1. Radiation environment estimates for Europa lander mission M. V. Podzolko 1, I. V. Getselev 1, Yu. I. Gubar 1, I. S. Veselovsky 1,2, A. A. Sukhanov 2 1 Skobeltsyn Institute of Nuclear Physics, Lomonosov Moscow State University, Russia 2 Space Research Institute (IKI), Russian Academy of Sciences, Moscow, Russia – Models of Jupiter’s radiation belts and satellite experiments near Jupiter – Charged particle fluxes and radiation doses in Europa’s orbit – Charged particle fluxes and radiation doses during the gravity assists near Jupiter – Radiation environment in the interplanetary and near-Earth’s part of the trajectory – Proposals for radiation environment control during the mission – 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. Equatorial profiles of the integral fluxes of E > 0.5, >2 and >10 MeV electrons and E > 2, >10 and >30 MeV protons at Jupiter. Charged particle flux and radiation dose equatorial profiles at Jupiter Equatorial profiles of radiation doses under 0.27, 1, 2.2 and 5 g/cm 2 shielding near Jupiter.

5. electrons protons total Renard et al., 04 mm Al g/cm 2 Doses under various shielding from electrons (dashed line), protons (dash-dot line) and total dose (solid line); also the doses from Renard et al. (2004) are given (triangles). Calculated radiation doses in Europa orbit: high hazard g/cm 2 radg/cm 2 rad 0.008.5·10 8 1.02.2·10 6 0.011.0·10 8 2.28.8·10 5 0.101.5·10 7 5.02.4·10 5 0.277.4·10 6 10.04.5·10 4 Total 2-month doses in Europa orbit (at 9.5 R J ) Integral fluxes of electrons (red) and protons (blue) in Europa’s orbit.

6. 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 altitude 5 g/cm 2, 100 km 2.2 g/cm 2, 100 km2.2 g/cm 2, 0 km 5 g/cm 2, 0 km

8. Optimal landing site and orbit The most hazardous region on Europa’s surface with regard to radiation dose is its trailing side along its orbital motion. But the doses there are ≈4–5 times lower than maximum value without Europa’s influence, i. e. 200–250 krad within 2 months behind 2.2 g/cm 2 ; and the doses also lower for about 3 times from the middle latitudes to the equator (≈100 krad behind 2.2 g/cm 2 during 2 months at the equator on the trailing side). More safe are the leading side and the high- altitude regions, the dose there appears to be <100 krad during 2 month behind 2.2 g/cm 2 (and <25 krad behind 5 g/cm 2 correspondingly). Orbital module will suffer higher radiation impact than the lander. The most safe 100 km orbit around Europa will be the polar orbit, but the spacecraft in it will receive ≈200 krad behind 2.2 g/cm 2 during 2 months. Thus we recommend the minimal shielding for the orbiter equivalent to 5 g/cm 2 Al; behind that shielding the spacecraft will receive ≈25 krad dose during 2 months in the polar orbit.

9. Dependence of a) total impulse dv 1 + dv 2, b) radiation dose during the 1st fly-by from the pericenter distance. Example of such trajectory: radiation dose 50 and 20 krad behind 2.2 и 5 g/cm 2 correspondingly; total impulse dv 1 + dv 2 + dv 3 = 2.5 km/s; duration <350 days, from which <20 days in Jupiter’s radiation belts. Optimal trajectory should include the 1st fly-by with a pericenter radius ≤1.5–1.8 R J and inclination i ≈ 40°; then gravity assists, using Ganymede; and final approach to Europa, preceded with a few circuits with an apocenter near Ganymede and the pericenter radii ≈ 12–13 R J. Radiation on the trajectory of gravity assists in the system of Jupiter

10. Radiation at the interplanetary part of the trajectory 5-year interplanetary proton spectra for the mission, calculated using simple model (Podzolko, Getselev, 2005, 2009). Upper and lower curves can be considered for optimistic and pessimistic upper estimate, under assumption that major part of the flight will take place during low Solar activity. Total dose <1 krad. “Galileo” was launched in October 1989 during the 22rd cycle maximum and received a dose up to 50 krad under 2.2 g/cm 2 during the interplanetary flight; although it amount to <10% from the dose of 600 krad it received in the system of Jupiter. Mission start in 2017–2020, flight to Jupiter 5–6 years. The 1st phase will include several gravity assists near Earth and Venus and last 2–2.5 years, during which the spacecraft will fly at 0.7–2 AU from the Sun. And then the final path to Jupiter.

11. 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.

12. 1.Radiation control onboard spacecraft: Spectrometer of linear energy transfer – dosimetry; – model of LET and dose distribution for Jupiter – need LET spectra to estimate the single event effects (SEE) frequency; – inverse task: discover or verify particles spectra from the measured LET spectra 2.For the orbiter module: Spectrometer of energetic electrons and protons – study energetic electrons and protons in Jupiter’s magnetosphere and interplanetary space; mass200–250 g power0.15–0.5 W telemetric information≤250 KB/day registered particles electrons: 0.1–50 MeV, protons: 1–150 MeV mass≈2.5 kg power≤5 W≤5 W telemetric information≤1 MB/day Proposals for radiation studying and environment control during the mission

13. Conclusions, discussion –Main radiation hazard for the mission will come from Jupiter’s radiation belts. Most of the radiation dose will be received in Europa’s orbit and on its surface from relativistic electrons –Relativistic electron flux and radiation doses distribution on Europa’s surface and in 100 km orbit have been modeled. Preferred regions for the landing are the leading side of the moon, the high-latitude near-equatorial regions – radiation doses there appear to be 10 times lower, than the maximum value without Europa’s influence. The most radiation-safe 100 km orbit around Europa is the polar orbit; but the doses in it will be higher, then on the surface – we suggest using the shielding equivalent to at least 5 g/cm 2 Al for the orbiter. – Gravity assists near Jupiter and its Galilean satellites is also connected with serious radiation hazard. We conclude, that the optimal trajectory with regard to minimizing both the energy consumption and the radiation dose should contain the 1st gravity assist near Jupiter with the pericenter closer than ~150 thousand km and an inclination of 40°; next the gravity assists using Ganymede, and possibly short final approach to Europa. – The doses on other parts of the trajectory will be considerably smaller. –Proposals have been made on radiation environment monitoring at all stages of the flight. –Working on Europa 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. –Improving the models of Jupiter’s radiation belts and magnetic field is still actual.