1 Status report and review of Electric Solar Wind Sail spacecraft propulsion NORDITA-Colloquium, Oct Pekka Janhunen Finnish Meteorological Institute, Helsinki (Kumpula Space Centre)‏

2 Collaborators (partial list)‏ Petri Toivanen, Risto Kurppa, Juha-Pekka Luntama, Arto Sandroos, Jouni Polkko, Walter Schmidt, Markku Mäkelä, Ari-Matti Harri (FMI), prof. Edward Haeggström, Henri Seppänen, Sergiy Kiprich, Jukka Ukkonen (Univ. Helsinki), Jouni Envall, Mart Noorma, Silver Lätt (Univ. Tarto), prof. Simo-Pekka Hannula, Yossi Ezer, Eero Haimi, Tomi Suhonen (TKK/MAT), prof. Aarne Halme, Tomi Ylikorpi (TKK/AUT), Pasi Tarvainen, Erkki Heikkola, Antti Niemistö (Numerola Oy), prof. Mikhail Zavyalov, prof. Slava Linkin, Pavel Tuyruykanov (IKI, Moscow), prof. Giovanni Mengali, Alessandro Quarta (Univ. Pisa), Lutz Richter, Olaf Krömer (DLR-Bremen), Greger Thornell, Henrik Kratz (ÅSTC, Uppsala). Acknowledgements: Mikhail Uspensky, Jyri Heilimo, Harri Haukka, et al. (FMI), Svenja Stellmann (now Bremen), Rami Vainio (Univ.Helsinki), Robert Hoyt (Tethers Unlimited Inc.), Giovanni Vulpetti (Rome), prof. Giancarlo Genta (Polytech.Torino), prof. Pierre Rochus, Yvan Stockman (Univ.Liege), Fredrik Bruhn (ÅAC, Uppsala), prof. Viktor Trakhtengerts(*), Andrei Demekhov (Nizhny Novgorod)‏

3 Outline Electric sail: what is it Physics of electric sail thrust prediction Technical status Potential applications Conclusions

4 Electric Solar Wind Sail Use solar wind for spacecraft propulsion Coulomb interaction between solar wind and long, thin, positively charged tethers (10-20 km, um wire, kV)‏ Centrifugal stretching, Potentiometer guiding & navigation High-performance, relatively simple, general-use propellantless propulsion technique First idea Janhunen 2004 In its present form Janhunen 2006‏

5 Electric sail, artist's view

6 Elementary process of Electric Sail Solar wind protons repelled by charged tether Proton pile-up on “dayside”, proton depletion on “nightside” Resulting electric field pushes charged tether outward from Sun (Thrust vectoring up to ~30 o possible, since force always perpendicular to tether)‏ Task: Predict electron sheath size and thrust per unit length on positively charged wire in flowing solar wind plasma

7 PIC simulation of electric sail (Janhunen and Sandroos, 2007)‏ Direct, time-dependent electrostatic particle simulation Includes potential ramp-up phase which creates population of trapped electrons Number of trapped electrons is independent of the speed at which the potential is ramped up Trapped electrons produce extra shielding which decreases E-sail thrust In purely 2-D case there is no apparent mechanism that could remove trapped electrons (except very slow, collisional ones)‏

8 PIC simulation of electric sail Challenges: – High e V0/Te ratio (V0 ~ 20 kV, Te ~ 10 eV)‏ – Logarithmic potential ==> multiscale

9 Removal mechanism for trapped electrons 2-D electron motion – 3 conserved quantities: angular momentum (cyl. symm.), parallel momentum (transl. symm.), energy (static)‏ 3-D electron motion – 1 conserved quantity: energy (static)‏

10 Removal mechanism for trapped electrons ‏Electron spirals along tether, is reflected from the tip Periodically visits vicinity of spacecraft At spacecraft, angular momentum and parallel momentum randomised

11 Removal mechanism for trapped electrons Finite probability that angular momentum becomes very small so that electron has change to hit tether wire and be lost Process is rare, but electrons are fast ==> lifetime order of minutes only This mechanism removes trapped electrons from vicinity of electric sail tethers ==> earlier PIC simulations underestimated the thrust!

12 “Electrosphere” principle Sheath around tether is empty of ions Assume it has some constant electron density (can be zero, in zeroth approximation)‏ Force balance (1/2)ε 0 E 2 = P dyn determines “electrosphere” size (assume zero n e for simplicity) and thrust per length: V 0 =20 kV, V 1 =1 kV, P dyn =2 nPa, R=100 m, r w =1 mm : dF/dz = 500 nN/m, > 5 times higher than from PIC

13 Thrust prediction: conclusion (Nearly) no trapped electrons Thrust ~500 nN/m at 1 AU at +20 kV, scales as 1/r Thrust vector can be turned +-30 o Accurate thrust prediction is still challenge to simulation Stationary solution exists, or to some extent turbulent? Electrosphere/electropause magnetosphere/magnetopause Electrosphere is nearly vacuum, as is the magnetosphere (in terms of pressure)‏

14 Possible caveat Choiniere & Gilchrist ( ) solved stationary Vlasov- Poisson system iteratively, with “inside-out” backward orbit, for charged tethers in various conditions For highly positively charged tether in flowing plasma, their code does not converge to a solution (Choiniere, priv.comm. 2009)‏ They suspect that stationary solution does not exist. Is this a problem? Could the thrust be less than predicted above? Not sure. But usually turbulence increases friction rather than decreases it. Maybe the wake is turbulent to some extent, but the effect on thrust is minor?

15 E-sail technology

16 Status of technology Dynamical simulation capability exists Short pieces of final-type tether (25/50 um Al wire) produced by ultrasonic wire-to-wire bonding (Univ. Helsinki)‏ ESTCube-1 LEO test satellite project going on (launch 2012; will measure E-sail force for the first time in orbit)‏ Rather detailed idea how to build 1 N, 100 kg device Things to accomplish: – Scale up tether production – Demonstrate reliable reeling of tether

17 E-sail tethers Ultrasonic bonding of um AlSi wire (Seppänen et al. 2009)‏ Wire-to-wire ultrasonic bonding not done earlier, developed for E-sail for the first time (Univ. Helsinki)‏ “Tether factory” under construction, first goal 10 m

18 ESTCube-1 test mission Measure E-sail effect in Low Earth Orbit (LEO)‏ Use relative speed of satellite as “solar wind” Measure change of tether spinrate after turning voltage on in sync with rotation Deploy 10 m tether 1 kg, 10x10x10 cm nanosatellite (Cubesat standard)‏ Launch 2012 Satellite builder and mission operator: Tarto Payload: FMI, DLR-Bremen, Univ. Helsinki, Univ. Jyväskylä

19 Collaborators FMI: General design, ESTCube-1 payload electronics Tarto Observatory/Tarto Univ.: ESTCube-1 test mission Univ. Helsinki/Electronics Research Lab: tethers DLR-Bremen: tether reels Univ. Jyväskylä: Electron gun for ESTCube-1 ÅSTC-Uppsala & Nanospace Inc.: Tether spin start&control Univ. Pisa: Mission orbital calculations

20 What could E-sail do For example, 100 tethers, 20 km each: 1 N thrust, scales as 1/r Propulsion system mass 100 kg: specific acceleration 10 mm/s 2 (would overcome solar gravity 6 mm/s 2 at 1 AU)‏ Impulse over 10 year lifetime corresponds to 100-ton chemical rocket 1000 times more efficient than chemical rocket, 100 times more efficient than contemporary ion engine... Non-Keplerian orbit missions, off-Lagrange space weather monitoring, sample return from Mercury, mission out of heliosphere, multi-asteroid touring mission...

21 E-sail for planetary targets Moon-Mars-Venus: almost no benefit Mercury: large benefit, 9 month traveltime, also sample return Asteroids: large benefit, multi-asteroid mission & sample returns possible J,S,U,N one-way: Traveltime 2,4,8,14 years if combined with small chemical burn for orbit insertion (Quarta et al. 2009), mPay=700 kg J,S,U,N sample return(?): Somewhat tricky, but should be possible Kuiper&Heliopause one-way: not difficult, v > 50 km/s

22 E-sail “modes of transfer” Outward motion always possible Inward motion possible (“tacking”), but time proportional to orbital period of starting planet – Towards-Sun motion fast enough only from asteroid belt and closer – Returning from giant planet needs small chemical burn to kick s/c towards inner solar system, after which E-sail can bring it to earth Non-Keplerian missions – Off-Lagrange point solar wind monitoring – Helioseismology of Sun's poles, from circular orbit placed above ecliptic plane E-sail does not work inside magnetospheres

23 E-sail navigation Toivanen & Janhunen (2009)‏ Accurate navigation possible, although solar wind varies – Debye length change partly cancels P dyn variations – Maximum power -strategy also partly compensates thrust variations – Leave modest power in reserve ==> accurately navigable Thrust magnitude and direction independently controllable Turning thrust off is easy (just turn off electron gun)‏

24 Far-fetched application(?)‏ Mission to another solar system: Accelerate probe to high speed (>0.01c) with beamed energy propulsion system (laser, microwave or E-sail)‏ Brake with E-sail in ISM before approaching remote star Sail with E-sail in star's asterosphere and study its planets Issues/questions: – Beaming system needs high power==>expensive – Erosion of E-sail tethers in fast flight? – How to return data?

25 Summary Electric sailing seems possible and practical We derived better thrust estimates and introduced electrosphere/electropause concepts It seems possible to build 1 N, 100 kg device – 100 times more efficient than ion engine (in terms of lifetime- integrated impulse per mass)‏ ESTCube-1 LEO test mission Applications are numerous and large