1. SKOBELTSYN INSTITUTE OF NUCLEAR PHYSICS, LOMONOSOV MOSCOW STATE UNIVERSITY, RUSSIA Radiation Conditions of a Mission to Jupiter ʼ s Moon Ganymede M. V. Podzolko, I. V. Getselev In cooperation with scientists from IAM, IKI and Russian Ganymede Lander Team – Strong charged particle fluxes and radiation doses in Jupiter’s magnetosphere – Radiation dose in Ganymede’s orbit: risks considerably lower, than near Europa – Radiation dose during gravity assists in Jupiter’s system: comparable, or several times higher, than during 2 months in Ganymede’s orbit. – Radiation doses during near-Earth and interplanetary flight: low, compared to doses in Jupiter’s near-planetary space –Conclusions, discussion

2. Magnetosphere and charged particles satellite measurements at Jupiter missiontimeorbitexperiments doses Pioneer 10Dec. 1973 Fly-by at 130 ths. km from Jupiter (2.8 R J ) Magnetic field, electrons: 0.06 to >35 MeV, protons: 0.6 to >80 MeV 1.5·10 6 rad on the surface, 4.5·10 5 rad at 3 mm Al Pioneer 11Dec. 1974 Fly-by at 43 ths. km (1.6 R J ), high incl. orb. 4.3·10 5 rad on the surface, 1.2·10 5 rad at 3 mm Al Voyager 1March 1979 Fly-by at 207 ths. km (4 R J ) Magnetic field, low-energy particles, electrons: 3–110 MeV, ions: 1–500 MeV/nuc ≈ 5·10 5 rad Voyager 2July 1979 Fly-by at 570 ths. km (9 R J ) UlyssesFeb. 1992 Fly-by at 378 ths. km (6.3 R J ), high incl. orb. Magnetic field, electrons: 0.03 to >170 MeV Ions: 0.05–75 MeV/nuc estim. 6·10 4 rad (inside?) Galileo1995–2003 35 highly elliptical orbital segments with r π typically 6–11 R J Magnetic field, electrons: 15 keV to >11 МeV, Ions: 10 keV to 200 MeV/nuc Designed for 150 krad at 2.2 g/cm 2, sustained >650 krad; “remarkably healthy”, but damaged some electronic systems CassiniNov. 2000 Fly-by at 10 mln. km (140 R J ) Magnetic field sync. w/Galileo, high-energy electrons (radiation spectrometer) New Horizons Feb. 2007 Fly-by at 2.3 mln. km (32 R J ) Local and remote (radio- spectrometer) magnetic field and charged particle measurements

3. N. Divine, Jupiter radiation belt models, Techn. Mem. 33-715, 1974, 13 p. /1st model, used Pioneer 10 data/ R. W. Fillius, C. E. McIlwain, A. Mogro-Campero, Radiation belts of Jupiter: a second look, Science, v. 188, 1975, p. 465–467. /An update of the model, using Pioneer 11 data/ N. Divine, H. B. Garrett, Charged particle distributions in Jupiter’s magnetosphere, J. Geophys. Res., v. 88, No 9, 1983, p. 6889–6903. /Model, based on the data from Pioneer 10, 11 and Voyager 1, 2/ M. H. Acuna, N. F. Ness, The main magnetic field of Jupiter, J. Geophys. Res., 81, 1976, p. 2917– 2922. /The “O4” Jupiter’s magnetic model, 15 Gauss coefficients/ I. V. Getselev, Yu. I. Gubar et al., Radiation conditions of the spacecraft flight in Jupiter’s near- planetary space, MSU, VINITI No 4636-84, 1984. (in Russian). I. V. Getselev, Yu. I. Gubar et al., Model of the radiation environment of Jupiter’s artificial satellites, MSU, VINITI No 8970B, 1985. (in Russian). H. B. Garrett, I. Jun, J. M. Ratliff, R. W. Evans, G. A. Clough, R. W. McEntire, Galileo interim radiation electron model, Publication 03-006, Jet Prop. Lab., California Inst. Tech., Pasadena, California, 85 p., 2003. /A revision of Divine-Garrett model for electrons at L = 8–16 using Galileo data/ H. B. Garrett, S. M. Levin, S. J. Bolton, A revised model of Jupiter’s inner electron belts: updating the Divine radiation model, Geophysical Research Letters, v. 32, L04104, 2005, 5 p. /A revision of Divine-Garrett model for relativistic electrons at L < 4 from Cassini and VLA synchrotron observations/ Models of Jupiter’s radiation belts

4. Charged particle flux and radiation dose equatorial profiles at Jupiter GanymedeIoAmaltheaEuropaCallisto [ 10 rad/day ] [ 15 krad/day ] [ 150 rad/day ] Equatorial profiles of integral fluxes of >0.5, >2 and >10 MeV electrons (upper left), >2, >10 and >30 MeV protons (upper right) and doses behind 1, 2.2 and 5 g/cm 2 (bottom) at Jupiter.

5. Doses under various shielding in Europa’s (solid line) and Ganymede’s (dash line) orbits. Calculated radiation doses in the orbits of Europa and Ganymede g/cm 2 EuropaGanymede 1.02.2·10 6 3.5·10 4 2.28.8·10 5 9.0·10 3 5.02.4·10 5 2.0·10 3 10.04.5·10 4 5.2·10 2 2-month doses in Europa’s and Ganymede’s orbits, rad. Integral fluxes of electrons in Europa’s (solid line) and Ganymede’s (dash line) orbits.

6. Computed electron fluxes on Europa’s surface and at 100 km altitude 50 MeV, 100 km 5 MeV, 100 km5 MeV, 0 km 50 MeV, 0 km

7. Radiation doses on Europa’s surface and at 100 km circular orbit 2.2 g/cm 2, 0 km 5 g/cm 2, 0 km

8. Ganymede: radiation risks considerably lower, compared to Europa Mission to Europa Radiation dose during 2 months behind 2.2 g/cm 2 : 1 Mrad Modeling the “shading” by Europa; optimal orbit and landing site: –100 krad at the surface on the leading side relative to Europa’s orbital motion, and the high-latitude regions; –250 krad in the 100 km orbit with inclination >60° Mission to Ganymede Radiation dose during 2 months behind 2.2 g/cm 2 : 10 krad – Radiation risk can be considerably lower.

9. General scheme of gravity assist maneuvering in the system of Jupiter Alexey Grushevskii et al., Keldysh Institute of Applied Mathematics, Moscow, Russia 1)First approach to Jupiter; firing the engines in the pericenter or using the gravity of one of its moons for transferring to highly-elliptical orbit of Jupiters’s artificial satellite; 2)Several gravity assists near Jpuiter’s large moons for lowering the spacecraft’s speed relative to Jupiter

10. Doses during the 1st fly-by and on the elliptical orbits around Jupiter Total impulse during the 1st circuit (upper plot) and radiation doze behind 2.2 g/cm 2 during 1st Jupiter’s fly-by (lower plot) for the orbit’s inclination i = 0, 20, 30 и 40°. The time of the 1st circuit was assumed equal to 90 days, the asymptotic speed – 6 km/s. The dependency of the radiation doses under 2.2 g/cm 2 during one full circuit of the orbit around Jupiter from the orbit’s apocenter, for the values of the pericenter r п = 10, 11, 12, 13 и 14 Jupiter radii (R J = 71490 km). dv p + dv a (Ganymede), km/s Dose under 2.2 g/cm 2, krad

11. Low radiation risk strategy of gravity assists in the system of Jupiter Rising the pericenter of each circuit up to Ganymede’s orbit radius plus the asymptotic distance by the impulse in apocenter. 00

12. NoDist. from Ganymede km r p, R J r a, R J Multiple of Ganymede circuits Ʃ (dv a ), km/s dv G,100km, km/s Ʃ (dv), km/s Time, days Dose for 2.2 g/cm 2, Krad 0’– straight entering of the orbit around Ganymede4.000 0.10 012.501.150 km/s in orbit’s pericenter1.00 1 12.50225.110.08332.8294.061160.93.60 2 65314.93167.43150.08842.6963.934268.53.86 3 94214.93133.38110.09462.5703.814347.44.15 4 66014.90105.0480.10412.4103.663404.94.46 5 72314.88 84.2560.11502.2353.501440.64.80 6176814.93 72.7850.12732.1053.382484.15.13 7125014.87 60.7240.14441.9283.222513.05.48 8 66314.76 47.6430.17661.6652.990534.95.87 9213014.83 40.312 × 2.50.20431.4712.824571.06.62 10204814.75 32.9320.24771.2292.626585.87.05 11161814.49 24.562 × 1.50.34020.9002.390608.08.00 Parameters of the considered series of gravity assists in Jupiter system Total dose for the suggested sequence of gravity assists behind 2.2 g/cm 2 amounts to 8 Krad.

13. Radiation on the interplanetary part of the trajectory Total differential solar (solid line) and galactic (dash line) proton spectra during the interplanetary part of the mission, using models (R. Nymmik, 1993; Tulka et al., 1997; ISO 15390, 2004;). Total dose behind 1–2 g/cm 2 <1 krad. Launch in 2020, approach to Jupiter in 6 years. The 1st phase includes 1 gravity assist near Venus and 2 – Earth and lasts 4 years, during which the spacecraft flies at 0.7–2 AU from the Sun. And then the final path to Jupiter.

14. Radiation during the flight in Earth’s radiation belts x z x y x z x y Trajectory of launching to the interplanetary space from the low-altitude Earth’s orbit (left plot) and gravity assist near Earth (right plot). Upper estimation of the integral spectra of protons (o) and electrons (□) during gravity assists near Earth (solid lines) and in the launch orbit (dashed lines). The radiation dose is <100 rad.

15. Conclusions, discussion –Main radiation hazard for the mission to Ganymede will come from Jupiter’s radiation belts. The major radiation dose under shielding >1 g/cm 2 will originate from relativistic electrons. –The radiation dose under 2.2 g/cm 2 during 2 months in the Ganymede’s orbit will amount to ≈10 krad. Thus the radiation risks for the mission to Ganymede can be considerably lower, than for mission to Europa, during which the spacecraft will receive a dose of several thousand krad behind the same shielding. – Radiation dose on the trajectory of gravity assists in Jupiter system will be comparable or several times higher, than during 2 months in vicinity of Ganymede – The trajectory of the flight in Jupiter system should whenever possible pass not closer than 11–12 Jupiter radii from the planet to lower the radiation risks. Thus for the gravity assists it is appropriate to use Ganymede and Callisto. –Radiation dose during one orbital circuit in the “outer region” of Jupiter’s magnetosphere at the dayside can amount to ≤150 rad. For the whole mission this value can reach up to 1–2 krad. This value is not negligible, but the main hazard still comes from the radiation belts. –The dose during near-Earth and interplanetary parts of the mission will be low, compared to that in Jupiter’s magnetosphere. –Working on Ganymede Lander project we have to solve the complex optimization task, simultaneously taking into account many factors: radiation, energy consumption, limits on the size and weight of the scientific equipment, data transfer and so on.