We studied a capability of Gaia to detect binary systems among small asteroids (with diameters ≲10km) by observing their photocenter oscillation. The closest binary systems with orbital periods about ...1 day or shorter show mostly a too low amplitude of the photocenter oscillation and Gaia will not be able to detect most of them with its expected astrometric performance. Wider binaries with orbital periods on an order of several days or longer shall be detectable with their amplitudes of the photocenter oscillation on an order of 10-times greater than the expected standard uncertainty of their Gaia astrometric measurements. A confusion of binaries with slowly rotating asteroids that show a rotational photocenter variation of a similar magnitude will not be significant unless the satellite is small or very large; in the range of medium-distance binaries (with the orbital periods on an order of several days), we shall be able to uniquely distinguish binaries with the diameter ratio D2/D1 between ∼0.1 and ∼0.95. Gaia will be the first survey to sample the largely unknown population of medium-distance binary systems among small main-belt asteroids where the present detection techniques (photometric and AO observations) are inefficient. A combination of Gaia binary observations with measurements with other techniques will be needed to eliminate existing degeneracy of the astrometric binary detection and to provide unique estimates of parameters of the binary systems.
► Gaia will detect asteroid binaries by measuring their photocenter oscillations. ► Medium-distance asteroid binaries will be sampled for the first time. ► Binaries can be distinguished from slowly rotating single asteroids.
The density measurements of Almahata Sitta ureilites reveal a bulk density of a arrow right 43.1g/cm3. This value, together with the 2008 TC3 asteroid shape model and albedo, was used to estimate the ...asteroid's mass. Based on the study of recovered meteorites and atmospheric entry observations Asteroid 2008 TC3 is compositionally heterogeneous and of low mechanical strength. Thus we consider the presence of significant macroporosity likely, lowering asteroid's bulk density compared to that of the Almahata Sitta ureilites. Most realistic albedos lie in a range of 0.09-0.2 and the presence of significant macroporosity leads to mass estimates below 20A-103 kg, which is lower than previously estimated. The presence of a non-ureilitic fraction and space weathering may affect the albedo and also influence the mass estimates. However, from current data it is not possible to quantify this effect.
The small binary asteroid (939) Isberga Carry, B.; Matter, A.; Scheirich, P. ...
Icarus (New York, N.Y. 1962),
03/2015, Letnik:
248
Journal Article, Web Resource
Recenzirano
Odprti dostop
•We study the small main-belt binary asteroid (939) Isberga.•Isberga is a Sq-type asteroid from near-infrared spectroscopy.•Mutual orbit, size ratio, and shape are derived from optical ...lightcurves.•Size is determined using mid-infrared interferometry with MIDI at VLTI.
In understanding the composition and internal structure of asteroids, their density is perhaps the most diagnostic quantity. We aim here at characterizing the surface composition, mutual orbit, size, mass, and density of the small main-belt binary asteroid (939) Isberga. For that, we conduct a suite of multi-technique observations, including optical lightcurves over many epochs, near-infrared spectroscopy, and interferometry in the thermal infrared. We develop a simple geometric model of binary systems to analyze the interferometric data in combination with the results of the lightcurve modeling. From spectroscopy, we classify Ibserga as a Sq-type asteroid, consistent with the albedo of 0.14-0.06+0.09 (all uncertainties are reported as 3-σ range) we determine (average albedo of S-types is 0.197±0.153, see Pravec et al. (Pravec et al. 2012. Icarus 221, 365–387). Lightcurve analysis reveals that the mutual orbit has a period of 26.6304±0.0001h, is close to circular (eccentricity lower than 0.1), and has pole coordinates within 7° of (225°, +86°) in Ecliptic J2000, implying a low obliquity of 1.5-1.5+6.0deg. The combined analysis of lightcurves and interferometric data allows us to determine the dimension of the system and we find volume-equivalent diameters of 12.4-1.2+2.5km and 3.6-0.3+0.7km for Isberga and its satellite, circling each other on a 33km wide orbit. Their density is assumed equal and found to be 2.91-2.01+1.72gcm-3, lower than that of the associated ordinary chondrite meteorites, suggesting the presence of some macroporosity, but typical of S-types of the same size range (Carry 2012. Planet. Space Sci. 73, 98–118). The present study is the first direct measurement of the size of a small main-belt binary. Although the interferometric observations of Isberga are at the edge of MIDI capabilities, the method described here is applicable to others suites of instruments (e.g., LBT, ALMA).
► Parameters of 18 binary inner main-belt asteroids estimated/constrained. ► Orbit poles of small binary asteroids concentrate at high ecliptic latitudes. ► The YORP tilt of spins of binary parent ...bodies or primaries proposed as the cause.
Our photometric observations of 18 main-belt binary systems in more than one apparition revealed a strikingly high number of 15 having positively re-observed mutual events in the return apparitions. Our simulations of the survey showed that it cannot be due to an observational selection effect and that the data strongly suggest that poles of mutual orbits between components of binary asteroids in the primary size range 3–8km are not distributed randomly: The null hypothesis of an isotropic distribution of the orbit poles is rejected at a confidence level greater than 99.99%. Binary orbit poles concentrate at high ecliptic latitudes, within 30° of the poles of the ecliptic. We propose that the binary orbit poles oriented preferentially up/down-right are due to either of the two processes: (i) the YORP tilt of spin axes of their parent bodies toward the asymptotic states near obliquities 0° and 180° (pre-formation mechanism) or (ii) the YORP tilt of spin axes of the primary components of already formed binary systems toward the asymptotic states near obliquities 0° and 180° (post-formation mechanism). The alternative process of elimination of binaries with poles closer to the ecliptic by dynamical instability, such as the Kozai effect due to gravitational perturbations from the Sun, does not explain the observed orbit pole concentration. This is because for close binary asteroid systems, the gravitational effects of primary’s irregular shape dominate the solar-tide effect.
Tumbling asteroids Pravec, P.; Harris, A.W.; Scheirich, P. ...
Icarus (New York, N.Y. 1962),
2005, 2005-1-00, 20050101, Letnik:
173, Številka:
1
Journal Article
Recenzirano
We present both a review of earlier data and new results on non-principal axis rotators (tumblers) among asteroids. Among new tumblers found, the best data we have are for 2002 TD
60, 2000 WL
107, ...and (54789) 2001 MZ
7—each of them shows a lightcurve with two frequencies (full terms with linear combinations of the two frequencies are present in the lightcurve). For 2002 TD
60, we have constructed a physical model of the NPA rotation. Other recent objects which have been found to be likely tumblers based on their lightcurves that do not fit with a single periodicity are 2002 NY
40, (16067) 1999 RH
27, and (5645) 1990 SP. We have done a statistical analysis of the present sample of the population of NPA rotators. It appears that most asteroids larger than
∼
0.4
km
with estimated damping timescales (Harris, 1994, Icarus 107, 209) of 4.5 byr and longer are NPA rotators. The statistic of two short-period tumblers (
D
=
0.04
and 0.4 km) with non-zero tensile strength suggests that for them the quantity
μ
Q
/
T
, where
μ is the mechanical rigidity,
Q is the elastic dissipation factor, and
T is a spin excitation age (i.e., a time elapsed since the last significant spin excitation event), is greater by two to four orders of magnitude than the larger, likely rubble-pile tumblers. Among observational conditions and selection effects affecting detections of NPA rotations, there is a bias against detection of low-amplitude (small elongation) tumblers.
Dimorphos, the secondary member of the binary asteroid (65803) Didymos, was impacted by NASA’s Double Asteroid Redirection Test (DART) spacecraft on September 26, 2022. Images taken with the DART’s ...DRACO camera before the impact showed that Dimorphos’s original shape was nearly rotationally symmetric, close to an oblate spheroid, and it was probably in a synchronous spin state. As such, it did not show a detectable secondary brightness variation before the DART impact in the integral light from the binary system. However, numerical impact models predicted that the DART impact could change both its shape and spin state, signatures of which could be detected with high-quality lightcurve observations. We have analyzed the best photometric observations of the Didymos–Dimorphos system that were taken during its favorable observing and geometric conditions in December 2022 and January 2023 and detected a significant secondary rotational lightcurve with amplitudes (in the total light from the primary and secondary) ranging from 0.008 to 0.031 mag at 7 distinct epochs. We estimate that the apparent cross section of Dimorphos varied with a relative amplitude (normalized to the mean cross section) between ±0.07 and ±0.24 over its rotation on the individual epochs. The observed changes of Dimorphos’s apparent cross-section amplitude over its rotation suggest an attitude instability, showing variations of Dimorphos’s obliquity. The lightcurve minima are approximately aligned, to within 30∘in mean anomaly, with the mutual events between the components of the binary system. The observations suggest that Dimorphos is in an excited, non-principal axis (NPA) spin state where, on average, it is tidally locked. By comparing the observations to high-fidelity simulations, we find that a moderate amount of NPA rotation in Dimorphos after the DART impact is required to reproduce the observed lightcurves. This NPA rotation is limited to around on-average synchronous or anti-synchronous configurations of Dimorphos. We also find Dimorphos elongations with the equatorial axis ratios a/b between 1.1 and 1.4 are generally consistent with the observed mean apsidal precession rate. We predict that when the ESA’s Hera spacecraft arrives to the Didymos–Dimorphos system in late 2026, it will still find Dimorphos in the NPA spin state, which places additional constraints on the operations of the spacecraft to achieve its objectives.
•Post-DART-impact rotational lightcurves of Dimorphos observed.•Dimorphos appears to be in an excited, non-principal axis spin state.•The Dimorphos spin state is on-average synchronous or anti-synchronous.•Dimorphos post-impact axis ratio a/b is between 1.1 and 1.4.•The ESA’s Hera s/c will probably still find Dimorphos in the excited spin state.
Asteroid pairs: A complex picture Pravec, P.; Fatka, P.; Vokrouhlický, D. ...
Icarus (New York, N.Y. 1962),
11/2019, Letnik:
333
Journal Article
Recenzirano
Odprti dostop
We studied a sample of 93 asteroid pairs, i.e., pairs of genetically related asteroids that are on highly similar heliocentric orbits. We estimated times elapsed since separation of pair members ...(i.e., pair age) that are between 7 × 103 yr and a few 106 yr. With photometric observations, we derived the rotation periods P1 for all the primaries (i.e., the larger members of asteroid pairs) and a sample of secondaries (the smaller pair members). We derived the absolute magnitude differences of the studied asteroid pairs that provide their mass ratios q. For a part of the studied pairs, we refined their WISE geometric albedos and collected or estimated their taxonomic classifications. For 17 asteroid pairs, we also determined their pole positions. In two pairs where we obtained the spin poles for both pair components, we saw the same sense of rotation for both components and constrained the angles between their original spin vectors at the time of their separation. We found that the primaries of 13 asteroid pairs in our sample are actually binary or triple systems, i.e., they have one or two bound, orbiting secondaries (satellites). As a by-product, we found also 3 new young asteroid clusters (each of them consisting of three known asteroids on highly similar heliocentric orbits). We compared the obtained asteroid pair data with theoretical predictions and discussed their implications. We found that 86 of the 93 studied asteroid pairs follow the trend of primary rotation period vs mass ratio that was found by Pravec et al. (2010). Of the 7 outliers, 3 appear insignificant (may be due to our uncertain or incomplete knowledge of the three pairs), but 4 are high mass ratio pairs that were unpredicted by the theory of asteroid pair formation by rotational fission. We discuss a (remotely) possible way that they could be created by rotational fission of flattened parent bodies followed by re-shaping of the formed components. The 13 asteroid pairs with binary primaries are particularly interesting systems that place important constraints on formation and evolution of asteroid pairs. We present two hypotheses for their formation: The asteroid pairs having both bound and unbound secondaries could be “failed asteroid clusters”, or they could be formed by a cascade primary spin fission process. Further studies are needed to reveal which of these two hypotheses for formation of the paired binary systems is real.
We obtained thorough photometric observations of two binary near-Earth asteroids (66391) Moshup = 1999 KW4 and (88710) 2001 SL9 taken from 2000 to 2019. We modeled the data and derived physical and ...dynamical properties of the binary systems. For (66391) 1999 KW4, we derived its mutual orbit's pole, semimajor axis and eccentricity that are in agreement with radar-derived values (Ostro et al., 2006. Science, 314, 1276–1280). However, we found that the data are inconsistent with a constant orbital period and we obtained unique solution with a quadratic drift of the mean anomaly of the satellite of −0.65 ± 0.16 deg./yr2 (all quoted uncertainties correspond to 3σ). This means that the semimajor axis of the mutual orbit of the components of this binary system, determined a = 2.548 ± 0.015 km by Ostro et al. (2006), increases in time with a mean rate of 1.2 ± 0.3 cm/yr.
For (88710) 2001 SL9, we determined that the mutual orbit has a pole within 10° of (L,B) = (302∘,−73∘) (ecliptic coordinates), and is close to circular (eccentricity < 0.07). The data for this system are also inconsistent with a constant orbital period and we obtained two solutions for the quadratic drift of the mean anomaly: 2.8 ± 0.2 and 5.2 ± 0.2 deg./yr2, implying that the semimajor axis of the mutual orbit of the components (estimated a ~ 1.6 km) decreases in time with a mean rate of −2.8 ± 0.2 or −5.1 ± 0.2 cm/yr for the two solutions, respectively.
The expanding orbit of (66391) 1999 KW4 may be explained by mutual tides interplaying with binary YORP (BYORP) effect (McMahon and Scheeres, 2010a. Icarus 209, 494–509). However, a modeling of the BYORP drift using radar-derived shapes of the binary components predicted a much higher value of the orbital drift than the observed one. It suggests that either the radar-derived shape model of the secondary is inadequate for computing the BYORP effect, or the present theory of BYORP overestimates it. It is possible that the BYORP coefficient has instead an opposite sign than predicted; in that case, the system may be moving into an equilibrium between the BYORP and the tides.
In the case of (88710) 2001 SL9, the BYORP effect is the only known physical mechanism that can cause the inward drift of its mutual orbit.
Together with the binary (175706) 1996 FG3 which has a mean anomaly drift consistent with zero, implying a stable equilibrium between the BYORP effect and mutual body tides (Scheirich et al., 2015. Icarus 245, 56-63), we now have three distinct cases of well observed binary asteroid systems with their long-term dynamical models inferred. They indicate a presence of all the three states of the mutual orbit evolution – equilibrium, expanding and contracting – in the population of near-Earth binary asteroids.
•Orbital vectors of two binary near-Earth asteroid were determined.•An orbit of the satellite of (66391) Moshup is expanding in time.•An orbit of the satellite of (88710) 2001 SL9 is shrinking in time.•The evolution of the orbits due to emission of thermal radiation (BYORP effect).
Abstract
We performed photometric observations of the binary near-Earth asteroid (65803) Didymos in support of the Double Asteroid Redirection Test (DART) mission that will test the Kinetic Impactor ...technology for diverting dangerous asteroids. It will hit the Didymos secondary, called Dimorphos, on 2022 September 26. We observed Didymos with 11 telescopes with diameters from 3.5 to 10.4 m during four apparitions in 2015–2021, obtaining data with rms residuals from 0.006 to 0.030 mag. We analyzed the light-curve data and decomposed them into the primary rotational and secondary orbital light curves. We detected 37 mutual eclipse/occultation events between the binary system components. The data presented here, in combination with 18 mutual events detected in 2003, provide the basis for modeling the Dimorphos orbit around the Didymos primary. The orbit modeling is discussed in detail by Scheirich & Pravec and Naidu et al. The primary light curves were complex, showing multiple extrema on some epochs. They suggest a presence of complex topography on the primary’s surface that is apparent in specific viewing/illumination geometries; the primary shape model by Naidu et al. (Icarus 348, 113777, 2020) needs to be refined. The secondary rotational light-curve data were limited and did not provide a clear solution for the rotation period and equatorial elongation of Dimorphos. We define the requirements for observations of the secondary light curve to provide the needed information on Dimorphos’s rotation and elongation when Didymos is bright in 2022 July–September before the DART impact.
► Limits on the physical properties of Asteroid 2008
TC
3 are presented based on its shape model and Almahata Sitta meteorite measurements. ► Results indicate the mass of Asteroid 2008 TC
3 to be ...probably below 20
×
10
3
kg. ► This estimate is significantly lower compared to previous studies.
The density measurements of Almahata Sitta ureilites reveal a bulk density of ∼3.1
g/cm
3. This value, together with the 2008 TC
3 asteroid shape model and albedo, was used to estimate the asteroid’s mass. Based on the study of recovered meteorites and atmospheric entry observations Asteroid 2008 TC
3 is compositionally heterogeneous and of low mechanical strength. Thus we consider the presence of significant macroporosity likely, lowering asteroid’s bulk density compared to that of the Almahata Sitta ureilites. Most realistic albedos lie in a range of 0.09-0.2 and the presence of significant macroporosity leads to mass estimates below 20
×
10
3
kg, which is lower than previously estimated. The presence of a non-ureilitic fraction and space weathering may affect the albedo and also influence the mass estimates. However, from current data it is not possible to quantify this effect.
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