A dedicated survey of fast-rotating near-Earth asteroids with the Two-meter Twin Telescope. I. Observational strategy and first results

Abstract

The rotational properties of small near-Earth asteroids (NEAs) provide crucial insights into their internal structure and collisional history. However, systematic surveys targeting metre- to decametre-sized bodies are rare, thus leaving their spin distribution poorly constrained. Our aim was to quantify the prevalence of fast rotation and characterise the spin-rate distribution of small NEAs to constrain their internal strength and evolution. We conducted a dedicated high-cadence photometric survey of 249 NEAs using the Two-meter Twin Telescope (TTT). Rotation periods and amplitudes were derived from dense time series to classify objects as fast or non-fast rotators. We determined the rotation period of 156 new fast rotators (P < 2.2 h), including 87 that rotate faster than 10 min. The prevalence of fast rotators increases with absolute magnitude: from 60.6-80.3 % for 22 < H < 24 to 77.3-89.4 % for 24 < H < 26 and 94.1-96.1 % for H > 26, indicating that fast rotation dominates in the small NEA population. Most objects spin faster than the gravity-defined limit; 98 targets require cohesive strengths exceeding that of weak rubble piles, and 22 are compatible only with compact, high-strength interiors. This is the first systematic survey targeting the rotation of such small NEAs, providing the largest homogeneous sample of fast rotators obtained by a single campaign. Our findings demonstrate that fast rotation is the norm for objects smaller than tens of metres, implying that modest cohesive strength is required to prevent their rotational disruption.

Figures (8)

• Figure 1: Distributions of observational parameters for the targets in our survey. The panels show (from left to right and top to bottom) geocentric distance $\Delta$, phase angle $\alpha$, absolute magnitude H, apparent V magnitude, sky-plane motion rate, and exposure time. Data obtained with the 0.8 m telescopes (TTT1/2, orange) and the 2 m TTT3 (dark blue) are shown separately.
• Figure 2: Rotation period as a function of diameter for the objects in this work. The left panel shows the full distribution, with asteroids from the Lightcurve Database (LCDB) and SsODNet with $U \geq 2$ in grey and near-Earth asteroids (NEAs) in orange. The 161 new NEAs with rotation periods derived in this paper are plotted in dark blue. Diameters were estimated from absolute magnitudes assuming a geometric albedo $p_V = 0.15$. Cohesive spin limits for weak rubble-pile material ($C\sim10$ Pa), lunar-like regolith ($C\sim1$ kPa), and a size-dependent monolithic strength ($C \propto D^{-0.5}$) are overplotted for reference. The right panel provides a zoomed-in image of the sub-140 m size range, where our survey is primarily focused.
• Figure 3: Representative examples of light curves without confirmed rotation periods, illustrating the types of partial or S/N-limited data included in the prevalence analysis.
• Figure 4: Histogram showing how the objects in our survey distribute by rotational classification as a function of absolute magnitude H. The blue bars indicate confirmed or tentative fast rotators, the orange bars show non-fast rotators, and the grey bars represent objects with insufficient information to determine their rotation state. The bottom horizontal axis displays the number of targets observed in each magnitude bin, whereas the top axis provides the corresponding diameter scale computed assuming an albedo of 0.15. The black line shows the fast-rotator fractions obtained from the LCDB and SsODNet ($U\geq2$), allowing a direct comparison with our sample.
• Figure 5: Synthetic light curve models in the $g$, $r$, $i$, and $z$ bands are shown for three representative fast-rotating near-Earth asteroids. The left panel shows the per filter rotational models with an example of multi-filter observing cycle, supposing an inter-image overhead of 5 s; the exposure windows in rotational phase are indicated at the top of the panel by two vertical markers delimiting the start and end of each exposure, connected by a dashed line, while the circles mark the phase centres. The right panel shows the distribution of the colour bias $(\mathrm{mag}_X-\mathrm{mag}_Y)_{\mathrm{meas}} - (\mathrm{mag}_X-\mathrm{mag}_Y)_{\mathrm{true}}$ for 5000 random realisations of this sequence. The underlying true colours are assumed to be $(g-r) = 0.45$, $(r-i) = 0.18$, and $(i-z) = 0.12$; these values are illustrative and only set the absolute colour scale, without affecting the shape of the bias distributions. The histograms are plotted with arbitrary normalisation since only the shape of the distributions are relevant.