16Hit1600
Information and IdeasVery hardInferring direction of a projection's error

Information and Ideas practice question

A satellite in low Earth orbit slowly loses altitude because it plows through the thin gas at the top of the atmosphere. The density of that gas at any given altitude is not constant: it swells when the Sun is magnetically active, so the eleven-year solar cycle continually changes the drag a satellite feels. Engineers tracked one satellite for a year near the low point of a cycle, during which it descended 6 kilometers, and extrapolated that rate to forecast the date it would reenter the atmosphere. Over the next three years, as solar activity climbed toward its peak, the same satellite descended 55 kilometers. It follows that the engineers' original forecast ______

Which choice most logically completes the text?

  1. A. placed the satellite's reentry later than it is now likely to occur, because the descent rate the forecast assumed was too slow.Correct
  2. B. placed the satellite's reentry earlier than it is now likely to occur, because the descent rate the forecast assumed was too fast.
  3. C. would have been accurate had it been based on several years of tracking near the same point in the solar cycle.
  4. D. relied on the assumption that solar activity would keep rising throughout the satellite's remaining lifetime.

Answer: A. placed the satellite's reentry later than it is now likely to occur, because the descent rate the forecast assumed was too slow.

The forecast extrapolated 6 kilometers per year, measured at solar minimum, when drag is weakest; actual descent then averaged over 18 kilometers per year. A rate that is too slow yields a reentry date too far in the future. B reverses the direction of the error, the most common trap here. C encodes the "more data is always better" error: additional years sampled at the same low-drag point in the cycle would reproduce the same unrepresentatively slow rate. D inverts the forecast's premise—extrapolating a single measured rate assumes conditions will stay as they were, and it was precisely the rise in solar activity that the forecast failed to anticipate.

Why the other answers are wrong

B. placed the satellite's reentry earlier than it is now likely to occur, because the descent rate the forecast assumed was too fast.
You have the direction backwards: the forecast assumed 6 km/year, but the satellite actually fell over 18 km/year, so the assumed rate was too slow and the predicted reentry date too late, not too early.
C. would have been accurate had it been based on several years of tracking near the same point in the solar cycle.
More years of tracking at the same low point in the solar cycle would just repeat the same unrepresentatively slow 6 km/year rate — the problem was when the data was collected, not how much of it there was.
D. relied on the assumption that solar activity would keep rising throughout the satellite's remaining lifetime.
Extrapolating a single measured rate assumes conditions stay the same; the forecast's flaw was precisely that it did not anticipate solar activity rising.

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