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 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 n-type p-type

Power At 1 AU, silicon solar panels can provide 0.04 A/cm 2 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 Pu-238 Radiator n p

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 PuO 2 in insoluable, ceramic form Get the launch right!

Power Typical RTG contains 11 kg PuO 2 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 –

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 –

What Next? “Stardust” comet and interplanetary material sample return –Launched Feb –Encounter with comet Wild 2 in Jan –Sample return 2006 –