# Astronautics MCQ (Multiple Choice Questions)

1. Who among the following great minds believed that Earth was the center of the universe?
a) Galileo
b) Aristarchus
c) Copernicus
d) Ptolemy

Explanation: Ptolemy was a strict advocate of the geocentric model of the universe. He, in fact, came up with a ‘circles-on-circles’ model (known as the ‘Deferent Epicycle’) in order to explain the observed motions of the heavenly bodies. When observations did not agree with geocentricity, Ptolemy added more and more complexity to the model to reconcile the two.

2. If the Earth were flat, then it would be difficult to account for _____________
a) the day-and-night cycle
b) the shadows cast by different spatially-separated objects
c) its uneven surface
d) the seasons

Explanation: In a flat Earth, the shadows of various objects would cast the same angle on the surface irrespective of where they are located. The actual case, however, shows us that at different locations on Earth, the shadows are cast at different angles. The day-and-night cycle can be explained simply by making the assumption that the flat-Earth itself rotates about an imaginary pivot (say the geometric center of the surface).

3. What causes weightlessness in Earth orbit?
a) the gravitational attraction from other external bodies
b) tangential velocity
c) continuous free-fall of objects in orbit
d) a lack of gravity at orbital altitudes

Explanation: Objects in orbit are continuously in free-fall and at the same time ‘missing’ the central body due to their tangential velocity. To understand how a free-fall is associated with weightlessness, consider the following situation: Imagine a person standing in an elevator stationed at the highest floor and, for some reason, the elevator cables suddenly fail and the elevator plummets to the ground. The person inside now has nothing to stand against and is literally in a state of free fall, experiencing weightlessness until the elevator impacts the ground. This analogy may be used to explain why astronauts aboard spacecraft encounter weightlessnesswhile in orbit.
At orbital altitudes, Earth’s gravity is just negligibly smaller than that on its surface, so a lack of gravity is not the correct explanation.

4. Which of the following tasks is easier to perform with Earth-orbiting spacecraft?
a) Earth-monitoring
b) Astronomical imagery
c) Sky surveys
d) Long-distance transportation

Explanation: Earth-monitoring is done more elegantly with orbiting spacecraft due to the fact that a large portion of our planet is visible from the satellite at any instant. Astronomical imagery from space may have its advantages but can nevertheless be outperformed using ground-based observatories. Long-distance transportation is highly infeasible with current technology and is limited to road, rail and air commute. Sky surveys on the other hand are better done from the surface since ground-based telescopes are rewarded with a much larger view of the sky.

5. Which is the first satellite launched into orbit around Earth?
a) Rohini
b) Aryabhata
c) Sputnik 1
d) Vanguard 1

Explanation: Sputnik 1 was the first artificial satellite around Earth which was launched way back in 1957 by the Soviet Union.

6. What defines the size of an elliptical orbit?
a) circumference
b) diameter
d) semi-major axis

Explanation: The ‘radius’ and ‘diameter’ become relevant only when talking about circular orbits, while the semi-major axis is unique to elliptical orbits and is what defines its size. The semi-major axis is nothing but half the length of the major axis, which is the line joining the opposite ends of the ellipse and passing through the two foci.

7. In an elliptical orbit, which of the following properties does not change its value throughout the orbit?
a) Angular velocity
b) Energy
c) Orbital height
d) Tangential velocity

Explanation: The energy in an orbit, which is the sum of the kinetic and potential energies of an orbiting body of unit mass, is always conserved. Close to the central body, the kinetic energy is more compared to the potential energy, while farther away, the potential energy is greater. In both cases, however, the sum of the kinetic and potential energies is constant.

8. What happens to spacecraft orbiting close to Earth (within 20,000 km)?
a) have a larger view of Earth
b) are on par with Earth’s rotation
c) move faster than Earth’s rotation
d) lag behind Earth’s rotation

Explanation: Satellites very close to Earth revolve with high orbital velocities (on the order of 8 km/s, or 28,800 km/hr). Earth’s rotation is fastest at the equator, which drifts at a modest speed of just 1670 km/h (slower than the much more rapid near-Earth spacecraft).

9. What is the primary advantage of geostationary orbit?
a) Better resolution for Earth-monitoring
c) Easy to reach
d) Larger field of view

Explanation: The biggest merit of geostationary orbit is that the satellite appears fixed above a given location on Earth, allowing for uninterrupted communication with a ground-station in the satellite’s field of view. This is because the satellite revolves with the same angular velocity as that of Earth’s rotation, with no relative motion between a point on the surface and the satellite.

10. What is the main disadvantage of a satellite constellation?
a) Tedious to keep track of each constituent satellite
b) Collisions
c) Cost of manufacturing
d) Launch costs

Explanation: The cost of manufacturing is very high for constellations. This is because designers need to build multiple satellites with the same scientific instruments rather than just one spacecraft.
All satellites can be made compact and accommodated as a single payload, so launch costs are not an issue. Analysts can plan out the constellation orbit(s) in such a way that a collision is next to impossible, so that isn’t a problem either. Keeping track of each satellite is accomplished quite easily with ground stations (the level of involvement is no higher than that for a single satellite).

11. Which of the following is not an orbital element?
a) Altitude
b) Declination
c) Argument of periapsis
d) Right ascension of ascending node

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.

12. Why is it important to know the position of a satellite in orbit?
a) To save money
b) To increase its lifespan
c) To decrease its lifespan

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.

13. Which among the following is not a source of Earth orbit perturbations?
a) Atmospheric Drag
b) Micrometeoroids
c) Lunar gravity
d) Solar gravity

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.

14. What do orbital perturbations affect?
a) satellite’s lifespan
b) satellite’s efficiency
c) orbital elements
d) orbital energy

Explanation: Perturbations affect the orbital elements and force the satellite into a slightly different orbit after a long period of time. This causes a change in some orbit parameters. The orbital energy is a constant value and does not vary even in the presence of perturbing effects.

15. Which of the following parameters varies in the case of nodal regression?
a) right ascension of ascending node
b) eccentricity
c) semi-major axis

Explanation: Since an orbit undergoing nodal regression precesses about Earth’s rotational axis, the nodes of the orbit keep on shifting, causing the ascending node (and the descending node too) to change position with time. This, by default, is accompanied by a change in the right ascension of the orbit. As a rule of thumb, the ascending node moves west for prograde orbits and east for retrograde orbits.

16. An elliptical orbit undergoes perigee precession at a rate of around 11 degrees per day. How long does it take for the argument of perigee to shift by 90 degrees?
a) 91.1 days
b) 8.18 days
c) 11 days
d) 90 days

Explanation: The time taken for the perigee to shift by 90 degrees = (90 degrees)/(11 degrees/day) = 8.18 days.

17. When talking about ‘third body perturbations’, which among the following does not fall under the designation of ‘third bodies’?
a) Asteroids
b) Planets
c) The Moon
d) The Sun

Explanation: Third body perturbations are only caused by massive planetary or moon-like objects that are large enough to exert a strong gravitational pull. Asteroids cannot be grouped under such a category.

18. How many times farther away from Earth is the Sun compared to the Moon?
a) 8000 times
b) 4000 times
c) 800 times
d) 400 times

Explanation: The Sun lies approximately 400 times farther away from Earth than the Moon. The Sun’s distance from our planet is approximately 150 million kilometers, while the Moon is around 380,000 kilometers away from Earth.

19. What is the effect of atmospheric drag on a satellite?
a) Increases the orbital height
b) Deforms the satellite
c) Lowers the orbital height
d) Increases the satellite’s lifespan

Explanation: Atmospheric drag acts continuously on a satellite and always opposes its forward motion, thereby snapping energy out of the orbit and lowering the satellite’s altitude.

## Chapterwise Multiple Choice Questions on Astronautics

Our MCQs focus on all topics of the Astronautics subject, covering all topics. This will help you to prepare for exams, contests, online tests, quizzes, viva-voce, interviews, and certifications. You can practice these MCQs chapter by chapter starting from the 1st chapter or you can jump to any chapter of your choice.

## 1. MCQ on Space Brief History

The section contains multiple-choice questions and answers covering the transition from the flat earth to the spherical Earth model, the shift from an Earth-centered to a sun-centered universe, and Newton’s law of universal gravitation.

## 2. Astronautics Questions on Basic Orbits

The section covers questions and answers on how spacecraft move in orbit, spacecraft mission analysis, basic orbits, types of orbits, and satellite constellations.

## 3. Astronautics MCQ on Real Orbits

The section contains MCQs on earth orbit perturbations, gravity anomalies, third-body forces, and aerodynamic forces.

If you would like to learn "Astronautics" thoroughly, you should attempt to work on the complete set of 1000+ MCQs - multiple choice questions and answers mentioned above. It will immensely help anyone trying to crack an exam or an interview.

Wish you the best in your endeavor to learn and master Astronautics!

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