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.

Answer Key

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 body	Its moon	Main body mass (kg)	Center to center	Time for one orbit	Moon's speed
Earth	Moon	6.0 × 10²⁴	384,000 km	27.3 days	1.0 km/s
Mars	Phobos	6.4 × 10²³	9,400 km	7.7 h	2.1 km/s
Pluto	Charon	1.3 × 10²²	19,600 km	6.4 days	0.2 km/s
Didymos	Dimorphos	5.4 × 10¹¹	1.19 km	11.9 h	0.00017 km/s

Table 2: Dimorphos Before and After the Crash
Measurement	Before	After
Time for one orbit	11 h 55 min	11 h 23 min
Center to center	about 1,190 m	about 1,150 m

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?

A. Phobos has more mass than our Moon, so it falls around Mars faster.
B. Phobos is about 40 times closer to its planet, where the pull is far stronger.
C. Mars is farther from the Sun, so the Sun pulls Phobos forward faster.
D. Mars spins faster than Earth, and its spin drags Phobos along with it.

Use Table 1. Why can Dimorphos stay in orbit while moving slower than a person walks?

A. Didymos has so little mass that its weak pull can hold only a slow-moving moon.
B. Dimorphos moves slowly because it is farther from Didymos than Phobos is from Mars.
C. Earth's gravity holds Dimorphos in place, so it does not need to move fast.
D. Dimorphos is too small to feel any gravity, so it drifts next to Didymos.

Use Table 2. Round the percent to the nearest tenth. (11 h 55 min is 715 minutes.)

After the crash, one orbit took 32 minutes less. That is about 4.5% shorter than before.

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

A. The crash sped Dimorphos up, so it now circles farther out from Didymos than before.
B. The crash added DART's mass to Dimorphos, so Dimorphos fell out of orbit and is drifting away.
C. The crash slowed Dimorphos, so Didymos's gravity pulled it into a slightly smaller, shorter orbit.
D. The crash turned off Didymos's gravity for a moment, so Dimorphos moved in a straight line.

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)
Run	Sideways speed (m/s)	What happened
1	0.1	Curved inward and hit Didymos
2	0.17	Circled at 1,190 m
3	0.21	Stretched oval, out to 3,800 m and back
4	0.3	Curved a little, then left for good
5	0.17, gravity off	Moved in a straight line

Use Runs 2 and 5. What do these two runs show?

A. Dimorphos needs both its sideways speed and Didymos's pull to keep circling.
B. Turning off gravity makes Dimorphos speed up and spiral away from Didymos.
C. Gravity pushes Dimorphos forward along its path, which keeps its speed at 0.17 m/s.
D. Dimorphos's speed keeps it circling, and gravity shrinks the circle by a few meters.

Match each run to the best explanation of its path.

Run 1 (0.1 m/s)C
Run 3 (0.21 m/s)B
Run 4 (0.3 m/s)A

A. Gravity bent its path, but it was too fast to be pulled back.
B. Gravity slowed it as it moved out, then pulled it back in.
C. It was too slow to miss Didymos as gravity pulled it inward.

Use Table 3.

In Run 3, the farthest point of the oval was 2610 m farther from Didymos's center than the starting point.

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?

A. Hit the main asteroid so its gravity turns off for a short time.
B. Hit the moon from the front to slow it down to about 0.1 m/s.
C. Hit the moon from behind to speed it up by about 0.13 m/s.
D. Hit the moon from the front to slow it by a tiny amount.

Use paragraphs 2 and 5. Sort each motion by the gravity that mainly holds it on its curved path.

Dimorphos going around once in about 11 hoursThe asteroid pair going around once in about 2.1 yearsThe Sun going around once in about 230 million yearsEarth going around once in about 1 yearDimorphos moving onto a slightly smaller orbit after the crash

Didymos's gravity	The Sun's gravity	Gravity of the Milky Way's mass
Dimorphos going around once in about 11 hours Dimorphos moving onto a slightly smaller orbit after the crash	The asteroid pair going around once in about 2.1 years Earth going around once in about 1 year	The Sun going around once in about 230 million years

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.

Claim: The student is wrong; Didymos's gravity is what keeps Dimorphos circling it. Evidence: When the simulator turned gravity off in Run 5, Dimorphos moved off in a straight line, but with gravity on at the same speed (Run 2) it circled at 1,190 m. Too slow (Run 1) and gravity pulled it into Didymos. After DART slowed it by a few millimeters per second, its orbit shrank from about 1,190 m to 1,150 m and got 32 minutes shorter. Reasoning: Didymos has little mass, so its pull is weak, and only a slow speed like 0.17 m/s is needed to stay in orbit. Gravity constantly pulls Dimorphos toward Didymos and bends its path into a loop. When its speed changed, gravity pulled it into a new, smaller orbit, which shows gravity controls its motion even though it is small and slow.