Antarctic long-duration balloon flights: JACEE experience

Adv. Space Res. Vol. 21, No. I. pp. 959-961, 1998 Q1998 COSPAR. Publishedby Elscvicr Science Ltd. All rights reserved Printedin Great Britain 0273-l 177/98 $19.00 + 0.00 PII: SO273-1177(97)01080-6

ANTARCTIC LONG-DURATION JACEE EXPERIENCE

BALLOON FLIGHTS:

R. Jeffrey Wilkes Department of Physics, Box 351560, University of Washington, Seattle, WA 98195, U.S.A.

ABSTRACT JACEE has had four successful long-duration balloon flights (LDBF) in Antarctica, in five atteqts. The latest flight, in December, 1995, provided 348 hours of exposure at approximately 5 g&m* residual overh&ca~, for an array of 6 emulsion chamber modules with ovea=aIlarea 100 x 120 cm*. Antarctic LDBF q have reached a degree of maturity which pezmits great advances in the rate of data aquisitim We will describe the gondola, data-logging onboard electronics, preparation and fIight procedures, and special aspects of fight operations in the Antarctic. 91998 COSPAR. Published by Elsevier Science Ltd. INTRODUCTION The JACEE (JapaneseAmerican Cosmic ray Emulsion chamber Expczimcnt) Collabcraticm was formed in 1979 to investigate primary cosmic ray compc&ion and spectra at the highest accessible enagias (Burnett 83, Asakimor 95, Cherry 95). Above the “knee” at lO”eV, all ~oftbeprimarycosmicrayspecavm have been made indirectly with ground-based &ectors. In order to directly observe cosmic ray fluxes at energies approaching the knee, exposure factors of m*-years are required. Detectors of large collecting arty weighing 1 ton or more, must he flown for hundreds of hours at altitudes exceeding 35 km (5 g/cm*), where the overburden is less than a typical mean free path for heavy nuclei. These requirements can be met at relatively low cost using large volume (up to 1 million m3) zero-pressure balloons. The cost of a balloon flight can be estimated most conservatively by simply dividing the US National Scientific Balloon Facility (NSBF) budge$ by the average number of flights per year: US$5OOK. The actual marginal cost per launch for expendables is closer to US$ lOOK.These figures are less by orders of magnitude than any orbital spacecraft program’s per-mission costs. Yet for many research areas, balloon flights are equivalent in value to space flight. Contempomry scientific ballooning technique reliably deli= flight durations over 200 hours, at nearly constant altitudes of up to 40 km (3 g/cm* overburden). Perforrmmce improvmts are due to a number of factors: enhanced quality control in ballmn production, more sophisticated onboard control and monitoring electronics, and availability of satellite commum‘cations and GPS navigation services. While mid-latitude flight paths are desirable for many applications due to their higher geomagnetic cutoff, polar routes eliminate many vexing diplomatic and logistical problems experienced in midlatitude operations during the mid-80s. JACEE detectors consist of one to six emulsion chamber (Strausz 83) rr&tles. Figure 1 shows a typical structure, used for the JACEElO Antarctic flight. Each module is a stack of nuclear emulsion plates, x-ray films, &&able plastic (e.g., CR-39) detector plates, and Pb sheets. The latter are used to compose an electromagnetic calorimeter which provides an estimate of particle energy; the emulsions and CR-39 layers provide charge identification. Methods for analysis of the data are described in (JACEE 83). Each module typically has mass about 110 kg, including its protective box, and external dimensions about 45 x 55 x 20 cm

R. J. Wilkes

PRIMARY ID SECTION Emulsion plates and CR39 TARGET SECTION 5 cycles: 0.3mm Pb emulsion 0.5mm spacer emulsion 0.5mm spacer emulsion

CALORIMETER 5 cycles: 1.Omm Pb 2 x-ray films emulsion

SECTION

12 cycles: 2.5mm Pb 2 x-ray films emulsion

ELECTRON

Fig. 1: JACEE- 10 emulsion

GAMMA

chamber

HADRON

structure.

The boxes are provided with lifting straps and can be handles by two men or a small winch. Table 1 shows a list of JACEE balloon flights and detector contents. At the beginning of the JACEE flight program, a long-duration balloon flight was defined as a turnaround flight exceeding 24 hr at float. JACEE has participated in the development of contemporary LDBF, serving as a willing guinea pig for new techniques and flight systems. It is remarkable that, despite acceptance of greater than normal flight and recovery risks, JACEE has lost only one payload in 15 years. (JACEE-6 and JACEE-9 experienced balloon failures at launch, but the payloads were promptly recovered and reflown.) LDBF IN ANTARCTICA Antarctica offers unique advantages as a site for LDBF (Wilkes 89). The massive Antarctic research effort operated by the US National Science Foundation (NSF) provides complete logistical support, including

Antarctic LDBF

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Table 1. JACEE balloon flights Flight

Date

Launch Site

.J ACEE-0 JACEE- 1 JACEE-2 JACEE-3 JACEE-4 JACEE-5 JACEE-6 JACEE-7 J ACEE-8 J ACEE-9 JACEE-10 JACEE-ll= JACEE-12 JACEF-13 J ACEE- 14

5179 9179 10180 6182 9183 10184 5186 l/87 2188 1o/90 12190 12193 l/94 12194 12195

Sanriku, Japan Palestine, Texas Palestine, Texas Greenville, S.C. Palestine, Texas Palestine, Texas Palestine, Texas Alice Spr., Australia Alice Spr., Australia Ft. Sumner, N.M. McMurdo, Antarctica McMurdo, Antarctica McMurdo, Antarctica McMurdo, Antarctica McMurdo, Antarctica

Altitude (mbar)

Duration (hours)

8.0 3.7 4.0 5.0 5.0 5.0 4.0 5.5 5.0 1.0 3.5 5.5 5.5 5.5 5.5

29.0 25.2 29.6 39.0 59.5 15.0 30.0 150.0 120.0 44.0 ‘04.0 714 209.0 303.0 348. I

Units(Area) (cm?)_. 1 (40x50, 4 (40x50) 4 (40X50) 1 (50x50‘) 4 (40x50) 4 (40x50) 4 (40x50) 3 (40x50) 3 (40x50) 4 (40x!%) 1(30X40) 6 (40x50) 6 (40x50 ) 6 (40x50) 6 (40x:50,

” (not recovered -- 1000 ft deep off Ross Island! )

transportation, housing, supplies and heavy equipment. As an international zone under the Antarctic Treaty, balloons can be launched, flown and recovered anywhere on the continent without diplomatic complications (the main obstacle to mid-latitude and Arctic flights). Flight paths are typically 90% over land, and most of the terrain permits landing of recovery aircraft, at least in principle. Figure 2 shows the flight path for J ACEE- I3, which was typical. Continuous sunlight during the Austral summer (the only season when LDBF launches can take place) reduces the diurnal sunrise/sunset altitude lluctuations typical of flights in lower latitudes. Antarctic flights thus require minimal ballasting (for example, essentially none was used in recent JACEE flights except to test the ballasting system), leaving the maxima1 weight for the scientific payload. Flight operations are conducted from Williams Field, near McMurdo Station, by a fully-equipped field team from NSBF, which supplies experimenters with a standard Science Interface Package (SIP) (Jones 89). The SIP provides telemetry and telecommand links to NSBF flight equipment, such as tracking systems, ballast boxes and housekeeping sensors, as well as providing a data and command interface to the user’s gondola. The user channels only permit 19.2 kbd data telemetry, however. Experiments like JACEE, in which data are passively recorded and telemetry requirements are minimal, arc ideal for Antarctic flights. Detectors with higher data-rate requirements must provide onboard data logging capability. Aircraft underfights can be arranged to provide data downloads at line-of-sight communications rates. A fully operational gondola must be delivered to the balloon facility in Texas by July, for flight the following December, to allow time for testing and integration of user and NSBF equipment. Sea shipments to New Zealand depart from Pt. Hueneme, California in late August. Small, lightweight, or time-sensitive components can be sent by air later if required. JACEE emulsion chambers are time-integrating track-sensitive detectors and so were assembled as late as possible (mid-October) and shipped to Christchurch, New Zealand by commercial air freight in early November. NSF maintains a logistical base at Christchurch from which US military aircraft transport personnel and equipment to McMurdo Station. Members of the field team must pass rigorous medical examinations, and are provided with all necessary special clothing and personal equipment while in transit at Christchurch.

962

R. J. Wilkes

Fig. 2: JACEE- 14 flight path (counterclockwise

on this map).

During Austral mid-summer, stratospheric flow in the polar region is predominantly along latitudinal lines, making it likely that the payload will return within easy reach of McMurdo Station after its trip around the South Pole. Polar stratospheric winds typically do not stabilize in this pattern until about the first week of December, which has been the earliest date for launches. Wind patterns are tested by tracking small “Pathfinder” balloons before scientific payloads are flown. Once launched, balloons have typically taken 200 to 350 hours to circumnavigate. Stratospheric winds slow down as the season progresses, so flights launched in late December or early January take longer to circumnavigate than earlier launches. The season for Antarctic aircraft activity runs from October through February. In early February, seasonal shutdown operations are underway, and it becomes difficult to book aircraft, which are busy returning field teams from remote sites, so balloon launches are not attempted after mid-January. By early February, the ice shelf has retreated sufficiently to allow ships bringing in heavy equipment for the next season (including truckloads of helium for the following year’s balloon flights) to reach McMurdo Station, In practice, then, the Antarctic balloon launch time window is about six weeks starting in early December. Normally there are only brief delays due to surface weather; launch delays thus far have more often been due to problems with experimenters’ equipment rather than weather.

Following launch and successful circumnavigation, the flight is terminated by radio command while within lineof-sight of a control station (which can be put aboard an aircraft, and thus operated virtually anywhere on the continent). Normally it is terminated when it has returned to within about 100 km of McMurdo, the maximum range for helicopters. Helicopter recovery is most convenient since they can land in terrain unsuitable for tixed wing aircraft. Helicopters were used for JACEE-10 and JACEE- 13, in the latter case having their range extended by previously established fuel caches (thanks to the Italian Antarctic program). For longer distances, Twin Otter aircraft can be used for smaller payloads (as with JACEE-I 2). For distances beyonld a few hundred km, or in cases where bulky gondolas cannot be broken down in the field, large LC-130 cargo aircraft must be used. Suitable landing areas for such large aircraft may not exist nearby, so it is useful to have a modular payload (like JACEE) that can be recovered in multiple helicopter or Twin Otter sorties. JACEE EXPERLENCE IN ANTARCTICA JACEE has had four successful circumpolar balloon flights in Antarctica. As shown in Table 1 and Figure 3, these flights already account for more than half of JACEE’s net exposure to date. The first Antarctic flight, JACEE-10 in 1990, was a test flight in which gondola space was shared with three other projects (Olson 95a, b). Two small emulsion chambers were flown and the data collected have already been analyn.ed and added to the cumulative database. In 1993, we had an exceptional opportunity: when two other projwts originally scheduled for flight were unable to prepare their payloads in time, the JACEE collaboration provided two complete gondolas equipped with six full-sized emulsion chambers each. The first payload, JACEE- 11, was lost in the Ross Sea after an otherwise perfect flight, due to mechanical failure in the termination mechanism, which cuts the payload and its parachute from the balloon upon command from the ground station. A thorough investigation by NASA revealed that parts used in the mechanism had not been machined to design specifications, and steps have been taken to prevent a recurrence. This was the only payload totally lost in ten Antarctic flights thus far. Our second 1993 flight, JACEE-12, was successful in all respects, and preliminary results from these emulsion chambers will be published soon. A third successful flight was performed in 1994, and another in 1995. Figure 4 compares the altitude profiles of an Antarctic flight (JACEE-10) and a tnidlatitude flight (JACEE-7). Note that the strong diurnal variation in altitude (even with the modest ballasting capability provided) for JACEE-8 is absent in the Antarctic flight: the small daily variation in <;olar altitude is reflected in a balloon altitude range of about 1 mbar.

JACEE Flights: Fraction of Exposure Factor

11% Mid-Latitude

1 q JACEE-IO->14 n JAC!BE-l->&9

Antarctica

Fig. 3: J.4CEE

net exposure

factor (m’-sr-set)

vs flight location.

-’

R. J. Wilkes

964

0

so

150

100

200

250

Tlh& HR

Fig. 4: JACEE-10

pressure

altitude

profile.

JACEE-11 and all subsequent flights used similar gondola designs (Figure 5). A simple, lightweight and very rugged framework provides protection for the emulsion chamber modules in case of an awkward landing or parachute drag. We provided our own onboard data logging computer to record pressure altitude and module temperatures, as well as to control a shifter mechanism (on JACEE-11 and 12) which moved upper emulsion layers out of registration when the gondola altitude was below 6 mbar. The computer system was an adaptation of electronics originally designed for DUMAND; a block diagram is shown in Figure 6 The JACEE data logger was interfaced to the NSBF SIP, allowing status monitoring and data downloading during flight. Antarctic flights differ from flights at other latitudes in two respects: the sun never sets (although its altitude varies a few degrees during the day), and the underlying terrain is snow-covered, providing substantial albedo. Thus thermal protection for temperature sensitive payload components is extremely important. Payload temperatures will tend to rise continuously, and can exceed typical electronics components’ temperature rating, as happened on the earliest flights. Figure 7 shows temperature records for sensors located at the bottom of an emulsion chamber modules in each of the four JACEE flights. No active cooling or radiators were employed; instead, passive means (paint and insulation) were used (Parnell 95). All parts of the gondola were painted white, the emulsion chamber section of the gondola was covered with silver&d Teflon and Tedlar insulation, and a barrier was provided between the chamber modules and the SIP below (a major source of heat due to electronics). Improvements were made each season, and as the Figure shows, by JACEE-14 the temperature graph showed a general tendency downward instead of upward. An important feature of JACEE is the modularity of the detector, and the expendability of the gondola. The gondola is relatively easy to disassemble in the filed (where typically one must work in severe temperatures, at a high equivalent altitude). In fact, we have rebuilt the gondola for each flight. Its design has been coded for numerically controlled machine tools and it can be reproduced quickly and relatively cheaply in our machine shop. CONCLUSIONS Antarctic flight opportunities have been very productive for the JACEE Collaboration. Flight times are very near ideal for our detectors, flight risk has turned out to be quite acceptable, and despite our initial misgivings, the rather heavy soft-particle background in our time-integrating detectors has been manageable. The main

Antarcttc LDBF

I I

EMC

EMC

I I

1

965

SIDE VIEW

I

3

TOP VIEW

5

( SHIFIER

CENTERED )

6

3

c

J ACEE GONDOLA

SHOWTNG SHIFIER

Fig. 5: Schematsc view of JACEE Antarctic gondola, showing shifter tray.

TEMPERATURE SENSOR

JACEE FLIGHT RECORDER

_----------_~

r-v I

(24 TOTAL)sip/+

] HF/Pl-T I

ANALOG BOARD

FAN- INS

CPU BOAR

Es@ ,

I \

I-

PRESS&hE SENSOR

I

t-____

I

I

Hiig

. . -I--..._

LAPTOPPC

___-.___-I

t SHIFTER MOTOR CONTROL

Fig. 6: JACEE onboard flight data recorder.

SIP

~grokd~bnly)

oc

-25

-20

5

10

15

0

I

50

I

100

I

150 200 Flight Time (hours)

I

JACEE Antarctic Temperature I

Profiles

250

I

300

JAC;E-11 JACEE-13

----.

350

attractions are the lack of diurnal altitude variation, resulting in greater payload capacity, and ehminating the need for a shifter mechanism, and the Pact that all logistical arrangements are made by professionals in the NSF/Antarctic Program. Results from these flights are already being produced (Cherry 95 1, From a scientific standpoint, our ideal flight path would be mid-latitude, especially the Alice Sprmgs route. Assuming balloon reliability can be maintainerl at the levels demonstrated in Antarctica, it should be possible IO circumnavigate the globe at 23 deg S, eliminating the duplicate logistical and diplomatic requirements of the Australia to South America route used previously. (Of course, one great advantage to Antarcti.ca. from the experimenters’ point of view, is that all such requirements arc provided and paid for by NSF/Polar Programs). If’ current NSBF developments in OZP balloons and autoballasting schemes arc successful. il should then be possible to provide adequate altitude stability (comparable to Antarctica) via ballasting. Al present there is community consensus [hat the greatest danger trj the US LDBF capability IS the lack ~)1 support for payload development. It would be truly ironic if, having painfully and slowly dcveioped this beautiful capability for providing long-duration, heavy-payload. ultra-high altitude Ilights, Iqulvalent 10 spaceflight for many applications. we were to lose it through lack of’ demand artificially induced by drying up the source. All of us with experience in the field know that ballooning is as much a craft as a science., and loss of experienced personnel through lack of funds or repeated frustration will have a devastating and probably fatal effecl on our capabilities in future. Scientific ballooning has the peculiar feature that the payloads typically far exceed the vehicle in cost. ctontrary to the customary situation with orbital flights. ‘There is a natural tendency to make program support proportional lo vchiclc cost. which is quite mapprcipri;itc in this case. and must he resisted.

ACKNOWLEDGMENTS Our experience with the NSBF field teams has been thoroughly satisfactory, and we extend our gratitude to all team members, particularly Mr. Steven Peterzen, whose personal exertions were in large part responsible for the success of our flights. We also wish to extend our thanks to the US NSF Office of Polar Progratns, and cspccially Dr. John Lynch, whose support and encouragement has been crucial to our work. Work described here was supported by NSF grant OPP-9220316. Tireless effort by W. Vernon Jones and his NASA colleagues on behalf of the LDBF program has been extremely valuable. This paper was prepared on behalf I):’ the entire JACEE Collaboration (,see (Asakimori OS) for a complete list), but any mistakes or omissions are my responsibility alone. I wish to thank Eric Zager. our most expcrienccd Antarctic field team member, !Or providing most of the ligures. REFEKEXES Asakimori. T., T. Burnett, M. Cherry, K. Chcvli, M. Christl, et al.. Z-‘roc..XXZVICRC’. vo’l. 2.. p. ?07 ( 1W5). Burnett, T., S. Dake, M. Fuki, J. Gregory, ‘T. Hayashi, P/ ~1.. Nuclear /rr.strumenf.rand Method,;. fG!S1, 583 (1986). Cherry, M., T. Asakimori, T. Burnett, K. Chevli, M. Christl, of ui., Proc-. XXIV ICRC, vol. 2, p. /‘1X (1995). W. V. Jones, Long-Duration Ballooning al Mid-Latitudes and in Antarctica, in A,r/rophy.rw IN Atrtarrtrc~~, eds. D. Mullan, M.Pomerantz and T. Stanev, p. 166, AIP. New York (1989). E. Olson, Proc. XXIV ICRC, vol. 2. p. 752 (1995a). E. Olson, “Results From An Antarctic Balloon Flight”,Phd Thesis, University of Washington ! 19951-t) S. C. Strausz, J. J. Lord, and R. J. Wilkes, Nuclear Tracks 7: ( 1983 ). Wilkes, R. J., T. Burnett, S. Dake, J. Derrickson, W. Fountain, er (11,JACEE Long-Duration Balloon Flights, in Astrophysir.r Itr AN tarctiw, eds. D. Mullan, M.Pomeranrz and ‘T. Stanev, p. 198. AIP. Yrw York (:989).