Orbits Flashcards
Between what altitude range are Low Earth Orbits
160-2000km
Between what altitude are Medium Earth Orbits (MEO)
2000 - 35,786km
In what altitude range are High Earth Orbit (HEO)
Anything above 35,786km
What are geosynchronous orbits (GSO)?
What is the special case of geosynchronous orbit?
Any orbit that matches the earth’s rotational period
which means having a semi-major axis of 42,164km
A geosynchronous orbit that is circular and on the equator is a geostationary orbit, because it would from the ground appear as if the satelite was not moving
Otherwise geosynchronous orbits form a figure of 8 in the sky swinging above and below the equator - thereby actually covering more of the Earth’s surface.
In what orbit would you find GPS systems?
At what altitude and why?
What is the minimum number of satellites needed (line of sight) for a position
Medium Earth Orbit (MEO)
at 20,200km
hence completing two complete orbits per sidereal day so that the satellites pass over the same location every day
four (currently 9 are visible at any time)
What is the most common form of transfer orbit?
Hohmann transfer orbit.
An orbital maneuver that moves a spacecraft from one circular orbit to another using two engine impulses. This maneuver was named after Walter Hohmann.
What do you call an orbit whose period is a rational multiple of the average rotation period of the body being orbited?
A synchronous orbit
A geosynchronous orbit is one where the orbiter moves around the earth in 1:1 ratio
A semi-synchronous orbit would have a ration of 1:2
What do you call a synchronous orbit which is highly eliptical and a high inclination.
Why would you have it?
In fact, what specifically will that inclination be, and why?
What is the name of the special case orbit where the orbit time is half the orbiting body’s rotation period
Tundra orbit
helps provide persistent coverage over high latitudes (that cannot be provided by geostationary or other circular geosynchronous orbits)
The inclination will be 63.4 degrees as this prevents precession, so that the main coverage will be over a consistent area
The special case, where the orbit is only 12 hours is called a Molniya orbit
What is a supersynchronous orbit
One with a period longer than the rotational period of the body being orbitted
What is a subsynchronous orbit
One just below a geosynchronous orbit, which means that the satellite will drift east
What is the graveyard orbit?
Why would a satellite be put there?
An orbit a few hundred kilometers above geosynchronous that satellites are moved into at the end of their operation
atellites are moved into such orbits to reduce the probability of colliding with operational spacecraft or generating space debris.
What is a Sun-synchronous orbit
What is the advantage of such an orbit?
It is a special case of polar orbit.
One where a satelite passes over any given point on the earth’s surface at the same local solar time
Such an orbit can place a satellite in constant sunlight and is useful for imaging, spy, and weather satellites. (It means that looking at the same place at the same time means you can effectively monitor change)
This consistent lighting is a useful characteristic for satellites that image the Earth’s surface in visible or infrared wavelengths (e.g. weather and spy satellites) and for other remote sensing satellites (e.g. those carrying ocean and atmospheric remote sensing instruments that require sunlight).
What is the typical altitude range for a sun-synchronous orbit
600-800km
therefore it is in LEO
What and when was the first hypervelocity satellite collision in low earth orbit?
For kudos at what altitude
In what year was the other famous source of space debris
When was the US anti-sat test, and what altitude was the target?
2009
Iridium 33 (US sat phone) and Kosmos 2251 (Strela class Russian mil comms)
at 789km
2007 Chinese anti-sat test (for detail, a FY1C polar orbit satellite was shot down at 865km altitude)
US test was in 1985 against a P78-1 solar observatory at 555km (some debris remained in orbit until after 1998 forcing NASA to enhance shielding on its planned space station)
What type of shielding does the ISS have to defend against space debri
Whipple Shielding
It has over 100 shield configurations
The Whipple shield or Whipple bumper, invented by Fred Whipple, is a type of hypervelocity impact shield used to protect manned and unmanned spacecraft from collisions with micrometeoroids and orbital debris whose velocities generally range between 3 and 18 kilometres per second (1.9 and 11.2 mi/s).
