THE ENVIRONMENT OF SPACE Col. John Keesee 1 Image courtesy of NASA. OUTLINE ? Overview of effects ? Solar Cycle ? Gravity ? Neutral Atmosphere ? Ionosphere ? GeoMagnetic Field ?Plasma ? Radiation 2 OVERVIEW OF THE EFFECTS OF THE SPACE ENVIRONMENT ? Outgassing in near vacuum ? Atmospheric drag ? Chemical reactions ? Plasma-induced charging ? Radiation damage of microcircuits, solar arrays, and sensors ? Single event upsets in digital devices ? Hyper-velocity impacts 3 4 ? Solar Cycle affects all space environments. ? Solar intensity is highly variable ? Variability caused by distortions in magnetic field caused by differential rotation ? Indicators are sunspots and flares Solar Cycle LONG TERM SOLAR CYCLE INDICES 5 ? Sunspot number R 10 (solar min) d R d 150 (solar max) ? Solar flux F 10.7 Radio emission line of Fe (2800 MHz) Related to variation in EUV Measures effect of sun on our atmosphere Measured in solar flux units (10 -22 w/m 2 ) 50 (solar min) d F 10.7 d 240 (solar max) SHORT TERM SOLAR CYCLE INDEX ? Geomagnetic Index A p – Daily average of maximum variation in the earth’s surface magnetic field at mid lattitude (units of 2 u 10 -9 T) A p = 0 quiet A p = 15 to 30 active A p > 50 major solar storm 6 GRAVITY force At surface of earth  G m 1 m 2 r 2  r G 6.672u 10 11 m 3 kg 1 2 2 E e 2 E e g sec m 9.8 R Gm g R Gm mf |  7 MICROGRAVITY 8 ? Satellites in orbit are in free fall - accelerating radially toward earth at the rate of free fall. ? Deviations from zero-g – Atmospheric drag – Gravity gradient – Spacecraft rotation (rotation about Y axis) – Coriolis forces 22 5.0 ZU a m AC x D ? ? ? ? ? 1 · ¨ ¨ ¨ ¨ ¨ ? §  222 2zwzywxxwx    22 ZZ zzxx  ZZ xzzoyzx   2 ATMOSPHERIC MODEL NEUTRAL ATMOSPHERE 9 ? Turbo sphere (0 ~ 120Km) is well mixed (78% N 2 , 21% O 2 ) – Troposphere (0 ~ 10Km) warmed by earth as heated by sun – Stratosphere (10 ~ 50 Km) heated from above by absorption of UV by 0 3 – Mesosphere (50 ~ 90Km) heated by radiation from stratosphere, cooled by radiation into space – Thermosphere (90 ~ 600Km) very sensitive to solar cycle, heated by absorption of EUV. ? Neutral atmosphere varies with season and time of day 10 Layers of the Earth’s Atmosphere TEMPERATURE MAGNETOSPHERE Pressure Molecular m ean free p ath EXOSPHERE Sunlit Spray region Warm region Maximum height for balloons THERMOSPHERE Aurora Aurora Airglow MESOSPHERE IONOSPHERE Noctilucent cloud D E F 1 F 2 Ozone region Sound waves reflected here Mother-of-pearl clouds Cirrus clouds Altocumulus clouds cumulus clouds Stratus clouds Tropopause STRATOSPHERE Mount Blanc Ben Nevis Temperature curve -100 0 C -50 0 C 0 0 C 50 0 C 100 0 C 1,000 mb 10 -8 mb 10 -6 cm 10 -4 cm 1cm 1km 100 km 100 mb 1 mb 10 -2 mb 10 -4 mb 10 -6 mb 10 -10 mb Miles 10,000 5,000 5,000 2,000 2,000 1,000 1,000 500 500 200 200 100 100 50 50 20 20 10 50,000 30,000 20,000 10,000 5 ,000 10 5 2 1 Kilometers Feet Mount Everest TROPOSPHERE DENSITY ALTITUDE MODEL Assume perfect gas and constant temperature n is number density (number/m 3 ) dpA - n m g A d h = o k is Boltzmann’s constant M is average molecular mass H ~ 8.4km h ~ 120km n = n o exp (-h/H) H { kT/mg (scale height) dh nkTd dh dp Tknp ? ? 1 · ¨ ¨ ? § dh nkTd nMg dh dp ? ? 1 · ¨ ¨ ? § dh KT Mg n dn p A dh p+dp 11 Atmospheric Gases ? At higher altitudes O 2 breaks down into O by UV ? Primarily O from 80 - 90 km to 500 km ? Hydrogen and Helium beyond 500 km ? Kinetic energy of O atom at 7.8 km/s ~ 5eV (enough to break molecular bonds ~1 - 2eV) ? O is highly reactive and destructive to spacecraft ? Temperature at LEO increases with altitude ? Atmosphere expands when heated by high UV (solar max) ? LEO densities ~ 10 8 particles/cm 3 12 ATMOSPHERIC MODEL 13 Most common Mass Spectrometer and Incoherent Scatter model - 1986 (MSIS - 1986) – Based on measured data – Requires A p , F 10.7 , month as input – Gives average values of n, n o , T, atomic mass as function of altitude – Instantaneous values can vary by factor of 10 http://nssdc.gsfc.nasa.gov/space/model/atmos/msis.html AERODYNAMIC DRAG Drag Ballistic coefficient U=density of the atmosphere=m o n o =16x1.67x10 -27 x10 13 =2.67x10 -13 kg/m 3 V=7.8km/s C D - Drag coefficient A - Cross sectional area D  1 2 Uv xv ( v v )CDA D m dv dt 'v 1 2 Uv 2 CDA m a ? ? ? o ? ? ? 't E m CDA a ? ? ? o ? ? ? 14 DRAG COEFFICIENTS Derived from Newtonian Aerodynamics. Depends on what air molecule does at impact – Reflected C D = 4 – Absorbed C D = 2 Since F = d(mv)/dt D = - F = - d(mv)/dt o m = U Av i dt C D = - 2 (v f -v i )/v i = 2 if v f = o in rarefied atmosphere = 4 if v f = - v i AdtV VVm AV D C i if D 2 2 2 1 2 1 U U ? ? 1 · ¨ ¨ ? §   A 15 TYPICAL DRAG PARAMETERS 16 E (kg/m 2 ) C D LANDSAT 25 - 123 3.4 - 4 ERS - 1 12 - 135 4 Hubble 29 - 192 3.3 - 4 90,000Kg Echo 1 0.515 2 Typically C D ~ 2.2 - 4 for spacecraft. (see SMAD Table 8.3) 'V over one year (E = 100 kg/m 2 ) h (km) 'V /year (m/s) 100 10 7 200 2 - 5 u 10 3 solar (min - max) 300 40 - 600 400 3 - 200 17 SATELLITE LIFETIMES Large variation depending on initial altitude and solar min/max condition (see SMAD Fig. 8 - 4) At LEO, design must compensate for effects of drag. MAGNETIC FIELD EFFECTS 18 ? Deflects charged particles/solar wind. – South Atlantic Anomaly ? Creates the structure of the ionosphere/plasmasphere – Magnetosphere – Van Allen radiation belts ? Direct effects on Spacecraft systems – Avionics - induced potential effects – Power - induced potential effects – GN&C - magnetic torquer performance, sizing – Structures - induced currents – TT&C - location of SAA GEOMAGNETIC FIELD 19 ? Earth’s Magnetic field comes from three sources – internal field (99%) ? currents inside the Earth ? residual magnetism of elements contained in crust – External field 1% ? Currents in the magnetosphere ?B i internal field varies slowly on the order of 100 years (0.05%/year.) ? Poles of magnetic field lie in Siberia and South Australia. 20 GEOMAGNETIC FIELD MAGNETOSPHERE 21 Magnetosphere (continued) 22 ? Earth’s field extends 10 Earth Radii (R H ) toward the sun - terminates at magneto pause ? Earth’s field slows and deflects solar wind – Compressed, heated, turbulent – Bow shock at about 14 R H ? Polar field lines are swept back in night-side tail – Does not close – Neutral sheet ? Surface of discontinuity in magnetic field implies current flow in the surface – Sunward magnetopause - eastward current flow across sub- solar point. – Neutral sheet current flow is westward across the tail EXTERNAL MAGNETIC FIELD 23 ?B e generated by ring currents and solar wind. Large variation with time – Milliseconds to 11-year cycle scales. ? Variations caused by – Magnetosphere fluctuations (geomagnetic storms) – Solar activity ? Geomagnetic storms dump large numbers of charged particles from magnetosphere into atmosphere – Ionizes and heats the atmosphere – Altitudes from 300 km to over 1000 km – Persist 8-12 hours after storm subsides GEOMAGNETIC COORDINATE SYSTEMS 24 Geomagnetic B - L B L=8 Greenwich meridian Geog r aphic nor th pole Geographic north pole Geographic Geomagnetic Solar-ecliptic Solar-magnetospheric Solar-magnetic Colatitude z z se x se x sm x sm y sm z sm z sm y sm T o sun T os u n y se y r r m z m r se x mx y m Magnetic colatitude East longitude T o nor th ecliptic pole Sun direction Several coordinate systems used in geomegnetism. Direction of geographic north pole Dipole axis direction Dipole axis Dipole axis Magnetic longitude φ φ se θ φ m φ se θ m B=.01 .02 .05 0.1 30 20 15 10 9 8 L=2 3 4 5 6 7 0 0 -90 -60 -30 1 2 3 4 0.2 .005 .002 .001 Field Strength (Oersteds) North Latitude (Degrees) Geocentric Distance (Re) GEOMAGNETIC FIELD Magnitude Formula/Models Tilted dipole (11q from geographic north) at LEO where M = 0.311 u 10 -4 =7.9u 10 15 T - m 3 International Geomagnetic Reference Field 1987 (IGRF1987) B i r,T m ,I m § ? ¨ · 1 ?  M r 3 3cos 2 T m § ? ¨ · 1 ?1 § ? ¨ · 1 ? 1/2 B r  M r 3 2cosT m B Tm  M r 3 sinT m B Im 0 TR e 3 25 FIELD VALUES ? Minimum (near equator) = 0.25 u 10 -4 T ? Maximum (near polar caps) = 0.50 u 10 -4 T ? Two peaks near north pole ? Two minimum near equator ? Largest minima is known as South Atlantic Anomaly – Much higher radiation exposure at LEO ? Geomagnetic storms impose variations of 0.01 u 10 -4 T 26 TOTAL FIELD INTENSITY 27 28 SOUTH ATLANTIC ANOMALY Reduced protection in SAA allows greater effect of high energy particles - electronic upsets, instrument interference. PLASMA EFFECTS OVERVIEW 29 ? Plasma is a gas made up of ions and free electrons in roughly equal numbers. ?Causes Elecromagnetic Interference Spacecraft charging & arcing Material effects ? Effects Avionics - Upsets from EMI Power - floating potential, contaminated solar arrays, current losses GN & C - torques from induced potential Materials - sputtering, contamination effects on surface materials PLASMA EFFECTS (cont.) ? Effects continued Optics systems - contamination changes properties of surface materials. Propulsion - Thruster firings change/shift the floating potential by contacting the plasma. 30 PLASMA GENERALIZATION 31 ? Plasma is caused by UV, EUV, X-ray photoelectric effect on atmospheric molecules. – Breaks diatomic molecule bonds. – Ejects electrons from outer shells. ? As UV, EUV, X-ray penetrate the atmosphere, ion density increases with atmospheric density until most UV, EUV have been absorbed (>60 Km altitude). Varies dramatically with altitude, latitude, magnetic field strength, time of day and solar activity. ? Electrically charged region of atmosphere is called the ionosphere. ? Gas in ionosphere is called ionospheric plasma. LEO PLASMA ENVIRONMENT 32 ? Balance between increasing density and increasing absorption leads to formation of ionization layers. F - layer 150 km - 1000 km E - layer 100 km - 150 km D - layer 60 km - 100 km ? Transition region from ion-free atmosphere to fully ionized region called the plasmasphere. ? Plasmasphere ion densities peak at 10 10 /m 3 to 10 11 /m 3 at 1000 km – Drops to 10 9 /m 3 at its boundary ? Outer boundary called plasmapause – Density drops to 10 5 /m 3 to 10 6 /m 3 – Height is ~ 4 RH between 0000 and 1800 hours – Expands to ~ 7 RH during the local dusk (dusk bulge) ELECTRON DENSITY 33 Solar Max Solar Min Daytime Electrons Nightime Electrons 10 1 100 1000 Altitude (km) 10 3 10 5 10 7 Density (cm -3 ) K p is Magnetic Activity Index PLASMAPAUSE HEIGHT VS LOCAL TIME 34 K p <1 K p =2 K p =4-5 K p =3 10 -2 N(H + ) (cm -2 ) OGO S, Nightside, 1968 1234567 10 0 10 2 10 4 L ION CONCENTRATIONS – Similar to neutral atmosphere D - layer NO + /O + E - layer O + F - layer O +/ H + -Daytime F layer density peaks at 10 12 /m 3 (300 km) -Nighttime F-layer density drops to 10 11 /m 3 (500 km) – Composition transitions from O + to H + 35 ION CONCENTRATIONS (cont.) 36 10 1 He++ O 2 + Altitude (km) Ion Concentrations (ions/cm 3 ) O + H + NO + 150 200 250 300 350 400 450 500 550 600 650 10 3 10 5 PLASMA TEMPERATURES Increases from ~100K at 50 - 60 km to 2000 - 3000K above 500 km Electron temperature T e = 4000K - 6000K Ion temperature T i = 2000K - 3000K Density much higher at solar maximum due to higher UV/EUV fluxes. 37 LEO PLASMA ENVIRONMENT MODELS International Reference Ionosphere (IRI) -Outputs - electron density n e - ion composition n i - Temperature T e , T i -Inputs (latitude, longitude, altitude, solar activity (R), time). Available at : http://nssdc.gsfc.nasa.gov/space/model/ionos/iri.html “Ionospheric models” Carlson, Schunk, Heelis, Basu 38 RADIO FREQUENCY TRANSMISSIVITY – Plasma transitions from a perfect conductor to perfect dielectric as a function of frequency. – Plasma frequency – Dielectric constant –ForZ >> Z pe the plasma appears like free space –ForZ~ Z pe electromagnetic waves cannot propagate ? Transmissions from below are reflected ? Transmissions from within are absorbed –ForZ!~ Z pe random variations in n e can cause random delays and phase shifts Z pe n e e 2 H o m § ? ¨ ¨ · 1 ? ? 1 2 H H o 1 Z pe Z § ? ¨ ¨ · 1 ? ? 2§ ? ¨ ¨ ¨ · 1 ? ? ? 39 SPACECRAFT CHARGING ? At LEO spacecraft become negatively charged – Plasma is dense but low energy – Orbital velocity is higher than ion thermal velocity – Lower than electron thermal velocity – Electrons impact all surfaces – Ions impact ram surfaces only ? Geo spacecraft charge during magnetospheric substorms between longitudes corresponding to midnight and dawn ? Biased surfaces (solar arrays) influence the floating potential 40 CHARGING EFFECTS ? Instrument reading bias ? Arcing-induced EMI, electronics upsets ? Increased current collection ? Re-attraction of contaminants ? Ion sputtering, accelerated erosion of materials Spacecraft must be designed to keep differential charging below the breakdown voltages or must tolerate the effects of discharges. 41 42 RADIATION ? Most radiation effects occur by energy depostion – Function of both energy, type of particle and material into which energy is deposited. ? Definitions 1 rad (Si) = 100 ergs/gm into Silicon 1 Cray (Si) = 1 J/kg into Si 1 rad (Si) = 10 -4 Cray Adapted from SMAD. 43 RADIATION DAMAGE THRESHOLDS In many materials the total dose of radiation is the most critical issue. In other circumstances the time over which the dose is received is equally important. Material Damage Threshold (rad) Biological Matter 10 1 -10 2 Electrical Matter 10 2 -10 4 Lubricants, hydraulic fluid 10 5 -10 7 Ceramics, glasses 10 6 -10 8 Polymeric materials 10 7 -10 9 Structural metals 10 9 -10 11 SPACECRAFT EFFECTS ? High energy particles travel through spacecraft material and deposit kinetic energy – Displaces atoms. – Leaves a stream of charged atoms in their wake. ? Reduces power output of solar arrays ? Causes sensitive electronics to fail ? Increases sensor background noise ? Radiation exposure to crews 44 HIGH ENERGY RADIATION ? Definition For Electrons E > 100 keV For protons and heavy ions E > 1 MeV ? Sources – Van Allen Belt (electrons and protons) (trapped radiation) – Galactic cosmic rays interplanetary protons and ionized heavy nuclei – Protons associated with solar proton events 45 VAN ALLEN BELTS ? Torodial belts around the earth made up of electrons and ions (primarily protons) with energies > 30 keV. ? Two big zones – Inner belt ~ 1000 Km 6000 km altitude ? Protons E > 10’s of MeV ? Electrons E ~ 1 - 10 MeV – Outer belt 10,000 - 60,000 km ? Electrons E ~ 0.04 - 4.5 MeV 46 VAN ALLEN BELTS (cont.) ? Sources – acceleration of lower-energy particles by magnetic storm activity – trapping of decay products produced by cosmic ray collisions with the atmosphere – solar flares 47 CONCENTRATION MECHANISM ? Earth’s magnetic field concentrates on large fluxes of electrons, protons and some heavy ions. ? Radiation belt particles spiral back and forth along magnetic field lines. – Ionizing radiation belts reach lowest altitude of the eastern coast of the eastern coast of South America (SAA). 48 (Image removed due to copyright considerations.) ELECTRON AND PROTON FLUXES 49 AP8Min Proton Fluxes (cm -2 s -1 ) 2x10 6 2x10 6 3x10 6 10 6 -2.5 z( R e ) x(R e ) 2.5 2.5 5.0 7.5 10.0 0 0 10 5 10 4 10 3 10 2 10 5 10 4 10 3 10 2 AE8Max Electron Fluxes (cm -2 s -1 ) 5 YEAR DOSE 50 10 2 10 2 10 3 10 4 5 YEAR DOSE, Rad (Si) ALTITUDE, nml DMSP Altitude (96 0 ) GPS Altitude (55 0 -63 0 ) Synchronous altitude (DSP, DSCS, Fltsatcom - 0 o ) 5Times synchronous alt Free space (Flare only) 0 0 63.4 0 90 0 Inclination Natural environment No operational satellites 10 5 10 6 10 3 10 4 10 5 10 6 TRAPPED RADIATION BELTS 51 10 4 10 8 10 7 10 6 10 9 5x10 8 N Electrons >40 Kev 1 2 3 10 2 10 1 10 3 Distance from center of earth (Earth Radii) Protons >100 Mev VAN ALLEN BELT RADIATION STABILITY ? Inner belt – Fairly stable with changes in solar cycle – May change by a factor of three as a result of geomagnetic storms loading in high energy electrons. ? Outer belt – Electron concentrations may change by a factor of 1000 during geomagnetic storms. ? Standard Models (AP8 protons) and (AE8 electrons) – Require B, L and whether solar min/solar max – Provide omni-directional fluxes of protons 50 keV < E < 500 MeV and electrons 50 keV < 7 Mev 52 53 SOLAR CELL DEGRADATION 0.4 NORMALIZED EFFICIENCIES 0 10 13 10 14 1 MeV ELECTRON FLUENCE (cm -2 ) Degradation caused by the radiation of InP, GaAs, conventional (8mil) Si, and thin (3 mil) Si solar cells. 10 15 10 16 0.6 0.8 1.0 InP GaAs/Ge S1 (2.6-3.1 mils thick) S1 (8 mils thick) GALACTIC COSMIC RAYS ? Primarily interplanetary protons and ionized heavy nuclei – 1 MeV < E < 1 GeV per nucleon Cause Single Event Upsets (SEU) ? Sources are outside the solar system – other solar flares – nova and supernova explosions – quasars 54 PARTICLE RANGE 55 Ranges of Protons and Electrons in Aluminum PARTICLE ENERGY (MeV) RANGE (cm) Electrons Protons 0.1 0.01 0.1 1 10 1 10 100 MAGNETIC SHIELDING 56 Magnetic Equator 12 Any Ion β 1147 MeV/n 2900 MeV/n 907 MeV 384 MeV 173 MeV 87 MeV 48 MeV 313 MeV/n 109 MeV/n 46 MeV/n 23 MeV/n 12 MeV/n 34 567 SOLAR PROTON EFFECTS ? Solar flares often eject high energy hydrogen and other nuclei – 1 MeV < E < 10 GeV/nucleon – At low energies the number can be much greater than galactic comic radiation level ? Solar events are sporadic but correlate somewhat with the solar cycle ? These events make a Mass Mission hazardous 57 PARTICLE ENERGY 58 Energy (MeV) 10 1x10 -6 Galactic Cosmic Rays Particles/ m 2 sec (MeV) ster Worst Case Solar Flare Event 1x10 -4 1x10 -2 1x10 2 1 1x10 4 1x10 6 1x10 8 100 1000 10000 100000 SOLAR PROTON DOSE 59 FEYNMAN MODEL Based on data from 1963 to 1991 60 1 Year 2 Year FLUENCE (cm -2 ) 10 9 0.001 0.01 0.1 1 10 10 10 11 PROBABILITY 3 Year 5 Year 7Year ELECTROMAGNETIC RADIATION ?Radio – 1 - 10 MHz galactic electromagnetic radiation – terminal noise – not significant for single event environment ? Visible/IR – solar flux – heating ? UV/EUV/X-ray – EUV @ 100 to 1000 ? is significant for surface chemistry 61 References ? Wertz, James R. and Wiley JH. Larson, Space Mission Analysis and Design, Third edition, Microcosm Press, El Segundo CA 1999 ? Pisacane, Vincenti and Robert C. Moore, Fundamentals of Space Systems, Oxford University Press, NY, 1994. ? http://nssdc.gsfc.nasa.gov/space/model/models_home.html ? http://nssdc.gsfc.nasa.gov/space/model/magnetos/igrf.html 62