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Themis family
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Title: Secular light curves of 25 members of the Themis family of asteroids, suspected of exhibiting low level cometary activity
Author: I. Ferrin, M. Perez, J. Rendon

From 1996 to 2015 sixteen main belt asteroids were discovered exhibiting cometary activity (less than one per year), all of them during searches at the telescope. In this work we will explore another way to discover them. We reduced 192016 magnitude observations of 165 asteroids of the Themis family, using data from the astrometric-photometric database of the Minor Planet Center, MPCOBS, and measuring the absolute magnitudes from the phase plots. 25 objects of 165 (15.2%), exhibited bumps or enhancements in brightness that might indicate low level cometary activity. Since activity repeats at the same place in different orbits and in many occasions is centered at perihelion, activity might be due to water ice sublimation. As of September 2016, there are 717768 asteroids listed in the MPC files. If we assume that we do not have any false positives and the above percentage can be extrapolated to the whole Main Belt, the number of potentially active asteroid gets to the very large number of ~111.000. This number is much larger than the ones predicted in previous surveys and indicates one of three scenarios: A) there are many false positives in our detections and the real number of active asteroid is much smaller than we found, implying that the MPC astrometric-photometric database is only astrometric and not photometric. B) The location of active asteroids is restricted to the Themis family and an extrapolation to the whole belt is not possible. Or C) there are few false positives in our candidates and the main belt actually contains many low level active asteroids undetected by current surveys. Case C) would imply that the main belt is not a field of bare rocks but a graveyard of extinct comets, changing our current paradigm of the main belt. So it is of the outmost importance to verify observationally our candidates, and determine which of these scenarios is valid.

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RE: Asteroid 24 Themis
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Asteroid (24) Themis is at Opposition in the constellation Ophiuchus on the 11th June 2015.

Magnitude: 11.5 
Distance to Earth: 2.262 AU 
Distance to Sun: 3.278 AU 

Themis110615 


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Ephemeris

Date       UT      R.A. (J2000) Decl.    Delta     r     El.    Ph.   Mag
2015 06 01 000000 17 27 06.7 -23 57 38   2.269   3.266  167.5   3.9  11.8
2015 06 02 000000 17 26 17.7 -23 57 11   2.267   3.267  168.6   3.5  11.7
2015 06 03 000000 17 25 28.2 -23 56 42   2.265   3.268  169.8   3.2  11.7
2015 06 04 000000 17 24 38.3 -23 56 11   2.264   3.269  170.9   2.8  11.7
2015 06 05 000000 17 23 48.1 -23 55 39   2.262   3.270  172.1   2.5  11.7
2015 06 06 000000 17 22 57.6 -23 55 05   2.262   3.271  173.2   2.1  11.7
2015 06 07 000000 17 22 06.9 -23 54 29   2.261   3.272  174.4   1.7  11.6
2015 06 08 000000 17 21 15.9 -23 53 52   2.261   3.273  175.5   1.4  11.6
2015 06 09 000000 17 20 24.8 -23 53 14   2.261   3.275  176.6   1.0  11.6
2015 06 10 000000 17 19 33.6 -23 52 33   2.261   3.276  177.7   0.7  11.5
2015 06 11 000000 17 18 42.4 -23 51 52   2.262   3.277  178.7   0.4  11.5
2015 06 12 000000 17 17 51.1 -23 51 09   2.263   3.278  179.2   0.2  11.5
2015 06 13 000000 17 16 59.9 -23 50 25   2.264   3.279  178.5   0.5  11.5
2015 06 14 000000 17 16 08.8 -23 49 39   2.265   3.280  177.4   0.8  11.6
2015 06 15 000000 17 15 17.9 -23 48 52   2.267   3.281  176.3   1.1  11.6
2015 06 16 000000 17 14 27.2 -23 48 04   2.269   3.282  175.2   1.5  11.6
2015 06 17 000000 17 13 36.7 -23 47 15   2.271   3.283  174.0   1.8  11.7
2015 06 18 000000 17 12 46.6 -23 46 25   2.274   3.284  172.9   2.2  11.7
2015 06 19 000000 17 11 56.8 -23 45 33   2.277   3.285  171.8   2.5  11.7
2015 06 20 000000 17 11 07.4 -23 44 41   2.280   3.287  170.6   2.9  11.7
2015 06 21 000000 17 10 18.5 -23 43 48   2.283   3.288  169.5   3.2  11.8
2015 06 22 000000 17 09 30.1 -23 42 55   2.287   3.289  168.3   3.6  11.8
2015 06 23 000000 17 08 42.2 -23 42 00   2.291   3.290  167.2   3.9  11.8
2015 06 24 000000 17 07 54.9 -23 41 05   2.295   3.291  166.1   4.3  11.8
2015 06 25 000000 17 07 08.3 -23 40 10   2.300   3.292  164.9   4.6  11.9
2015 06 26 000000 17 06 22.4 -23 39 14   2.305   3.293  163.8   4.9  11.9
2015 06 27 000000 17 05 37.1 -23 38 18   2.310   3.294  162.7   5.3  11.9
2015 06 28 000000 17 04 52.7 -23 37 22   2.315   3.295  161.6   5.6  11.9
2015 06 29 000000 17 04 09.0 -23 36 26   2.321   3.296  160.5   5.9  11.9
2015 06 30 000000 17 03 26.1 -23 35 30   2.326   3.297  159.3   6.2  12.0


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Asteroid (24) Themis
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Asteroid (24) Themis is at Opposition in the constellation Leo on the 13th March 2014

Magnitude: 10.6
Distance to Earth: 1.789 AU
Distance to Sun: 2.783 AU



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RE: Asteroid 24 Themis
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Asteroid (24) Themis is at Opposition on the 1st December, 2012.

Magnitude: 11.0
Distance to Earth: 1.988 au 
Distance to Sun: 2.974 au

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Ephemeris

Date       UT      R.A. (J2000) Decl.    Delta     r     El.    Ph.  Mag
2012 11 30 000000 04 34 53.6 +22 37 44   1.991   2.977  177.5   0.8  11.1
2012 12 01 000000 04 33 59.7 +22 36 07   1.990   2.975  178.6   0.4  11.0
2012 12 02 000000 04 33 05.7 +22 34 29   1.988   2.974  179.4   0.2  11.0
2012 12 03 000000 04 32 11.7 +22 32 48   1.988   2.973  178.6   0.5  11.0
2012 12 04 000000 04 31 17.6 +22 31 07   1.987   2.972  177.5   0.8  11.1
2012 12 05 000000 04 30 23.7 +22 29 23   1.987   2.971  176.3   1.2  11.1
2012 12 06 000000 04 29 29.9 +22 27 39   1.987   2.970  175.0   1.6  11.1
2012 12 07 000000 04 28 36.4 +22 25 54   1.987   2.968  173.8   2.0  11.2
2012 12 08 000000 04 27 43.1 +22 24 08   1.988   2.967  172.6   2.4  11.2
2012 12 09 000000 04 26 50.1 +22 22 21   1.989   2.966  171.4   2.8  11.2
2012 12 10 000000 04 25 57.5 +22 20 33   1.990   2.965  170.2   3.2  11.2
2012 12 11 000000 04 25 05.4 +22 18 45   1.991   2.964  169.0   3.6  11.3
2012 12 12 000000 04 24 13.8 +22 16 57   1.993   2.962  167.7   4.0  11.3
2012 12 13 000000 04 23 22.8 +22 15 09   1.995   2.961  166.5   4.4  11.3
2012 12 14 000000 04 22 32.5 +22 13 21   1.997   2.960  165.3   4.8  11.3
2012 12 15 000000 04 21 42.8 +22 11 33   2.000   2.959  164.1   5.2  11.4
2012 12 16 000000 04 20 53.9 +22 09 45   2.003   2.958  162.9   5.6  11.4
2012 12 17 000000 04 20 05.8 +22 07 58   2.006   2.957  161.7   6.0  11.4
2012 12 18 000000 04 19 18.6 +22 06 12   2.010   2.955  160.5   6.4  11.4
2012 12 19 000000 04 18 32.3 +22 04 27   2.013   2.954  159.3   6.8  11.4
2012 12 20 000000 04 17 46.9 +22 02 42   2.017   2.953  158.1   7.1  11.5
2012 12 21 000000 04 17 02.6 +22 01 00   2.022   2.952  156.9   7.5  11.5
2012 12 22 000000 04 16 19.4 +21 59 18   2.026   2.951  155.7   7.9  11.5
2012 12 23 000000 04 15 37.2 +21 57 38   2.031   2.950  154.5   8.2  11.5
2012 12 24 000000 04 14 56.2 +21 56 00   2.036   2.948  153.4   8.6  11.5
2012 12 25 000000 04 14 16.3 +21 54 24   2.042   2.947  152.2   9.0  11.6
2012 12 26 000000 04 13 37.7 +21 52 50   2.047   2.946  151.0   9.3  11.6
2012 12 27 000000 04 13 00.3 +21 51 18   2.053   2.945  149.8   9.7  11.6
2012 12 28 000000 04 12 24.2 +21 49 49   2.059   2.944  148.7  10.0  11.6
2012 12 29 000000 04 11 49.4 +21 48 22   2.065   2.943  147.5  10.3  11.6
2012 12 30 000000 04 11 15.9 +21 46 58   2.072   2.942  146.4  10.7  11.6
2012 12 31 000000 04 10 43.8 +21 45 36   2.079   2.940  145.2  11.0  11.7


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Asteroid (24) Themis
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Title: Limits to Ice on Asteroids (24) Themis and (65) Cybele
Authors: D. Jewitt, A. Guilbert-Lepoutre

We present optical spectra of (24) Themis and (65) Cybele, two large main-belt asteroids on which exposed water ice has recently been reported. No emission lines, expected from resonance fluorescence in gas sublimated from the ice, were detected. Derived limits to the production rates of water are < 400 kg/s (5{\sigma}), for each object, assuming a cometary H2O/CN ratio. We rule out models in which a large fraction of the surface is occupied by high albedo ("fresh") water ice because the measured albedos of Themis and Cybele are low (0.05 - 0.07). We also rule out models in which a large fraction of the surface is occupied by low albedo ("dirty") water ice because dirty ice would be warm, and would sublimate strongly enough for gaseous products to have been detected. If ice exists on these bodies it must be relatively clean (albedo >0.3) and confined to a fraction of the Earth-facing surface <10%. By analogy with impacted asteroid (596) Scheila, we propose an impact excavation scenario, in which 10 m scale projectiles have exposed buried ice. If the ice is even more reflective (albedo >0.6) then the timescale for sublimation of an optically thick layer can rival the 10^3 yr interval between impacts with bodies this size. In this sense, exposure by impact may be a quasi steady-state feature of ice-containing asteroids at 3 AU.

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RE: Asteroid 24 Themis
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Asteroid Themis has 'frosted surface'

Scientists have detected water-ice on the surface of an asteroid.
The first-time observation was made on 24 Themis, a huge rock that orbits almost 480 million km out from the Sun.
The researchers say that ice is not stable in such circumstances and has to be being replenished by some means - perhaps from inside the object.

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Asteroid ice hints at rocky start to life on Earth

A slushy cócktail of water-ice and organic materials has been directly detected on the surface of an asteroid for the first time. The finding strengthens the theory that asteroids delivered the ingredients for Earth's oceans and life, and could make astronomers rethink conventional models for how the Solar System evolved.
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Asteroid isn't just a dry heap of rubble

Two independent teams have found what may be the first direct evidence of water ice on the surface of an asteroid. The discovery lends support to the idea that asteroids could have helped deliver water to the early Earth.
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