A familiar asteroid mystery just became more complicated
Jupiter’s Trojan asteroids have long been treated as valuable clues to the early solar system, but a new study suggests the smaller members of that population may not behave the way astronomers expected. Universe Today reports that researchers in Japan examined small Trojan asteroids and found that they do not display the same clear color split seen in larger objects. Instead of resolving a longstanding question, the result appears to have created a new one.
The older puzzle is straightforward in outline. Among larger Trojan asteroids, astronomers have identified two broad color groupings: “red” and “less red.” Those labels are not cosmetic. They are linked to different asteroid types and, by implication, to different surface compositions or histories. Red Trojans are typically associated with D-type asteroids, which are described in the source text as extremely dark and thought to be rich in complex organic molecules. Less red Trojans are more likely to be P-type or C-type, although the text notes that P-types may have more in common with D-types than C-types do, apart from their distinctly less red spectral slope.
Because Trojans are often treated as time capsules from the solar system’s formative era, this color distinction matters. If large asteroids separate cleanly into different spectral groups, that can hint at differences in origin, composition, or environmental processing. The expectation might be that smaller members of the same population would preserve some version of that pattern. The new work suggests otherwise.
What the researchers did
Studying small Trojan asteroids is technically difficult. Universe Today notes that one major challenge is rotation. Smaller asteroids tend to spin quickly, and astronomers need images taken in different wavelengths to build up an accurate spectral profile. If the asteroid rotates too much between exposures, the data can effectively sample different sides of the object and distort the final picture.
To address that problem, the researchers used the final run of the Suprime-Cam instrument on the 8.2-meter Subaru Telescope in Hawaii. According to the source text, Suprime-Cam offered one advantage especially relevant to this project: it could change filters faster than its successor, Hyper Suprime-Cam. That shorter filter-switching time reduced the amount of asteroid rotation between observations, improving the odds of obtaining cleaner color measurements from fast-spinning small bodies.
The team identified 120 small Trojan asteroids and narrowed that set to 44 unbiased samples ranging from roughly 3 kilometers to around 16 kilometers in diameter. That size range is important because it pushes the investigation into a part of the Trojan population that is harder to characterize than the larger objects that have historically defined the red-versus-less-red discussion.
