Held by Gravity: Rubble-Pile Asteroids and a Racing Star

Use asteroid spin and escape-speed data to model what gravity holds together.

Part 1: Rubble piles and spin

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

Loose Rock, Here and in Space

1In the mountain town of Copper Notch, a week of heavy spring rain loosened a slope above the highway, and a landslide covered one lane with boulders, gravel and mud. A few days later, eighth grader Thandiwe Morrell went to a talk at the town library by Dr. Joaquín Ellery, a planetary scientist who grew up in the valley. He began with the landslide. Gravity pulls every rock on a slope downhill. Dry, loose rock can rest on a slope up to about 35° steep, but on a steeper slope gravity's pull wins and the rock slides.

2Then he turned to asteroids. Pictures from spacecraft show that many asteroids are rubble piles: boulders, gravel and dust resting together with gaps between them. One of these is Bennu, an asteroid about 490 m wide. When a spacecraft touched Bennu's surface to collect a sample, its arm sank into the loose rock like a hand pushed into a ball pit. Each body also has an escape speed, the speed a rock tossed straight up needs to fly off into space and not fall back. On Bennu, that speed is only about 0.2 m/s, slower than a slow walk.

3Asteroids spin, some slowly and some fast. As a pile spins faster, the rocks at its equator need a stronger inward pull to keep them moving in their circle. Telescope surveys have measured how fast thousands of asteroids spin, and Table 1 shows the fastest spin found in each size group. Pictures and radar show that some small asteroids are single solid boulders, and the strength of the rock holds each one together.

4Gravity pulls every piece of a body toward the body's center. Dr. Ellery compared Bennu with larger bodies in Table 2. Ceres, the largest object in the asteroid belt, is round, while Bennu is shaped like a lumpy spinning top. The same force reaches much farther. It holds the planets in their orbits around the Sun and holds the Sun and billions of other stars together in the Milky Way galaxy.

5Near the center of the Milky Way, astronomers have tracked a star called S2 as it circles an unseen object once every 16 years. At its closest, S2 is about 120 AU from that object (1 AU is Earth's distance from the Sun), and it moves at about 7,700 km/s. By comparison, Mercury, the fastest planet, moves at most about 59 km/s, when it is about 0.3 AU from the Sun.

Table 1: Fastest Spin Found in Each Size Group (simplified survey data)
Width of asteroid	Asteroids measured	Fastest spin found
Under 150 m	1,000	1 turn in 3 minutes
150 m to 1 km	2,500	1 turn in 2.2 hours
1 km to 5 km	3,000	1 turn in 2.2 hours
5 km to 20 km	1,500	1 turn in 2.3 hours

Use paragraph 2. Which observation is the best evidence that Bennu is a loose pile of rubble, not one solid rock?

A. Bennu spins as it travels, like the other asteroids in the surveys.
B. Bennu is about 490 m wide, far wider than the Copper Notch landslide on the highway.
C. A spacecraft's arm sank into Bennu's surface like a hand into a ball pit.
D. A pebble moving faster than 0.2 m/s could fly off Bennu into space.

Use Table 1. One hour is 60 minutes.

The fastest turn in the 150 m to 1 km group takes times as long as the fastest turn in the under 150 m group.

Which statement best describes the pattern in Table 1?

A. The wider the asteroid, the faster its fastest spin, from 2.3 hours down to 3 minutes.
B. Small and large asteroids have about the same fastest spin, close to 3 minutes.
C. Above about 150 m wide, the fastest spin stays near 2.2 hours, whatever the size.
D. The fastest spin gets about 2 hours slower with each larger size group in the table.

Use paragraph 3. Why do rubble-pile asteroids seldom spin faster than about one turn in 2.2 hours?

A. Rubble piles are too wide to spin quickly, since their gaps catch on dust in space.
B. The Sun's gravity pulls back on any asteroid that spins fast, which slows it to 2.2 hours.
C. A faster spin would make each rock heavier, so the rocks would sink deep inside the pile.
D. A faster spin would fling loose rocks off the equator, since the pile's gravity is too weak.

Use Table 1 and paragraph 3. Why can some asteroids under 150 m wide turn once in just a few minutes?

A. Small asteroids have stronger gravity than large ones, so they hold their rocks better.
B. Many of them are likely single solid rocks, held together by the strength of the rock.
C. Small asteroids were measured more often, so the surveys found faster spins for them.
D. Small asteroids are loose heaps of fine gravel and dust, which can spin very fast.

Over millions of years, sunlight can slowly speed up the spin of a small asteroid. Suppose a 1 km rubble pile speeds up until it turns once every 2 hours. What will most likely happen?

A. Its gravity will grow stronger to keep up with the faster spin.
B. Rocks near its equator will likely start to fly off into space.
C. It will be flung out of its orbit and leave the solar system.
D. It will press itself into one solid rock and keep speeding up.

Part 2: From asteroids to the galaxy

Use Table 2 and paragraphs 4 and 5 to answer questions 7 to 11. Then use the whole page for question 12.

Table 2: Four Bodies and Their Escape Speeds
Body	Width (km)	Escape speed (m/s)	Shape
Bennu (asteroid)	0.49	0.2	Lumpy spinning top
Ceres (dwarf planet)	940	510	Round
The Moon	3,475	2,380	Round
Earth	12,742	11,200	Round

Use Table 2.

A rock must move times as fast to escape Ceres as to escape Bennu.

Which statement best describes the pattern in Table 2?

A. The wider the body, the faster a rock must go to escape its gravity.
B. The escape speed is about the same number as the width in kilometers.
C. Narrow bodies have higher escape speeds, since their rocks sit closer.
D. The round bodies have escape speeds close to one another, near 500 m/s.

Use Table 2 and paragraph 4. Which explanation best fits why Ceres is round but Bennu is lumpy?

A. Ceres is closer to the Sun, so the Sun's gravity shaped it into a ball.
B. Ceres spins faster than Bennu, and the fast spin rounds off its corners.
C. Ceres has enough mass that its gravity pulled its pieces into a ball.
D. Ceres is made of one solid rock, which wore smooth as it moved through space.

10.

Use Table 1 and the passage. Sort each object by what mainly holds it together.

A 4 m boulder in space that turns once every minuteBennu, a pile of boulders and gravel with gaps between themCeres, a round body 940 km wideA 50 m asteroid that turns once every 5 minutesA 2 km pile of loose rock that turns once every 6 hours

Mainly its own gravity	Mainly the strength of solid rock

11.

Use paragraph 5. At its closest, S2 is about 400 times as far from the object it circles as Mercury is from the Sun, yet it moves more than 100 times as fast. Which explanation fits best?

A. The object S2 circles has far more mass than the Sun, so it pulls far harder.
B. S2 has more mass than Mercury, and objects with more mass orbit faster.
C. The galaxy's center has weaker gravity than the Sun, so S2 can speed up.
D. S2 is closer to its center than Mercury is to the Sun, so it is pulled harder.

12.

A town resident says, "Gravity matters for planets, but a small asteroid like Bennu is held together by its rocks sticking to each other. How fast it spins has nothing to do with gravity." Make a claim about this idea. Use evidence from both tables and the passage, and explain your reasoning.