Mercury Environmental Specification – Issue 3.0 Mercury Environmental Specification (Part II) BepiColombo Definition Study prepared by J. Sørensen & H. Evans Space Environment and Effects Analysis Section ESA/ESTEC/TOS-EMA reference SCI-PF/BC/TN/02 issue 3.0 revision 0 date of issue 9 February 2004 1 Mercury Environmental Specification – Issue 3.0 CHANGE LOG date issue revision pages Reason for change 10/12-01 1.0 0 All First issue 1/8-02 2.0 0 5,7 New trajectory 9 Update of solar constant (ref. part I of the specification) 21,22,24,27-35 Updated radiation analysis (applying inverse square scaling for all energies) 39-42 Case 2 deleted, cruise phase fluences updated 61-67 Appendix A deleted 30/1-04 3.0 0 2-7, 19-39, New trajectory and launch date 42-43 2 Mercury Environmental Specification – Issue 3.0 Table of Content Chapter 1. Introduction 2. Solar and Planetary electromagnetic radiation environment 3. Plasma 4. Energetic Particle Radiation 5. Particulates 6. Contamination 3 Mercury Environmental Specification – Issue 3.0 4 Mercury Environmental Specification – Issue 3.0 1 Introduction The Space Environment can cause severe problems for any space system including BepiColombo on its mission to the planet Mercury. Proper assessment of the potential effects is an essential part of the engineering process leading to the construction of any element of the spacecraft. It is important that this is taken into account from the earliest phases of a project when consideration is given to mass budget, protection, component selection policy, etc. As the design of an element is developed, further engineering iteration is normally necessary with more detailed analysis. This document is Part II of the Mercury Environmental Specification prepared for the BepiColombo definition study. The specification is organised as follows: · Part I contains physical and dynamic properties, thermal environment, surface properties and morphology specifications (document SCI-PF/BC/TN/01). · Part II contains electromagnetic, plasma and energetic particle radiation environment, micro-meteoroids (particulates) and contamination specifications (this document). This document is intended to assist the developers of instruments for BepiColombo in assessing the effects of the space environment on their systems. The document is based on the ECSS-E-10-04 Space Environment Standard, from which most of the background information has been taken (ECSS is a cooperative effort of the European Space Agency, National Space Agencies and European industry associations for the purpose of developing and maintaining common standards). The ECSS- E-10-04 standard can also be accessed via the WWW site http://www.estec.esa.nl/wmwww/wma/. The Mission BepiColombo consists of 2 satellites to be launched in May 2012 to explore the planet Mercury: 1. A Planetary Orbiter (400x1500km polar orbit, 3-axis stabilised) 2. A Magnetospheric Satellite (400x12000km polar orbit, spinning 15rpm) Both share the same carrier to Mercury, where the satellites are deployed to their respective orbits. The transfer phase to Mercury starts with 3 to 6 loops in an Earth orbit (with apogee 450000km, the perigee gradually rising from 200km to 10000km and inclination 51.6deg). This is followed by a lunar flyby, after which the actual journey to Mercury will start in July 2012. The duration of the transfer is 4.35 years in addition to the up to 0.20 years in the initial loops in an Earth orbit. Figure 1 gives an impression of the trajectory of the transfer phase in terms of the distance to the sun as a function of time. The duration of the operational phase is assumed to be 2 years, giving a total duration of 6.55 years for the mission. 5 Mercury Environmental Specification – Issue 3.0 The Space Environment In general, when assessing the effects of the space environment on an instrument, the following environments should be included: · Solar and Planetary Electromagnetic Radiation · Neutral Atmosphere · Plasmas · Energetic Particle Radiation · Particulates · Contamination · Thermal 1) For the environment around Mercury only very little is known about many of these components. Indeed part of the purpose of the mission is to investigate the nature of several of these environments around Mercury. Each component of the space environment is treated separately, although synergies and cross- linking of models are specified. The natural environment is described together with the general models in use and principles for determining the local induced environment. Mercury has a very tenuous atmosphere. The gaseous environment of the planet is best described as an exosphere i.e. a medium so rarified that its neutral constituents never collide. The neutral atmosphere is described in part I of the specification. Each of the other environments are treated in separate chapters. Many of the models mentioned are also installed in the Space Environment Information System (Spenvis), which can be accessed via the WWW site http://www.spenvis.oma.be/spenvis/. For many of the associated analyses it is necessary to take the geometry of the spacecraft into account, but such geometrical analyses are out of the scope of this document. 1) Although some data is repeated here for completeness, the thermal environment is described in part I of the specification. 6 Mercury Environmental Specification – Issue 3.0 Bepi Colombo 2012 trajectory 1.2 1.0 0.8 0.6 0.4 Distance from the SUN [AU] 0.2 0.0 4500 5000 5500 6000 6500 Time since 1/1-2000 [days] Figure 1: BepiColombo trajectory 7 Mercury Environmental Specification – Issue 3.0 8 Mercury Environmental Specification – Issue 3.0 2 Solar and Planetary electromagnetic radiation environment 2.1 Introduction The BepiColombo spacecraft will on their journey to Mercury and in the orbit around the planet receive electromagnetic radiation from several external sources. The largest source is the direct solar flux. This will be present during the entire mission. The fraction of incident sunlight that is reflected off a planet is termed albedo. When in orbit around Mercury this will also contribute to the flux received by the spacecraft. The albedo value depends mainly on the sunlit part of the planet, which the spacecraft can see. Albedo radiation has approximately the same spectral distribution as the sun. It is highly variable across the planet and depends on surface properties. A third source is the planet infrared radiation. The planet-emitted thermal radiation has a spectrum of a black body and also varies across the planet. The electromagnetic radiation varies with solar activities, which is highly variable over a solar cycle. The given data are mainly average values. For thermal analyses or certain special applications more detailed treatment is required. For these, chapter 4 of part I of the specification shall be used. 2.2 Solar electromagnetic radiation Direct solar flux The direct solar flux is inverse proportional to the square of the distance to the Sun. At the Earth’s distance the value is fairly constant, but since Mercury is in a slightly elliptical orbit (eccentricity 0.206) the value will vary considerably through Mercury’s orbit. The following values for the electromagnetic radiation shall be used (solar energy that falls on a unit area of surface normal to the line from the Sun, per unit time): Average solar irradiance at the Earth (the solar constant) 1366 W/m2 Average solar irradiance at Mercury 9116 W/m2 Aphelion solar irradiance at Mercury (at 0.47AU) 6272 W/m2 Perihelion solar irradiance at Mercury (at 0.31AU) 14448 W/m2 (the average solar irradiance at the Earth has an uncertainty of about ±3W/ m2) Solar spectrum The solar spectrum shall be approximated by a black body curve with a characteristic temperature of 5780K (this is the temperature that at 1AU gives the value of the solar constant reported above). A space sink temperature of 3 K shall be assumed. The UV portion (wavelength, l,< 300 nm) of the electromagnetic spectrum is of particular importance in determining effects of solar radiation on material properties. The integrated irradiance of the near UV electromagnetic radiation flux (180 nm < l < 400 nm) is approximately: Average near UV at the Earth 118 W/m2 Average near UV at Mercury 787 W/m2 9 Mercury Environmental Specification – Issue 3.0 Aphelion near UV at Mercury 542 W/m2 Perihelion near UV at Mercury 1248 W/m2 The far UV portion (l < 180 nm) contributes about 0.023 W/m2 at the Earth’s distance (0.15 W/ m2 at Mercury). Certain parts of the spectrum are varying very much, both, over the 27-day solar rotation period and over the 11-year solar cycle. This variation ranges from about 50 % for the near UV part to a factor 2 for the UV and far UV portions and can reach orders of magnitude for flare X-rays. Average and worst case irradiance levels for the high-energy spectrum are summarized in Table 1. The average values for the Earth’s distance were taken from [1]. Table 1: High-energy solar electromagnetic flux Type Wavelength At the Earth’s distance At Mercury’s distance (nm) Average Flux Worst-Case Flux Average Flux Worst-Case Flux (W/m2) (W/m2) (W/m2) (W/m2) Near UV 180-400 118 177 787 1872 UV < 180 2.3 ´10-2 4.6 ´10-2 0.15 0.49 UV 100-150 7.5 ´10-3 1.5 ´10-2 5 ´10-2 0.16 EUV 10-100 2 ´10-3 4 ´10-3 1 ´10-2 4 ´10-2 X-Rays 1-10 5 ´10-5 1 ´10-4 3 ´10-4 1 ´10-3 Flare X-Rays 0.1-1 1 ´10-4 1 ´10-3 7 ´10-4 1 ´10-2 For design purposes the worst-case values of Table 1 shall be used.