I/GCSE Physics - Satellites

Geostationary Orbit

• Geostationary Orbit is one in which the satellite is always in the same position with respect to the rotating Earth, which is a concept of I/GCSE Physics. The satellite orbits at an elevation of approximately 35,790 km because that produces an orbital period (time for one orbit) equal to the period of rotation of the Earth (23 hrs, 56 mins, 4.09 secs). By orbiting at the same rate, in the same direction as Earth, the satellite appears stationary (synchronous with respect to the rotation of the Earth).
• Geostationary satellites provide a "big picture" view, enabling coverage of weather events. This is especially useful for monitoring severe local storms and tropical cyclones. Because a geostationary orbit must be in the same plane as the Earth's rotation, that is the equatorial plane, it provides distorted images of the polar regions with poor spatial resolution. 
• Geostationary satellites are used for weather forecasting, satellite TV, satellite radio and most other types of global communications.

Polar Orbit

• Polar-orbiting satellites provide a more global view of Earth, circling at near-polar inclination (the angle between the equatorial plane and the satellite orbital plane -- a true polar orbit has an inclination of 90 degrees). Orbiting at an altitude of 700 to 800 km, these satellites cover best the parts of the world most difficult to cover in situ (on site). For example, McMurdo, Antartica, can be seen on 11-12 of the 14 daily NOAA polar-orbiter passes.
• These satellites operate in a sun-synchronous orbit. The satellite passes the equator and each latitude at the same local solar time each day, meaning the satellite passes overhead at essentially the same solar time throughout all seasons of the year. This feature enables regular data collection at consistent times as well as long-term comparisons. The orbital plane of a sun-synchronous orbit must also rotate approximately one degree per day to keep pace with the Earth's surface.  Polar orbits are often used for earth-mapping, earth observation, and reconnaissance satellites, as well as for some weather satellites. The disadvantage to this orbit is that no one spot on the Earth's surface can be sensed continuously from a satellite in a polar orbit.

Hubble Space Telescope

General information

Organization

NASA / ESA / STScI

Launch date

24 April 1990, 8:33:51 am EDT

Launch vehicle

Space Shuttle Discovery, (STS-31)

Mission length

19 years, 6 months, and 13 days elapsed

Deorbited

~ 2013–2021^[1][2]

Mass

11,110 kg (24,500 lb)

Type of orbit

Near-circular low Earth orbit

Orbit height

559 km (347 mi)

Orbit period

96–97 minutes

Orbit velocity

7,500 m/s (25,000 ft/s)

Acceleration due to gravity

8.169 m/s² (26.80 ft/s²)

Location

Low Earth orbit

Telescope style

Ritchey-Chretien reflector

Wavelength

Optical, ultraviolet, near-infrared

Diameter

2.4 m (7 ft 10 in)

Collecting area

4.5 m² (48 sq ft)^[3]

Focal length

57.6 m (189 ft)

• Hubble Space Telescope had an initial price tag was $1.5 billion (US dollars). Moreover, the cost did not stop there, but continued to balloon throughout the next several years. By 1992 costs had increased to $2.5 billion ⁽⁶⁾. By 1999, approximately $3.8 billion had been invested ⁽⁷⁾. By the time the HST is retired in another ten years, the estimated total cost will be about $6 billion. Spread out over its ten years of development and twenty years of operation, however, the cost is negligible. According to Ed Weiler, the HST space science chief, that amount "...equates to about two cents per week per American taxpayer over that period of time" ⁽⁸⁾. When put into the proper perspective, the HST would be a bargain at twice the price.

What happens to satellites when they have come to the end of their life?

• Before the satellite runs out of fuel, the ground station performs one last maneuver to send the satellite to a place called (the satellite graveyard), it’s an orbit around the earth used to retire old useless satellites & they become junk on space , a growing problem , this is when in it come to satellites using the geostationary orbit , for satellites using lower orbits they may be put on re-entry course so they burn before they reach the earth.
• When satellites reach the end of their mission, satellite operators have the option of de-orbiting the satellite, leaving the satellite in its current orbit or moving the satellite to a graveyard orbit. Historically, due to budgetary constraints at the beginning of satellite missions, satellites were rarely designed to be de-orbited. One example of this practice is the satellite Vanguard 1. Launched in 1958, Vanguard 1, the 4th manmade satellite put in Geocentric orbit, was still in orbit as of August 2009.
• Instead of being de-orbited, most satellites are either left in their current orbit or moved to a graveyard orbit. As of 2002, the FCC now requires all geostationary satellites to commit to moving to a graveyard orbit at the end of their operational life prior to launch.

That's the end of the topic!

Drafted by Kin (Physics)