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
• Limited power solved in two ways– Large receiving stations– Directional microwaves
• Round-the-clock communication possible by the DSN
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
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.
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
Instrumentation
• Detectors
• Remote sensing
• Other systems– Basics of Space Flight Chapters 11, 12, 13
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
Remote Sensing
• Imagers
• Spectrometers– Remotely measure compositions
• Polarimeters– Determine the size, composition and
structures of particles in, for example, planetary rings
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
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)
Power
• Solar Panels– Utilise photovoltaic effect across a
semiconductor junction– Usually gallium arsenide or silicon
http://www.iclei.org/efacts/photovol.htm
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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!
Power
• Radioisotope Thermal Generators (RTGs)– Use thermoelectric effect– Heat provided by decay of radioactive isotopes, usually Pu-238
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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/
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
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/
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/
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/