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Scientists study unusual places to learn about the early solar system. One key location is the collection of Trojan asteroids. These space rocks share the orbit of Jupiter. Some lead the giant planet, and others follow it. They act like cosmic time capsules, containing material from the solar system's distant past. For decades, astronomers have observed a distinct pattern among the larger Trojan asteroids. They separate into two distinct color groups. Scientists informally label these groups as the 'reds' and the 'less reds.' A research team from Japan sought to solve this color mystery. They decided to focus on much smaller asteroids. Their investigation, published in January 2024, did not solve the old puzzle. Instead, it revealed a new and unexpected one. The smaller Trojans do not show the same two-color pattern at all.
The color differences in larger asteroids are not random. They correspond to different asteroid 'types.' These types are categories based on an asteroid's composition. For example, asteroids in the 'red' group are typically D-types. These are very dark objects. Scientists believe they are rich in complex organic molecules. The 'less red' asteroids are usually P-types or C-types. P-types share some features with D-types, but they reflect light that is less red. C-type asteroids are carbon-rich. They are common in the main asteroid belt between Mars and Jupiter. These scientific classifications should apply to asteroids of any size. However, measuring the color of a small, distant asteroid is very difficult. One major challenge is their fast rotation. Smaller asteroids tend to spin much faster than larger ones. To get an accurate color measurement, astronomers must take several images using different wavelengths of light. They then average the data to account for surface differences. If an asteroid spins too quickly during this process, each filter captures a different side. This leads to an incorrect calculation of its overall color.
The Japanese research team solved this problem with a specific instrument. They used the Suprime-Cam on the 8.2-meter Subaru Telescope in Hawai'i. They selected this camera for a critical reason. It could change its light filters extremely fast. This speed was essential. It allowed the astronomers to take images through different filters in a very short time. This minimized how much an asteroid could rotate between readings. The team identified 120 small Trojan asteroids. They refined their list to 44 unbiased samples. These asteroids ranged in diameter from about 3 kilometers to 16 kilometers. For each one, they cycled through two filter changes in less than an hour. This rapid process let them capture data from all sides of these fast-spinning rocks. They could then determine the asteroids' true colors with confidence.
The new data revealed a striking and unexpected feature. Typical larger Trojan asteroids fall neatly into one of two color groups: red or less red. The smaller asteroids showed no such split. Their color data formed a smooth, continuous curve. There was no clear separation into two groups. The researchers also checked if size was a factor. When they divided the small asteroids into 'red' and 'less red' groups, the size ranges in each group were statistically the same. This finding directly contradicts decades of observations of larger Trojans. It also challenges long-standing theories about how these asteroids formed and evolved.