4. Discussion4.1. Stable and unstab

4. Discussion
4.1. Stable and unstable orbits
Our results confirm that most NEAs move in stable orbits over the time scale of of our computations, as shown by the parameters fq=0 and/or fa=0. On the other hand, most JFCs are found to move in unstable orbits, thus showing values for fq,fa>0. We remind the reader that our conclusions were derived for bodies with , namely that are potentially able to experience close encounters with Jupiter. Yet, a few NEAs are found to move in cometary orbits. These NEAs in Cometary Orbits (NEACOs) have unstable orbits with fq≳0.25 and fa≳0.06 and mix quite well with the JFCs in the plots of Fig. 5,Fig. 6 and Fig. 8. Table II brings the list of the most prominent NEACOs found in our sample. The times,tNEA, elapsed since these bodies were transferred from a large-q orbit (>2.5 au) to its current NEA orbit are relatively short, of the order of several hundreds to a couple thousands yr (i.e. ∼80–400 revolutions). These short residence times in an orbit with are consistent with the physical lifetimes estimated for typical kilometer-size JFCs ( Fernández et al., 2002 and Di Sisto et al., 2009). We also have some other potential NEACOs (though of lower probability than the ones of Table II): 2010 LR68 and 2012 MA7, for which we find fq∼0.2.
Table II.
NEACOs.
Object H Ra (km)
fq fa tNEA (yr)
1997 SE5 14.8 3.25–2.30 0.628 0.285 365
2000 DN1 19.8 0.33–0.23 0.307 0.172 850–1450
2001 XQ 19.2 0.43–0.30 0.306 0.061 1400
2002 GJ8 19.4 0.39–0.28 0.292 0.077 670
2002 RN38 16.9 1.24–0.87 0.470 0.209 485
2003 CC11 19.1 0.45–0.32 0.775 0.324 400
2003 WY25b
20.9 0.20–0.14 0.480 0.164 600–2800
2009 CR2 16.7 1.36–0.96 0.333 0.154 610
2011 OL51 19.8 0.33–0.23 0.601 0.227 870–1640
a
The radii were computed by assuming geometric albedos pV=0.05–0.1.
b
It was identified with Comet D/1819 W1 (Blanpain) and given the permanent name 289P/Blanpain.
Table options
Tancredi (2014) has recently proposed a list of Asteroids in Cometary Orbits (ACOs), considering as a key criterion to define the cometary nature the Minimum Orbital Intersection Distance (MOID) parameter, namely the minimum distance between the orbits of the object and that of Jupiter. Tancredi identifies 331 ACOs from which 32 are in our NEA sample, including our 8 NEACOs of Table II. On the other hand, Tancredi includes 24 additional objects for which we find asteroid-like orbits, in most cases with fq=0 or close to zero. Object 2003 EH1 ( Fig. 12, right) is a good example of an object included in Tancredi’s ACO list that suffers many close encounters with Jupiter, though they are not able to break the regular pattern of its motion through most of the integration time. Therefore, even though the MOID parameter may be useful for a quick diagnostic, it is still necessary to perform direct integrations to know more accurately the degree of stability (or instability) of the orbit.
4.2. The physical nature of objects approaching the Earth
Since stable orbits mean that objects may remain bound to small-q orbits over time scales , such objects will have to be able to withstand the intense solar radiation without significant erosion. On the other hand, active comets lose copious amounts of material in every passage, so they cannot survive for too long in the Earth’s neighborhood as active bodies. Their orbits must accordingly be unstable, with short residence times in the near-Earth region before their discovery.
Among the NEOs, we can find different compositions that might be grouped into:
1)
Rocky bodies: They are devoid of volatile material and their internal strength is rather high. They can withstand close passages by the Sun without losing measurable amounts of material, as is observed in many asteroids passing at heliocentric distances

Results (English) 1:

4. Discussion4.11. Stable and unstable orbitsOur results confirm that most NEAs move in stable orbits over the time scale of of our computations, as shown by the parameters fq = 0 and/or fa = 0. On the other hand, most JFCs are found to move in unstable orbits, thus showing values for fq, fa> 0. We remind the reader that our conclusions were derived for bodies with, namely that are potentially able to experience close encounters with Jupiter. Yet, a few NEAs are found to move in cometary orbits. These NEAs in Cometary Orbits (NEACOs) have unstable orbits with fq≳0.25 and fa≳0.06 and mix quite well with the JFCs in the plots of Fig. 5, Fig. 6 and Fig. 8. Table II brings the list of the most prominent NEACOs found in our sample. The times, tNEA, elapsed since these bodies were transferred from a large-q orbit (> 2.5 au) to its current NEA orbit are relatively short, of the order of several hundreds to a couple thousands year (i.e. ∼80–400 revolutions). These short residence times in an orbit with are consistent with the physical lifetimes estimated for typical kilometer-size JFCs (Fernández et al., 2002 and Di Sisto et al., 2009). We also have some other potential NEACOs (though of lower probability than the ones of Table II): 2010 LR68 and 2012 Mail, for which we find fq∼0.2.Table II.Neacos.Object H Ra (km)fq fa tNEA (year)1997 SE5 14.8 3.25–2.30 0.628 0.285 365to DN1 19.8 0.33–0.23 0.307 0.172 850–14502001 XQ 19.2 0.43–0.30 0.306 0.061 14002002 GJ8 19.4 0.39–0.28 0.292 0.077 6702002 RN38 16.9 1.24–0.87 0.470 0.209 4852003 C1 19.1 0.45–0.32 0.775 0.324 4002003 WY25b20.9 0.20–0.14 0.480 0.164 600–28002009 CR2 16.7 1.36–0.96 0.333 0.154 6102011 OL51 19.8 0.33–0.23 0.601 0.227 870–1640aThe radii were computed by assuming geometric albedos pV = 0.05–0.1.bIt was identified with Comet D/1819 W1 (Blanpain) and given the permanent name 289P/Blanpain.Table optionsTancredi (2014) has recently proposed a list of Asteroids in Cometary Orbits (ACOs), considering as a key criterion to define the cometary nature the Minimum Orbital Intersection Distance (MOID) parameter, namely the minimum distance between the orbits of the object and that of Jupiter. Tancredi identifies 331 ACOs from which 32 are in our NEA sample, including our 8 NEACOs of Table II. On the other hand, Tancredi includes 24 additional objects for which we find asteroid-like orbits, in most cases with fq = 0 or close to zero. Object 2003 EH1 (Fig. 12, right) is a good example of an object included in Tancredi's ACO list that suffers many close encounters with Jupiter, though they are not able to break the regular pattern of its motion through most of the integration time. Therefore, even though the MOID parameter may be useful for a quick diagnostic, it is still necessary to perform direct integrations to know more accurately the degree of stability (or instability) of the orbit.4. to. The physical nature of objects approaching the EarthSince stable orbits mean that objects may remain bound to small-q orbits over time scales, such objects will have to be able to withstand the intense solar radiation without significant erosion. On the other hand, active comets lose copious amounts of material in every passage, so they cannot survive for too long in the Earths neighborhood as active bodies. Their orbits must accordingly be unstable, with short residence times in the near-Earth region before their discovery.Among the NEOs, we can find different compositions that might be grouped into:1)Rocky bodies: They are devoid of volatile material and their internal strength is rather high. They can withstand close passages by the Sun without losing measurable amounts of material, as is observed in many asteroids passing at heliocentric distances < 0.25 au (Jewitt, 2013). Collisions with meteoroids may be the only agent capable of triggering (dusty) activity.2)Rocky-acqueous bodies: The matrix is built with the mineral (refractory) component, but they are carbon-rich and contain some water, either under the form of hydrated silicates, or even as water ice buried in their interiors. The mineral matrix provides a moderate internal strength, capable of withstanding close passages by the Sun with losses of material limited to the outer layers. These bodies may display some activity under the intense Sounds radiation, so they may become "rock comets" (Jewitt and Li, 2010). Their progenitors may be the so called main-belt comets (MBCs) and their source region may be the outer asteroid belt where water molecules in the protoplanetary disk could have condensed into ice or bound to silicates. Spectroscopic observations of the MBCs, 133P/Elst–Pizarro and 176P/LINEAR, did not show the CN emission band at ∼3800 Å, setting an upper limit to the gas production rate ∼3 orders of magnitude below that found for typical JFCs observed at similar heliocentric distances (Licandro et al., 2011). These observations show that whereas free sublimation of water ice is the main activation mechanism of comets near the Sun, other mechanisms that require little (or no) water sublimation are responsible for activating MBCs.3)Icy bodies: The matrix is built as a very loose aggregate of ice and dust particles, so their bulk density and internal strength is very low (Don, 1990 and Sosa and Fernández, 2009). They lose appreciable amounts of material in each passage by the Suns vicinity by sublimation and frequent outbursts and breakups, so a typical one-km size nucleus has a very short physical lifetime. There has been a long discussion on whether they transit through stages of dormancy before final disintegration into meteoritic dust, or if they pass straight from an active nucleus to dust (cf. Fig. 1). These objects formed in the trans-Jovian region where they have remained until present in cold reservoirs: the trans-Neptunian region and the Oort cloud.4. e. Thermally-induced activity of rocky-acqueous bodies close to the SunThe proximity to the Sun (distances) can raise the surface temperature of a NEA with a visual geometric albedo pv∼0.05 to about 1000 K. Expected physical effects are thermal fracture induced by strong temperature gradients, and dehydration in case the object contains minerals with bound water molecules, or OH radicals (Jewitt and Li, 2010). These effects may be the source of some activity as, for instance, the release of water molecules as the chemically-bound water in hydrated silicates is progressively lost, starting at temperatures of about 600 K, and the ejection of dust particles that get enough energy to escape from the asteroid upon thermal fracture. Jewitt (2013) searched for activity in 2002 PD43 with negative results, which might suggest that it is thermally resistant, perhaps with a composition rich in silicates like pyroxenes, olivines and metal, similar to the asteroids predominant in the inner belt of taxonomic classes S or M.On the other hand, if 2003 EH1 has released material to give raise to the Quadrantid meteor stream, its composition might be different, perhaps rocky-acqueous as discussed before. Their different semimajor axes: 2.5 au for 2002 PD43, and 3.2–3.4 au for 2003 EH1, may give support to the idea of a different geochemical composition. If we assume that most of the mass of the Quadrantid was released when 2003 EH1 had a very small q (≲0.15 au), then the age of this meteor shower would be around 1500 year. This is about three times the age estimated by Jenniskens (2005) based on the dispersion of the orbital parameters of the Quadrantid shower with respect to those of 2003 EH1. This discrepancy might be explained either because of an underestimation of the age of the breakup that generated the Quadrantid, or because the breakup took place after the object reached the minimum q. For instance 500 year ago 2003 EH1 had. As regards the physical nature of 2003 EH1, it has a typical asteroid orbit (cf. Fig. 12, right), so it may come from the outer asteroid belt and has a rocky-aqueous composition. Its activity might arise from thermal fracture and dehydration when the body's surface attains temperatures close to 1000 K (Jewitt and Li, 2010).(3200) Phaethon may be another example of an active asteroid whose activity is triggered by thermal fracture and/or decomposition of hydrated silicates upon close approach to the Sun (). Li and Jewitt (2013) reported about one magnitude brightening at its 2009 and 2012 perihelion passages, and attributed it to the ejection of dust by the mechanism mentioned before. The visible and near infrared reflectance spectrum of this object is found to be similar to that produced by aqueously altered samples of CI/CM carbonaceous chondrites and hydrated silicates (Licandro et al., 2007). de León et al. (2010) found that (2) Pallas is the most likely parent body of Phaeton based on spectroscopic similarities between the latter and several members of the Pallas family, as well as numerical simulations that show a dynamical pathway through which fragments of Pallas can reach Phaethon-like orbits.4.4. Taxonomic types and albedos among NEAs of our sampleUnfortunately, the information available on these two physical parameters for our NEA sample is very scant. Only a small fraction of them have known albedos and/or taxonomic types. The collected data are shown in Table I, which have been drawn from DeMeo and Binzel (2008). Even though this is a small fraction of our total sample, it gives us a hint of the taxonomic types prevailing among NEAs with. These are types C, D, and P, that correspond to low-albedo (around 0.05–0.06) primitive material, and linear spectra over visible wavelengths with neutral to red slopes. These taxonomic types prevail among the asteroids in the outer belt and the Jupiter's Trojans (DeMeo and Binzel, 2008 and DeMeo and Carry, 2013). These photometric features have also been associated with dead comets (Jewitt, 2002 and Licandro et al., 200 8)

Results (English) 3:

4。
4.1讨论。稳定的和不稳定的轨道，我们的结果证实，大多数近地小行星
移动稳定轨道的时间尺度上的计算，如图所示的参数FQ = 0和/或FA = 0。另一方面，大多数的JFCs被发现在不稳定的轨道移动，从而显示值的FQ，发> 0。我们提醒读者，我们的结论是衍生体，即可能与木星亲密接触的经验。然而，一些近地小行星被发现在彗星的轨道移动。这些近地小行星彗星的轨道（neacos）与FQ≳0.25和FA≳0.06加在图5的图的JFCs很不稳定轨道，图6和图8。表二neacos带来最突出的列表在我们的样本中发现的。时代，tnea，因为这些尸体被从轨道转移到大Q（> 2.5金）到目前的NEA轨道相对较短，数百的顺序的一对千年（即80的400次革命∼–）。这些短的停留时间在一个轨道与与物理中典型的公里大小一致的估计（蕨类áJFCs费尔南德斯等人。，2002、二Sisto等人。，2009）。我们也有其他一些潜在neacos（虽然低的概率比表II中的2010和2012）：lr68 ma7，其中我们发现FQ∼
0.2。表二。
neacos
对象。H RA（公里）
FQ足总TNEA（年）1997 14.8 3.25 SE5
–2.30 0.628 0.285 365 2000 19.8 0.33–
DN1 0.23 0.307 0.172 850 1450 2001 19.2 0.43 XQ–
–0.30 0.306 0.061 1400 2002 19.4 0.39
gj8–0.28 0.292 0.077 670 2002 16.9 1.24 0.87
rn38–0。470 0.209 485 2003 19.1 0.45 CC11
–0.32 0.775 0.324 400 2003 20.9 0.20
wy25b
–0.14 0.480 0.164 600 2800 2009 16.7 1.36 CR2–
–0.96 0.333 0.154 610 2011 19.8 0.33
ol51–0.23 0.601 0.227 870 1640

–一半径的假设的几何反照率PV = 0.05–0.1计算
。B
经鉴定与彗星的D / 1819 W1（白朗盼）和给定的永久名字289p /白朗盼

表选项。迪（2014）最近提出了一个清单，在彗星轨道的小行星（ACOS），考虑定义彗星性质的最小轨道相交的距离的一个关键指标（MOID）参数，即最小距离物体的轨道与木星。该标识331 ACOS，32在我们的样本包括8 neacos NEA，表II。另一方面，和包括24个额外的对象，我们发现小行星的轨道，在大多数情况下，FQ = 0或接近零。对象2003 EH1（图12，右）是对象的一个很好的例子包括Tancredi的ACO列表遭受许多木星的近距离接触，虽然他们不能够打破规律的运动通过大部分的积分时间。因此，即使模具参数可以用于快速诊断是有用的，仍然有必要进行直接集成到知道更准确的稳定度（或不稳定）的轨道
4.2。对象的自稳定的轨道意味着对象可能还是会在小Q的轨道时间尺度接近地球
物理性质，这样的对象必须能够承受强烈的太阳辐射没有明显的侵蚀。另一方面，活跃的彗星失去大量的材料在每一个通道，所以他们无法生存在地球附近太长为主动体。他们的轨道必须是不稳定的，短的停留时间在地球附近的地区在他们发现
。在近地天体，我们可以发现不同的组合物，可以分为：1）

岩石体：它们缺乏挥发性物质，其内部的强度是相当高的。他们能承受关闭通道的太阳没有一定数量的材料，如在许多小行星在以太阳为中心的距离小于0.25金通过观察（Jewitt，2013）。碰撞陨石可能是唯一能够引发剂（灰尘）活动。2）岩石含水

身体：矩阵与矿建（耐火材料）组成，但他们是富碳，含有水，水合硅酸盐的形式下，甚至冰埋在其内部。矿物基质提供适度的内部力量，能承受关闭通道的太阳与损失的材料限于外层。这些机构可以强烈阳光的照射下显示的一些活动，所以他们可能会成为“摇滚彗星”（2010 Jewitt和李，）。他们的祖先可能是所谓的主带彗星（MBCS），其源区可能是外小行星带，在原行星盘的水分子会凝结成冰或绑定到硅酸盐。的多光谱观测，133p /埃尔斯特–皮萨罗176p /线性，没有3800显示CN∼Å发射带，设置对产气速率∼3个数量级低于典型的JFCs类似日心距离（利坎德罗观察上限等人。，2011）。这些观察表明，而水无冰的升华是接近太阳的彗星主要激活机制，其他的需要很少的机制（或没有）水升华是负责激活MBCS。3）

冰冷的尸体：矩阵是一个非常松散的聚集体的冰和尘埃粒子，所以他们的密度和内部强度很低（Donn，1990和索萨和蕨类áá，2009）。他们失去了大量的材料在各个通道通过升华和频繁爆发和分手太阳的附近，所以一个典型的一公里大小的核有一个很短的物理寿命。有一个是否运输阶段的休眠前的最后解体成陨石尘长时间的讨论，或如果他们从一个活动的核直尘（参见图1）。这些对象在跨区域形成木星在他们直到冷藏：海王星区
4.3奥尔特云。热引起的岩石水体接近太阳
接近太阳活动（距离）可以提高NEA与视觉的几何反照率PV∼0.05表面温度约1000 K。预期的物理效应是由强烈的温度梯度引起的热断裂，脱水的情况下，该对象包含与结合水分子的矿物质，或OH自由基（2010 Jewitt和李，）。这些影响可能是一些活动，源，释放的水分子的化学结合水的水合硅酸盐逐渐失去，开始在大约600 K的温度下，和尘埃颗粒，得到足够的能量来躲避小行星在热断裂弹射。Jewitt（2013）寻找2002 pd43阴性结果的活动，这可能表明，它是耐热的，也许一个富含硅酸盐橄榄石和辉石等组成，金属，类似于小行星的分类类S或M
另一方面内带为主，如果2003 EH1发布材料引起的象限仪座流星雨，其成分可能不同，也许石水讨论之前。不同的半长轴：2.5金2002 pd43，和3.2 3.4金2003 EH1–，可以支持一个不同的地球化学组成的想法。如果我们假设大多数的象限仪座大众发布2003 EH1的时候有一个非常小的Q（≲0.15金），然后该流星雨的年龄将在1500岁。这是约三倍的年龄估计杰尼斯肯斯（2005）基于与2003 EH1的象限仪座流星群的轨道参数的分散性阵雨。这种差异可能被解释是因为与产生一破碎的年龄低估，或者因为分手后发生对象达到最小Q。比如500年前，2003 EH1有。至于2003 EH1的物理性质，它具有典型的小行星轨道（参见图12，右），所以它可能来自外小行星带，岩石的含水组合物。它的活动可能会产生热断裂和脱水时身体的表面达到的温度接近1000 K（2010 Jewitt和李，）。（3200）法厄同的可能是另一个活跃的小行星的活动所引发的热断裂和/或水合硅酸盐分解在接近太阳（）。李Jewitt（2013）报道了约一个数量级在2009和2012点光亮的通道，并归因于它的尘埃弹射之前提及的机制。可见光和近红外反射光谱的对象是相似的，产生的水可改变样品的CI /厘米（利坎德罗碳质球粒陨石和水合硅酸盐等人。，2007）。德乐óN等人。（2010）发现（2）雅典娜是基于对帕拉斯家族后者和几个成员之间的光谱相似的辉腾最可能的母体，以及数值模拟，显示一个动态的途径通过片段蝮蛇可达法厄同就像
4.4轨道。在我们的样本
不幸的近地小行星分类类型和反照率，提供的信息对这些物理参数的个数，我们都非常缺乏样本。只有一小部分人知道反照率和/或分类类型。收集到的数据在表III所示，已从DeMeo和宾佐绘制（2008）。虽然这是我们的总样本的一小部分，它给了我们一个暗示的分类类型盛行的近地小行星带。这些都是类型C，D，P，对应的反射率较低（约0.05–0.06）原始的材料，在可见光波长的线性光谱与中性红的斜坡。这些分类类型为外部带小行星和木星的木马在（DeMeo和宾佐，2008套带，2013）。这些光度特征也被与死亡相关（Jewitt彗星，利坎德罗等人，2002和2008）。