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Asteroid 1862 Apollo
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Title: A thermophysical analysis of the (1862) Apollo Yarkovsky and YORP effects
Authors: Ben Rozitis, Sam R. Duddy, Simon F. Green, Stephen C. Lowry

Near-Earth asteroid (1862) Apollo has strong detections of both orbital semimajor axis drift and rotational acceleration. We produce a unified model that can accurately match both observed effects using a single set of thermophysical properties derived from ground-based observations, and we determine Apollo's long term evolution. We use light-curve shape inversion techniques and the ATPM on published light-curve, thermal-infrared, and radar observations to constrain Apollo's thermophysical properties. The derived properties are used to make detailed predictions of Apollo's Yarkovsky and YORP effects, which are then compared with published measurements of orbital drift and rotational acceleration. The ATPM explicitly incorporates 1D heat conduction, shadowing, multiple scattering of sunlight, global self-heating, and rough surface thermal-infrared beaming in the model predictions. We find that ATPM can accurately reproduce the light-curve, thermal-infrared, and radar observations of Apollo, and simultaneously match the observed orbital drift and rotational acceleration using: a shape model with axis ratios of 1.94:1.65:1.00, an effective diameter of 1.55 ± 0.07 km, a geometric albedo of 0.20 ± 0.02, a thermal inertia of 140 +140/-100 J m-2 K-1 s-½, a highly rough surface, and a bulk density of 2850 +480/-680 kg m-3. Using these properties we predict that Apollo's obliquity is increasing towards the 180 degree YORP asymptotic state at a rate of 1.5 +0.3/-0.5 degrees per 10^5 yr. The derived thermal inertia suggests that Apollo has loose regolith material resting on its surface, which is consistent with Apollo undergoing a recent resurfacing event based on its observed Q-type spectrum. The inferred bulk density is consistent with those determined for other S-type asteroids, and suggests that Apollo has a fractured interior.

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Yes, your visit will be quick as you whip by planet Earth today, and you aren't getting close enough for us to get a good look unless we have a pretty good telescope in our backyard. But every time an asteroid comes close, we start to wonder when we will get hit.
No one is worried about 1862 Apollo hitting Earth -- its approach puts it 6.84 million miles away as it passes by. So named because it was the 1,862nd asteroid to be discovered, 1862 Apollo travels with a tiny moon as well.

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1862 Apollo is a Q-type asteroid, discovered by Karl Reinmuth in 1932, but lost and not recovered until 1973. It is named after the Greek god Apollo.
It is the namesake of the Apollo asteroids, and the first one discovered, although because it was lost for a time its asteroid number (1862) is higher than that of some other Apollo asteroids such as 1566 Icarus. It is also a Venus- and Mars-crosser asteroid.
It was the first asteroid recognized to cross Earth's orbit (although the earlier-discovered 887 Alinda is now known to do so as well).

Source

Date__(UT)__HR:MN     R.A._(ICRF/J2000.0)_DEC  APmag  S-brt            delta      deldot    S-O-T /r    S-T-O

 2007-May-08 00:00     21 29 08.81 -40 07 00.9  13.55   4.99 .071759358703919  -1.8630735  95.0364 /L  80.9371
 2007-May-09 00:00     22 13 51.24 -38 12 47.8  13.82   5.14 .071527246356745   1.0632135  87.1557 /L  88.7819
 2007-May-10 00:00     22 55 02.97 -35 22 09.7  14.17   5.29 .072973244256696   3.9157014  79.4031 /L  96.4788
 2007-May-11 00:00     23 30 56.06 -31 56 57.9  14.58   5.45 .076002571606023   6.5226747  72.1407 /L 103.6658
 2007-May-12 00:00     00 01 06.34 -28 20 03.2  15.04   5.61 .080440016511120   8.7818283  65.6133 /L 110.0980
 2007-May-13 00:00     00 26 02.77 -24 48 34.0  15.50   5.78 .086073810032344  10.6667903  59.9279 /L 115.6685


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The constant bombardment of billions of tiny particles from the Sun is shaping the Solar System, studies have shown.
As the fine solar shower rains down on objects, such as asteroids, it can steadily alter their orbit and spin.
Although the mechanism that describes the effect has been known for many years, it has never been seen.
Now, separate studies published in the journals Nature and Science have observed and measured the tiny stellar shoves on two spinning asteroids.
They reveal that both are gradually starting to spin faster and faster, which could eventually create new Solar System landmarks.

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An international research team led by Academy Research Fellow Mikko Kaasalainen has found an asteroid whose rotation receives an extra kick from solar radiation. The asteroid 1862 Apollo's diameter is about 1.5 km, it has a small moonlet, S/2005 (1862) 1, and its orbit crosses that of the Earth. The team reconstructed Apollo's shape and determined its rotational state using brightness measurements from several years. They found that Apollo's rotation speed steadily increases, and showed that this is due to the re-radiation of solar energy from its surface. The study was published in Nature online on 7 March.

Apollo's rotation period is slightly over three hours, and it decreases only by four thousandths of a second per year, so the analysis required accurate mathematical methods. Because of the acceleration, Apollo is likely to break apart or radically change its figure in the future. It may already have done so earlier, and its present moonlet may be a remnant of such a breakup.
The study confirms that non-gravitational forces are important in the dynamical evolution of asteroids. Re-radiation of solar energy acts as a propulsion engine on the asteroid's surface. There are two coupled manifestations of this phenomenon: the one changing the orbit (the Yarkovsky effect), and the one changing the spin state (the Yarkovsky-Radzievskii-O'Keefe-Paddack or YORP effect). The study confirmed the latter, and the former was detected by radar in 2003. Non-gravitational orbital and spin changes can be significant or even critical over long time intervals. They affect the motion of asteroids that may collide with the Earth. The phenomenon can also be used to estimate the masses of asteroids. Apollo is now the first object larger than one kilometre across for which the propulsion effect has been detected.
Academy Research Fellow Mikko Kaasalainen works in the Centre of Excellence in Inverse Problem Research of the Academy of Finland at the Department of Mathematics and Statistics of the University of Helsinki. The CoE develops and applies mathematical methods in data analysis in various fields from biology to space research. Dr. Kaasalainen coordinates an international solar system research and observation network with researchers from Europe, America, Asia, and Australia. The study published in Nature was carried out by scientists from Finland, Czech Republic, the United States, and Ukraine.

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