# Interplanetary Trajectories Questions and Answers – Station Keeping

This set of Spaceflight Mechanics Multiple Choice Questions & Answers (MCQs) focuses on “Interplanetary Trajectories – Station Keeping”.

1. What is station keeping?
a) Inserting a spacecraft in a parking orbit
b) Mainting fixed distance between two spacecrafts
c) Maintating a station for observing spacecraft
d) Inserting spacecraft to monitor interplanetary objects

Explanation: Station-kepping is the process of keeping a spacecraft/satellite at the fixed distance from another spacecraft. Various maneuvers have to be made to keep this distance fixed.

2. Which of these does not hamper the process of station keeping?
b) Third-body perturbation
c) Oblatenes
d) Spacecraft’s cross-sectional area

Explanation: In order to maintain the constant distance between the two spacecrafts’ it is essential to counter the perturbation forces. These are caused due to solar radiation presssure, gravitational field from the moon and sun and earth’s oblateness.

3. Why is it essential to keep the spacecraft’s re-entry orbit in sync with the earth’s rotation?
a) Prevent orbital period shortening
b) Prevent it to fall at the wrong location
c) Prevent drag
d) Prevent issues due to pertubation

Explanation: When a spacecraft re-enters the earth’s atmosphere, it is essential to keep the orbit in sync with the earth’s rotation else the orbital time period will increase.

4. Which of these Lagrange points pose a problem for station keeping due to its divergent nature?
a) L4, L5
b) L1, L2, L3
c) L6
d) L7

Explanation: Lagrage point L1, L2 and L3 are considered to be unstable and the orbit near it is known as halo orbit. When a satellite is perturbed even slightly near these points, they do not return to the initial position because of its unstable nature. This this poses problems for station keeping.

5. What is the reason for the ISS to lose orbital energy in LEO orbit?
a) Gravity
b) Atmospheric drag
c) Centripetal force
d) Centrigula force

Explanation: The International Space Stations orbits the Earth in the Low Earth Orbit. It is at a distance of 330-410 km. Due to the presence of atmospheric drag, ISS is continuously losing its orbital energy which is countered by providing thrust to maintain the orbit.

6. Out-of- plane maneuvers have to made by the observation spacecrafts to compensate for the third-body gravitational force.
a) True
b) False

Explanation: Earth observation spacecrafts are usually at an altitude of 700 km where there is negligible atmospheric drag. In order to maintain its ground track it is essential to carry out out-of-plane maneuver in order to counter the inclination change caused by third- body gravitational force (moon, sun).

7. How much delta-v is required by the sun-synchronous spacecrafts to compensate the inclination change caused by sun’s gravitational force?
a) 0.2 m/s
b) 1-2 m/s
c) 10-15 m/s
d) 8-10 m/s

Explanation: The sun-synchronous spacecrafts ae under a continuous influence of sun’s gravitational force which causes a change in inclination of the spacecraft’s orbit. To compensate for this change, delta-v of the order 1-2 m/s has to be applied.

Sanfoundry Global Education & Learning Series – Spaceflight Mechanics.

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