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## astrophysics

### (10 points)4. Series 32. Year - P. V-1 in the space

The interstellar space is not empty but contains an insignificant amount of mass. For simplicity, assume hydrogen only and look up the required density. Could we build a spaceship that would „suck in“ the hydrogen and would use energy from it? How fast/large would the spaceship have to be in order to keep up the thermonuclear fusion only from the acquired hydrogen? What reasonable obstacles in realization should we consider?

crypto-facism → Red Dwarf → drive → thrust → V-1 and the circle closes

### (12 points)1. Series 32. Year - E. hourly

Measure the length of one day. However, there is a limitation: one continuous measurement can't take longer than one hour. For the sake of statistical accuracy, though, do repeat your measurements multiple times.

Jachym had an hour until deadline

### (10 points)1. Series 32. Year - P. terrible cold

Some nebulas constituted of a gas from stars, e. g. Bumerang, have lower temperature than the Cosmic Microwave Background (CMB), hence are technically colder than space. How is this possible? Try to determine a condition for a gas ejected by a hot star to cool down below the temperature of the CMB.

Karel wasn't satisfied with the claim that the temperature everywhere in space is at least that of the CMB.

### (9 points)6. Series 31. Year - P. universe expansion compensation

According to the current observations and cosmological models, it seems that our Universe is expanding and the rate of expansion is accelerating. What if that wasn't the case? What if the Universe stayed the same, but the physical laws/constants were changing so that it would seem like the universe is expanding, the way we observe it? Describe as many laws that would need to change.

Karel was intrigued whether one can compensate the expansion of universe.

### (9 points)4. Series 31. Year - P. Voyager II and Voyager I live!

We have a satellite and we want to launch it out of the Solar System. We launch it from Earth's orbit so that after some corrections of the trajectory it gets a velocity which is higher than the escape velocity from the Solar System. What is the probability that the satellite will collide with some cosmic material with higher diameter that $d=1 \mathrm{m}$ before leaving the Solar System.

Karel was wondering why NASA doesn't consider this possibility…

### (3 points)2. Series 31. Year - 2. solar power plant

The solar constant, or more accurately the solar irradiance, is the influx of energy coming from the Sun at the distance where Earth is. It technically doesn't have a constant value, but let's suppose it is approximately $P = 1{,}370\,\mathrm{W\cdot m^{-2}}$. Also, suppose that Earth's orbit is circular and its axis of rotation is tilted with respect to the normal of the orbital plane by $23.5\dg $. What would be the maximum power captured by a solar panel of area $S= 1\,\mathrm{m^2}$ at the summer and winter solstice, if the panel lies flat on the ground in Prague (latitude $50\dg $ N)? Ignore the effects of any obstructions or the atmosphere.

Karel watched Crash Course Astronomy

### (6 points)2. Series 31. Year - 3. observing

What fraction of a spherical planet's surface cannot be seen from the stationary orbit above the planet? (A stationary orbit is one where the satellite stays fixed above a certain point on the planet.) The density of the planet is $\rho $ and its rotation period is $T$.

Filip went through the unseen competition problems.

### (6 points)2. Series 31. Year - 4. nuclear waste no more

Imagine we have a thing (e.g. a nuclear waste container) and we want to get rid of it. We transfer the object to a circular orbit around the Sun at the same distance as Earth, but far enough from Earth to ignore its gravitational influence. Which of these methods of the objects disposal would require the least amount of energy and thus would be the most efficient?

- Throw it into the Sun. Getting it to the solar surface would be sufficient to burn the object.
- Transfer it to a circular orbit in the Asteroid belt (located between the orbits of Mars and Jupiter).
- Get it out of the Solar System completely.

Karel thought about what exactly is SEO and discovered this problem.

### (7 points)1. Series 31. Year - 5. planetary colonization

You have probably already thought about whether there are alien civilizations in the universe. As a general rule, the bigger a star the higher is its radiation power and the shorter its life. Let's focus on two stars, where one has twice the power output than the other. If the band where life is possible is given by the steady state temperature of a black body orbiting there, which has a lower and upper limit (that are the same for each and every system), around which star is this life-supporting band wider? How many times larger it will be in comparison to the other star?

Karel often procrastinates on Youtube.