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Dec.14, 2015 Lecture in “ and Systems” Tokyo University

Overview of Sail Propulsion for Space Flight

Ikkoh FUNAKI Japan Aerospace Exploration Agency (JAXA)

1 Abstract

Currently, three kinds of sail propulsion concepts are studied and they are called as solar sail, , and . Since these sail propulsion concepts do not require , but instead, the of the solar or the is used to accelerate a . Such propellant-less propulsion system may be the dream for spacecraft engineers because engineers are willing to improve maneuverability by increasing specific (Isp), but propellant-less propulsion may provide infinite Isp! In this lecture, research status of sail propulsion for space flight is overviewed.

2 Outline 1. Introduction

2. Solar Light Sail ()

3. Solar Wind Sail (Magsail)

4. Another Solar Wind Sail (Electric sail)

5. Future Evolutions of Sail Technology

6. Summary 3 1. Introduction

4 Deep Space Propulsion Using

Artificial B-field by Spacecraft

Solar Wind Light from Magnetic acceleration the Sail (Magsail)

Mirror

Solar Ligh Sail Electric Sail production by light pressure Solar Wind Sail Accelerated by the solar wind dynamic pressure 5 2. Solar Light Sail (Solar Sail)

6 Principle of Solar Light Sail

Thrust per unit (pressure): P

Maximum Pressure for perfect reflection: W P = 2 (2.1) 0 c α 1368[W ] ≈ 2 3×108 [m / s] ≈ 9.12 [µN / m2 ]

The solar intensity W at 1 AU is 1368 [W/m2], and c is the .

7 Theory of Solar Light Sail • The pressure P on a flat perfect reflector is given by: 2 2 cos α cos α 2 P = P0 = 9.120 µN m (2.2) R2 R2 where R the distance from the Sun's center in AU, and α is the angle between Sun and sail normal. • Using an efficiency factor, η, of typically about 0.90, pressure, characteristic acceleration (ac), and sail loading, σ, are related by:

P0 ×η 9.120 ×η 8.28 2 ac = = = mm s (2.3) σ σ σ Sail loading σ is total divided by sail area, and σ is given in [g/m2].

8 Orbital Transfer by Solar Light Sail (1) Outward Spiral Case

9 Orbital Transfer by Solar Light Sail (2) Inward Spiral Case

10 Orbital Transfer by Solar Light Sail (3) Trajectory

days Sun

days

J.L. Wright, Space Sailing 11 Orbital Transfer by Solar Light Sail (4) Saturn Flyby Trajectory

days

days Sun

J.L. Wright, Space Sailing 12 History of Solar Light Sail

• predicted in 1908 the possibility of solar distributing spores across interstellar distances, the concept of . He apparently was the first scientist to state that light could move objects between stars. • (Tsander) published a technical paper that included technical analysis of solar sailing. Zander wrote of "using tremendous of very thin sheets" and "using the pressure of to attain cosmic velocities”. • J.D. Bernal wrote in 1929, "A form of space sailing might be developed which used the repulsive effect of the sun's rays instead of wind. A space vessel spreading its large, metallic wings, acres in extent, to the full, might be blown to the limit of Neptune's . Then, to increase its speed, it would tack, close-hauled, down the gravitational field, spreading full sail again as it rushed past the sun.” • The first formal technology and design effort for a solar sail began in 1976 at Jet Propulsion Laboratory for a proposed mission to rendezvous with Halley's . WIKIPEDIA 13 Key Technologies of Solar Light Sail • Sail deployment – 33m x 33m sail is required for 10mN thrust – 105m x 105m sail is required for 100mN thrust Usually, 100m-sq sail or larger is necessary for 1,000-kg interplanetary spacecraft. • Sail material lightweight, therefore ultra thin (~µm) film is necessary 2 2 – σ ~ 8 g/m is required to obtain ac~ 1 mm/s . In this case, weight is 80 kg for 100m-sq sail. – So far, 5 µm thick Mylar sail material achieves mass ~ 7 g/m2, aluminized films have a mass as much as 12 g/m2. • – achieved by a relative shift between the craft's center of pressure and its Sail Configurations WIKIPEDIA 14 Flight Tests of Small-scale Solar Sails

Year Country Flight Goal Result 1993 Znamiye -- Deployment of “shade” Success – but not a sail outside of 1999 Russia Znamiye-2 Deployment test from Collided with s/c Progress during deployment 2002 TPS Sub-orbital deployment test 3rd stage failed to separate 2004 Japan Sub-orbital deployment tests Success 2006 Japan Deployment test in orbit on Did not fully deploy Astro-F 2005 TPS – Fully controlled flight in Rocket failed during 1st stage Earth orbit 2008 U.S. Nanosail-D: Sail orbital deployment in Rocket failed to deliver to orbit atmosphere as a brake 2010 Japan IKAROS: interplanetary flight June – successful deploy first solar sail in space

TPS: 15 JAXA IKAROS IKAROS : Interplanetary Kite-craft Accelerated by Radiation Of the Sun

Piggybacked on the Climate Orbiter (Planet-C) Mission: June 2010 16 JAXA IKAROS 14 m

Deploying Sequence Tip Mass Tether Steering Device

Vary reflectance 17 Future Plans for Solar Light Sail

Mission Who Size Status Mass Launch

COSMOS 1 TPS 150 sq m Launch failure 130 kg 2005 Nanosail D MSFC 9 sq. m Launch failure 4.5 Kg 2008 IKAROS JAXA 200 sq. m In orbit – success 2010 In orbit – partly Nanosail D-II MSFC 20 sq. m 4.5 kg 2010 success Lightsail – 1 TPS 32 sq. m In development 4.5 kg EADS - Cubesail 25 sq. m In planning/dev. 5 kg Surrey CU Cube Sail 20 sq. m In planning 5 kg Aerospace Lightsail - 2 TPS 100 sq. m In planning 10 kg CU Ultra Sail 100 sq. m In planning 20 kg Aerospace 1,208 sq m MSFC In development 32 kg 2015 (38mx38m)

TPS: The Planetary Society MSFC: NASA Marshall Space Flight Center 18 3. Solar Wind Sail (MagSail)

19 The Idea of Solar Wind Sail, Magnetic Sail (=Magsail)

Top View Solar Wind Side View Streamlines Superconducting Coil Coil

Solar Blocking size (L) ~60 km Wind () accele ration

Magnetic Support System Field Lines and Payload

20 History and Status of Magsail Research

• R.M. Zubrin and D.G. Andrew proposed Magnetic sail for earth escape, interplanetary travel, and interstellar flight in 1989-1991. • Due to some technical issues, Magsail never flew in space. • In 2001, R. Winglee and his colleagues started Mini- magnetospeheric Propulsion (M2P2) as an way to compactly implement the concept of Magsail. • From 2006, JAXA, in collaboration with Japanese universities, continues theoretical and experimental investigations on Magsail and its derivatives. The first laboratory experiment of Magsail was conducted to confirm theoretically derived thrust formula. Also, small flight demonstration of Magsail is being proposed. 21 Plasma Flow and Thrust of MagSail

Magnetosphere Characteristic Length of MagSail

or magnetic cavity (Stand-off distance, L0 ) 2 µ M Polar 6 0 Cusp L0 = (3.1) 8 2 n m u 2 π i M:dipole moment 2 nmiu :solar wind dynamic pressure Magnetic Cavity L0 is derived from pressure balance: 2 2B µ M € 2 ( mp ) B 0 L Loop Current nmi u = and mp = 3 0 (coil of MagSail Spacecraft ) 4 L 2µ 0 π 0 Thrust of Magsail (Drag )

Solar Wind Plasma 1 characteristic area € F C €u 2 S Magnetos pheric Boundary = d ρsw sw Magnetospheric() Bounary 2 (Magnetopause) solar wind dynamic pressure non-dim. Thrust coefficient (a function of L , ρ , u ) Plasma Flow and B-field of 1 0 sw sw = C ρ u 2 π L2 (3.2) MagSail d 2 sw sw ( 0) 22

€ Thrust by Magsail and Solar Light Sail • The dynamic pressure P of a solar wind at 1 AU is given by: 1 P = ρu2 0 2 1 2 ≈ 1.67×10−27 × 5×106 × 5×105 Pa (3.3) 2 ( ) ( ) ≈ 1 nPa when solar wind number density 5x106 particle/m3 and velocity 500 km/s are assumed.

• P0 by the solar wind (~1 nPa) is very small in comparison with the solar light pressure (~9 µPa at 1 AU). Hence, sunlight has thousands of times more momentum (per unit area) than the solar wind. Therefore, a magnetic sail must deflect a proportionally larger area of the solar wind than a comparable solar sail to generate the same amount of thrust. • However, Magsail need not be as massive as a solar sail because the solar wind is deflected by a instead of a large physical sail. So, in

some cases, Magsail might have competitiveness against solar light sail. 23 Theory of Magsail (cont.) Effect of L (Stand-off Distance) on Thrust (F) R. Zubrin proposed 20-N-class Magsail (for 100-km-diameter magnetosphere) that is obtained with B-field by superconducting coil of 5mm in diam., 5000 kg weight. 20-N-class (Zubrin) 100 Solar Wind Side View 10 Streamlines Coil 1 1 F C u2 L2 0.1 = D ρ sw (π ) 2 L # # r & 2 & F (Thrust), N 0.01 Li Solar CD = 3.6exp% −0.28% ( ( $ $ L ' ' Wind 0.001 accele 0 20€ 40 60 80 100 ration L (stand-off distance of magnetic cavity), km € Thrust of Pure MagSail Magnetic Field Lines Design Target: 1-N-class Sail (for 1,000-kg-spacecraft)

Definition of L 24 Typical MagSail Experiment in Laboratory

Plasma plume is φ700mm at the coil position

Solar Wind Simulator 70mmφ Coil L φ18mm coil simulating MagSail Solar

Wind Magnetosphere (Mag. cavity)

0.6m Magnetopause

Hydrogen 0.4g/s,

PFN1: 4kV, 1T at the center of the 70mmφ coil 25 Direct Thrust Measurement of MagSail in Laboratory

Laser Disp. Sensor

0.40

0.20 MagSail Operation in Laboratory Thrust Stand mm

0.0

• -0.20 0.4g/s, PFN1: 4kV • ballistic pendulum method Displacement, • 1.9T at the center of the coil • supported by 4 wires -0.40 0 5 10 15 20 • Discharge duration:0.8 ms, • laser displacement sensors Time,sec • Time resolution:5 µs 26 Thrust Characteristics of MagSail 4 - F 3 Cd = 1 2 Hybrid Simulation ρu S particle/ fluid 2 sw 2 by Fujita/Kajimura Coefficient), non-dimenaional thrust coef.

(Thrust Experiment € 1 Cd

0 0.001 0.01 0.1 1 10 rLir /Lmiui =Li =(non-dim. ion Larmor radius L eBmp L at magnetopause) 71000 7100 710 71 7 27 L (Magnetic Cavity Size in Space), km space € Small Magsail Demonstration Plan Outline: • 300kg(wet) class small engineering spacecraft • inserted into Earth escape orbit at C3≒0 by Japanese Small Launcher (Epsilon) or as a secondary payload of heavy (H-IIA)

Objectives: superconducting • Cooling and start-up of superconducting coil coil in an orbit departing the Earth • 1mN-class thrust production when operating the superconducting coil, as a spacecraft bus and result of solar wind to magnetosphere solar paddle interaction • Testing attitude control and guidance of Spacecraft and Mission Image of Magsail in deep space Magsail Demonstration Mission 28 4. Another Solar Wind Sail (Electric Sail)

29 Principle of Another Solar Wind Sail, Electric Sail

Solar Wind

+ +

sheath ~ 10 m

thin wire ~ 5 µm biased to +20-40 kV solar wind

Solar Wind Repelling Scheme Electric Sail Spacecraft Concept The electric sail consists of a number of thin, long and conducting tethers which are kept in a high positive potential by an onboard electron gun. The positively charged tethers repel solar wind protons, thus deflecting their paths and extracting momentum from them. Janhunen, Electric Sail HP 30 Principle of Electric Sail (cont.) Electric Solar Wind Sail • To capture solar wind momentum, Coulomb • Uses interactionsolar wind between for spacecraft solar wind propulsion and long, • Coulombthin, interaction positively charged between tethers solar (10-20 wind km, and long,25-50 thin, µm wire,positively +20-40 charged kV) is used. tethers (10-20• Thrust km, 25-50per unit µ lengthm wire, is 500+20-40 nN/m kV). Up to 1 N thrust (scales ~ 1/r) is expected for 100- • Up to 1 N thrust (scales ~ 1/r) from 100- 200 kg unit 200 kg unit • A way to deploy the tethers is to rotate the Remote Unit • Powerspacecraft consumption and have low and the centrifugalscales ~ 1/r force² Auxiliary Tether • Baselinekeep approach them stretched. uses non-conducting Auxiliary• By fine-tuning Tethers to the stabilise potentials flight of individual without activetethers control and thus the solar wind force individually, the attitude of the spacecraft • “Remotecan beUnits controlled.” at tips contain auxtether reels and spinup propulsion/spin control 6 31 Status of Electric Sail Research

• P. Janhunen (Kumpula Space Centre, ) proposed the concept in 2004. • Until recently, theoretical model and spacecraft design were studied by Janhunen’s group. • E-sail effect in is to be demonstrated onboard ESTCube-1 , which was launched on May 7 2013. – ESTCube-1 will deploy a 10 m long tether and charge it up which allows us to measure the ESTCube-1 (1-kg nano- E-sail force exerted on the tether sat) was the first satellite by the ionospheric ram flow to test electric sail. acting on the satellite. WIKIPEDIA 32 5. Future Evolutions of Sail Technology

33 Solar Sails: Limitations and Future Prospects

1. Thrust is produced only near the Sun – Large acceleration is possible for inner exploration. Therefore, spacecraft can be put into an orbit of Mercury, Venus and Mars via solar sails. – Acceleration is NOT enough for orbital transfer in the outer solar system beyond . This means that solar sail propulsion is effective for 1) flyby missions targeting at outer planets in the solar system, or 2) for solar system escape (with nuclear source) when enough acceleration is provided in the inner solar system region.

34 Solar Sails: Limitations and Future Prospects (cont.) 2. Solar sails’ thrust as well as electric power are severly limited. To overcome electric power limitation, in particular in the outer solar system, the usage of sail (SPS) is planned for future Jupiter-Trojan mission. To overcome thrust limitation for outer planet exploration, aeroassist maneuver or the usage of electric propulsion (in combination with SPS) may be effective. 3. Rapid solar system escape would be another target mission with solar sails. Drastic acceleration in the inner solar system may be possible by Magsail’s derivatives (MPS) or by Electrostatic sail. 4. The usage of sail technology for far future long-distance missions is also discussed. 35 JAXA Solar Power Sail (SPS) Concept

A solar power sail combines a solar sail ( propulsion) with solar sail (50mx50m) additional electric power generation solar cells capability of flexible solar cells attached to the sail membrane. Solar power sail spacecraft is powered by the hybrid propulsion of solar photon acceleration and highly efficient ion engines driven by the large electric power supply from the spacecraft body flexible solar cells. This in turn, with leads to flexible and efficient orbital Solar Power Sail Spacecraft Concept control capability. JAXA JSPEC HP

36 JAXA Solar Power Sail Mission Proposal

!

Fig. 2. Overview of solar power sail mission sequence (left) and position of Jovian Trojan at Lagrange points L4/L5 associated with Sun-Jupiter Jupiter-Trojansystem (right). Sample Return Mission (2020~) to the Earth. Trojan asteroids are located at Lagrange points ! (RCS) that functions at very low R. Funase,L4 and O. Mori, L5 et associated al., ISTS2013-d-20 with the Sun-Jupiter system (Fig. 2). temperatures 37 JAXA plans to initiate the project in a few years and expects ! fuel cells integrated with propulsion system the launch in around 2020. ! EDVEGA technique using hybrid propulsion system This paper analyzes the feasibility of this Trojan ! ultra stable oscillator for 1-way range or Very Long exploration mission using the solar power sail technology. In Baseline Interferometry (VLBI) precise orbit section 2, we provide an overview of the mission, some of the determination new technologies to be demonstrated in this mission, and ! Ka-band communication for interplanetary missions other scientific objectives. In section 3, we perform trajectory Utilizing the large power supply from the large, ultralight

design and system feasibility analysis, focusing on the mission thin-film solar cells, the spacecraft will drive its ultra-high Isp until the rendezvous with the asteroid. In section 4, we discuss ion engines even at the Jupiter distance (>5 AU) in order to

the feasibility of the sample return mission. In section 5, we rendezvous with a Trojan asteroid. The intended Isp of the ion present the conclusions of this study. engines will be 6,000 - 10,000 s, almost up to 3.3 times as efficient as existing contemporary ion engines3). A very large 2. Jupiter and Trojan Asteroid Exploration Using Solar solar power sail will be deployed and extended to drive the ion Power Sail engine. The area of the sail will be over 10 times larger than on IKAROS. The EDVEGA technique utilizing the highly 2.1. Mission Overview efficient electric propulsion system aims at increasing the The current scenario for the Jovian Trojan asteroid payload mass for outer solar system exploration. These exploration mission consists of five phases. The first phase is technologies enable flexible orbital maneuverability even in an EDVEGA (Electric Delta-V Earth Assist)4) phase outer planetary regions of the solar system without relying on to efficiently increase the departure velocity relative to the nuclear technology. Earth. The second phase is a transfer phase from the Earth to The RCS that functions at very low temperatures (below Jupiter followed by a Jupiter swingby. The third phase is a -40 °C) is a technology that will drastically reduce heater rendezvous with one of the Trojan asteroids that is at wattage in outer solar system environments with very low Lagrange points L4 or L5 associated with the Sun-Jupiter solar radiation intensity. The fuel cells integrated with the system (Fig. 2). Utilizing the large power supply from the RCS is a technology to realize very light-weight power large, ultralight thin-film solar cells, the spacecraft will drive storage with high power density. These two technologies are

its ultra-high (Isp) ion engines even at the key to advancing the bus technologies for outer solar system Jupiter distance (>5 AU) in order to rendezvous with a Trojan exploration. asteroid. The fourth and fifth phase is capturing of an asteroid 2.3. Scientific Missions sample and bringing it back to the Earth, respectively. In addition to the technology demonstrations, there are We expect the launch in around 2020 and the overall other new and innovative science purposes carried by this mission period will be around 10 years. spacecraft5). 2.2. Technology Demonstration Missions Cruising science This mission will carry the following new technology During the mission’s long cruising period to the Trojan demonstrations, which are required for future solar system asteroid via the main asteroid belt and Jupiter, various voyages: astronomical observations will be performed that are only ! fabrication of a large-membrane space structure and its possible in deep space. These include: (1) a long baseline deployment strategy gamma ray burst detection and polarimetry of highly energetic ! hybrid propulsion using both photon acceleration and light sources, (2) infrared survey observation for the dark age ultra-high performance ion engines stars in the early universe in the whole sky beyond the ! ultralight thin-film solar cells foreground zodiacal light that originates primarily from the

2 JAXA Magnetoplasma Sail (MPS) Concept

(3) magentosphere (MPS spacecraft) is accelerated space B-field magentosphere(magnetic cavity)

L spacecraft with coil solar wind (1) B-field by coil thrust

space B-field flow path

MPS spacecraft plasma jet 1 spacecraft plasma jet 2 Thrust by MPS: F ≈ Cd ρswuswπ L (2) plasma is released to inflate the 2 magnetic field (6.1) 38 Magnetoplasma Sail (MPS) Research at JAXA

2007 2015 ~2020 ~2025

Fundamental Research Space Demonstration Future Missions by 1. Thrust Control Technology of Magsail/MPS Magsail/MPS

(C)JAXA/NEC (C)JAXA/NEC Scale-model experiment Numerical simulation 2. Conceptual Study of MPS Technology Space Deep space Mission MPS Spacecraft Demonstration based on Plasma Sail, ‒ Spacecraft wet mass 2025 or later) ~300kg ‒rapid Jupiter/Saturn ‒ In-space testing of Flyby ‒ Magsail (coil) ‒rapid Solar system 3. R&D of Superconducting technology escape Coil System ‒ MPS technology thrust production capability 39 192 R.L. FORWARFarD Future InterstellarJ . SPACECRAFT Mission by Sails If th e phase error i s t o b e less than ?r/4, then th e su m o f lens errors must b e less than

(25)

This means that th e in-plane variations o f th e ring dimensions VELOCITY AFTER should b e controlled t o 40 YEARS 0.21 C sf Ar<— =56 cm (26) 8r MARCH-APRIL 1984 INTERSTELLAR TRAVEL USING LASER-PUSHEFig. 3 InterstellaD LIGHTSAILr rendezvouSs mission-acceleration phase. 193 and the out-of-plane variations to mitment to build a large array of solar-powered lasers in space. 30 km DIAMETER Th e diffraction limited spot size o f th e 1000 k m diameter (27) 71-TON 4r At this distance, the spot size d' of the laser beaPAYLOAm Da sSTAG focuseE d para-lens at 4.3 light years is by the para-lens with diameter D of 1000 km isDECELERATES A T 0. 2 g STOPS I N 1 YEAR We se e from these numbers that because th e focal length o f AT a CENTAURI th e lens system i s very longd'=2.4s\/D=98km, th e wavefront i s practically plane (36) d'=2.4s\/D = 3. (33) a s i t passes through th e lens, s o there i s n o concern fo r out-of- plane "flapping" o f th e lens. A simple slow rotation t o keep s o that nearly al l th e laser power i s still being captured b y th e whicth e we b structurh is leses tauthatn an d th e axi thse o f th e len diametes linedr u p o n th e of the , all the laser 3.6-km-diam. sail. The 10lase0 kmr DIAMETEis theRn turned off and the in- power i s thus striking th e lightsail an d th e acceleration 714-TON target star should provide sufficient stability. To keep the terstellar probe coasts to DECEits targetL STAGE, reaching ot Centauri at 4.3 remains constant during th e boost phase. Under a constant light years' distance i n 4 0 years from launch. variations i n ring radius belo2 w 0. 5 m will b e slightly more ACCELERATES A T 0.02 g acceleratiodifficultn, buoft b0.0y 5"tuning m/s ," ththee stretctime hi t otakef thes spokto travee wirel sth wite h FLYS B Y a CENTAURI The Mini-Mag Orion interstellar distance o f 4. 3 light years i s 4 0 years. A t th e en d o f this adjustable ballast weights along th e strands, i t should b e Fig. 4 InterstellaInterstellar rendezvour Rendezvous mission-deceleratios Mission n phase. constanpossiblt acceleratioe to maken th periode ring,s th e lightsaicircularl ha s treacheo the drequire th e d degree. If th e discoveries made b y th e flyby probe generate interest concept, a hybrid accelerated velocit(Noty eo thaf 0.2t i1t isc .onl Thye thrapidle variatioy travelinn in thg elightsai ring radiul is sno abouw apt -the inInterstellar further exploration rendezvouss of the target system mission, the nex t tophas a-e of proachinmean gtha a Centaurt needis an d no w mus correctiont decelerate. An. overall expansion or con- interstellar exploration would b e t o send a larger, unmanned by beamed pellet , and Th e tractiolightsainl o f th e i s buillens i s t i n tw o equivalensections, a n t t o a outeslighr tdoughtnut shift i n foca-l durinspacecrafCentaurig th e launct t o hrendezvou phas usinge i s formidabls a t th e targe laser-pushedt estar an d explore i t n lightsail decelerated with a magnetic sail. shapelengthd ring, whic, anhd isa neasil inney rcompensate circular sectiod for.n )30 km in diameter. detail. It might seem that since all the laser can do is push the mac 16 This 30 km payload section of the sail has a mass of 71 metric lightsailR. L. ForwardP=——, it woul (1984)d= no4.3xl0t be possiblW=43,OOOTWe to use a solar system-base(37)d D.G. Andrew (2003/2007) 40 tons, includingOne-Wa a science ypayloa Interstellad o f 2 6 metrir Flybc tonsy Prob. Th e e laser to stop the sail at the target system. By separating the remainingThe firs, ring-shapet interstellad r"decel missio"n tstago bee sizehasd this ea near-termmass of 71, one4 - lightsail into two parts, however, and using one part to reflect metriwayc tons, unmanned, or ten time, flybs yth probe smallee missior payloan t o th e nearesd "stage"t star. .This Atth e lasethisr rat lighe to bacf acceleratiok toward th e nsola ther lightsaisystem,l thawiltl retrodirectereach a velocitd y of Thmissioe centran will payloal b e limited dsectio i n acceleration of then b y th e saimaximul is detachem d from the hallasef th e speer lighd o f tligh ca n b e uset i n 1. 6 dyears t o decelerat. W e coule th e dothe continur portioe ac - n o f th e largetemperaturr stage ane dtha turnet th e saidl ca n takearoun. I f w e assumd so etha a maximut its reflectinm g surface celeratinlightsail.g t o highe14'27 r cruise velocities, bu t fo r this preliminary faceoperatins the reflecting temperaturg surface Tofe o f600°K the ring-shape, a reflectancd portioe 77 of n0.82 (se, ean analysiA t launchs we wil, th e lightsail assuml fo r a rendezvoue a 0.5 c cruiss missioe velocityn will. hav(There e are also an initial diameter d of 100 km. With an areal density $ of 0.1 Figabsorptanc. 4). At a time ae o 4.f 30.135 year, sa earliern emissivit, they lase e ofr 0.06powe, ran frod amn thareae l going t2o be severe problems with radiation and damage from soladensitr systey mof wa s upgrade sail, structured t o 2 6 T W (ther,e an ar e 3 7 yeard payloas t o ge d /3 of 0.1 g/m2, then the interstellag/m , th e masr particles o f th e lightsais anl i s 78 5 dmetri dustc atonst thes. I f lightsaie speeds.l ) readthermally fo r this yincreas limiteed i n power)acceleratio. Th e strongen is r laser beam Atechnologt a velocity v o f 0.is 5assume c , th e relativistid toc havfactoe rimproved so that the back of travels across th e space t o th e larger ring sail. Th e increased th e sail ha s microstructure that produces th e effect o f a nearly 2 A T^ black body, then the averag2 2e of the sail will be power raises th e acceleration o f th e rin2g sail t o 0. 2 m/s , an d y=(l-V /C )*-0.87 (38) a= — —— = 0.36 m/s = 0.036 (28) about 0.50 (0.95 o n on e side an d 0.06 o n th e other). it continues ct o apgain speed. The light reflected from the ring sail is focused onto the smaller sail, now some distance is Maximua 1397o correctiom temperature-limiten to the massd oacceleratiof the lightsain isl noanwd the aging behindIf. w e assumThe lighe a minimat reboundl weighs frot probm the ewit 30-kh a totam lsail mas, sgivin m o f g it a of the crew, while the Doppler effect has lowered the 1000 k g (roughly one-third each o f sail structure an d /t T4 momentum push opposite to its velocity, and slowing it down. frequency an d energa = — ——y o f th e lase=3.0r beam/sm2 = 0. 3 b y th e grati r a vo i t i e s (34) Sincpayload)e the smalle, thenr th e diametesail is 1/1r d o f th e sai0 l i s the mass of the larger one, its c ot{3 deceleration rate i s 2. 0 m/s2, o r 0. 2 g . Th e light flux o n th e smaller sail ha s increased considerably, bu t i s only two- (29) With this improvement in maximum allowed acceleratio(39)n thirds of the maximum light flux that the sail can handle. capability, thermal considerations ar e n o longer limitations, 2 At a deceleration rate o f 2. 0 m/s , th e time required t o Thusbu t maximu, if we wanm lasert to providpowere alimitation constants an d accelerationmission tim, the e laser The power needed to push this 1000-kg sail at the ac- considerations are. Data should be returned from the mission bring the payload sail to2 a stop in the a Centauri system is power would have to be increased from 43,000 TW at the start onlyceleratio 1 year, makinn a o f 0.36g a m/sectotal tri i s p time o f 4 1 years. Th e distance t o 75,00in les0s T W o r morthane a t th e en d o f 5acceleratio0 years. Sincn phasee 4.3 year. s are required to radio the the tw o portions o f th e sail have separated during th e Thinformatioe distance ncovere backd fro durinm a Centaurig the acceleratio, an d a numbern o f yearperios t o d is only 0.4 explore all the planets in the three-star system, only about 43 deceleration period, with one portion increasing its speed an(30d ) light years, s o th e remainder o f th e 10-light year trip t o th e leaving the star system, and2rj the other decreasing its speed and outskirtyears sshoul of ed Eridan be speni att gettin0.5 cg wil therel tak. eThi 20s yearin turs eartn implieh clocs kan stopping, i s only 0.12 light years. Th e spot size fo r th e ring timaverage an d 17.3e triyearp velocits crewy cloc o f 0.10k th e speetimed. o f A t 0. 4 lighlightt c . years from sail Whillens ewit thihs ias diametea great dear olf o10f lase0 kmr power at a distanc, it is wele ol fwithi 0.12n lighfuturt e th e star. T, th e o320-km-diam minimize th. erendezvou laser powes portior neededn o f th e sai, lw i s e will assume a yearcapabilitiess is only 2. 7Highe km, rs opowe all rth levele lighs ttha fron mthi ths ear largee generater sail di sby detacheconstand an d th e deceleratiot acceleration ntechniqu over thee describewhole distanced previousl, rathey r than a high-power acceleration period followed by a coast period. capturethe dSpac b y th e 3 0 k m diametee Shuttlr payloae rocketd sais l adurint liftoffg th e entir. Ife the acceleration is is applied. The 320-km-rendezvous sail has2 a mass of 7850 deceleratiomaintainen periodd fo r three. years, th e will attain metriAssuminc tonsg, o f whican acceleratioh about 290n 0a metri of 0.0c ton5 m/ss i s th e mas, s o f th e e laser power Therthe velocite are many y versions of this basic concept. One of them crerequirew an d habitatd to, pussuppliesh a 10, an d exploratio0 km diameten /Thir lightsail wits i s h a mass of uses higher initial laser power t o ge th e sail u p t o th e 0. 2 c 785 metric tons i s 7 more than sufficient t o land o n an d explore a number o f coast velocity soonerv = at; th=e 3.4xl0laser is m/sthen =used to launch othe(31r ) planets, especially since the rendezvous sail can use the probes while th e first coasts ou t o th e target star. Al l versions starlight from e Eridani for transportation within th(35e ) needat furthethe distancr studye . planetary system. Th e incident laser power needed t o decelerate this 320-km- Roundtrip Journey b y Interstellar Lightcrafm t diam(see. Figlightsai. 3).l a t th e Ithermallt should yb elimite noted aacceleratiot this poinnt o f 3. 0 the total power If the reports from the interstellar rendezvous probes are m/soutpu2 is tone-tent o f th e entire worlh thed i s initiaaboult 1 launcTW. hThi powes amounr or 4,30t o f 0 TW. The favorable, then the nex= t0.1 phas 7 lighte woul yearsd be to send a human crew(32) 245-km-diamlaser power. spois not sizt triviae of the lasel ran bead mwil focusel requird by the e1000 a significan- t com- on an interstellar exploration journey. Although a typical km-diam. lens at 10.8 light years is smaller than the 320-km- interstellar mission will literally last a lifetime, and a diam. sail, so all the transmitted in the main beam are reasonable case could b e made fo r a one-way journey,27 collected b y th e lightsail. increasing the capabilities of the laser lightsail system will get The power emitted by the laser system based on Earth will the crew to its objective and back as fast as possible. have to be significantly larger than 4,300 TW. First, there is More than just th e nearest star system will b e ultimately the 16% of the laser power that is outside the central spot in explored, so we will design the laser lightsail system to allow even a diffraction-limited system. Then there is the 11% loss roundtrip capability ou t o 1 0 light years, e.g., t o e Eridani a t of laser energy through th e central hole o f th e decelerator- 10.8 light years (see Fig. 5) . W e will assume th e diameter o f stage ring sail, plus th e 13% excess relativistic mass o f th e sail the lightsail at launch to be 1000 km, the same size as the at the start of the deceleration period. Finally, there is the transmitter lens. With an areal density of 0.1 g/m2 it will mass decrease in energy of the photons by the Doppler redshift. 7.85X107 kg, or 78,500 metric tons. The crew must be This Doppler shift varies with time an d i s complicated b y th e transported a s fast a s possible, s o w e will accelerate a t th e reflection of the laser light from the moving of the ring thermally limited acceleration o f 3. 0 m/s2. Th e power needed sail. A t th e beginning o f th e deceleration period, th e 0.5-c 6. Summary

41 Sail Propulsion for Deep Space Flight is overviewed.

• Solar Sail (thrust based on light pressure) – firstly demonstrated as IKAROS by JAXA – Further increase in size is required for practical deep space mission IKAROS • Magsail (thrust based on solar wind pressure) – Never flew, no feasible design was shown – small demonstration is being proposed – magnetosphere deployment (or expansion) should be further studied to obtain large acceleration Magsail • Electric Sail (thrust based on solar wind pressure) – a new concept recently proposed – small demonstration is in progress Electric Sail Some hybrid/future concepts are also studied for future deep space missions 42