Space exploration - Answer key

Year 10 answers

Fluency

No atmospheric blurring or absorption, so sharper images and access to wavelengths (X-ray, UV, infrared) that ground telescopes cannot detect.
Earth’s atmosphere absorbs X-rays completely; they must be observed above it.
Infrared.
Pulsars, radio galaxies, cool dust clouds that hide star formation, and distant quasars. Radio also penetrates dust.
Hubble Deep Field (distant galaxies); cosmic microwave background by WMAP and Planck; exoplanet atmospheres by Kepler and James Webb. (Any reasonable answer.)

Fluency

For every action there is an equal and opposite reaction. A rocket pushes hot exhaust gases backwards; the gases push the rocket forwards.
$\sim 7.8$ km/s (about $28\,000$ km/h).
An orbit with a $24$ -hour period so the satellite appears fixed above one spot on the equator; altitude is fixed by the requirement that orbital period equals one Earth day.
Both the station and the astronauts are in continuous free-fall; no surface pushes up on the astronaut, so they feel weightless.
Escape velocity is the minimum speed needed to leave Earth’s gravity altogether ( $\sim 11.2$ km/s). Orbital velocity only needs to keep an object in free-fall around Earth.

Fluency

Yuri Gagarin, 1961.
1960s to early 1970s (1969 - 1972).
Weather: GOES, Himawari (any). Navigation: GPS (US), Galileo (Europe), GLONASS (Russia), BeiDou (China).
USA (NASA, SpaceX), Russia (Roscosmos), China (CNSA). (Any two.)

Reasoning

Reaching orbital speed (~ $7.8$ km/s) requires so much $\Delta v$ that most of the launch mass must be propellant; additionally, the fuel itself must be lifted up through the lower atmosphere, requiring yet more fuel.
There is no air in space, so the engine must carry its own oxygen (or equivalent oxidiser) to burn the fuel.
Each satellite gives only a sphere of possible positions. Three satellites intersect to two points; a fourth resolves ambiguity and corrects the receiver’s clock.
Human radio transmissions (mobile phones, Wi-Fi, TV) are radio noise that swamps faint astronomical sources. Remote sites minimise interference.
Mars: longer duration (~2-3 years), cosmic radiation outside Earth’s magnetic field, much bigger distance, communication delay of $\sim 20$ minutes each way, no rescue possible. Far greater risks than a few-day Moon mission.

Problem solving

Distance from Earth’s centre: $6\,400 + 20\,200 = 26\,600$ km. Signal time $= \dfrac{20\,200}{3.0 \times 10^5} \approx 0.067$ s (from satellite to ground, approximate).
Time $= \dfrac{384\,000}{5\,500} \approx 69.8$ h $\approx 2.9$ days. (Real Apollo missions took about 3 days.)
$d = ct = 3.0 \times 10^5 \times 0.12 = 36\,000$ km. Matches the geostationary altitude.
Mars and Earth move on different orbits; a minimum-energy (Hohmann) transfer is only possible when the two planets are in the right relative positions, which happens every $\sim 26$ months.

Reasoning

$\Delta v$ : required velocity change; $v_e$ : exhaust velocity; $m_0$ : initial mass (with fuel); $m_f$ : final mass (after fuel burn). The ratio $m_0/m_f$ grows exponentially with $\Delta v$ , so a modest increase in required $\Delta v$ multiplies the fuel (and initial mass) needed.
Each stage can be optimised for its task and discarded when empty, reducing the mass that subsequent stages must accelerate. Monolithic designs carry the mass of empty tanks all the way, wasting fuel.
Scientific: humans can adapt, improvise, and collect diverse samples faster than robots. Economic: robots cost orders of magnitude less and need no life support. Ethical: subjecting humans to unknown radiation and travel risks raises serious responsibility questions; arguments exist on both sides. (Any reasoned answer.)
A kinetic impactor or gravity tractor could be sent to nudge the asteroid’s trajectory. Over many orbits, a tiny $\Delta v$ translates to a large miss distance; $20$ years of warning makes this feasible. Short warning would require much more energy and may be impossible.