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The Juno Mission to Jupiter – A Pre-Launch Update

Rick Nybakken Jet Propulsion Laboratory, California Institute of Technology

4800 Oak Grove Drive, M/S 321-360 Pasadena, CA, 91011

818-354-6672 nybakken@jpl.nasa.gov

Abstract—Juno, the second mission within the New Frontiers Program, is a Jupiter polar orbiter mission that will return high-priority science data relevant to multiple divisions within NASA’s Science Mission Directorate.12 Juno is currently in system integration/test in Denver and ships to Astrotech in Titusville, Florida in April 2011 to conduct final checkout in preparation for integration with the launch vehicle and subsequent launch in August 2011.

This paper builds upon the Juno mission overview paper published after the Project PDR (2009 IEEE Aerospace Conference – paper #1582 that provided an comprehensive description of Juno’s science objectives, and the instrument payload, spacecraft design including solar arrays, radiation vault, stellar reference unit, and the mission/operations design following arrival at Jupiter) and reviews Juno’s current project status, provides a description of the Juno mission through Jupiter arrival, summarizes mission and spacecraft design changes that have occurred since the Juno Project level PDR and discusses some of the technical and management challenges that the Juno team has encountered in keeping Juno successfully on track for launch in August.

TABLE OF CONTENTS

1. INTRODUCTION .................................................................1 2. HISTORY ............................................................................2 3. MISSION OVERVIEW & CHANGES SINCE PDR ....................2 4. SPACECRAFT OVERVIEW & CHANGES SINCE PDR ............3 6. TECHNICAL & MANAGEMENT CHALLENGES ...................7 7. CONCLUSION .....................................................................7 ACKNOWLEDGEMENT ..........................................................8 BIOGRAPHY ..........................................................................8

1978-1-4244-7351-9/11/$26.00 ©2011 IEEE. 2 IEEEAC paper #1179, Version 8, Updated January 12, 2011

1. INTRODUCTION

The Juno Mission to Jupiter (Figure 1) will launch in August 2011, perform two deep-space maneuvers approximately 13 months after launch, perform an Earth-gravity assist approximately 26 months after launch, and achieve Jupiter orbit insertion (JOI) in 2016 after a five-year journey (Figure 2). Juno’s scientific objectives remain unchanged since the Juno baseline requirements were established in 2008 (and covered in the 2009 paper) and to meet these objectives, Juno is carrying nine instruments with 29 instrument sensors distributed on the forward and aft decks of the spacecraft. Eight of these are science instruments and the ninth is a four-color camera being flown for education/public outreach purposes. In addition to the complexities involved in developing and delivering one Education/Public Outreach (E/PO) and eight science instruments, Juno is also the first solar powered mission to Jupiter. To ensure that the mission will have adequate power while in orbit around the gas giant, the team completed extensive solar cell performance analysis, testing and modeling to characterize the solar array performance under low-intensity, low-temperature (LILT) conditions in Jupiter’s radiation environment. Juno also completed extensive analysis and test to quantify design changes needed to allow both electronics and materials to perform as required in Jupiter’s charged particle and magnetic field environments. Some of the challenges involved in the above are described later in this paper.

A brief history of the project to date will be provided

Figure 1 – Juno looks deep inside Jupiter to unlock

the secrets of solar system formation.

Figure 2 – Juno in orbit around Jupiter.

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followed by brief descriptions of the Juno mission and spacecraft. Then, the changes in the mission design will be discussed followed by a discussion of spacecraft design changes. Finally, a summary is included of some of the more interesting technical and management challenges that the Juno team has faced in keeping the project on track for the August 2011 launch.

2. HISTORY

Juno was selected in May 2005 as the 2nd mission in NASA’s New Frontiers Program within NASA’s Science Mission Directorate (SMD). Juno was originally scheduled for a June 2009 launch opportunity but was subsequently slipped to a July 2010 launch (upon selection) and, ultimately, an August 2011 launch (within the 1st year following selection) due to factors not specifically related to Juno itself. This resulted in an extended Phase B period (almost 3 years) that allowed the project to dedicate much more effort to requirements and preliminary design maturation, as well as to focus on key risk reduction activities. As a result, Juno had a very successful Preliminary Design Review (PDR) and obtained authorization from NASA Headquarters to proceed into Phase C effective September 1, 2008.

Since the PDR, Juno has successfully completed the Critical Design Review (CDR) and the System Integration Review (SIR) and obtained authorization (soft gate) from NASA Headquarters to proceed into Phase D (also known as Assembly, Test, and Launch Operations - ATLO). Juno

started ATLO (on schedule) on April 1, 2010 and has subsequently integrated and tested most of the avionics and instruments. As of January 2011, Juno is on track to start system acoustic and thermal vacuum testing leading to the Pre-Ship Review (PSR) in March 2011. In April 2011, Juno will ship to the Astrotech (Titusville, FL) facility just outside Kennedy Space Center (KSC) to begin final integration activities leading to launch in August 2011. For reference, the project Phase B/C/D schedule is shown in Figure 3.

3. MISSION OVERVIEW & CHANGES SINCE PDR

This section will address the key characteristics of the Juno launch, cruise and JOI mission design and a description of all mission changes since the Juno PDR.

Juno launches in August 2011 on an Atlas V 551 (with a 5m short payload fairing) with arrival at Jupiter after a 5-year cruise (the mission trajectory is shown in Figure 4). Juno uses an Earth Gravity Assist (EGA) trajectory (technically a ∆V-EGA 2+ trajectory) to reach Jupiter in 59 months. Arrival at Jupiter in July 2016 is before the September 2016 solar conjunction with the Period Reduction Maneuver (PRM) occurring after conjunction. It is pertinent to note that all the science orbits, the de-orbit and Jupiter impact are also completed before the next solar conjunction in October 2017.

Juno’s launch period is 22 days (opening on 8-5-11 and closing on 8-26-11) and it has been optimized not only to

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RFIS 4/6/10 Done TLGA 7/30/10 Done X-Band MGA 7/30/10 Done HGA 10/5/10 Done LGAs 7/30/10 Done

Figure 3 – Juno Phase B/C/D Project Schedule.

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maximize the injected mass into the Jupiter science orbits, but also to provide enough contingency days for the deep space maneuvers (DSM) throughout the launch period. The launch window ranges from 30 to 90 minutes throughout the launch period. Juno is also performing a study to determine conditions under which the launch period might be lengthened 2-3 days (a critical consideration for a planetary launch).

Ground station coverage during launch is provided primarily by the Deep Space Network (DSN) station at Canberra with additional coverage by European Space Agency (ESA) antennas at Perth and New Norcia in western Australia. Prior to separation, the communications system is set to standby and the spacecraft is spun up to 1.4 rpm. Following separation (SEP) from the launch vehicle, the solar arrays are deployed at SEP + 3.5 minutes. This timing provides a good balance between telemetry acquisition priorities and staying well within solar array deployment torque margins that were established in ground thermal test. The deployments of the three solar array wings are staggered by 1 second with nominal deployment for all three wings complete within 2 minutes. Instrument low voltage checkouts are scheduled to begin after the first trajectory correction maneuver (TCM) nominally at L+20 days while instrument high voltage checkouts occur after L+90 days.

Two DSMs are planned 13 months after launch to establish the coarse targeting for the Earth Flyby (EFB). Of the thirteen TCMs planned, the DSMs are the only ones with a significant deterministic component. A side benefit of the DSM’s is that they are also used as rehearsals for the JOI main engine (ME) burn including use of the toroidal low gain antenna (TLGA), communication tones, and dual 70-m DSN station coverage.

The EFB occurs 26 months after launch and it not only provides the critical gravity assist but also introduces an opportunity for a rehearsal of a Microwave Radiometer (MWR) orbit and perijove science pass. The MWR receivers however will not be operating due to concerns about potential damage from Earth based transmissions. Operational scenarios for the other instruments are being examined subject to temperature constraints for electronics inside the vault as well as overall power constraints. Both the Earth and the Moon are candidate observational targets during the EFB and approach period.

JOI, the 2nd critical event of the mission, occurs 59 months after launch (the 1st critical event of the mission), and is executed via a ME burn of approximately 30 minutes duration. Communications coverage is via tones using the TLGA due to burn vector constraints. This burn, performed with Juno spun up to 5 rpm, allows the spacecraft to be captured into a 107 day capture orbit about Jupiter and establishes the orbital geometry for Juno’s science orbits. This long capture orbit provided a substantial ∆V savings (compared to a direct JOI entry into an 11-day orbit) and creates an opportunity to gain valuable early orbital operations experience for both the spacecraft and the instruments. To decrease the orbital period to that needed for the science orbits, a PRM burn of 37 minutes is planned to establish the 10.9725 day science orbit duration that creates the specific perigee timing to coincide with the Goldstone DSN passes (critical because of the Ka-Band coverage needed for the gravity science passes).

Orbits 1-2 contain orbital trim maneuvers (OTM), a PRM clean-up maneuver and final instrument checkouts to ensure readiness for the upcoming science orbits. Orbits 3-33 are the science orbits and are broken up into five MWR orbits (orbits 3 and 5 through 8), 25 Gravity Science orbits and 1 spare science orbit. Orbit 34 contains the final maneuver of the mission – a de-orbit burn which ultimately sends Juno into Jupiter for planetary protection purposes.

Key mission design changes since PDR are listed in Table 1. The rationale and benefits of each change are included.

4. SPACECRAFT OVERVIEW & CHANGES SINCE

PDR

This section will review how key aspects of the spacecraft design have changed since PDR. A summary of these changes is listed in Table 2 and specifics associated with

Figure 4 – Juno’s EGA Trajectory for August 2011 Launch.

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vault and solar array design changes are further discussed below. Figure 5 contains a rendering of Juno in the cruise/science configuration while Figure 6 illustrates each of Juno’s 9 instruments and their location on the spacecraft.

Figures 7 and 8 are photos showing the partially integrated Juno spacecraft bus and solar array wing #1 (with the magnetometer instrument boom) in assembly.

Vault Design Changes

Several detailed radiation models were run taking maximum advantage of the shielding provided by adjacent boxes in the vault and more refined models of the spacecraft structure – this allowed the thickness of the vault walls to be individually optimized for the required radiation shielding while realizing some helpful mass reductions (vault mass was reduced from approximately 180 kg at PDR to 157 kg). Also, the vault wall material was changed to titanium (from tantalum face-sheet over honeycomb core) to improve manufacturability and simplify any last minute design changes as the detailed design was completed. Electronics in the vault are exposed to total ionizing dose (TID) levels of ≤ 25 krads vs. as much as 300 krads on the upper deck (behind 100 mils of aluminum) or 50 Mrads on the

(unshielded) solar cell cover glass.

Solar Array Design Changes

The three solar array wings have been enlarged since PDR with the surface area now approximately 60 m2 (50 m2 active). The re-sizing (with associated mass impacts) was necessary because of updates in the power model from the 2008 cell radiation/LILT test cycle, maturation of loss factors used in the power model, finalization of the string/circuit designs for each array, and the decision at CDR to include an additional 9% margin (above required design margins) in the power model to account for any changes that might occur due to the 2009-2010 cell radiation/LILT test cycle and measurements from the flight panels themselves (all now completed). The arrays, which contain a total of over 18,000 cells, will produce > 400W end-of-mission at Jupiter (where the sun intensity is approximately 8% of that received at Mars and 4% of that received at Earth).

Table 2. Key Spacecraft Design Changes made since PDR

Table 1. Juno Mission Design Changes Since PDR

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Figure 5 – Rendering of Juno Spacecraft During Cruise

Figure 6 – Juno Science and E/PO Instrument Suite

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Figure 8 – Solar Array Wing 1 (with Magnetometer Boom) during Assembly in Denver

Figure 7 – Juno Spacecraft Bus in Assembly in Denver

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6. TECHNICAL & MANAGEMENT CHALLENGES

In this section, the project’s experience with a number of technical and management challenges is discussed. Many of these were identified early on Juno as shown by one of the early lists of challenges presented during Phase B in 2007.

- First use of solar power at Jupiter - 6-year mission life, with one year operations at

Jupiter - Radiation environment - Magnetics cleanliness requirements - Electronics vault - Electromagnetic Interference/Electromagnetic

Compatibility (EMI/EMC) environment - Large number of payload instruments - High random vibration environment due to Launch

Vehicle (LV) acoustics - Stellar Reference Units that will meet mission

needs - Command and data handling (C&DH) subsystem

modifications - Maintaining as much spacecraft and instrument

inheritance as possible - Large and widely dispersed project teams - Project execution within cost constraints

Tables 3 and 4 list some of the key technical and

management problems encountered on Juno. The project’s approach to mitigate is discussed for each challenge listed and the project’s assessment is listed for each major challenge and/or problem encountered. Many of the key challenges received a lot of focused and consistent technical/management attention from the beginning of the project and this had a significant effect on the outcome. For instance, the stellar reference unit (SRU) was clearly identified as the top risk in the Juno Phase Concept Study Report (March 2005) and although it was listed as a Project Manager (PM) top concern at the PDR, it was not at the Project CDR due to the effectiveness of the mitigation activities. However, despite significant attention from the project (and the extended Phase B), some of the issues commonly seen on flight projects also were encountered on Juno (i.e. late avionics and instrument deliveries).

7. CONCLUSION

The Juno mission has successfully progressed through the critical design phase, has completed flight system integration, and is preparing for system level acoustic and thermal vacuum testing. In April 2011, Juno will ship from the flight system integrator (Lockheed Martin-Denver) to the Cape to complete final preparations for launch in August 2011. The history of the project since the Project PDR is briefly described including a list of spacecraft and

Table 3 – List of Key Technical Challenges and Problems Encountered on the Juno Project

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mission design updates. In addition, the project’s experience with key technical and management challenges/problems encountered is described.

Having successfully dealt with many anticipated (as well as unexpected) challenges, the project is on track for launch from Cape Canaveral Air Force Station in August 2011.

ACKNOWLEDGEMENT

This research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.

BIOGRAPHY

Rick Nybakken is the Deputy Project Manager for the Juno mission at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California. He has worked on Juno since February 2006 when Juno was still scheduled for a 2010 launch.

Prior to this position, Mr. Nybakken has served JPL in numerous capacities

including Deputy Manager of JPL’s Mission Assurance Division, Technical Manager for the Mars Reconnaissance Orbiter Flight System Contract, Task Manager for Advanced Transponder Development, Project Element Manager for the SeaWinds Electronics Subsystem, Project Element Manager for the QuikScat Radar Electronics Subsystem, and Technical Manager for the Cassini Antenna Subsystem.

Mr. Nybakken received his B.S degree from California Polytechnic State University. He has been awarded the NASA Exceptional Achievement Medal for his work on QuikScat and received numerous NASA Group Achievement Awards.

Table 4 – List of Key Management Challenges and Problems Encountered on the Juno Project