Orbital Mechanics In Low Earth Orbit at Universität Stuttgart | Flashcards & Summaries

Lernmaterialien für Orbital Mechanics in Low Earth Orbit an der Universität Stuttgart

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Spacecraft Charging

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Definition: Spacecraft charging is the process by which a spacecraft or selected component accumulates an electrical charge from its environment


It is independent of the characteristics of the space environment:

  • location of the spacecraft
  • solar activity
  • geomagnetic activity
  • solar electron flux density

Absolute: the S/C accumulates a net charge relative to its environment -> potential difference

Differential: different portions of the S/C charge to different potentials

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TESTE DEIN WISSEN

Primary Objectives for FIPEX on ISS

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  • time-resolved measurement for validation of models of the earths upper atmosphere
    • contribution to global climate models
    • reference for the prediction of material degradation
  • investigation of contamination issues / gas-surface-interactions under extreme rarefied flow conditions
  • complete space qualification according to ECSS-standards at an university in Europe with high involvement of students
Lösung ausblenden
TESTE DEIN WISSEN

Combinations of EUV proxies

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TESTE DEIN WISSEN

Problem: a single spectral line or proxy is clearly not sufficient to represent variability at the different wavelengths within the EUV spectrum that affect the thermosphere

Solution:

  • Using a combination of proxies for representing EUV input in the thermosphere has been the strategy in preparation for the JB2006 density model
Lösung ausblenden
TESTE DEIN WISSEN

Effects of atmospheric drag on the satellite orbit

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  • Causes all low Earth objects to spiral downward and eventually re-enter in the most dense layers of the atmosphere
  • because of the exponential decrease of the densiy with altitude, the drag force in an elliptical orbit is strongest at perigee.  Drag force causes kinetic energy to be transformed into heating, causing a decrease of size and eccentricity of the orbit
  • Close to the end of its lifetime, when the orbit becomes nearly circular, the rate of decay of the orbits semi-major axis quickly accelerates up to more than 30km/day as the density and drag force along the trajectory rapidly increases
Lösung ausblenden
TESTE DEIN WISSEN

Diurnal and seasonal variations

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TESTE DEIN WISSEN
  • uneven heating by the sun of the spherical Earth causes the day-to-night variation in density
  • Variation is also subject to seasonal changes, due to the movement of the sub-solar point as a result of the seasonally changing sun-earth geometry
  • these variation result in a daytime density bulge:
    • maximum is following the sub-solar point with a time-lag of about two hours
    • the density smoothly decreases to a minimum on the night side
  • because heating generally causes the entire thermosphere to expand, the difference can be around a factor of 5 at 500km and higher!
Lösung ausblenden
TESTE DEIN WISSEN

Solar activity variations

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TESTE DEIN WISSEN
  • the level of solar radiation in the peak wavelengths of the visible spectrum is quite constant
  • thermosphere absorbs only radiation at EUV and X-ray wavelengths, which can be highly variable

Solar minimum

  • during minima of the 11-year solar cycle period
    • fewer sunspots
    • EUV output by the sun is quite stable and at a low level

Solar Maximum

  • during solar cycle peak years, the EUV output is much higher and variable die to
    • more rapid changes in the suns magnetic field
    • effect of solar rotation on the restricted view of the suns surface from the earth

The density can vary by a factor of up to 20 at altitudes >500km

Lösung ausblenden
TESTE DEIN WISSEN

Geomagnetic activity variations

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TESTE DEIN WISSEN
  • active regions are related to solar flares and coronal mass ejections, which hurl great quantities of charged particles from the Sun into space
  • near the earth, these particles are deflected by the Earths magnetic field
  • a fraction can enter through the polar cusps and via reconnection processes, causing geomagnetic storms
  • during these storms, energy is deposited in the polar thermo-and ionosphere over a short period of time
  • these sudden and short-lived events cause density variations of up to one order of magnitude
Lösung ausblenden
TESTE DEIN WISSEN

Semi-annual variations

Lösung anzeigen
TESTE DEIN WISSEN
  • the semi-annual variation leads to density changes of about 30%, with maxima around April and OCtober
  • during periods of low solar acitivity, it is dominant cause of changes in density
  • a possible explanation involves an increased mixing of the thermospheric species due to the uneven heating of the hemispheres at the solstices
  • this results in raised molecular oxygen and nitrogen and reduced AO densities, which lowers the mean density scale height and causes a compression of the atmosphere
Lösung ausblenden
TESTE DEIN WISSEN

Horizontal winds in the thermosphere

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TESTE DEIN WISSEN
  • dominant motion is the co-rotation with the earth
  • this co-rotation is evidently transferred to the outer gas envelope first by friction from the earth's surface and subsequently by frictional forces between the individual gas layers
  • for heights near 300km, co-rotations of 500m/s are reached at the equator
Lösung ausblenden
TESTE DEIN WISSEN

Diurnal (tidal) wind circulation

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TESTE DEIN WISSEN
  • between the day and the night sides, there exists considerable temperature, density and pressure differences
    • pressure bulge exissts in the afternoon sector at equatorial latitutdes
    • low pressure region is shifted by 12 hours
  • total pressure difference is sufficient to generate significant balancing winds. due to its 24-hour periodicity, these are also referred to as the tidal winds
  • it generally follows the pressure gradient and can reach velocities as high as 200m/s at mid-latitudes
Lösung ausblenden
TESTE DEIN WISSEN

Height dependency of thermospheric winds

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TESTE DEIN WISSEN
  • Viscosity dependence
    • viscosity coefficient increases with the temperature so that the viscous forces can smooth out velocity differences more effectively with increased height -> wind velocity in the upper thermosphere tends to approach a constant limiting value
  • Low direction
    • the mass flow directed towards the equator in the upper thermosphere is balanced by a poleward-directed mass flow in the lower thermosphere. Because of increased densities, the return flow velocities in the lower thermosphere are much smaller
  • seasonal winds
    • superimposed on the flow is a wind circulation from the summer to the winter hemisphere
Lösung ausblenden
TESTE DEIN WISSEN

Influence of geomagnetic storms on global wind patterns

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TESTE DEIN WISSEN
  • at high altitudes, where strong electric fields drive a polar convection pattern of ions, which in turn spins up the neutral wind
  •  heating at high latitudes by dissipation of electrical currents creates pressure gradients that drive equatorial winds
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Beispielhafte Karteikarten für deinen Orbital Mechanics in Low Earth Orbit Kurs an der Universität Stuttgart - von Kommilitonen auf StudySmarter erstellt!

Q:

Spacecraft Charging

A:

Definition: Spacecraft charging is the process by which a spacecraft or selected component accumulates an electrical charge from its environment


It is independent of the characteristics of the space environment:

  • location of the spacecraft
  • solar activity
  • geomagnetic activity
  • solar electron flux density

Absolute: the S/C accumulates a net charge relative to its environment -> potential difference

Differential: different portions of the S/C charge to different potentials

Q:

Primary Objectives for FIPEX on ISS

A:
  • time-resolved measurement for validation of models of the earths upper atmosphere
    • contribution to global climate models
    • reference for the prediction of material degradation
  • investigation of contamination issues / gas-surface-interactions under extreme rarefied flow conditions
  • complete space qualification according to ECSS-standards at an university in Europe with high involvement of students
Q:

Combinations of EUV proxies

A:

Problem: a single spectral line or proxy is clearly not sufficient to represent variability at the different wavelengths within the EUV spectrum that affect the thermosphere

Solution:

  • Using a combination of proxies for representing EUV input in the thermosphere has been the strategy in preparation for the JB2006 density model
Q:

Effects of atmospheric drag on the satellite orbit

A:
  • Causes all low Earth objects to spiral downward and eventually re-enter in the most dense layers of the atmosphere
  • because of the exponential decrease of the densiy with altitude, the drag force in an elliptical orbit is strongest at perigee.  Drag force causes kinetic energy to be transformed into heating, causing a decrease of size and eccentricity of the orbit
  • Close to the end of its lifetime, when the orbit becomes nearly circular, the rate of decay of the orbits semi-major axis quickly accelerates up to more than 30km/day as the density and drag force along the trajectory rapidly increases
Q:

Diurnal and seasonal variations

A:
  • uneven heating by the sun of the spherical Earth causes the day-to-night variation in density
  • Variation is also subject to seasonal changes, due to the movement of the sub-solar point as a result of the seasonally changing sun-earth geometry
  • these variation result in a daytime density bulge:
    • maximum is following the sub-solar point with a time-lag of about two hours
    • the density smoothly decreases to a minimum on the night side
  • because heating generally causes the entire thermosphere to expand, the difference can be around a factor of 5 at 500km and higher!
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Q:

Solar activity variations

A:
  • the level of solar radiation in the peak wavelengths of the visible spectrum is quite constant
  • thermosphere absorbs only radiation at EUV and X-ray wavelengths, which can be highly variable

Solar minimum

  • during minima of the 11-year solar cycle period
    • fewer sunspots
    • EUV output by the sun is quite stable and at a low level

Solar Maximum

  • during solar cycle peak years, the EUV output is much higher and variable die to
    • more rapid changes in the suns magnetic field
    • effect of solar rotation on the restricted view of the suns surface from the earth

The density can vary by a factor of up to 20 at altitudes >500km

Q:

Geomagnetic activity variations

A:
  • active regions are related to solar flares and coronal mass ejections, which hurl great quantities of charged particles from the Sun into space
  • near the earth, these particles are deflected by the Earths magnetic field
  • a fraction can enter through the polar cusps and via reconnection processes, causing geomagnetic storms
  • during these storms, energy is deposited in the polar thermo-and ionosphere over a short period of time
  • these sudden and short-lived events cause density variations of up to one order of magnitude
Q:

Semi-annual variations

A:
  • the semi-annual variation leads to density changes of about 30%, with maxima around April and OCtober
  • during periods of low solar acitivity, it is dominant cause of changes in density
  • a possible explanation involves an increased mixing of the thermospheric species due to the uneven heating of the hemispheres at the solstices
  • this results in raised molecular oxygen and nitrogen and reduced AO densities, which lowers the mean density scale height and causes a compression of the atmosphere
Q:

Horizontal winds in the thermosphere

A:
  • dominant motion is the co-rotation with the earth
  • this co-rotation is evidently transferred to the outer gas envelope first by friction from the earth's surface and subsequently by frictional forces between the individual gas layers
  • for heights near 300km, co-rotations of 500m/s are reached at the equator
Q:

Diurnal (tidal) wind circulation

A:
  • between the day and the night sides, there exists considerable temperature, density and pressure differences
    • pressure bulge exissts in the afternoon sector at equatorial latitutdes
    • low pressure region is shifted by 12 hours
  • total pressure difference is sufficient to generate significant balancing winds. due to its 24-hour periodicity, these are also referred to as the tidal winds
  • it generally follows the pressure gradient and can reach velocities as high as 200m/s at mid-latitudes
Q:

Height dependency of thermospheric winds

A:
  • Viscosity dependence
    • viscosity coefficient increases with the temperature so that the viscous forces can smooth out velocity differences more effectively with increased height -> wind velocity in the upper thermosphere tends to approach a constant limiting value
  • Low direction
    • the mass flow directed towards the equator in the upper thermosphere is balanced by a poleward-directed mass flow in the lower thermosphere. Because of increased densities, the return flow velocities in the lower thermosphere are much smaller
  • seasonal winds
    • superimposed on the flow is a wind circulation from the summer to the winter hemisphere
Q:

Influence of geomagnetic storms on global wind patterns

A:
  • at high altitudes, where strong electric fields drive a polar convection pattern of ions, which in turn spins up the neutral wind
  •  heating at high latitudes by dissipation of electrical currents creates pressure gradients that drive equatorial winds
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