This set of Astronautics Multiple Choice Questions & Answers (MCQs) focuses on “Earth Orbit Perturbations”.
1. Which of the following is not an orbital element?
a) Argument of periapsis
b) Right ascension of ascending node
c) Declination
d) Altitude
View Answer
Explanation: Declination is similar to latitude, and is therefore an angular measure North or South of the equator. Declination is often used for objects that are not attached to Earth and has nothing to do with orbits.
2. Which of the following parameters never stays constant as long as a satellite orbits the Earth?
a) Inclination
b) Right ascension of ascending node
c) True anomaly
d) Orbital energy
View Answer
Explanation: True anomaly is the angular position of a satellite with respect to the periapsis (or ‘perigee’ in this case). As the satellite keeps orbiting our planet, its true anomaly is always changing.
3. The right ascension of ascending node is undefined for _____________
a) polar orbits
b) equatorial orbits
c) high inclination MEO
d) low-inclination MEO
View Answer
Explanation: The right ascension of ascending node defines the orientation of an orbit and is the angular distance of the ascending node from the x-axis (which points toward the vernal equinox) of the ECI (Earth Centered Inertial) coordinate system. For equatorial orbits, the ‘ascending node’ is non-existent, hence making the right ascension an undefined parameter.
4. For equatorial elliptical orbits, which of the following orbital elements becomes invalid?
a) Argument of perigee
b) Inclination
c) Orbital height
d) True anomaly
View Answer
Explanation: The argument of perigee is an angle measured from the ascending node to the perigee. For an equatorial orbit, there is no ascending node (or descending node for that matter) since the orbiter does not move North or South, but rather stays in the equatorial plane. The absence of a node makes the argument of perigee lose its importance.
5. An idealized orbit which does not undergo any perturbing effects is also called a _____________
a) Lagrangian orbit
b) Keplerian orbit
c) Newtonian orbit
d) circular orbit
View Answer
Explanation: A Keplerian orbit is an idealized orbit which consists of only two bodies: a massive central object and a smaller orbiter. The system is governed only by the gravitationally attraction between the two bodies and does not take into account any sort of perturbation.
6. Why is it important to know the position of a satellite in orbit?
a) For uplink and downlink
b) To decrease its lifespan
c) To increase its lifespan
d) To save money
View Answer
Explanation: Ground-stations and satellites must periodically maintain contact with each other in order to fulfill the mission objective(s). If we consider a satellite built for scientific purposes, all its gathered data must be sent back to us for analysis. In addition, ground-stations must often command the satellite to perform a certain maneuver (pointing its sensors towards a particular region, say). To do this, we need to know as well as predict the position of the spacecraft in order to establish this line-of-sight communications link that facilitates exchange of information. Only when the expected time of entry of a satellite into the range of a ground station is known can we uplink (ground-station to satellite) commands or intercept a downlink (satellite to ground-station) during this brief window.
7. Failure to take the orbital perturbation effects into consideration will lead to _____________
a) deviation from the ideal orbit
b) change in orbital energy
c) longer lifespan
d) satellite damage
View Answer
Explanation: Perturbing effects tend to change the original planned orbit of a satellite into a slightly different trajectory, and these errors build up over time. The orbital energy stays constant and has nothing do with such perturbations.
8. Longitude of periapsis is used instead of argument of periapsis for _____________
a) polar circular orbits
b) polar elliptical orbits
c) equatorial circular orbits
d) equatorial elliptical orbits
View Answer
Explanation: As discussed earlier, the argument of periapsis becomes invalid for equatorial orbits. Instead, another parameter – the longitude or periapsis – is used, which is measured from the vernal equinox (also called the ‘First Point of Aries’).
9. Which among the following is not a source of Earth orbit perturbations?
a) Lunar gravity
b) Solar gravity
c) Atmospheric Drag
d) Micrometeoroids
View Answer
Explanation: Micrometeoroids are miniscule fragments of extraterrestrial origin that continuously rain down on Earth. These chunks do not directly affect the trajectory of spacecraft and are not a source of orbital perturbations.
10. Secular perturbations are ___________ in nature.
a) periodic
b) insignificant
c) cumulative
d) brief
View Answer
Explanation: Secular perturbations refer to effects which are long-lasting and cumulative. The deviations from an ideal orbit due to secular perturbations alone are small at first, but continuously build up over time and cause large errors over a significant duration.
Sanfoundry Global Education & Learning Series – Astronautics.
To practice all areas of Astronautics, here is complete set of Multiple Choice Questions and Answers.
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