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Bits and Pieces

Bits and Pieces. Spacecraft Systems. Propulsion Already discussed Communications Science instruments Power. Communications. A number of issues: Limited power Large distances Reception Multiplexing. Communications. Typical spacecraft transmission power ~20 W

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Bits and Pieces

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  1. Bits and Pieces

  2. Spacecraft Systems • Propulsion • Already discussed • Communications • Science instruments • Power

  3. Communications • A number of issues: • Limited power • Large distances • Reception • Multiplexing

  4. Communications • Typical spacecraft transmission power ~20 W • Limited power solved in two ways • Large receiving stations • Directional microwaves • Round-the-clock communication possible by the DSN

  5. Communications • How about at the spacecraft? • Cannot have huge antennae - typically 5m diameter • High transmission power from Earth • Highly sensitive amplifiers, narrow band-pass, phase locking, low data rates all used • See “Basics of Space Flight Chs. 10, 11

  6. Communications • Two principal types of spacecraft antennae: • High gain antennae provide primary communications • Highly directional, high data rates possible • Low gain antennae provide wide angle coverage at the expense of gain • Low pointing accuracy needed, hence can be used for initial contact or in the event of problems. Low data rates only.

  7. Communications • Spacecraft receivers/transmitters • Many spacecraft use the “S” or “X” bands (~ 2 and 5 GHz respectively) • See “Basics of Spaceflight” p. 101 • DS1 testing a “Ka” band transponder (~20 GHz) • Advantages; smaller, more directional (hence less power), less suceptible to poor ground station conditions - e.g., bad weather

  8. Instrumentation • Detectors • Remote sensing • Other systems • Basics of Space Flight Chapters 11, 12, 13

  9. Detectors • Charged particle detectors • Measure composition and distribution of interplanetary medium • Plasma detectors • Measure interactions of solar wind with planetary magnetic fields • Dust detectors • Magnetometers

  10. Remote Sensing • Imagers • Spectrometers • Remotely measure compositions • Polarimeters • Determine the size, composition and structures of particles in, for example, planetary rings

  11. Other Systems • Data recording • Record data for later playback • Tape recorders used, now being replaced by high capacity solid-state memories • Fault protection • “Default” procedures to re-establish contact with Earth, etc. if something goes wrong • Redundancy - Duplication of important systems

  12. Power • Typical spacecraft (e.g., Voyager, Galileo etc.) require 0.3-2.5 kW, over possibly decades! • Two currently available methods for long-term power • Photovoltaic cells (solar panels) • Radioisotope Thermal Generators (RTGs)

  13. n-type p-type Power • Solar Panels • Utilise photovoltaic effect across a semiconductor junction • Usually gallium arsenide or silicon http://www.iclei.org/efacts/photovol.htm

  14. Power • At 1 AU, silicon solar panels can provide 0.04 A/cm2 at 0.25 V per cell. GaAs is more efficient. • Solar power can, in practice, be used out to the orbit of Mars. • Output degrades by about 2% per year due to radiation damage - faster if there is high solar activity!

  15. n Radiator Pu-238 p Power • Radioisotope Thermal Generators (RTGs) • Use thermoelectric effect • Heat provided by decay of radioactive isotopes, usually Pu-238

  16. Power • Special issues relating to RTGs • Safety • They cannot “explode” • Design ensures RTG units remain intact, even after re-entry and impact in the event of an accident • PuO2 in insoluable, ceramic form • Get the launch right! • http://www.jpl.nasa.gov/cassini/rtg/

  17. Power • Typical RTG contains 11 kg PuO2 fuel, producing about 300 W of electricity from about 400 W of heat. • Total mass about 60 kg. • Decay rate of about 1-2% per year • e.g., Voyager RTGs provided 470 W at launch (1977), now provide 330 W • Probably still good for at least another 20 years

  18. What Next? • “DS 1” • Launched October 1998 • Test of new technologies, for example... • ion engine • autonomous navigation and operations • Ka band transponder • Asteroid and comet encounters • Solar wind studies • http://nmp.jpl.nasa.gov/ds1/

  19. What Next? • Continued Mars campaign • Launches at each opportunity • 2001 (orbiter) • 2003 (orbiter and rover) • 2005… (orbiters, landers, rovers, sample return…) • Failure of the 1998/99 missions raised a few questions • http://mars.jpl.nasa.gov/

  20. What Next? • “Stardust” comet and interplanetary material sample return • Launched Feb. 1999 • Encounter with comet Wild 2 in Jan. 2004 • Sample return 2006 • http://stardust.jpl.nasa.gov/

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