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AOO-36982

AIAA2WO-3870

Future Application ofEiectrodynamic Tethers tor Propulsion

Andrew SantangeloThe Michigan Technic CorporationHolland, Michigan

Les JohnsonNASA Marshal] Space Flight CenterHuntsville. Al

36th AIA A/ASME/SAE/ASEEJoint Propulsion Conference & Exhibit

16-19 Julv 2000/HuntsvilIe, Alabama

For permission to copy «r ti> rt-pubHsh, ««ita« the American Institute of Aeronautics and AstronauticsI8»l Alexander Bdl Drive, Suite 5W, Hssttm, W. 2«I*> 1-4344,

(c)2000 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization.

Future Application of Electrodynamic Tethers for PropulsionAndrew Santangelo'

The Michigan Technic CorporationHolland, Michigan

Les JohnsonNASA Marshall Space Flight Center

Huntsville, AL v/s>

AbstractA key goal of space exploration is to improve the specific Impulse and mass fraction forthe orbit transfer of satellites. One of the aspirations of the space tether community is todevelop an Electrodynamic (ED) tether facility to place satellites in orbit, change theirorbit and/or remove them from orbit without the use of chemical propellants. In thispaper we will discuss applications of ED tethers including Orbit Transfer Vehicles,satellite deorbit, reboost of the ISS, and missions to Jupiter. Two missions are discussed,which when successfully completed will signify the start of commercial application of thisinnovative method of transportation.

I. Introduction

Since the 1960's there have been at least17 tether missions. In the 3990's severalimportant milstones were reached,including the retrieval of a tether inspace (TSS-1, 1992), successfuldeployment of a 20 km long tether inspace (SEDS-1, 1993), and operation ofan electrodynamic tether with tethercurrent driven in both directions —power and thrust modes (PMG, 1993).Future tether mission include ProSEDS,an ED Tether Propulsion experiment andthe STEP-AIRSEDS mission, which willdemonstrate ED propulsion in spaceover a one year period, including boost,

deboost, station keeping and planechanges. From theoretical analyses andpreliminary plasma chamber tests, baretethers (exposed metal wire tethers)appear to be very effective anodes forcollecting electrons from the ionosphereand, consequently, attaining highcurrents with relatively short tetherlengths. A predominantly unisulatedconducting tether, terminated at one endwith a plasma contactor, can be used asan electromagnetic thruster. Apropulsive force of F = IL x B isgenerated on a spacecraft/tether when acurrent, I, from electrons collected in aspace plasma, flows down a tether oflength, L, due to the emf induced in it by

1 President/CEO, Professional Member, Chairman of the AIAA MichiganSectionCopyright © 2000 The American Institute of Aeronauticsand Astronautics, Inc. All rights reserved.

(c)2000 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization.

the geomagnetic field, B. Preliminarytests indicate that a thin unisuiated wirecould be 40 times more efficient as acollector vs. the previous system used(TSS-1).

In this paper we will discuss applicationsof ED Tethers. Two missions are alsocovered in this paper, ProSEDS andSTEP-AIRSEDS, which whensuccessfully completed will signify thestart of commercial application of thisinnovative method of transportation.

II. Applications of ED Tethers

Electrodynamic tethers have manypotential applications of interest toNASA and the commercial spaceindustry. They range from satellitedeobit and propeilantless orbit transfervehicles to reboost of the InternationalSpace Station and missions to Jupiter.

Orbit Transfer Vehicles (QTV)An electrodynamic tether upper stagecould be used as an orbital tug to movepayloads within low earth orbit (LEO)after insertion. The tug wouldrendezvous with the payload and launchvehicle, dock/grapple the payload andmaneuver it to a new orbital altitude orinclination within LEO without the useof boost propellant. Figure 1 presents anartists conceptual view of an OTV. Thetug could then lower its orbit to

rendezvous with the next payload andrepeat the process. Such a system couldconceivably perform several orbitalmaneuvering assignments withoutresupply, making it low recurring costspace asset. This capability is ofparticular interest within NASA, in thatit may provide a way to allow repeatedaccess to staging orbits for integrationand assembly of human explorationvehicles for missions beyond LEO.

The performance of a 10 kW, 10 kmtether system for altitude changes isillustrated in Figure 2, The same systemcan be used to change the orbitalinclination of a payload as well. Figure3 can be used to determine the availableinclination change for a particularspacecraft and payload mass by dividingthe 'specific inclination rate' indicatedby the total system mass as a givenaltitude.

Figure 1. The OTV at work

(c)2000 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization.

..„0.9 -

-0.8 -

Q-2 -

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:rJpigE• I "':""'-•-•:--.

I ' I • I ' I ' I ' I ' I 'X) 400 600 800 1000 1200 1400

Altitude (km)

Figure 2. The performance of an electrodynamic tether thruster varies with altitude in theionosphere (where i is the orbital inclination).

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|240:f200 -

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it eo -

1 120 -

| 80 -

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400 600 800 1000Altitude (km)

1200 1400

Figure 3. The performance of an electrodynamic tethers thruster for inclination changeapplications depends strongly on the initial orbital inclination (U).

Satellite DeorbitWith the flight of the Propulsive SmallExpendable Deployer (ProSEDS)experiment in late 2000, the capability todeorbit payioads with a passiveelectrodynamic tether will bedemonstrated. A ProSEDS-derivedsystem could be used operationally toextend the capability of existing launchsystems by providing a propeliantless

system for deorbiting spent stages orsatellites that have reached the end oftheir orbital life. For the former case, thelaunch service provider need not carryadditional fuel for the soon-to-be-required deorbit maneuver, thusallowing all the onboard fuel to be usedfor increasing the vehicle's performance.Similarly, satellites thus equipped couldsafely deorbit at their end of life without

(c)2000 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization.

using precious onboard propellant. Bothof these applications would heip reducethe increasing threat posed by orbitaldebris.

Reboost of the International SpaceStationAn electrodynamic tether system couldbe used on the International SpaceStation (ISS) to supply a reboost thrustof 0.5-0.8N, thus saving up to 6000kg ofpropellant per year. The reduction ofpropellant needed to reboost the ISSequates to a $1-2B savings over it's 10year lifetime. Other advantages of usingthe electrodynamic tether on ISS are thatthe microgravity environment ismaintained and external contaminantsare reduced.

The value in an electrodynamictether reboost system lies in its ability tocouple power generation with thrust.Heretofore the electrical and propulsionsystems have been effectively totallyseparate entities. Outfitting ISS with anelectrodynamic reboost tether severs themost critical and constrainingdependency on Earth - propellantresupply. The Station can supply itsown power but not its own propellant.Without an electrodynamic tether, thespecter of SkyLab and the words"reentry" and "atmospheric burnup" willforever haunt the minds of anyone whohas an interest in the program. Add atether and some additional storagecapacity for supplies, and suddenly a oneyear interval between visits to theStation becomes conceivable.

Even if the current frequency ofresupply flights to the Station ismaintained, with an electrodynamictether the Station Program has the optionto trade kilowatts for increased payloadcapacity. Resupply vehicles can deliver

useful cargo like payloads, replacementparts, and crew supplies rather thanpropellant. Within the range of 5 to 10kW, a crude approximation of 1,000 kgof user payload gained per kW expendedper year appears reasonable.

Station users have been allocated aminimum of 180 days of microgravityper year. Current planning essentiallyhalts science activity during reboostmaneuvers. Low thrust electrodynamictether reboost could be performed overlong duration, as opposed to shortduration, high thrust propulsivemaneuvers. The 0.5 to 0.8 N thrustprovided by a 10km tether more thancounteracts the Station's atmosphericdrag on a daily basis. Recent analysisindicates that an electrodynamic tethercan compensate for the drag while it isoccurring, without disrupting themicrogravity environment.

Jovian MissionsFollowing the successful Galileomission, there is considerable interest ina follow-on mission to Jupiter and itsmoon, Europa. Due to low solarluminosity sun, radioactivethermoelectric generators (RTG) were

Figure 4. Reboost of the ISS using anED Tether.

(c)2000 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization.

used for electrical power by Galileo andin all past deep space missions. Thefinite risk of releasing plutonium into theterrestrial environment may rule outRTGs on future missions. Thepossibility of using solar panels forelectrical power generation hasimproved in recent years, withimprovements in this technology,however, the high levels of radiation inthe Jovian system are expected to rapidlydegrade the effectiveness of solar arrays,as a result of extended exposure.Extended operations in the Joviansystem, or around any planet, alsotypically require use of an expendablepropellant for orbital maneuvering. Thismay lead to high "wet" spacecraft massat launch and/or limited lifetime onorbit. It is for these reasons and becauseof the strong magnetic field and rapidplanetary rotation that electromagnetictethers are being considered for use inthe Jovian magnetosphere. Preliminaryanalysis indicates that a megawatt ofpower can be theoretically generated bya 10 km tether in near Jovian space.Specifically, such a tether operating nearthe planet would experience inducedvoltages greater than 50,000 volts,currents in excess of 20 Amperes,generate approximately 1 megawatt ofpower and experience more than SON ofthrust! Needless to say, this would posesignificant engineering challenges formission planners.

III. Missions

In order to fully develop the systemswhich will be utilized in future EDTether supported missions, two keymissions are required to test,demonstrate, understand and develop EDTether Technologies and methods of

applications. These MissionsProSEDS and STEP-AIRSEDS.

are

A. ProSEDSLate in the winter of 2001 NASA MSFCwill test in space an electrodynamictether which will passively deboost asatellite back to Earth. The ProSEDSmission (Propulsive Small ExpendableDeployer System) will be the firstmission to demonstrate the basic conceptof an electrodynamic tether propulsionsystem. The ProSEDS mission willdeploy 5 km of bare wire plus 10 km ofSpectra tether from a Delta II upperstage to achieve -0.4 N of drag thrust,thus de-orbiting the stage. Theexperiment will use a predominantly'bare' tether for current collection in lieuof the endmass collector and insulatedtether approach utilized on the TSS-1missions. ProSEDS will also utilizetether generated current to providelimited space craft power. Thedemonstration mission will last less thanone week. Figure 6 shows a conceptualdrawing of ProSEDS and diagrams thebasic principle.

Figure 5. An ED Tether equippedsatellite orbiting Jupiter.

(c)2000 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)1 Sponsoring Organization.

long-life tether

Figure 6. Operation of the ProSEDS EDTether System.

B. STEP-AIRSEPSThe STEP-AIRSEDS Mission representsa new and innovative method of spacetransportation — the use of a conductingtether, solar power and the Earth'smagnetic field to move a satellite up,down and change planes without the useof chemical propellants in Low EarthOrbit. The STEP-AIRSEDS mission is ademonstration mission to fully study theperformance and operationcharacteristics of an Electrodynamic(ED) tether propulsion system for arange of flight operations.

The primary objectives of the STEP-AIRSEDS mission are:

(1) Demonstrate the predictableEarth maneuvering capability ofelectrodynamic tethers

(2) Demonstrate electrodynamictether system dynamic stabilityduring nominal tether operation

(3) Demonstratetechnology

(4) Characterize the performanceof electrodynamic propulsionsystems and their operationallimits (dynamic andelectrodynamic)

(5) Demonstrate low poweralternate electron emissiontechnology requiring noconsumables

The STEP-AIRSEDS Satellite consistsof two units tethered together via a 10km Long Life ED Tether. Larger thanProSEDS, the total mass of the satellitewith tether is approximately 1000 kg; thetotal length of each unit is 2 m with a 0.9m diameter. The Upper Unit of STEP-AIRSEDS drives the boost operations ofthe satellite; the Lower Unit drives thedeboost operations. The tether itselfconnecting the units will support up to a1,000 Watt load and a peak current of 10Amps. The tether deployer andseparation systems are located in theUpper Unit. Both units will haveduplicate suites of instrumentation tomonitor and study the local environment,the orientation and position of the endmasses and the overall performance ofthe system. Figure 9 presents an artist'sconceptual design of STEP-AIRSEDSduring deployment operations.

STEP-AIRSEDS InnovationsDue to the "newness" of the program,many innovations will be flown on theSTEP-AIRSEDS mission. Theseinnovations will allow ED TetherPropulsive systems to be commerciallyviable. Listed below are a few of thenew innovations of the STEP-AIRSEDSmission.

6

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ED TetherThe Long Life ED tether physically andelectrically connects the STEP-AIRSEDS units. The purpose of thetether is to constrain the units, collectelectrons from the surrounding plasma,and carry the current. Similar toProSEDS, the ED tether has an exposedsegment for current collection anddeboost operations. However, being ahybrid system, a segment of the tether isalso insulated to support boostoperations. Hence we are able tosupport boost and deboost operationsfrom one single tether, reducingcomplexity and cost. The Long Life EDTether is also designed to meet thefollowing requirements:

Survivability requirements• Maintain 80% of performance

level at End-of-Life due to AOdegradation.

• 95% survivability over 1 year formicrometeoroid/orbital debrisimpacts

Electrical requirements• Partially exposed for collection,

partially insulated• With stand a 1 kW power level• Specific resistance less than 15

ohms/km• Maximum tether current of 10

ampsPhysical requirements• Static strength of 15 N• Dynamic strength of 150 N• Smooth splicing

Tether ObserverDuring deployment, boost/deboost andother modes of operations, the EDTether will experience perturbationswhich, if unchecked, will drive the tetherlibration unstable. Control systems willbe utilized to maintain stability and

dampen tether perturbations. However,to control the motion of the tether it isrequired to observe the tether. In thepast the only methods available was tomeasure tether deployment rates and tomeasure the tether tension; in the case ofTSS-1 and -1R the shuttle crew servedas a supplemental observer. Eithermethod only inferred the tether shape toindicate possible instabilities developingin the system. In order to providecomplete observability two additionalsystems have been introduced — a tetheroptical observer and a GPScommunications system.

The GPS communications system willtell us the overall system librations andthe position the Upper and Lower Unitsrelative to one another. Acommunication link between the unitstransmitting the GPS data will allow thedeployer and other on board controlsystems to control the system in-planeand out-of-plane Sibration angles. Thetether optical observer will allow thesatellite and ground observers todetermine the exact shape of the tetherhence allowing the controllers to notonly control tether motion but alsodetermine if unstable modes aredeveloping.

FEACAn essential requirement for the EDtether system is to "close the circuit"with the ionosphere at each end of thetether. At the negatively charged end(lower end for deboost, upper end forreboost) electron emission issignificantly more efficient than ioncollection. Two techniques will be usedfor this purpose. The first and primarymethod uses a hollow-cathode plasmacontactor (HCPC) which is consideredthe state-of-the-art. A second, but

(c)2000 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)1 Sponsoring Organization.

important electron emission method willbe based on field-emitter array cathode(FEAC) technology. Successfuldemonstration of the FEAC will allowfor a truly "propellantless" OrbitTransfer Vehicle.

FEACs are small silicon semiconductorwafer chips consisting of millions of tinyconductive points placed below similarlysmall conducting holes (gate) whichforms an accelerating grid over the tips(see Figure 6-21 and Figure 6-22).Because of the small dimensionscommon to semiconductor processing,low bias potentials can provide the10s V/m electric fields between the gridand points needed to generate electronfield emission of microamp levelcurrents from each point. With up tomillions of points in a few squarecentimeters it should be possible togenerate the several amps of currentneeded for STEP-A1RSEDS without thepower required to heat a cathode andionize a gas nor the associated gas tanksand plumbing of an HCPC. FEACs offerthe possibility to simplify certainelectrodynamic tether applications.However, they have not beendemonstrated in space. The principalquestions to be tested in-space are (i) theability of FEACs to emit high currentsinto the space environment and (ii) theirsurvivability in the non-ultrahighvacuum spaceflight conditions around atypical spacecraft.

emitter cone

gate electrodeoxide insulationsubstrate

Figure 7. Schematic of a Single Elementof a Field Emitter Array Cathode(FEAC).

Figure 8. SEM photograph of SRI RingCathode developed for theARPA/NRL/NASA VacuumMicroelectronics Initiative (Emissiongated rf amplifier), courtesy of CappSpindt. These arrays were not resistivelyprotected nor coated, but neverthelessproduced 0.67 uAJtip @ a gate voltageof 70 V in a power tube (klystrode)environment.

ED Tether Deployer SystemThe ED Tether Deploy er System isdesigned to be integrated either with arange of unmanned launch vehicles orthe International Space Station. Thedeployer system is designed to supportED tether lengths up to 15 km anddeployment loads up to 500 N. Thedeployer system consists of thefollowing key elements: the canister, thereel and reel mechanisms, the tetherguide, ED tether material, the brakesystem and the interface module.Because of the stiffness of the tether, theinertia of the reel, and friction the tetherwill impart during deployment fromcontact with surfaces of the deployer athruster based separation system will beutilized to draw the ED tether from thedeployer for the first kilometer ofdeployment. The thruster system will beused to create a sufficient separationdistance until the gravity gradient candeploy the balance of the tether withoutthe use of the cold gas thrusters. Thecanister of the deployer is designed to

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contain the tether and provideenvironmental and thermal protectionprior to deployment.

The deployer system will have a motorto spin the tether reel to maintain theproper deployment profile. This motorwill be used for varying the rates ofdeployment and recovery. The brakingsystem shall be used to slow down orstop the deployment process by applyingpressure to the spool and hence slowingdown the tether itself. Both the spooland guide will be locked down prior tolaunch preventing any prematureunwinding. The winding pattern of thetether is not determined yet but thedeployer will be capable of parallel touniversal style winding patterns. Thetether guide will traverse the length ofthe spool and not be fixed at one end likethe ProSEDS deployer design. TheProSEDS design of a fixed spool createsa twist in the material as it is pulled fromthe spool. When working with the new,innovative tether designs twisting is notacceptable, as it will cause unnecessarystressing and possible failure of all or aportion of the tether. Therefore STEP-AIRSEDS will have a rotating spool. Itis also assumed that the ED tether willbe hardwired to the spool and that allelectrical connections/isolations will becontrolled with a surface contactbetween the spool edge and the rest ofthe deployer system. Lastly the groundand/or shuttle crew can actively controlthe deployer system, i.e. start/stop thedeployment process and control systemlibration

STEP-AIRSEDS Flight PlansThe STEP-AIRSEDS satellite is deployedto an initial altitude of 400 km from aTaurus 2210 Launch Vehicle. Onceseparated from the launch vehicle and

after a complete systems check in orbit,the satellite will split into two segmentsand deploy the 10 km Long Life EDTether.

Eight flight profiles were developed forthe STEP-AIRSEDS mission to meet themission objectives. The goal of theproject was to select a range of modes ofoperation for a range of altitudes to testand demonstrate the full capabilities ofthe STEP-AIRSEDS satellite throughoutLow Earth Orbit (LEO). Table Ipresents the planned Flight Profiles tomeet the mission objectives. The tablepresents the start and end altitude orinclination change for each altitude andthe duration of each flight profile, ifknown.

Table 1. STEP-AIRSEPS Flight PlanFlight Plan

Number1233a44a56

Start (km)400650400

Maximum +1°700

Maximum - I ° i350

350x1100

Duration,2-body

End (km) (days)650400700

plane change350

vlane change350 x 1100

350

TBDTBD

26.0TBD

20.0TBDTBDTBD

TBD - To Be Determined

Flight profiles 1 and 3 represent boostoperations; flight profiles 2 and 4represent deboost operations. If theseflight profiles are executed successfullythey will satisfy objectives 1 and 4.Flight profiles 3a and 4a represent planechange operations and if executedsuccessfully will satisfy objectives 1 and4. Flight Profiles 5 and 6 are ellipticalorbits maneuvers which allow us tounderstand the performance of an EDtether at higher LEO altitudes and tounderstand the performance of the EDtether systems in elliptical orbits.Conducting all flight profiles will allowus to demonstrate electrodynamic tether

(c)2000 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization.

system dynamic stability during nominaltether operations and characterize theperformance of electrodynamicpropulsion systems and their operationallimits (dynamic and electrodynamic).

The nominal mission life for STEP-AIRSEDS is one year. If the ED tetheris not cut and is the entire system isoperational by the end of the nominalmission time period the team will havedemonstrated long-life tether technologyin an actual space environment. Note:not all the flight profiles need beconducted to meet this objective.

Current Research

Work on STEP-AIRSEDS began inMarch 1999. Initial funding covered theconceptual study of the satellite and itssystems. In August 1999, NASAMSFC, TMTC and its subcontractorscontinued development of the STEP-AIRSEDS mission. During the Phase Bperiod several studies have beencompleted or are currently underdevelopment including: ED tether designand definition, launch vehicle selection,mission definition, near Earth spaceenvironment studies including tethersurvivability, preliminary tetherperformance and dynamic studies, EDtether operations and control, anddeployer requirements definition. Otherresearch done has focused onpreliminary satellite subsystem designand requirements definition, includingpower systems, instrumentation, weightcontrol planning, thermal analysis andsubsystems, and the attitude control anddetermination system. Phase B researchis currently being conducted by TMTCand its subcontractors and NASAMSFC. Launch is targeted for the2004/2005 time frame.

IV. Conclusion

STEP-AIRSEDS, and the ProSEDSexperiment, will lead to a range offollow-on ED Tether commercialapplications. Table 2 presents theSTEP-AIRSEDS characteristics inrelation to the applications with interestin the characteristic. The table showsqualitatively the correlation between theSTEP-AIRSEDS and applicationcharacteristic (High, Medium, Low, andN/A).

At the successful conclusion of theProSEDS and STEP-AIRSEDS missions,ED tethers will be ready for primetime.During the next 4 years critical groundand space testing of tether systems willbe conducted with the goal ofdeveloping a system for commercialapplication. Once testing has beencompleted, all prerequisites necessaryfor LEO space operations will be met.And then ED Tether Propulsion will beready to usher in the next century offlight — in space.

IV. References

[11 NASA MSFC & TMTC, STEP-AIRSEDS Phase A Design Study, vl.O,TMTC, Holland, Michigan July, 1999.

12} Johnson et al., "ElectrodynamicTethers for Reboost of the InternationalSpace Station and SpacecraftPropulsion," AIAA 96^250, 1996AIAA Space Programs andTechnologies Conference, September24-26, 1996, Huntsville, Alabama.

PI NASA and TMTC, STEP-AIRSEDSTether Vibration Attenuation Plan,TMTC-S A-PB-TV APv 1.1, The

10

(c)2000 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization.

Michigan Technic Corporation, Holland,Michigan, April 2000.

14] NASA and TMTC, STEP-AIRSEDSTether Definition Report, TMTC-SA-PB-TDvl.O, The Michigan TechnicCorporation, Holland, Michigan, April2000.

[5j Peiaez, E. C. Lorenzini, O. Lopez-Rebollal and M. Ruiz, "A new kind ofdynamic instability in electrodynamictethers." Proceedings of the AAS/A1AASpace Flight Mechanics Meeting, PaperAAS 00-190, January 23 - 26, 2000,Clearwater, Florida.

Table 2. STEP-AIRSEDS Characteristics in Relation to Proposed FutureApplications of ED Tethers.

STEP-AIRSEDS CHARACTERISTICAPPLICATIONS WITH INTEREST IN

THE CHARACTERISTIC

Long life tetherTether dynamic stability and controlPowered BoostPowered DeboostPrecision Orbital ManeuveringStationkeepingFEAC DemonstrationRendezvousabilityInclination Change

De-OrbitHigh

MediumN/A

MediumLowLowHighN/ALow

Large Spacecraft orISS Stationkeeping

HighHighHighLow

MediumHighLowHighLow

Orbit TransferVehicle

HighHighHighHighHighHigh

MediumHighHigh

JovianMissions

HighHighHighHighHigh

MediumHighLowLow

Figure 9. STEP-AIRSEDS during tether deployment.