Mississauga Astronomical Society

54rd Meeting

Speaker's Night

 

Day:                January 13, 2006

 

Speaker:     Richard Matthews

                  

                              

 

NASA Video of Stardust Space Probe

Randy Attwood showed a NASA video describing the Stardust Comet Sample Return Mission.  The Stardust spacecraft was launched in 1999.  After 3 loops around the Sun, it encountered Comet p/Wild2, imaged the nucleus and collected comet tail particles on an aerogel collector.  The probe is scheduled to return to Earth in Utah in the early morning of January 15.  The video described the mission and recovery team in Utah.  

 

 

An Overview of Artificial Satellites

Richard Matthews a B.Sc. in civil engineering has worked at the Canadian Space Agency and then in Cambridge with a company making parts for the Iridium satellites.  He spoke about satellites and their orbits.

 

Satellites are man made devices in orbit around the Earth or other solar system bodies. There are none yet outside the solar system although Voyager 1 and 2 are on their way there.  The three major types by orbit are low Earth (LEO), geostationary (GEO) and interplanetary.  To describe the orbit one needs to know the semimajor axis, eccentricity, inclination, augment of periapsis, time of periapsis passage and longitude of the ascending node, as well as whether the orbit is prograde or retrograde.

 

LEO satellites are generally below 500 km altitude and are mostly for Earth based communication, but also scientific, military and Earth observation.  Examples include the space shuttle, ISS, Mir, Hubble. These satellites can be single or in a constellation such as the Iridium group.   Advantages of the LEO are low cost, good views over a relatively small portion of the Earth, above Earth for good scientific observations.  With precession, coverage changes with each orbit so that most of the planet can be covered (except the poles).  Problems are small coverage area, non-consistent positioning, and atmosphere drag requiring regular boosting.  In addition, the LEO environment is hostile with debris and oxygen.      

 

Richard Matthews studied the Long Duration Exposure Facility (LDEF), released by the space shuttle in April 1984 and retrieved in January 1990, only 2 months before re-entry and burnup.  The LDEF was designed to investigate the LEO environment.  With an orbital velocity of 26,000 kph, debris in LEO, such as dust, paint flakes, tools and fragments of old satellites have a huge impact energy.  The relative velocities of impacting particles is important. There are over 10,000 objects larger than 10 cm currently being tracked by US radar.  Manned craft must avoid these and are armored, giving protection against impactors up to 10 mm.  The danger zone is from 10 to 100 mm because these are too small to be seen.  Although the probability of impact over 5 years is low but possible, it adds up as the years go by. 

 

Atomic oxygen, O+ is created by high energy radiation which splits the oxygen molecule.  It is very reactive with anything prone to oxidation, and affects what materials can be used on satellites.

 

Iridium is an example of a constellation of satellites, put in orbit for a specific purpose; in this particular case for a telephone network connecting phones anywhere on the Earth.  There are 6 satellites in each orbit, with a total of 66 in 11 orbits.  Each satellites “talks” to the one ahead, behind and on each side. 

 

GEO is circular at 36,000 km above the Earth’s surface over the equator.  A satellite in such an orbit essentially stays above the same spot all the time, with its period equal to Earth’s rotation, and thus ideal for direct broadcasting.  Due to the altitude, there is no concern about O+  or debris.  However the satellite is susceptible to solar flares.    “Slots” in GEO are very desirable especially above populated areas but more energy is required to reach such a high altitude and there is no possibility for repair.  Fuel enables the satellite to maintain its position in the orbit. 

 

Interplanetary satellites are in orbit around another planet or moon or in a transfer orbit wherein a planet can boost the velocity of the probe.  Interplanetary orbiters include Cassini, Voyager, Galileo, Messenger, New Horizons.  Examples of landers are Huygens, Spirit and Opportunity, Pathfinder. 

 

In a sun-synchronous orbit, the satellite orbits the Earth (or Mars in the case of the Mars Global Surveyor), such that it crosses the same point at the same time each day.  In  Lagrange points,  3 objects orbit together at the vertices of an equilateral triangle such as SOHO at the L1 point, and WMAP at L2.  In a polar orbit, the satellite is in a LEO with an inclination approaching 90o, and covering most of the planet’s surface and thus useful for remote sensing.  However, it also requires more fuel to get there.  Richard ended up by describing the Cassini spacecraft in detail. 

 

 

Submitted by Chris Malicki, Secretary  Chris Malicki, Secretary                               back to M.A.S. meeting reports page
Mississauga Astronomical Society