Self-Assessment Quiz: Why Is Space So Hard?

Twenty questions to check your grasp of why spaceflight is difficult: orbital speed, the rocket-equation preview, the space environment, microgravity, and the no-repair rule. Answer each before opening the key. Aim for 16 or more. Where a number is needed, use $g_0 = 9.81\ \text{m/s}^2$ and $R_E = 6{,}371\ \text{km}$.

Question 1

The Kármán line, the conventional boundary of space, is at an altitude of about:

A) 10 km B) 100 km C) 1,000 km D) 10,000 km

Question 2

The single biggest reason reaching orbit is hard is:

A) space is extremely far away B) the enormous sideways speed required to stay in orbit C) the intense cold of space D) radiation from the Van Allen belts

Question 3

Orbital velocity in low Earth orbit is approximately:

A) 0.34 km/s B) 2.8 km/s C) 7.8 km/s D) 78 km/s

Question 4

A launch vehicle must supply roughly how much delta-v to reach low Earth orbit?

A) 7.8 km/s B) 9.4 km/s C) 11.2 km/s D) 25 km/s

Question 5

Roughly what fraction of an orbital rocket's lift-off mass is propellant?

A) ~25% B) ~50% C) ~90% D) ~99.9%

Question 6

Escape velocity from Earth's surface is about:

A) 7.8 km/s B) 9.4 km/s C) 11.2 km/s D) 42 km/s

Question 7

Astronauts float inside the ISS because:

A) there is no gravity at that altitude B) they are in continuous free fall, orbiting together with the station C) the ISS is beyond the reach of Earth's gravity D) the station's spin creates weightlessness

Question 8

Compared with the surface, the local gravitational acceleration at the ISS (~400 km) is about:

A) 0% B) 10% C) 50% D) 89%

Question 9

"Microgravity" most accurately describes:

A) a place where gravity is exactly zero B) the weak gravity found far from any planet C) the condition of free fall, in which felt weight is nearly zero D) gravity produced artificially by rotation

Question 10

In the vacuum of space, a spacecraft sheds heat mainly by:

A) convection B) conduction to the surrounding air C) radiating infrared light D) evaporation

Question 11

The Van Allen belts are:

A) rings of orbital debris around Earth B) regions of energetic charged particles trapped by Earth's magnetic field C) layers of the upper atmosphere D) the faint rings around Earth analogous to Saturn's

Question 12

The temperature of deep space (the cosmic microwave background) is about:

A) 0 °C B) −40 °C C) −270 °C D) −460 °C

Question 13

A one-gram fleck of debris is a serious threat to a satellite mainly because:

A) it is chemically corrosive B) at orbital closing speeds its kinetic energy is enormous (energy scales with speed²) C) it is strongly magnetic D) it blocks the satellite's solar panels

Question 14

The "no-repair rule" — that most spacecraft can never be physically serviced — most directly drives:

A) higher exhaust velocities B) redundancy, extensive pre-launch testing, and design margins C) larger payload fairings D) lower orbital altitudes

Question 15 (True/False, justify)

"There is essentially no gravity in space." True or false? Justify in one sentence, with a number.

Question 16 (True/False, justify)

"Making a rocket ten times larger, keeping the same proportions, lets it reach a higher delta-v." True or false? Explain briefly.

Question 17 (True/False, justify)

"Because kinetic energy grows with the square of speed, a tiny particle moving at orbital velocity can carry far more energy than its small mass suggests." True or false? Justify.

Question 18 (Short answer)

In one or two sentences, explain the idea that "orbit is sideways, not up," and state the approximate speed of a low orbit.

Question 19 (Short answer)

Name the two recurring themes introduced in this chapter and give a one-line meaning of each.

Question 20 (Short answer)

A spacecraft's sunlit and shadowed faces can differ by hundreds of degrees at the same instant. Why doesn't the heat simply even out across the vehicle, the way it would for a warm object on Earth?

Answer Key

Q Ans Note
1 B The Kármán line is at 100 km — space is close; altitude is the cheap part.
2 B Orbit is about sideways speed (~7.8 km/s), not distance.
3 C $v = \sqrt{\mu/r} \approx 7.8\ \text{km/s}$ in LEO.
4 B ~9.4 km/s: orbital speed plus gravity and drag losses.
5 C Mass ratio ~16 (kerosene) means ~94%; ~90% is the round figure.
6 C $v_{esc} = \sqrt{2}\,v_{orbit} \approx 11.2\ \text{km/s}$ from the surface.
7 B Free fall: station and crew fall around Earth together, so nothing pushes them to a floor.
8 D $g = \mu/r^2 \approx 8.7\ \text{m/s}^2 \approx 89\%$ of surface gravity.
9 C It is a state of falling, not an absence of gravity (residual ~$10^{-6}g$).
10 C No air means no convection or conduction to air; only radiation remains.
11 B Trapped protons (inner) and electrons (outer); discovered 1958 (Explorer 1).
12 C 2.7 K ≈ −270 °C. (−460 °F is the same temperature in Fahrenheit — a trap.)
13 B $E = \tfrac12 m v^2$; at ~10 km/s a 1 g fleck carries ~50 kJ.
14 B If you cannot fix it, you must build it not to break — redundancy, testing, margin.
15 False Gravity at the ISS is ~89% of its surface value; floating is free fall, not absent gravity.
16 False Delta-v depends on the mass ratio $m_0/m_f$, not absolute size; scaling up changes nothing.
17 True Energy $\propto v^2$, so at orbital speed even a small mass carries surprisingly large energy.
18 — An orbit is falling toward Earth while moving sideways fast enough to keep missing it; ~7.8 km/s in LEO.
19 — (1) Tyranny of the rocket equation — delta-v costs exponentially more fuel; (2) space is unforgiving — vacuum, radiation, thermal, debris, and no repair mean everything must work.
20 — In vacuum there is no air to convect heat and even out temperatures; heat moves only by (slow, direction-dependent) radiation, so the two faces stay far apart until engineered otherwise.

Target score: 16/20. Below that, reread the sections flagged below before moving to Chapter 2.

Topics to review by question

Questions Topic Section
1, 2, 3, 18 Orbit is speed, not height §1.1
4, 5, 16 The delta-v number and the rocket-equation preview §1.1–1.2
6 Escape velocity and the gravity well §1.5
10, 11, 12, 13, 20 The space environment §1.3
7, 8, 9 Microgravity is free fall §1.4
14 The no-repair rule and reliability §1.4
15, 17 Common misconceptions §1.3–1.4
19 The two themes (synthesis) §1.2, §1.5