🔭 astrophysics

NEO Colors from The Mission Accessible Near-Earth Object Survey (MANOS)

This study presents griz colors for 189 near-Earth objects from the MANOS survey, revealing a significant size-dependent compositional trend where the abundance of S-complex (ordinary chondrite-like) objects decreases from approximately 65% at kilometer scales to one-third at sizes below 50 meters, a finding best explained by a compositional gradient in the NEO population rather than observational biases or surface modification processes.

Original authors: Nicholas Moskovitz, Theodore Kareta, Samantha Hemmelgarn, Hannah Zigo, Maxime Devogèle, Audrey Thirouin, Katie Breeland-Newcomb, Brian Burt, Annika Gustaffson, Mitchell Magnuson, Michael Mommert, Davi

Published 2026-03-18

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Nicholas Moskovitz, Theodore Kareta, Samantha Hemmelgarn, Hannah Zigo, Maxime Devogèle, Audrey Thirouin, Katie Breeland-Newcomb, Brian Burt, Annika Gustaffson, Mitchell Magnuson, Michael Mommert, David Polishook, Robert Schottland, Brian Skiff, Cristina Thomas, Mark Willman

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the Solar System as a bustling cosmic highway. Most of the traffic consists of asteroids, but a special group called Near-Earth Objects (NEOs) occasionally drifts close to our neighborhood. Some of these are harmless tourists, while others are potential hazards that could crash into Earth.

This paper is a report from a team of astronomers (the MANOS survey) who went on a "color photography" mission to take a closer look at 189 of these space rocks. Their goal? To figure out what these asteroids are made of and how their composition changes as they get smaller.

Here is the breakdown of their findings, explained with some everyday analogies:

1. The Challenge: Taking a Photo of a Spinning Top

Imagine trying to take a photo of a spinning top with a camera that can only take one color at a time. You take a red photo, then you have to wait a few seconds to switch the lens and take a blue photo, then wait again for a green photo.

If the top is spinning fast, it might be facing a different angle when you take the blue photo than when you took the red one. This means your "color" measurement (how red vs. how blue it looks) could be totally wrong because the object moved or changed brightness while you were switching lenses.

The Fix: The astronomers realized that for about half of their asteroids, this "spinning top" problem was messing up their data. They developed a new way to mathematically "correct" the photos, essentially rewinding the clock to figure out exactly how bright the asteroid was at the exact moment each color photo was taken.

The Result: Without this correction, they were accidentally misidentifying dark, carbon-rich asteroids (C-types) as something else. It's like mistaking a charcoal briquette for a piece of dark chocolate just because the lighting changed while you were looking at it.

2. The Big Discovery: Size Matters

Once they fixed the "spinning" errors, they combined their new data with older surveys to look at asteroids ranging from huge (kilometers wide) to tiny (the size of a house).

They found a surprising pattern: As asteroids get smaller, their "flavor" changes.

The Big Guys (Kilometer-scale): These are mostly made of ordinary rock, similar to the S-type asteroids. Think of these as the "standard issue" space rocks, like the common granite found in a driveway.
The Little Guys (Meter-scale): As you look at smaller and smaller asteroids, the number of these "standard rock" types drops by half! Instead, you start seeing more X-types (which are often dark, primitive, or metallic) and other rare types.

The Analogy: Imagine a bag of mixed nuts. If you look at the whole bag, it's mostly peanuts (S-types). But if you only look at the tiny, broken crumbs at the bottom of the bag, you find a lot more cashews and almonds (X-types) than you expected. The "recipe" of the asteroid population changes depending on the size of the piece.

3. Why Does This Happen? (The Detective Work)

The team played detective to figure out why the small asteroids are different. They ruled out several suspects:

Suspect 1: The "Source" (Where they came from). They thought maybe the small ones came from a different part of the asteroid belt. Verdict: Unlikely. The small ones seem to come from the same place as the big ones.
Suspect 2: The "Sunburn" (Thermal effects). Maybe the sun cooks the small ones differently? Verdict: No, the small ones aren't getting any hotter than the big ones.
Suspect 3: The "Tidal Shake" (Earth's gravity). Maybe Earth's gravity shakes the small ones apart, revealing fresh insides? Verdict: Probably not enough to explain the whole trend.
Suspect 4: The "Dust" (Surface texture). Maybe the small rocks have coarser dust that looks different? Verdict: Doesn't quite fit the data.

The Real Culprit: The "Family Tree" (Composition Gradient)
The most likely explanation is that the small asteroids are the "children" of very specific, young asteroid families in the main belt.

Think of the main asteroid belt as a giant family reunion. Some families are old and have been around for billions of years (producing the big, common S-type rocks).
Other families are very young (created by recent collisions). These young families are still full of tiny fragments (the meter-sized rocks).
The data suggests that the tiny fragments coming from these young families have a different chemical makeup than the older, larger rocks. It's like a family recipe that changes slightly with every generation.

4. Why Should We Care?

This isn't just about trivia; it matters for two big reasons:

Meteorites vs. Reality: When we find meteorites on Earth, they are mostly "ordinary chondrites" (S-types). But this study suggests that the actual population of small asteroids hitting our atmosphere is much more diverse. Our collection of meteorites is biased because the atmosphere acts like a filter, burning up the weird, dark, or fragile ones before they hit the ground. We are only catching the "tough" ones.
Planetary Defense: If we ever need to deflect an asteroid, we need to know what it's made of. A fluffy, porous rock needs a different deflection strategy than a solid metal one. Knowing that the smallest, most frequent impactors might have different compositions helps us build better defense models.

Summary

The astronomers fixed a "camera glitch" caused by spinning asteroids and discovered that small asteroids are chemically different from big ones. They aren't just tiny versions of the big rocks; they are a distinct group, likely representing the fresh, young debris from recent collisions in the asteroid belt. This changes how we understand the ingredients of our Solar System and how we prepare to protect Earth from space rocks.

1. Problem Statement

Near-Earth Objects (NEOs) are critical for understanding planetary formation, resource utilization, and planetary defense. However, the compositional distribution of NEOs, particularly those smaller than 100 meters (sub-km), remains poorly constrained due to observational biases. Existing surveys have historically focused on larger, brighter objects, leaving a gap in knowledge regarding the "top of the atmosphere" population—the direct parent bodies of meteorites.

Furthermore, a significant methodological challenge exists in deriving accurate colors for small, rapidly rotating asteroids. Most surveys use non-simultaneous multi-band exposures. If an asteroid's rotation period is comparable to the time required to cycle through filters (typically 10–30 minutes), rotational lightcurve variations can introduce systematic errors in derived colors, leading to incorrect taxonomic classifications. The paper addresses the need for a robust, large-scale color survey of small NEOs that accounts for these lightcurve effects to determine if the compositional distribution of NEOs changes with size.

2. Methodology

Observational Campaign:

Survey: The Mission Accessible Near-Earth Object Survey (MANOS).
Targets: 189 individual NEOs (199 sets of color measurements) selected for low $\Delta v$ (mission accessibility) and small sizes ( $H > 20$ , corresponding to diameters $< \sim 300$ meters).
Facilities: Data collected between 2014 and 2025 using three 4-meter-class telescopes:
- Lowell Discovery Telescope (LDT), Arizona.
- Southern Astrophysical Research (SOAR) Telescope, Chile.
- Mayall 4-m Telescope, Kitt Peak, Arizona.
Instrumentation: SDSS $griz$ filter set ($0.47, 0.62, 0.75, 0.89$ $\mu$ m).

Data Reduction and Lightcurve Correction:

Strategy: To mitigate errors from non-simultaneous exposures, the team employed an interleaved reference filter strategy (e.g., $r-g-r-i-r-z-r$ ) to track brightness variations.
Correction Methods:
- For 49 objects, dedicated lightcurve observations (approx. 1 hour) were taken prior to color sequences. These were fitted with Fourier series to correct the color measurements.
- For the remaining sample, polynomial fits were applied to the reference filter data within the color sequence to interpolate magnitudes.
Classification: Colors were converted to solar-corrected reflectance and classified using the Bus-DeMeo taxonomic system. To handle the low spectral resolution of 4 bands, similar types were grouped into 11 broad classes (A, C, D, K, O, Q, R, S, T, V, X).

Validation:

Internal consistency checks were performed on objects observed on multiple nights.
Cross-validation was conducted against new and archival spectroscopic data (Gemini, IRTF, MITHNEOS, NEOSHIELD2).

3. Key Contributions

Lightcurve Impact Analysis: The study provides a rigorous demonstration that failing to correct for rotational lightcurve variations in non-simultaneous color surveys introduces systematic biases. Specifically, uncorrected data artificially enhances the fraction of C-complex objects by $\sim 19\%$ in the test sample.
Size-Dependent Taxonomic Trends: By combining MANOS color data with visible-wavelength spectra from MITHNEOS and NEOSHIELD2, the authors constructed the most comprehensive size-dependent taxonomic distribution of NEOs to date (covering $H=13$ to $H=29$ ).
Mission Accessibility Data: The paper provides physical properties (colors, taxonomic types, rotation periods, and diameters) for 7 NEOs with $\Delta v < 4$ km/s, including the enigmatic D-type 2022 BX5 and the potential Moon impactor 2024 YR4.

4. Key Results

A. Lightcurve Variability Effects

Of the 49 objects with measured lightcurves, 65% would have switched taxonomic types if lightcurve corrections were not applied.
The lack of correction led to a systematic overestimation of C-type asteroids. This suggests that future large-scale surveys (e.g., LSST) must account for lightcurve variability to avoid misinterpreting population statistics.

B. Size-Dependent Compositional Gradient

S/Q vs. X Complex: A clear trend was identified where the fraction of S-complex (ordinary chondrite-like) and Q-type objects decreases with increasing absolute magnitude $H$ $H$ (decreasing size), while the fraction of X-complex objects increases.
- At $H \approx 14$ (km-scale), S+Q types constitute $\sim 70\%$ of the population.
- At $H \approx 28$ (meter-scale), S+Q types drop to $\sim 35\%$ .
Statistical Significance: Linear regression confirms this trend is significant ( $> 4\sigma$ ).
Stability of Other Types: The fractions of C, D, V, and other types remain relatively constant across the size range, suggesting the shift is specific to the S/Q to X transition.

C. Specific Objects of Interest

2022 BX5: A D-type object with the reddest colors of any known NEO, suggesting an outer Solar System origin or a connection to D-type interlopers in the inner Main Belt. Pre-discovery recovery attempts in archival VST images were unsuccessful.
2024 YR4: Confirmed as an S-complex object with a $\sim 20$ -minute rotation period. While an Earth impact in 2032 is ruled out, a 4% impact probability with the Moon remains.

5. Significance and Implications

Rejection of Alternative Explanations:
The authors systematically ruled out several mechanisms for the observed size-dependent trend:

Source Region: While small NEOs are dynamically linked to the $\nu_6$ resonance (typically S-type rich), the trend persists, suggesting source region alone does not explain the diversity.
Thermal Modification/Tidal Resurfacing: These mechanisms affect the S:Q ratio (weathering) but do not explain the total S+Q fraction drop.
Discovery Bias: The trend is opposite to what would be expected from albedo-based discovery biases (which favor high-albedo S-types at small sizes).
Shock Darkening: Unlikely to affect the population at the scale required to explain the trend.

Preferred Explanation: Compositional Gradient
The results support a model where the NEO population has a compositional gradient dependent on size:

Large NEOs ( $H < 20$ ): Derived from a mix of Main Belt families, dominated by S-complex (ordinary chondrite) material.
Small NEOs ( $H > 25$ ): Dominated by the youngest Main Belt collisional families (e.g., Veritas, Polana, Eos) which have steep size-frequency distributions. These families are compositionally diverse and less dominated by S-types.
Meteorite Connection: This implies that the "top of the atmosphere" population (meter-scale impactors) is dominated by non-ordinary chondrite types. The current meteorite collection is biased toward S-types because atmospheric filtering preferentially destroys non-ordinary chondrite material or because the flux of S-type meteorites is currently high due to specific young families.

Impact on Planetary Defense:
Understanding that small NEOs have different compositions (and thus likely different densities and structural strengths) than their larger counterparts is crucial for impact risk assessment models. Deflection strategies must account for these size-dependent physical properties.

Future Work:
The paper calls for spectral observations of objects with $H > 27$ to further constrain the transition point of this compositional gradient and refine the connection between meteorites and their parent bodies.