Download ASTR 2020, Spring 2015 Professor Jack Burns Final Exam

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ASTR 2020, Spring 2015
Professor Jack Burns
Final Exam
May 4, 2015
Student ID Number
INSTRUCTIONS: Closed books, one page (2 sides) of notes allowed, calculators may
be used, strictly individual effort. WRITE your name and student ID number on this
page BEFORE you begin the exam.
The exam consists of 20 multiple choice questions worth 2 points each, and 6 short
answer questions worth 10 points each. Please allocate your time accordingly among
these parts of the exam.
Multiple Choice. In questions 1-20, choose the best answer (1 pt). Then explain your
reasoning in 1-2 complete sentences, including why these statements are correct or
incorrect (1 pt). So, a correct answer and correct explanation is worth a total of 2 pts.
1. Why are astronauts weightless in the International Space Station?
a. because the Space Station is traveling so fast
b. because the ISS is constantly in free-fall around Earth
c. because there is no gravity in space
d. because the Space Station is moving at constant velocity
e. because the ISS is decelerating toward the Earth
A: b – We feel weight because there is something opposing the pull of gravity on
us. The ISS is in free-fall, so there is no opposing force on the astronauts, and thus
they are weightless.
2. Imagine another solar system, with a star of the same mass as the Sun. Suppose a
planet with a mass twice that of Earth (2MEarth) orbits at a distance of 1 AU
(astronomical unit) from the star. What is the orbital period of this planet?
a. 1 year
b. 6 months
c. 2 years
d. 4 years
e. It cannot be determined from the information given.
A: a – The orbital period of a planet depends on the orbital distance and the mass
of the star, both of which are the same in this hypothetical system as in our solar
system.
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A 3. Which of the following best explains why the Moon's orbital period and rotation
period are the same?
a. The Moon was once closer to Earth, but the force of gravity got weaker as
the Moon moved farther away.
b. The law of conservation of angular momentum ensured that the Moon
must have the same amount of rotational angular momentum as it has of
orbital angular momentum.
c. The Moon once rotated faster, but tidal friction slowed the rotation period
until it matched the orbital period.
d. The equality of the Moon's orbital and rotation periods is an extraordinary
astronomical coincidence.
A: c – This is called tidal locking. The same effect has caused all of the moons
that we have discussed in this class to be tidally locked to their planets.
4. Why do astronomers need different telescope designs to observe across the
electromagnetic spectrum?
a. New telescopes incorporate new technology to increase their efficiency.
b. Telescopes have to adapt to the greater distortion of the atmosphere at
shorter wavelengths.
c. Light pollution is worse at far infrared wavelengths than visible
wavelengths.
d. Astronomers and engineers enjoy the challenge of making new telescope
designs.
e. Photons of different energy behave differently and require different
collection strategies.
A: e – For example, while visible light can be reflected using standard mirrors,
such mirrors would fail to reflect x-rays. Instead, grazing incidence mirrors are
used for x-rays. On the other end of the spectrum, we use arrays of antennae to
study radio waves.
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A 5. A larger telescope will always have a better angular resolution than a smaller
telescope when observing at the same wavelength.
a. True
b. False
!
A: a – The formula for angular resolution is 𝜃 = 206,265 arcsec × !. If the
wavelength stays the same, and D increases, the angle will decrease, which is a
better angular resolution.
6. What is the primary reason why the Pluto New Horizons flyby mission is cheaper
than a Pluto orbiter?
a. The flyby can use less expensive cameras than the orbiter.
b. The flyby is easier to design than the orbiter.
c. The fuel needed for an orbiter to slow down when it reaches Pluto is very
expensive in and of itself.
d. The fuel needed for an orbiter to slow down when it reaches Pluto adds a
lot of weight to the spacecraft.
e. The question is incorrect; in general, orbiters are cheaper than flybys.
A: d – If the spacecraft were to arrive at Pluto in any reasonable timeframe, it
would be going much much faster than velocity to orbit the planet. The fuel
necessary to slow the spacecraft down upon arrival would add enormously to the
fuel needed to do the initial launch.
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A 7. Which is the densest planet in the solar system?
a. Mercury
b. Venus
c. Earth
d. Mars
e. Jupiter
A: c (a is also acceptable) – Terrestrial planets are denser than jovian planets,
which are mostly made of gas. Earth is the densest, at 5.52 g/cc. Mercury has an
iron core that is a much larger fraction of its volume than the other terrestrial
planets, meaning that it is made up of the densest materials. However, due to its
larger mass, Earth experiences greater gravitational compression, making it
denser than Mercury, which is 5.43 g/cc.
8. Which of the following is not a characteristic of the outer jovian planets?
a. They have very few, if any, satellites.
b. They all magnetospheres.
c. They are primarily made of hydrogen and helium.
d. Their orbits are separated by relatively large distances.
e. They all have rings.
A: a – On the contrary, jovian planets have numerous satellites. Jupiter and Saturn
both have more than 60 moons each. This is due to their stronger gravitational
pull.
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A 9. Which of the following made important early discoveries of the interaction
between the Sun’s solar wind and the Earth’s magnetosphere?
a. Project Mercury
b. Sputnik
c. Project Gemini
d. Explorer 1
e. Apollo 11
A: d – While Sputnik was the first artificial satellite, it did not have any scientific
equipment. When the US launched their first satellite, Explorer 1, it discovered
the Van Allen belts.
10. Which of the following carried a probe that landed on Titan and discovered lakes
of liquid methane?
a. Curiosity
b. Hubble
c. Cassini
d. MAVEN
e. Voyager
A: c – The probe was named Huygens. It used Titan’s atmosphere to break and
was the first probe to land on the surface of Titan.
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A 11. Allison is floating freely in her spacecraft, and you are accelerating away from her
with an acceleration of 1g. How will you feel in your spacecraft?
a. You will be floating weightlessly.
b. You will feel weight, but less than on Earth.
c. You will feel weight, but more than on Earth.
d. You will feel the same weight as you do on Earth.
e. You will feel yourself pressed against the back of your spaceship with
great force, making it difficult to move.
A: d – No matter whether the acceleration of 1g is coming from the Earth’s
gravity or from the acceleration of a spacecraft, it will feel the same.
12. The theory of General Relativity proves that wormholes like those used in recent
science fiction movies such as Interstellar cannot exist.
a. True
b. False
A: b – General Relativity has solutions which would allow for the existence of
wormholes. While it doesn’t prove that they do exist, it doesn’t prove that they
cannot exist.
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A 13. What happens to energy in the convection zone of the Sun and Jupiter?
a. Energy slowly leaks outward through the diffusion of photons that
repeatedly bounce off ions and electrons.
b. Energy is produced in the convection zone by nuclear fusion.
c. Energy is transported outward by the rising of hot gas and the sinking of
cooler gas.
d. Energy is consumed in the convection zone by the creation of electrons
and positrons.
e. Energy is conserved so while the gas moves up and down, there is no net
transport of energy.
A: c – Option (b) is the core. Moving outward, next comes (a), which is the
radiation zone. After the radiation zone comes the convection, which is (c). Just as
in a pot of water on your stove, hot material rises and gives off some of its energy.
This cools the material, and so it sinks, where it is heated and starts the process
over.
14. Which internal energy source is the most important in continuing to heat the
terrestrial planets today?
a. radioactivity
b. accretion
c. differentiation
d. tidal heating
e. gravitational contraction of the cores
A: a – Radioactivity takes a long time to cease. On the other hand, accretion and
differentiation, which heated terrestrial planets early on in their formation, have
since finished.
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A 15. Which of the following best describes why the smaller terrestrial worlds have
cooler interiors than the larger ones?
a. They were cooler when they formed.
b. The smaller ones are farther from the Sun.
c. They have relatively fewer radioactive elements.
d. They have relatively more surface area compared to their volumes.
e. They had more volcanic eruptions in the past, which released their internal
heat.
A: d – A planet’s surface area determines how quickly it can radiate energy away.
Its volume determines how much energy it needs to radiate away in order to cool.
Thus the ratio of surface area to volume tells us the rate at which a planet cools.
This ratio is greater for smaller planets, and so they cool faster, meaning it takes
them less time to cool.
16. What are the conditions necessary for a terrestrial planet to have a strong
magnetic field?
a. a molten metallic core only
b. fast rotation only
c. a rocky mantle only
d. both a molten metallic core and reasonably fast rotation
e. both a metal core and a rocky mantle
A: d – For a planet (or moon) to have a global magnetic field it needs a fluid
conducting material that is rotating. In the case of terrestrial planets, this material
is a molten metal core.
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A 17. What are greenhouse gases?
a. gases that absorb visible light
b. gases that absorb ultraviolet light
c. gases that absorb infrared light
d. gases that transmit visible light
e. gases that transmit infrared light
A: c – Planets give off infrared light. The greenhouse gases absorb and re-radiate
this IR light, slowing its escape from the planet and thus warming the planet’s
surface.
18. Uranus and Neptune have methane clouds but Jupiter and Saturn do not. Which
factor explains why?
a. Temperatures on Jupiter and Saturn are too high for methane to condense.
b. Water in the atmospheres of Jupiter and Saturn has combined with
methane to dilute methane to only a trace.
c. The rapid rotation of Jupiter and Saturn prevents methane clouds from
forming.
d. The stronger gravity on Jupiter and Saturn pulls methane downward so
that it can't form clouds.
A: a – The composition of clouds is dependent on what gases are allowed to
condense. The farther you get from the Sun, the colder it gets. Once you get to
Uranus and Neptune, it is finally cold enough for methane to condense.
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A 19. Which statement about asteroids is not true?
a. Many but not all orbit the Sun in the asteroid belt.
b. Some are more like loosely bound piles of rubble than solid chunks of
rock.
c. Most asteroids are not spherical in shape.
d. If we could put all the asteroids together, they would make an object about
the size of Earth.
e. NASA is currently planning to retrieve an asteroid and bring it back to an
orbit around the Moon.
A: d – Combining all of the asteroids wouldn’t even result in a small terrestrial
planet, much less an Earth-size one.
20. Most of the exoplanets discovered around other stars
a. are more massive than Earth and orbit very far from the star.
b. are more massive than Earth and orbit very close to the star.
c. are less massive than Earth and orbit very far from the star.
d. are less massive than Earth and orbit very close to the star.
e. are found around neutron stars.
A: b – Planets that are closer to their stars take less time to orbit, and so are easier
to find. More massive planets are also easier to find. For instance, in the transit
method, a more massive planet would block more light. In the Doppler method, a
more massive planet would pull on the star more.
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A Short Answer Questions 21-26: Please answer the following questions in a few
sentences. Be sure to write legibly. Also, use sketches, if helpful, in addition to the text.
Please be brief. Literacy and clarity count! Each short answer is worth 10 points.
21. In your opinion, why do we explore space? Give at least 3 reasons to justify the
expense and risks associated with NASA’s space program.
Example: Not only does space exploration excite our natural human curiosity and
desire to explore, it also results in more tangible benefits. The requirements for space
exploration lead to advances in medicine, engineering, and more. Space explorers
require precision instruments in order to function and record scientifically useful data.
These instruments developed for these purposes often have uses here on Earth that
were never dreamed of until a NASA mission required the instrument. Finally, in a
more distant look at the future, humanity may need to depend on resources beyond
Earth for its survival.
22. You have just been put in charge of planning the next robotic mission to Europa.
Discuss the advantages and disadvantages of using a Hohmann Transfer Orbit
trajectory versus a Gravitational Slingshot trajectory to reach the Jupiter system.
A: The Hohmann transfer orbit requires the least fuel among trajectories that depend
on burning fuel for all of their velocity changes. However, we can save on fuel by
using the angular momentum of planets to speed up the spacecraft. Gravitational
slingshot maneuvers do just that, but at the cost of time. In order to use a gravitational
slingshot, the spacecraft must first go to a planet to slingshot around. Since this is a
robotic mission, time doesn’t need to be optimized for above all else. Therefore a
gravitational slingshot trajectory is desirable, for an object as far away as Europa.
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A 23. Why is long-distance, interplanetary space travel dangerous for humans and their
spacecraft, electronics, and sensors?
a. Discuss the environment in interplanetary space and the source of these
problems.
b. What strategies might mitigate these problems?
A: Interplanetary space is full of radiation from our Sun’s solar wind and from
Galactic cosmic rays. For humans, the danger that this radiation presents is in the
form of the breaking of DNA. In the case of electronics, the particles can impact and
damage circuitry. Furthermore, spacecraft and sensors can be damaged by high-speed
impacts with dust, which is present in interplanetary space.
Solar wind radiation can be protected against through the right selection of materials,
with possibilities including certain plastics and water. In the case of human
exploration, the crust of the destination planet can be used for protection.
24. Do you think interstellar travel will ever be feasible? Using the physics principles
and laws described in class and in the reading this semester, discuss the
opportunities and challenges of an interstellar voyage to an exoplanet.
Example: Even given that we manage to not destroy ourselves, I do not suspect
interstellar travel will be possible. The main hurdle that we face is the time required.
In order to traverse the immense distances between stars in any reasonable amount of
time, we must travel at near the speed of light. This requires truly spectacular
amounts of energy. Proposed ideas for where to get this energy include tapping the
energy of stars, light sails, and anti-matter rockets.
If wormholes exist, as General Relativity allows for, this gives us a way out of the
velocity requirements. However, unless the wormholes were pre-existing, this does
not get us out of major energy requirements.
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A 25. Earth and Venus both presumably had similar gases outgassed from their
volcanoes. Explain how their atmospheres ended up so different.
A: The main factor in this is that Venus is closer to the Sun than the Earth. This led to
a runaway greenhouse effect. The runaway greenhouse effect evaporated all of the
water on Venus, creating an atmosphere much thicker than Earth’s. On the Earth,
rains remove from the atmosphere much of the CO2 and other gases released by
volcanism. However, it was too warm for Venus to rain, and so Venus has very high
levels of CO2 in its atmosphere.
26. The Moon and Mars are two potential destinations for human exploration over the
next 2-3 decades.
a. Why are the lunar farside and the lunar poles potentially interesting
destinations for near-term exploration?
b. What might we learn from further human expeditions to the Moon’s
surface that would be useful in preparing for a long-duration crewed
mission to Mars?
A: There are permanently shadowed craters at the lunar poles that contain water ice.
On the lunar farside is found the South Pole Aitken Basin, the oldest impact crater.
The farside is also radio-quiet, making it an excellent location for looking at the
Universe at the radio end of the light spectrum. These reasons, among others, make
the lunar farside and poles intriguing places to explore.
Sending humans to the lunar surface will allow us to gather more information on how
the human body responds to gravity on objects significantly smaller than Earth – a
category which also includes Mars. It will also require us to develop radiation
shielding, a requirement for any long-duration human mission. Finally, it would be an
opportunity to learn how to mine resources such as water and metals.
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