gies, are recovered, refurbished and reused to ad-dress important scientific questions. In certaindisciplines, they have been targeted as the idealplatform for moving the field forward. Exoplan-etary studies is one example of such usage.

Exozodiacal dust. Balloon payloads are anexcellent platform for the study of exozodiacaldebris. The bright zodiacal light in our solarsystem is produced by the scattering of sunlightoff 1-200 µm dust grains that are continually re-plenished by outgassing comets and colliding as-teroids. Exozodiacal dust is the dominant as-trophysical background that needs to be char-acterized and quantified in order to successfullyimage exoplanets. Furthermore, coronagraphicstudies of debris disk morphology are impor-tant in answering fundamental questions such ashow circumstellar disks evolve and form plane-tary systems. These structures are accessible toballoon-borne measurements employing meter-class telescopes, and as these debris discs are rel-atively bright, observations can be made in day-light.

Exoplanetary transit observations. Thephase curve of an exoplanet describes changesof its brightness during an orbital period. Spec-troscopic observations of exoplanetary transitscan be used to determine the inhomogeneouschemical composition and cloud coverage of theplanet. The value of spectroscopic observa-tions of phase curves for transiting planets to de-termine their atmospheric composition, thermalstructure and dynamics of exoplanets has beendemonstrated with both ground and space-basedobservations.

However, further progress requires precisionspectroscopic observations of complete phasecurves which are very difficult to obtain with-out continuous observations. Such spectroscopicphase curves could be obtained during an en-tire orbital period from long-duration balloonsfor short period transiting planets; such mea-surements are not possible from Low Earth Or-bit (LEO) satellite platforms due to orbital andscheduling constraints.

Stellar Astrophysics. The major impedi-ment to high precision IR photometry is watervapor; thus stratospheric balloons, which floatabove the tropopause and its associated coldtrap, are the ideal platform for near to mid in-frared photometric measurements. Specifically,high precision photometric measurements, cur-rently of limited precision for the ∼ 10,000brightest stars due to saturation in high sensitiv-ity modern surveys, are needed to complementthe very high resolution space and ground basedmeasurements, which are necessarily of limitedsensitivity, used to compare with physics-basedmodels of exoplanet atmospheres, cool stars, andevolved stars. These photometric measurementscan easily be obtained from a high altitude bal-loon.

Terahertz astrophysics. The TeraHertz (THz)portion of the electromagnetic spectrum (1-10THz) provides us with a powerful window intocosmic evolution. THz photons arriving at Earthprovide valuable insights into everything fromthe birth and death of stars to the cataclysmicevents associated with the origin of galaxies andthe Universe itself. Many of the THz photonswe observe are emitted by the gas and dust be-

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tween the stars, that is, the interstellar medium(ISM). At THz frequencies we can observe pho-tons associated with the ISM of our own galaxy,the Milky Way, as well as from the ISMs of dis-tant galaxies. Today the ISM accounts for only1% of the total gravitational mass of the MilkyWay, with the balance being in stars ( 9%) anddark matter ( 90%). However, in the beginning,most of the Galaxy’s baryonic mass was in ISM.It was, and continues to be, the mass and en-ergetics of the ISM that is a principal driver ofgalactic evolution.

The evolution of THz astronomy has beendriven largely by two factors; 1) atmosphericabsorption of THz light and 2) the availabil-ity of detector technology. Water vapor in theEarth’s atmosphere is a very efficient absorberof THz photons Therefore, THz observations arebest conducted from high altitude balloon-bornetelescopes or space-based telescopes. Utilizingrecent breakthroughs in detector technologies,such observatories have or will serve as powerfulprobes of the life cycle of the ISM, both in ourown Milky Way and beyond. Due to their abilityto take heavy payloads to the edge of space, highaltitude balloons allow the possibility of realiz-ing large aperture telescopes ( > 5 meters) withsophisticated cryogenic instruments at a fractionof the cost and time associated with achievingsimilar capabilities on an orbital mission. Withsuch observatories it will be possible to probe theintricacies of star and planet formation, as wellas the origins of galaxies.2.3 High Energy Astrophysics. High energyastrophysics is the study of the extreme Uni-verse. The x-ray and gamma-ray sky shineswith non-thermal emission from high-energy en-vironments, such as jets, AGN, black holes,GRBs, and x-ray binaries. Balloon-borne mea-surements in this energy range require high al-titudes to reduce atmospheric absorption, andlong exposure times to account for the high at-mospheric backgrounds and low source fluxes.

MeV Astrophysics. Technology advance-ment is key in this relatively unexplored en-ergy range. The MeV sky was first opened byCGRO/COMPTEL in the 1990s,6 but many in-teresting science topics remain, from gamma-ray

line measurements of Galactic nucleosynthesisand positrons annihilation to continuum studiesof high energy events and sources such as pul-sars, high red-shift blazers, and GRBs.7, 8 Notonly has the balloon platform enabled the neces-sary technology development in this challengingenergy range over the past decade, but with therealization of NASA’s new Super Pressure bal-loon, ultra-long duration flights will soon allowfor sufficient exposure to accomplish competi-tive science goals at the relative low cost of a bal-loon mission. Additionally, the newly availablesouthern hemisphere launch location in Wanaka,New Zealand, allows for exposure of the Galac-tic Center region, which is an important MeVscience target, especially for studies of positronannihilation.

Hard x-ray and gamma-ray polarization mea-surements. Polarization measurements in thehard x-ray and gamma-ray regime are just be-ginning to be realized, and balloon-borne instru-ments are at the forefront of this novel tech-nique.9–11 These measurements give unique in-sight into high energy phenomena with a diag-nostic of the emission mechanisms and sourcegeometries for GRBs, galactic black holes, AGNand accreting pulsars. Recent measurementsof hard x-ray and gamma-ray polarization fromballoon-borne instruments have demonstratedthis technology,12 and the timing is optimal tocomplement the upcoming IXPE satellite mis-sion, which will perform polarization measure-ments at lower energies. Long duration balloonflights are generally required for high-energy po-larization measurements to allow for sufficientexposure of these sources and at the same timegive a higher probability of catching a flaringor transient source. The Wallops Arc SecondPointer (WASP) has enabled much of this sci-ence in the hard x-ray regime.2.4 Observational Cosmology.2.4.1 The Cosmic Microwave Background.In the post-Planck era, and in anticipation ofthe ambitious next generation of ground-basedCMB projects, the scientific ballooning plat-form represents a unique and enabling scientificopportunity. Broadly speaking, the measure-ments enabled fall in two classes. At frequencies

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above 220 GHz, near the peak of the CMB sig-nal and where Galactic polarized dust emissiondominates, the advantages of the stratosphericplatform over the best terrestrial sites are over-whelming. The lack of atmospheric contamina-tion and the ability to cover a significant fractionof the full sky allows balloon borne instrumen-tation access to the largest angular scales, whichare of critical importance both for characterizingthe epoch of reionization and for probes of pri-mordial gravitational waves.2.4.2 Cosmic Concordance. Cosmologicalobservations at high- and low-redshift , includ-ing measurements of the Hubble parameter andof the large scale distribution of matter, provideindependent tests of the standard cosmologicalmodel. The unique capabilities of the HubbleSpace Telescope have proven invaluable inthe development of this field. Long-durationmid-latitude Super Pressure Balloon flights offertantalizing opportunities for expanding well be-yond the capabilities of HST from the near-UVto near-IR wavelengths, offering space-qualitydiffraction limited wide-field imaging andspectroscopy. The relatively rapid developmentcycle of the balloon payloads is well poisedto take advantage of the exponential growth inthe imaging technology that is transforming thefield. Importantly, these capabilities will notbe provided in the coming generation of largemissions, including JWST and WFIRST.

3 EARTH SCIENCEThe climate structure of the Earth is under-

going rapid and unprecedented changes. Thesechanges have global impacts, demanding criti-cally needed studies and forecasts of: (1) hurri-cane formation, intensity and trajectory, (2) therate of sea level rise, (3) climate-forced changesin the large-scale dynamics of the atmosphere,(4) wildfire risk, (5) shortwave forcing of theclimate that is the dominant uncertainty in cli-mate forecasts, (6) drought and agricultural pro-duction, (7) severe storm initiation, developmentand geographic coverage, (8) Arctic sea andglacial ice breakup, (9) UV radiation increasesover the central United States in summer, (10)risk of flooding, (11) increases in CO2 and CH4

emission globally, and (12) cirrus cloud, pyrocu-mulus, and volcanic eruptions impact on cli-mate.3.1 Response of Large Scale AtmosphericDynamics to Carbon Dioxide and Methane.As greenhouse gases increase in Earth’s atmo-sphere, the stratosphere is predicted to have twodistinct responses. First, as the surface warms,the stratosphere cools, and both observations andmodels show that stratospheric cooling is a ro-bust response to increasing radiative forcing bygreenhouse gases. Second, chemistry-climatemodels universally predict that the Brewer-Dobson circulation (BDC), the primary circula-tion pattern in the stratosphere, should be accel-erating in response to greenhouse gas increases.The BDC is responsible for transporting tracegases, such as water vapor and ozone, that arecritical for radiative forcing and impact surfaceweather and climate. The acceleration of theBDC predicted by models would be a positivefeedback on the increased radiative forcing bygreenhouse gases. However, rates and patternsof the BDC inferred from observations are notconsistent with each other or with model predic-tions, raising important questions.

A balloon-borne stratospheric Earth-observing payload can systematically andcomprehensively map the details of this dy-namical structure globally employing an arrayof high spatial resolution in situ measurementsof short, medium and long-lived atmospherictracers in combination with highly sensitivedynamical observations of three-dimensionalvelocity fields, wave-breaking structures, mo-mentum transport and heat transport as well asradiance divergence observations. These are thedetailed, high spatial resolution observationsthat are critically needed to quantitatively estab-lish the dynamical structure of the atmosphererequired to develop tested and trusted forecastmodels of climate response to rapidly increasedforcing by carbon dioxide and methane.3.2 Terrestrial Gamma-ray Flashes (TGF).Intense sub-millisecond bursts of energetic pho-tons in excess of tens of MeV have beenobserved from terrestrial sources by satellitesfor 25 years. These Terrestrial Gamma-ray

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Flashes are localized in regions of high thunder-storm activity and are thought to be caused bybremsstrahlung of electrons in the atmosphereaccelerated by the high electric fields in thunder-storms and associated lightning. Recent groundexperiments have been able to correlate TGFevents with local lightning events, but the de-tails of the gamma-ray generation mechanismand the relationship between ground and satel-lite observations remain uncertain. Balloon pay-loads, possibly hand launched, would be capableof traveling over thunderstorms, intercepting theupward beam of TGF gammas providing a linkbetween local and satellite observations and newconstraints on gamma-ray burst generation mod-els.

4 PLANETARY SCIENCEAt the time of this writing NASA has no pro-

posal opportunities that could fund balloon mis-sions in planetary science. This obstacle is spe-cific to the Planetary Science Division amongthe four divisions within the Science Mission Di-rectorate. It does not mean that planetary sci-entists could not benefit from balloon missions– on the contrary, the recent GHAPS/SIDT re-port (Gondola for High Altitude Planetary Sci-ence/Science Instrument Definition Team) out-lines many planetary investigations in which aballoon mission would yield low-hanging fruit.Of the roughly 200 important questions listedin the at least 44 that are clearly addressablefrom balloon platforms, generally because of ac-cess to key wavelengths or diffraction-limitedimaging at UV and visible wavelengths, whereground-based adaptive optics (AO) systems havepoor Strehl ratios.

The atmospheric windows for observation be-yond the visible-light bands are crucial to plane-tary science in the near-infrared and thermal in-frared, as well as the near-ultra-violet. Here weshow a comparison of transmission curves forthree altitudes, including balloon-borne systems.4.1 Atmospheric Dynamics. The gas giants(Jupiter and Saturn), the ice giants (Uranus andNeptune), Mars, Titan and Venus all have ob-servable clouds that can be tracked to study at-mospheric dynamics. There is a rich historyof dynamical studies on these objects [Refs],

either from spacecraft or from high-acuity as-sets like Keck/AO or HST (Hubble Space Tele-scope). Because different wavelengths sounddifferent altitudes, it has been useful to combineHST imaging (at λ = 0.2− 1µm) with ground-based adaptive optics (typically λ = 1−2.5µm).One problem has been the availability of observ-ing time: the total number of days available onKeck or HS may be less than the number of daysneeded for an investigation (e.g., weeks of cloudtracking to characterize planetary scale waves orother long-lived phenomena).

Tracking Venus’s cloud tops at UV wave-lengths. The two most recent missions to Venus,VEx (Venus Express) and Akatsuki, were bothequipped with UV cameras to image Venus at283 (sensing SO2) and 365 nm (the unknownUV absorbers). Both missions could imageVenus’s full disk at these wavelengths with spa-tial resolutions of about 20 km, although VEx’seccentric polar orbit only obtained full-disk im-ages of Venus’s southern hemisphere and Akat-suki’s eccentric equatorial orbit provides irregu-lar sampling with coarser resolutions over mostof its orbit.

A balloon-borne terrestrial 1-m aperture tele-scope has a diffraction-limited resolution of 75mas at 365 nm, commensurate with a spatial res-olution of 30 km on Venus from a distance of0.55 AU. Larger apertures could actually exceedthe spacecrafts’ resolutions. A Venus balloonmission could carry additional UV filters (e.g.,with sensitivity to both SO2 and SO), and, unlikethe spacecraft missions, it could obtain images at10-min intervals over up to a 100-day baselineto help characterize Hadley cell-like circulation,convection, gravity waves (some launched byVenus’s mountains) and planetary scale waves.

Tracking Venus’s lower and middle clouddecks. Venus’s CO2 atmosphere has transmis-sion windows at 1.74 µm and 2.2 - 2.6 µm, wherethermal emission from the hot surface and low-est scale heights of the atmosphere show revealclouds from 48 - 55 km as silhouettes. Cloudtracking in these wavelength windows deter-mines wind fields that constrain Venus’s circula-tion, and the distribution of cloud opacities, par-ticle sizes and chemical composition constrain

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the coupled radiative-convective-chemical pro-cesses that control Venus’s atmosphere. At 1.74and 2.3 µm, the diffraction limit of a 1-m aper-ture is 440 and 580 mas, respectively – hardlybetter than good ground-based sites. Image se-quences from the IRTF can resolve cloud mo-tions with errors at the 3 m/s level in good seeingconditions, but the goal should be 0.5 m/s errors,in order to resolve meridional winds that are ex-pected to be ∼1 m/s. A large aperture – about3-m in diameter – and a 6-hr observing intervalare necessary to achieve the 0.5 m/s goal.4.2 Comets. Comets in UV and IR Wave-lengths. The composition of cometary jets canhelp determine where a comet formed in theearly solar nebula, based on the idea that dif-ferent volatiles condensed at different distancesfrom the Sun. Two wavelengths are particularlyuseful: OH lines at 308 - 311 nm, which serve asa proxy for water production, and the CO2 bandat 4.3 µm. The former can be observed fromthe ground, but with telluric extinctions around50%; the latter cannot.

Cooled OTAs for IR Wavelengths. Heat trans-fer in the stratosphere is dominated by radiativeand conductive processes. There is an advan-tage to reducing the thermal emission from thetelescope optics and the OTA enclosure: on theBOPPS mission, thermal emission from the pri-mary and secondary mirrors accounted for ap-proximately two-thirds of the background countsat 4.3 µm, despite the fact that the primary mir-ror was coated with low-emissivity gold. Werecommend studies on ways to cool the OTA,beginning with sun- and earth-shields. Prelimi-nary thermal models suggest that passive shield-ing can reduce the daytime temperature of a tele-scope by 100 K, which translates to nearly a fac-tor of 6x reduction in thermal background froma telescope’s own mirrors.4.3 Asteroids and Trans-Neptunian ObjectSatellites. Satellite detections. Satellite detec-tions and their orbit characterizations can poten-tially (a) determine the mass of the central body,(b) constrain formation scenarios and (c) rule outclasses of events (e.g., impacts, close encoun-ters) in an object’s history. These detections canbe challenging, since they often entail identifica-

tions of faint objects adjacent to bright objects.This means that the PSF halo is of paramountimportance. Cloud tracking (or other imagingof low-contrast fields) is relatively tolerant ofpower in a PSF’s halo, as long as the PSF’score is narrow. Satellite detection require lowpower in the PSF halo to ensure that the satelliteis not swamped by light from the central body.The best wavelength region (in terms of the bestSNR) is often in visible wavelengths because ofthe peak in solar flux, but can be in specific ab-sorption bands if the central object is covered incertain constituents (e.g., water ice).

5 HELIOPHYSICSBalloon experiments have a rich history in

both solar and magnetospheric physics, with sig-nificant contributions in science areas rangingfrom solar flares to particle precipitation intoEarth’s upper atmosphere to measurements oflarge-scale magnetospheric electric fields. In ad-dition to producing important stand-alone sci-ence, these experiments have worked in tandemwith larger NASA missions to augment their sci-ence return and have contributed significantly tothe development of Explorer-class satellite mis-sions.5.1 Solar Physics. Balloons can carry largeand powerful optical telescopes high abovethe atmosphere where they make ultra high-resolution observations of the Sun at 50 kmspatial scales, 3x better than the state-of-the-artHinode space mission.13 Balloon-borne exper-iments also make measurements at UV wave-lengths that are unreachable from the ground dueto atmospheric absorption.

Gamma-ray observations of the Sun requireheavy instruments, making balloons an idealplatform. Gamma-ray imaging indicates thatsolar flares accelerate and transport ions differ-ently from electrons. The angular resolution ofthe current state-of-the-art balloon experiment issignificantly better than any spacecraft observa-tory to date (e.g., RHESSI). Long-duration bal-loon flights are critical for catching infrequenttransient events such as solar flares. Recommen-dations for solar physics.5.2 Particle Precipitation into Earth’s atmo-sphere. Particles with energies spanning more

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than four decades - from auroral electrons (100eV - 10 keV) to relativistic electrons from theVan Allen belts (100 keV - MeV) - rain downonto Earth’s upper atmosphere from space. Au-roral precipitation controls the nighttime con-ductivity of the ionosphere while relativistic pre-cipitation plays an important role in the dynam-ics of the Van Allen radiation belts and mayalso be important for the creation of HOx andNOx that destroy ozone. Electron microburstprecipitation was discovered with balloons14 andis known to be an important loss mechanismfor the radiation belts. These rapid bursts ofprecipitation provide an ideal target for study-ing the physics of non-linear wave-particle in-teractions in space plasmas. Balloons can eas-ily distinguish the spatial and temporal varia-tions of these bursts which is difficult from aLEO satellite. Experiments are under develop-ment to perform high-resolution imaging of rela-tivistic electron precipitation, only possible froma balloon platform. These measurements will,for the first time, quantify the distribution of mi-crobursts scale-size, revealing important infor-mation about the connection with plasma wavespatial scales near the equator.5.3 Thermospheric Winds. Joule heating inthe high latitude regions of Earth has an impor-tant impact on the global circulation of the ther-mosphere, a part of Earth’s upper atmosphere.The magnetosphere, driven by the solar wind, isnow understood to be an important source of en-ergy input to the thermosphere. However, the de-tails of the energy transfer process are not well-quantified, and the effects of geomagnetic ac-tivity, season, and geomagnetic latitude are notwell-studied. Balloon-borne experiments canmonitor thermospheric neutral winds by mea-suring the Doppler-shift in airglow emissions ofatomic oxygen at 630nm. Such experimentswill help quantify the coupling of the magne-tosphere, ionosphere and thermosphere and willimprove thermospheric density models.5.4 Large-scale Magnetospheric ElectricField. Unlike the magnetospheric magneticfield, which penetrates to the ground, thedynamics of the large scale electric field inthe ionosphere and magnetosphere cannot be

deduced from the ground. A BARREL-likeflotilla capable of measuring vector electricfields would allow for an instantaneous determi-nation of the large scale magnetospheric electricfield. There is no other cheap way to get direct,real- time, mapped-in-situ, large scale magne-tospheric electric fields with 1 to 10 secondresolution, and global, instantaneous coverage.Magnetospheric science is a system scienceoften requiring multi-point measurements andcoordinated measurements from different plat-forms. This holds true for studies of the globalelectric field, but also for other areas of researchincluding particle precipitation, and ionosphericstudies.

6 EDUCATION & OUTREACHIn a December 2018 report by the Commit-

tee on STEM Education (COSTEM) of the Na-tional Science & Technology Council, ChartingA Course for Success: America’s Strategy forSTEM Education, three major objectives wereidentified for NASA: 1) foster STEM ecosys-tems that unite communities, 2) increase work-based learning and training through educator-employer partnerships, and 3) encourage trans-disciplinary learning. While ballooning con-tributes to all three objectives it is the last onewhere ballooning has the most impact on STEMeducation. In fact, the report highlighted aNASA balloon platform as an exemplary ex-ample of transdisciplinary learning Thus, in ad-dition to supporting multiple significant scien-tific objectives, ballooning contributes to theaerospace workforce development pipeline, en-hancing public awareness of NASA programs,and training the next generation of scientists andengineers.6.1 The Workforce Development Pipeline.Ballooning is a unique NASA capability thatsupports workforce development from K-12through early career faculty and engineers. Sci-entific ballooning provides high quality, experi-ential projects for students from K-12 throughgraduate school that enables interdisciplinarySTEM workforce training consistent with theCOSTEM report objectives assigned to NASA.In addition, nation-wide programs making useof the established National Space Grant College

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and Fellowship network, provide special oppor-tunities to not only support workforce develop-ment but to also convey the advantages of scien-tific ballooning to new and large audiences.6.2 Enhance the Public Awareness of Bal-looning. For the August 21, 2017 North Amer-ica total solar eclipse 54 teams from 32 statesinvolving over 900 students were positionedacross the path of totality from Oregon to SouthCarolina and flew payloads to an altitude upto 100,000 feet to downlink HD video of themoon’s shadow on Earth that was live streamedover the internet. This effort, organized by theMontana Space Grant Consortium was an amaz-ing success and was featured as part of NASA’seclipse coverage, which reached 600 millionpeople. These kinds of projects engage the pub-lic, as well as students, exciting their imagina-tion, and encouraging them to learn more aboutNASA ballooning.6.3 Training Next Generation Scientists &Engineers. Balloon platforms are a critical ve-hicle for supporting PI-class academic researchdue to the A) short time scale between concep-tion and flight results, B) relatively low cost, andC) recoverability. This combination of charac-teristics enables universities to support researchprograms that develop near-space flight hard-ware while engaging future scientists and en-gineers. As most balloon experiments are re-covered and reflown the acceptable risk on in-dividual balloon payloads should be high rela-tive to spacecraft. By accepting risk on balloonpayloads, quality assurance requirements are re-duced which, in turn, will decrease developmenttime and overall cost.

Recently, however, there is growing concernthat safety and quality assurance requirementsare increasing in the direction toward spacecraftmission requirements. If this trend is allowed tocontinue it is likely that the project time of de-velopment and cost will increase while access tothe instrument by untrained students or early ca-reer faculty will decrease. These trends wouldhave an adverse impact on training the next gen-eration of scientists and engineers.

7 BALLOON TECHNOLOGIESOver time the sophistication and complexity

of balloon payloads has steadily increased, insome instances potentially yielding scientific re-turns that can rival or exceed what can be accom-plished in a much more expensive orbital mis-sion. What ultimately limits the type and qual-ity of the science that can be performed froma balloon platform is the altitude of the carrierballoon. In particular, high energy astrophysicsexperiments often require both high altitude andhigh suspended weight, as well as long flighttimes to increase the likelihood of detections.Similar statements can also be made for over-coming the absorptive and scattering/seeing ef-fects of the atmosphere for payloads operatingin the UV and optical. In the infrared and far-infrared, it is atmospheric water vapor that lim-its sensitivity. In all cases, the deleterious effectsof the atmosphere decrease exponentially withheight, making even modest increases in the al-titude limits of carrier balloons capable of yield-ing significant increases in science return.

NASA qualified balloons come in two types,the standard zero-pressure balloon (ZPB) andthe super pressure balloon (SPB). In the case ofa ZPB, once it reaches its target altitude excesshelium is vented, such that the balloon becomesin pressure equilibrium with its surroundings.Diurnal temperature variations then can lead tosubstantial altitude excursions which may ne-cessitate the use of expendables (ballast and he-lium), limiting the time at float for a ZPB to ap-proximately 1 to 2 months. In the case of anSPB, the balloon is made strong enough suchthat it can maintain a specific volume even whenthe helium within it is at a higher pressure thanthe surrounding atmosphere. This ability pro-vides volumetric margin against changes in thetemperature of the ambient atmosphere and canhold the altitude of the balloon constant withoutthe use of expendables, thereby dramatically in-creasing an SPB’s time at float (3+ months).

ZPBs can take as much as 4 tons into the lowstratosphere ( 100,000 ft), but less than a ton intothe high stratosphere (> 160,000 ft). To date,SPBs have only achieved modest sizes and sus-pended payload weight, 2 tons, restricting their

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practical use to modest altitudes ( 110,000 ft). Inorder to realize the full scientific and cost sav-ing potential of scientific ballooning, the devel-opment of larger zero pressure and super pres-sure balloons should be considered a high prior-ity. Even now the demand for balloon flights out-strips the ability of NASA to launch them, eitherfrom mid latitude sites (e.g. Ft. Sumner, Pales-tine, or Wanaka) or polar regions (e.g. Antarc-tica and Sweden). At any given time, there isoften a backlog of missions waiting for flight,which both delays the delivery of science to thecommunity and increases its associated cost.

8 FINDINGS/RECOMMENDATIONS

The Balloon Program analysis group makesthe following findings and recommendations forNASA for the next decade of the scientific bal-loon program. These are presented in the or-der of the preceding sections, in which the dis-ciplines were ordered alphabetically, not rankedby relative importance.

1. In this document, as we have noted in the in-troduction, the PAG has chosen not to advocatefor a specific payloads to address the importantNASA science questions across all of the Sci-ence Mission Directorate outlined above. ThePAG does wish to emphasize the critically im-portant role that NASA balloon missions playin generating cutting-edge science on short-timescale and at modest cost, in addition at advanc-ing technologies and training future leaders forNASA space missions.

2. The PAG finds that several areas of scien-tific research would be greatly augmented withthe development of one or more diffraction-limited near-UV, visible, near-IR, and/or thermalIR telescopes of up to 3 m on a balloon-borneplatform. At IR wavelengths, there is a particu-lar need for large aperture telescopes, althoughnot necessarily ones with diffraction-limited im-age quality. The PAG recommend an active de-velopment program in three key areas to achievethis goal: thermal stabilization of optical assem-blies, wavefront sensors to generate real-time in-flight wavefront errors, and a wavefront correc-

tion scheme (e.g.,deformable mirrors) to com-pensate for optical assembly aberrations.

3. The PAG commends the Balloon Programfor the development of the Wallops Arcsec-ond Pointing System, and recommends build-ing on this success to provide even better stabi-lization for potential observatory-class balloon-borne telescopes.

4. The PAG commends the Balloon Programfor the continuing success and scientific impactof the Antarctic Long Duration Balloon (LDB)program flown out of McMurdo Station. ThePAG recommends unwavering NASA supportfor this flagship program and its associated fa-cilities near Williams Field, Antarctica. Antarc-tica represents a unique resource and environ-ment for investigations across several disciplinesand the opportunities afforded by the LDB meritsuch support. NASA should deploy a third pay-load building and commit to the resources neces-sary to sustain a three-large-payload per seasonlaunch rate.

5. The PAG welcomes recent increases inthe NASA Astrophysics Research and Anal-ysis funding and recommends that this in-creased funding for balloon payloads be ad-vanced through the next decade given the needfor increased payload complexity to address thescientific challenges summarized here.

6. The PAG commends the NASA AstrophysicsDivision and Balloon Program for developingand flight-qualifying the 60 Mcf conventionalballoon and maturing the 18 Mcf super-pressureballoon, and the new launch facility in Wanaka,New Zealand, which supports SPB launches.The PAG recommends continued support for thegrowth of this facility, including a new payloadintegration building that could accommodatemultiple payloads. These developments will en-able new science investigations from Wanakawith science returns comparable to significantlymore costly space flight missions, and comple-menting to NASA flagship missions.

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7. The PAG recommends that NASA shouldprovide a large-scale class of balloon investiga-tion ( $15-20M over 5 years) for highly mer-itorious and high-impact investigations. Thisnew class is of particular importance for long-duration and ultra-long duration payloads thatlaunch from Antarctica and Wanaka, NZ.

8. The PAG finds that a North American launchsite that can provide reliable night-time launchopportunities is a high priority not only for As-trophysics balloon payloads, bu other disciplinesas well. The PAG recommends that NASA iden-tity and develop alternative launch sites in addi-tion to Palestine, TX, and Ft. Sumner, NM.

9. The PAG recommends that NASA continueto advance the lift capability and float altitudeof super pressure balloon to the point where itis commensurate with current zero-pressure bal-loon capabilities.

10. The PAG finds that commercial providersoffer launch opportunities with station keepingand trajectory modification capabilities that maycomplement the NASA balloon program. Thesesystems presently have small allowable payloadsizes and weight, and lower altitudes than theNASA balloon program provides. While thesecapabilities do not currently meet Astrophysicspayload requirements, they could be of use forEarth Science missions, and the PAG recom-mends that NASA engage with such providersand study opportunities that they may enable.

11. The PAG finds that while payload downlinkbandwidth is improving, but is not keeping stepwith science needs. The science return of anymission can be significantly enhanced by higherdata downlink capability, avoiding the possibil-ity of data vault loss as a mission failure mode.By the end of the next decade, payloads willlikely be capable of producing tens of Terabytesof science data in a 30-day flight. The PAG rec-ommends that NASA pursue a cummunicationdownlink goal of 10 MB/s average through aflight, enabling a higher science return at lowermission risk.

12. The PAG recommends that NASA shouldengage with the National Space Grant Collegeand Fellowship program to continue and expandstrong support for student training ballooningprograms that support the workforce develop-ment pipeline at all levels including K-12, uni-versity students, and in-service teachers.

13. The PAG recommends that NASA engagewith stakeholders interested in the April 8, 2024North American total solar eclipse as early aspossible to assess scientific, workforce develop-ment, and public engagement projects, as wellas payload weight classes including heavy pay-loads, and potential launch sites.

14. The PAG recommends that balloon projectsafety and performance standards applied to bal-loon missions should be reviewed at a level sig-nificantly below spacecraft standards. Such stan-dards should be relevant to the project, avoid du-plication of effort, be consistent with the needto provide novice personnel the flexibility tohave direct involvement in the payload success.They should be associated with relevant, simple,concise, and straight forward best practices andsafety training materials.

15. The PAG finds abundant evidence that ex-pansion, not contraction, of the present portfo-lio of balloon flight options for both launch lo-cation and duration is important to the contin-ued health of NASA astrophysics research, andthe training of new investigators at every level.The PAG recommends that NASA increase thecapacity of launch facilities, and more impor-tantly and with the highest priority, the numberof ground crews that can support them. NASAmust also continue operations from locationsthat support research in auroral and radiationbelt physics, and high latitude magnetosphere-, ionosphere-,and thermosphere-coupling, whichare compelling scientific drivers in heliophysicsand require flights at magnetic latitudes rangingfrom 55-70 degrees (e.g., Kiruna, Sweden).

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REFERENCES[1] The current draft of the PAG report is available at https://www.phys.hawaii.edu/

~gorham/Post/Roadmap2018/BalloonRoadmapDraft.pdf. In addition, a compilation ofrelevant documents and community input can be found at https://www.phys.hawaii.edu/~gorham/Post/Roadmap2018/BalloonRoadmappers.html.

[2] M. G. Aartsen et al., Phys. Rev. Lett 117, 241101 (2016).

[3] Constraints on the ultra-high energy cosmic neutrino flux from the fourth flight of ANITA, P.W. Gorham, et al. [ANITA collaboration] (2019), arXiv:1902.04005.

[4] P. Gorham, et al. [ANITA Collaboration], “Observation of an Unusual Upward-goingCosmic-ray-like Event in the Third Flight of ANITA,” https://arxiv.org/abs/1803.05088.

[5] C. Walker, Terahertz Astronomy, 2016, (CRC Press: Boca Raton, FL).

[6] Instrument Description and Performance of the Imaging Gamma-Ray Telescope COMPTELAboard the Compton Gamma-Ray Observatory, V. Schönfelder and H. Aarts and K. Bennettand H. de Boer and J. Clear and W. Collmar and A. Connors and A. Deerenberg and R. Diehland A. von Dordrecht and J. W. den Herder and W. Hermsen and M. Kippen and L. Kuiperand G. Lichti and J. Lockwood and J. Marci and M. McConnell and D. Morris and R. Muchand J. Ryan and G. Simpson and M. Snelling and G. Stacy and H. Steinle and A. Strong andB. N. Swanenburg and B. Taylor and C. de Vries and C. Winkler, Ap. J. Suppl. 86, 657, 1993.

[7] The 2016 Super Pressure Balloon flight of the Compton Spectrometer and Imager,. Kierans,C. A. and Boggs, S. E. and Chiu, J.-L. and Lowell, A. and Sleator, C. and Tomsick, J. A.and Zoglauer, A. and Amman, M. and Chang, H.-K. and Tseng, C.-H. and Yang, C.-Y. andLin, C.-H. and Jean, P. and von Ballmoos, P., Gamma-Ray Astrophysics in Multi-WavelengthPerspective, Proceedings in 11th INTEGRAL Conference, 2016.

[8] Development of the balloon-borne sub-MeV gamma-ray Compton camera using an electron-tracking gaseous TPC and a scintillation camera, K. Ueno and T. Mizumoto and K. Hattoriand N. Higashi and S. Iwaki and S. Kabuki and Y. Kishimoto and S. Komura and H. Kuboand S. Kurosawa, Journal of Instrumentation 7, 1088, 2012.

[9] The Advanced Scintillator Compton Telescope (ASCOT) balloon payload, P. F.Bloser,T. Sharma,J. S. Legere,C. M. Bancroft,M. L. McConnell,J. M. Ryan,A. M.Wright, https://doi.org/10.1117/12.2312150, 2018; https://ui.adsabs.harvard.edu/abs/2018Galax...6...30F.

[10] The PoGO+ Ballon-Borne Hard X-ray Polarimetry Mission, Friis, M. and Kiss, M. andMikhalev, V. and Pearce, M. and Takahashi, H., arxiv:1803.02106, 2018.

[11] The continued development of a low energy Compton imager for GRB polarization studies,Mark L. McConnell,Peter F. Bloser,Jason S. Legere,James M. Ryan,Lorraine Hanlon,SheilaMcBreen,Alexey Uliyanov, Proceedings Volume 10699, Space Telescopes and Instrumen-tation 2018: Ultraviolet to Gamma Ray; SPIE Astronomical Telescopes + Instrumentation,2018, Austin, Texas, United States, 2018.

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[12] Results From the Antarctic Balloon Flight of the Hard X-ray Polarimeter X-Calibur, Kislat,F. and Abarr, Q. and Bose, R. and Braun, D. and de Geronimo, G. and Dowkontt, P. and Er-rando, M. and Gadson, T. A. and Guarino, V. and Heatwole, S. E. and Iyer, N. and Kiss, M.and Kitaguchi, T. and Krawczynski, H. and Kushwah, R. and Lanzi, J. and Li, S. and Lisalda,L. and Okajima, T. and Pearce, M. and Peterson, Z. and Rauch, B. and Stuchlik, D. and Taka-hashi, H. and Tang, J. and Uchida, N. and West, A. ,American Astronomical Society MeetingAbstracts,2019, https://ui.adsabs.harvard.edu/abs/2019AAS...23421508K

[13] Committee on NASA’s suborbital research capabilities, N. R. C. (2010). Revitalizing NASA’sSuborbital Program: Advancing Science, Driving Innovation, and Developing a Workforce.The National Academies Press, Washington, D. C.

[14] Anderson, K. A., and Milton, D. W. ( 1964), Balloon observations of X rays in theauroral zone: 3. High time resolution studies, J. Geophys. Res., 69( 21), 4457-4479,doi:10.1029/JZ069i021p04457.

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