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Nudging a Moonlet: Gravity and the Asteroid Impact Test

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Nudging a Moonlet: Gravity and the Asteroid Impact Test

Use orbit data and simulator runs to explain how gravity holds a tiny moon in place.

Part 1: Four moons and one small push

Read the passage, Table 1 and Table 2, then answer questions 1–6.

The Planning Briefing

1Saoirse Nakamura-Ruiz spent a week of her summer in a space science program for eighth graders. On the last day, the group sat in on a planning briefing with Dr. Emeka Lindahl, a scientist who helps plan spacecraft visits to asteroids. His team is getting ready for data from a follow-up spacecraft that will study a pair of asteroids up close. Before the spacecraft arrives, the team plans which measurements to take first, so Dr. Lindahl walked the students through what is already known about the pair and asked them to explain it.

2The pair is called Didymos and Dimorphos. Didymos is a rocky asteroid about 780 m wide, and Dimorphos, about 150 m wide, circles it like a tiny moon. Together they travel around the Sun about once every 2.1 years. Dr. Lindahl compared Dimorphos with three other moons in Table 1. Dimorphos moves at about 0.17 m/s, which is much slower than a person walks.

3In 2022, a NASA spacecraft called DART, with a mass of about 580 kg, crashed into Dimorphos at about 6 km/s on purpose. The test asked a planetary defense question: could hitting a small asteroid change its path? Telescopes on Earth timed Dimorphos as it passed in front of and behind Didymos, before and after the crash (Table 2). The crash slowed Dimorphos down by only a few millimeters per second.

4Next, Dr. Lindahl showed runs from his team's orbit simulator. In each run, Dimorphos started 1,190 m from the center of Didymos, moving sideways at a different speed (Table 3). Everything else stayed the same, including the masses of both asteroids. The simulator then drew the path Dimorphos would follow for several days. In one extra run, the team switched gravity off to see what Dimorphos would do with no pull at all.

5At the end, he zoomed out on the screen. The Sun holds Didymos, Dimorphos, Earth and every other planet in their orbits. The Sun itself travels at about 230 km/s around the center of the Milky Way galaxy and takes about 230 million years to go around once. The galaxy holds hundreds of billions of stars and huge clouds of gas and dust.

Table 1: Four Moons and What They Orbit
Main bodyIts moonMain body mass (kg)Center to centerTime for one orbitMoon's speed
EarthMoon6.0 × 10²⁴384,000 km27.3 days1.0 km/s
MarsPhobos6.4 × 10²³9,400 km7.7 h2.1 km/s
PlutoCharon1.3 × 10²²19,600 km6.4 days0.2 km/s
DidymosDimorphos5.4 × 10¹¹1.19 km11.9 h0.00017 km/s
Table 2: Dimorphos Before and After the Crash
MeasurementBeforeAfter
Time for one orbit11 h 55 min11 h 23 min
Center to centerabout 1,190 mabout 1,150 m
1.
Use Table 1. Mars has less mass than Earth, yet Phobos moves about twice as fast as our Moon. Which explanation fits the data best?
Question 1 options
2.
Use Table 1. Why can Dimorphos stay in orbit while moving slower than a person walks?
Question 2 options
3.
Use Table 2. Round the percent to the nearest tenth. (11 h 55 min is 715 minutes.)

After the crash, one orbit took minutes less. That is about % shorter than before.

4.
Use paragraph 3 and Table 2. Which explanation of the change in Dimorphos's orbit fits the data best?
Question 4 options

Part 2: The orbit simulator and the bigger picture

Use Table 3 and paragraphs 4 and 5 to answer questions 5–10.

Table 3: Simulator Runs (start 1,190 m from Didymos's center)
RunSideways speed (m/s)What happened
10.1Curved inward and hit Didymos
20.17Circled at 1,190 m
30.21Stretched oval, out to 3,800 m and back
40.3Curved a little, then left for good
50.17, gravity offMoved in a straight line
5.
Use Runs 2 and 5. What do these two runs show?
Question 5 options
6.
Match each run to the best explanation of its path.
7.
Use Table 3.

In Run 3, the farthest point of the oval was m farther from Didymos's center than the starting point.

8.
A planetary defense team wants to push a future asteroid moon onto a slightly smaller orbit, as DART did. Based on Table 2 and Table 3, which plan fits best?
Question 8 options
9.
Use paragraphs 2 and 5. Sort each motion by the gravity that mainly holds it on its curved path.
10.
A student in the program says, "Dimorphos is so small and moves so slowly that gravity does not really matter for it. It just floats along next to Didymos." Make a claim about this idea. Use evidence from Table 2 and at least two runs in Table 3, and explain your reasoning about gravity.