THE ENVIRONMENT OF SPACE Image courtesy of NASA. Col. John Keesee 1 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 Solar Cycle • 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 4 LONG TERM SOLAR CYCLE INDICES • Sunspot number R 10 (solar min) d R d 150 (solar max) • Solar flux F10.7 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/m2) 50 (solar min) d F10.7 d 240 (solar max) 5 SHORT TERM SOLAR CYCLE INDEX • Geomagnetic Index Ap – Daily average of maximum variation in the earth’s surface magnetic field at mid lattitude (units of 2 u 10-9 T) Ap = 0 quiet Ap = 15 to 30 active Ap > 50 major solar storm 6 GRAVITY m m G 1 2 r force 2 r 1 G 6.672u 1011m3kg At surface of earth Gm Gm f m e g e | 9.8 m g 2 2 sec 2 RE RE 7 MICROGRAVITY • Satellites in orbit are in free fall - accelerating radially toward earth at the rate of free fall. • Deviations from zero-g § · ¨ ¸ ¨CD A¸ 2 2 – Atmospheric drag x 0.5¨ ¸U a Z ¨ m ¸ ¨ ¸ – Gravity gradient © ¹ x xw2 x yw2 z 2zw2 – Spacecraft rotation x xZ2 z zZ2 (rotation about Y axis) – Coriolis forces x 2zZ y o z zxZ 8 ATMOSPHERIC MODEL NEUTRAL ATMOSPHERE • Turbo sphere (0 ~ 120Km) is well mixed (78% N2, 21% O2) – Troposphere (0 ~ 10Km) warmed by earth as heated by sun – Stratosphere (10 ~ 50 Km) heated from above by absorption of UV by 03 – 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 day9 Miles 10,000 5,000 5,000 MAGNETOSPHERE 2,000 2,000 Pressure Layers of the 1,000 Molecular mean free path 1,000 10-10 500 mb EXOSPHERE 100 km Spray region 500 Sunlit 10-8 Earth’s Aurora F2 mb 1km 200 THERMOSPHERE IONOSPHERE F1 -6 200 10 E mb 100 -4 Aurora 10 Atmosphere 100 D mb Airglow -2 50 MESOSPHERE 10 1cm Noctilucent mb cloud 50 Warm region Maximum Sound waves 1 Ozone region mb -4 20 height for reflected here 10 balloons cm 100 20 10 Mother-of-pearl mb clouds 50,000 STRATOSPHERE Tropopause 10 30,000 Cirrus 20,000 Mount clouds Everest 5 10-6 Altocumulus cm 10,000 TROPOSPHERE clouds Mount Temperature curve Blanc 2 cumulus 5 ,000 clouds Ben Stratus 1 Nevis clouds Feet 1,000 Kilometers mb -1000C -500C 00C 500C 1000C TEMPERATURE 10 DENSITY ALTITUDE MODEL Assume perfect gas and constant temperature p § · A d ¨nkT ¸ dp ¨ ¸ p n k T © ¹ dh dh dh p+dp n is number density (number/m3) dpA - n m g A d h = o § · d ¨nkT ¸ dp ¨ ¸ k is Boltzmann’s constant nMg © ¹ dh dh M is average molecular mass dn Mgdh n KT H ~ 8.4km h ~ 120km n = noexp (-h/H) H { kT/mg (scale height) 11 Atmospheric Gases • At higher altitudes O2 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 ~ 108 particles/cm3 12 ATMOSPHERIC MODEL Most common Mass Spectrometer and Incoherent Scatter model - 1986 (MSIS - 1986) – Based on measured data – Requires Ap, F10.7, month as input – Gives average values of n, no, 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 13 AERODYNAMIC DRAG 1 v D Uv xv ( )CDA Drag 2 v dv D m dt ª º 1 CDA 'v Uv 2 « »'t 2 ¬«m ¼» ªm º Ballistic coefficient E « » ¬«CDA ¼» U=density of the atmosphere=mono =16x1.67x10-27x1013=2.67x10-13kg/m3 V=7.8km/s CD - Drag coefficient A - Cross sectional area 14 DRAG COEFFICIENTS Derived from Newtonian Aerodynamics. Depends on what air molecule does at impact – Reflected CD = 4 – Absorbed CD = 2 Since F = d(mv)/dt D = - F = - d(mv)/dt o § · A m¨V V ¸ ¨ f i ¸ C D © ¹ D 1 2 1 V 2Adt UV A U i 2 2 m = U Avidt CD = - 2 (vf -vi)/vi = 2 if vf = o in rarefied atmosphere = 4 if vf = - vi 15 TYPICAL DRAG PARAMETERS E (kg/m2) CD 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 CD ~ 2.2 - 4 for spacecraft. (see SMAD Table 8.3) 'V over one year (E = 100 kg/m2) h (km) 'V /year (m/s) 100 107 200 2 - 5 u 103 solar (min - max) 300 40 - 600 16 400 3 - 200 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. 17 MAGNETIC FIELD EFFECTS • 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 18 GEOMAGNETIC FIELD • 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 •Bi internal field varies slowly on the order of 100 years (0.05%/year.) • Poles of magnetic field lie in Siberia and South Australia. 19 GEOMAGNETIC FIELD 20 MAGNETOSPHERE 21 Magnetosphere (continued) • Earth’s field extends 10 Earth Radii (RH) toward the sun - terminates at magneto pause • Earth’s field slows and deflects solar wind – Compressed, heated, turbulent – Bow shock at about 14 RH • 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 22 EXTERNAL MAGNETIC FIELD •Be 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 23 GEOMAGNETIC COORDINATE SYSTEMS Geomagnetic B - L 30 20 15 4 Geographic Geomagnetic 10 9 z Geographic m north pole 8 z Magnetic 3 colatitude Colatitude θ θ Dipole axis m Geographic north pole r m 2 Greenwich r meridian y y B m x x m φ φ East longitude Magnetic longitude m 1 L=8 Solar-ecliptic Solar-magnetospheric Geocentric Distance (Re) ecliptic pole 0 zse To north 0 L=2 3 4 5 6 7 zsm Dipole axis φ se r se 0.2 y yse sm 0.1 x se φ x sm se To sun Tosun .05 Solar-magnetic .02 zsm (Oersteds) Dipole axis Field Strength direction B=.01 .005 Direction of .002 .001 geographic north pole ysm -90 -60 -30 x sm Sun direction North Latitude (Degrees) Several coordinate systems used in geomegnetism. 24 GEOMAGNETIC FIELD Magnitude Formula/Models Tilted dipole (11q from geographic north) 1/2 § · M § 2§ · · B ¨r,T ,I ¸ ¨3cos ¨T ¸1¸ at LEO i© m m¹ r3 © © m¹ ¹ where M = 0.311 u 10-4 TR3e =7.9u 1015 T - m3 B M2cosT r r3 m B MsinT Tm r3 m B 0 Im International Geomagnetic Reference Field 25 1987 (IGRF1987) 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 SOUTH ATLANTIC ANOMALY Reduced protection in SAA allows greater effect of high 28 energy particles - electronic upsets, instrument interference. PLASMA EFFECTS OVERVIEW • 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 29 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 • 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).