**5. Implications of the results**

#### **Young planetesimals were wet**

Because many of these bodies began accreting well outside the snowline [Lunine, 2006; Ciesla and Charnley, 2006], they contain considerable quantities of water as ice grains, much like comets. However, can this water be retained to be incorporated into the growing Earth? In a dense nebula, the light of the protosun is unlikely to drive the loss of volatiles from a small planetesimal within the snowline as our sun does today. Instead, volatile loss was more likely controlled by the internal heat generated within the planetesimals via radioactive decay or by accretional impact. For small planetesimals that coalesce from even smaller bodies in similar orbits, impact energy is likely to be localized and relatively insignificant on the global scale. Such collisions resulted in planetesimals that we call comets when they now arrive from the outer solar system, and are unlikely to result in extensive volatile loss in the inner solar system.

The concentration of radioactive elements initially available for incorporation into the terrestrial planets depends to a large degree on how they were added to the system. If injection of such material initiated the collapse of the nebula [Wadhwa et al., 2006], or if they were simply present in the collapsing molecular cloud that formed our solar system [Chabot and Haack, 2006], then all planetesimals would accrete from roughly the same mix of material. If the short-lived radioactive elements were injected into the nebula at some time after nebular collapse [Wadhwa et al, 2006], then later formed planetesimals could contain higher fractions of these heat sources than planetesimals formed from the less radioactive solids in the molecular cloud core, and these younger bodies would therefore evolve faster than those formed earlier. However, even small (5 km) planetesimals that are enriched in 60Fe and 26Al require from a few hundred thousand to several tens of millions of years to reach their maximum internal temperatures [Das and Srinivasan, 2007] and become totally dehydrated. If such bodies contained substantial quantities of water as ice, then heating would be slower due to the reduced concentration of radioactive heat sources, and the slow loss of volatiles would allow these planetesimals to sweat, thus efficiently losing heat from their interiors.

While planetesimals heat slowly [LaTourette and Wasserburg, 1998; Huss et al., 2006], protoplanets form rapidly in a runaway growth process caused by the increased gravitational cross sections of larger planetesimals [Wetherill and Stewart, 1989; 1993]. It has been suggested that terrestrial planets may have formed within 10 million years of nebular collapse [Jacobsen, 2003; Jacobsen et al., 2009] and that core formation on the Earth occurred less than 20 million years later [Nichols, 2006]. In other words, the Earth may have formed from very young planetesimals that had not yet had a chance to lose any significant quantity of ice or water of hydration due to radioactive decay driven heating. In this scenario, a proto-Earth may have formed directly from the planetesimals whose ice content is listed in Table 1. This would result in a planet with much too much water, rather than too little. Given other considerations, the rocky fraction of the planet would most likely need to be comparable to the present mass of the Moon or Mars, and so the proto-Earth may have been nearly twice as massive if much of the incoming ice remained trapped within the growing protoplanet, on its very wet surface, or within its massive, water - rich atmosphere.

#### **Composition of planetesimals**

52 Space Science

there were no small bodies present at 1 A.U. that began accretion beyond the snowline to be incorporated into these planetesimals, then the proto-Earth would have accreted considerably more water than is assumed in models that begin with planetesimals of chondritic composition.

Because many of these bodies began accreting well outside the snowline [Lunine, 2006; Ciesla and Charnley, 2006], they contain considerable quantities of water as ice grains, much like comets. However, can this water be retained to be incorporated into the growing Earth? In a dense nebula, the light of the protosun is unlikely to drive the loss of volatiles from a small planetesimal within the snowline as our sun does today. Instead, volatile loss was more likely controlled by the internal heat generated within the planetesimals via radioactive decay or by accretional impact. For small planetesimals that coalesce from even smaller bodies in similar orbits, impact energy is likely to be localized and relatively insignificant on the global scale. Such collisions resulted in planetesimals that we call comets when they now arrive from the outer solar system, and are unlikely to result in extensive

The concentration of radioactive elements initially available for incorporation into the terrestrial planets depends to a large degree on how they were added to the system. If injection of such material initiated the collapse of the nebula [Wadhwa et al., 2006], or if they were simply present in the collapsing molecular cloud that formed our solar system [Chabot and Haack, 2006], then all planetesimals would accrete from roughly the same mix of material. If the short-lived radioactive elements were injected into the nebula at some time after nebular collapse [Wadhwa et al, 2006], then later formed planetesimals could contain higher fractions of these heat sources than planetesimals formed from the less radioactive solids in the molecular cloud core, and these younger bodies would therefore evolve faster than those formed earlier. However, even small (5 km) planetesimals that are enriched in 60Fe and 26Al require from a few hundred thousand to several tens of millions of years to reach their maximum internal temperatures [Das and Srinivasan, 2007] and become totally dehydrated. If such bodies contained substantial quantities of water as ice, then heating would be slower due to the reduced concentration of radioactive heat sources, and the slow loss of volatiles would

allow these planetesimals to sweat, thus efficiently losing heat from their interiors.

While planetesimals heat slowly [LaTourette and Wasserburg, 1998; Huss et al., 2006], protoplanets form rapidly in a runaway growth process caused by the increased gravitational cross sections of larger planetesimals [Wetherill and Stewart, 1989; 1993]. It has been suggested that terrestrial planets may have formed within 10 million years of nebular collapse [Jacobsen, 2003; Jacobsen et al., 2009] and that core formation on the Earth occurred less than 20 million years later [Nichols, 2006]. In other words, the Earth may have formed from very young planetesimals that had not yet had a chance to lose any significant quantity of ice or water of hydration due to radioactive decay driven heating. In this scenario, a proto-Earth may have formed directly from the planetesimals whose ice content is listed in Table 1. This would result in a planet with much too much water, rather than too little. Given other considerations, the rocky fraction of the planet would most likely need to be comparable to the present mass of the Moon or Mars, and so the proto-Earth may have been

**5. Implications of the results Young planetesimals were wet** 

volatile loss in the inner solar system.

The composition of the rocky component of the planetesimals formed via gas-drag mediated accretion should be roughly chondritic. There is no mechanism for metal-silicate fractionation to operate on the dust grains and boulders that would be accreted via this mechanism. Although the overall composition of the planetesimals formed in this scenario bear more resemblance to what we currently call comets than to modern asteroids, for reasonable values of the nebular mass, the majority of the water accreted into these planetesimals originates in the inner solar system within a few A.U. of the snowline. For this reason, the planetesimals would not be expected to have the high D/H ratios or any significant content of volatile organic materials found in modern Kuiper-belt or Oort cloud comets.

As both the refractory composition of the planetesimals and the isotopic composition of the water would be "normal" when compared to typical models for the formation of the Earth, the only real compositional difference between the scenario described above and current models for the origin of the terrestrial planets is in the total quantity of water that might have been accreted. While current models often require the delivery of water at the end of the accretion process, this scenario requires the loss of water from the proto-Earth to be compatible with the composition of the modern Earth.

We do not have samples of the population of planetesimals that accreted to form the terrestrial planets in our modern meteorite collection [Drake and Righter, 2002]. Such planetesimals would have lost their ice and much of their water of hydration several billion years ago [Nuth, 2008]. The residual dehydrated body that began with more ice than dust and thus contained a smaller radioactive heat source than that which produced the large scale melting and differentiation of Ceres, Vesta and other asteroids would also be much more fragile than such solid rocks, much like loosely compressed sandstone. Collisional processes over the lifetime of the solar system would have gradually reduced the surviving number of these fragile bodies to an insignificant fraction of the asteroid population. It is therefore very likely that we do not have any representative samples of the planetesimal population that contributed to the formation of the terrestrial planets in our modern meteorite collections.

#### **Loss of volatiles from the Earth**

We expect that large quantities of water may have been lost from the growing proto-Earth due to impact induced heating, especially considering the lower escape velocity of the less massive, but rapidly growing protoplanet. We must also assume that the rocky terrestrial interior would remain at least fully saturated by water dissolved within the rocks and magma. In fact, given the overburden and likely difficulty of escape to the surface, we would expect that water vapor would become supersaturated within the proto-planet, and that any terrestrial proto-planet in the late stages of accretion would have a thick, watervapor-laden atmosphere that should undergo some loss of water back to space. In the case of the Earth it is likely that this entire atmosphere, as well as any nascent hydrosphere, was lost during the impact of the Mars-scale body that formed the Moon [Cameron and Benz, 1991; Canup and Asphaug, 2001; Canup, 2004]. In addition, such a large impact would dehydrate a significant fraction of the terrestrial mantle as well as virtually all of the material from the impactor that might fall back onto the surface of the Earth. The Late Heavy Bombardment would have further dehydrated the terrestrial crust and uppermost

Why Isn't the Earth Completely Covered in Water? 55

exchange reactions mediated by a moderately long lived disk. Such a disk and its isotopic exchange efficiency would be greatly enhanced by increased water content in the proto-Earth. Depending on the accretional loss of water during the initial accretion of both bodies, there could be several tenths of an Earth mass of water vapor in this cloud. Although most of this water would be lost from the system due to the high temperature of the debris disk, this same high temperature would ensure the equilibration of the silicates in the cloud. If the oceans of the larger proto-Earth dominate the contribution to this disk, then the silicate particles and SiO vapor in the debris cloud will be equilibrated with the Earth before the

Some planetesimals in the early solar system accreted from wide feeding zones and took a long time to heat to sufficiently high temperatures to lose the volatiles and ices they initially contained. Models for the formation of the terrestrial planets suggest that planets grew by the aggregation and growth of proto-planets that themselves grew quickly via runaway accretion from planetesimals in narrow feeding zones. Because these planetesimals had not yet had time to warm to the stage where they would lose a large fraction of their water and volatiles prior to their aggregation into proto-planets, many of those bodies that accreted should have contained reasonably large quantities of ice. The initial composition of the Earth contained more than enough water to form the modern hydrosphere depending on the position of the snowline, the fraction of planetesimals formed via gravitational instabilities or turbulent aggregation and the overall mass of the solar nebula. Even with substantial accretional loses and atmospheric erosion during the series of giant collisions between proto-planets that formed the terrestrial planets, more than enough water should have been available on and within the Earth to account for several modern oceans, especially when one includes the contributions from the

We do not need to wonder where the Earth's water came from; it clearly arrived with the planetesimals accumulated by the proto-Earth during the accretion process. Instead we should be asking how all of the initial water was lost from the Earth and what the consequences of these possible loss mechanisms are. Modeling the formation of the Earth and the other terrestrial planets from modern (dry) meteoritic matter, even carbonaceous meteorites, is not appropriate, and is certainly inconsistent with results one gets by following a planetesimal from its origin beyond the snowline, into the accreting planet. What effect might such large quantities of water have had on the geochemical differentiation of proto-planets, or of the Earth prior to the Moon forming event? Could a more massive but water filled proto-Earth better account for the properties of the Earth-Moon system during and after the giant collision with a Mars sized body (e.g., Pahlevan and Stevenson, 2007)? No one has yet investigated such

Blum, J. 1990, in Formation of the Stars and Planets, and the Evolution of the Solar System,

cloud coalesces into the Moon.

decreasing, but continuous infall of comets to modern times.

possibilities: There is much more work still to do.

Canup, R.M. 2004 Icarus. 168. 433–456.

ed. B. Battrick, (Noordwijk: ESTEC), 87. Blum, J., and Wurm, G. 2000, Icarus, 143, 138 – 146. Cameron, A.G.W. and Benz, W. 1991 Icarus, 92, 204–216. Canup, R.M. and Asphaug, E. 2001 Nature. 412,708–712.

**7. References** 

**6. Conclusions** 

mantle due to impact induced heating. Meanwhile the planetesimals impacting the Earth at this late stage would have had 500 million years since their accretion to lose a large fraction of their own volatile content, probably resulting in a net loss of water from the Earth.

If runaway growth morphed into oligarchic growth and only formed proto-planets in size ranges somewhere between Ceres and the Moon, then it may be much easier to explain volatile loss during the growth of the Earth. Because the initial proto-planets would be small, they would more easily lose volatiles that reached the surface than would a 90% finished Earth. In addition, a large number of collisional aggregation events are required to increase the mass of the Earth to its present value (~6 x 1027g) starting from a population of much smaller objects (~1026g). Each of these events would strip away any nascent hydrosphere on the colliding proto-planets, and could partially dehydrate the interior of the resultant body. However, if the interior of the growing proto-Earth remained near saturation (~3% water by mass) throughout the accretion process, the resulting planet would have many times more water than is needed to form the current hydrosphere (<0.025% of the Earth by mass). In fact, the above statement would still be true even if 95% of the water contained in all of the saturated proto-planets that accreted to form the Earth were lost in the growth process.

#### **Water on the newborn Earth**

Comets still impact the Earth today (e.g., Tunguska) and the frequency of such impacts was higher in the first billion years of Solar System history [Chyba, et al., 1994]. Comets therefore must have contributed some fraction of the water currently on the modern Earth. But we submit that a significant hydrosphere and a very wet atmosphere already existed on the proto-Earth prior to the Moon forming event, due to the accretion of ice laden planetesimals. Since up to 3% water can be dissolved in the modern terrestrial mantle at equilibrium [Righter, 2007] and even more water could have been present "in transit" through the mantle and crust as water worked its way up to the surface from the deeper planetary interior, we see no possible alternative but to accept that the early Earth had quite a large complement of water and may have been a bit more massive than expected when the moon forming collision occurred.

One might ask what the modern Earth would have been like if the Moon-forming event and the Late Heavy Bombardment had never occurred. The Earth's surface is already 75% ocean even though water comprises much less than 0.1% of the Earth's total mass. Had the early Earth not lost its original wet atmosphere, hydrosphere and some very large fraction of the water dissolved in its upper mantle, the entire surface of the Earth might today be covered by water to depths of at least several hundred miles, assuming that natural atmospheric erosion would have eliminated a substantial fraction of the initially accreted ice. Even with the moon forming event and Late Heavy Bombardment, the interior of the planet should still be rich in dissolved water and hydrogen: dissolved hydrogen could certainly be plentiful in the Earth's core, and most mantle magmas should be fully saturated in water.

#### **Water equilibrates oxygen isotopes in the Earth-Moon system**

If the proto-Earth and the "Mars-size impactor" were both equilibrated bodies containing significant reservoirs of liquid water prior to impact, the energy of the collision would vaporize any oceans on the surfaces of the bodies while simultaneously equilibrating the isotopic composition of both bodies in the debris cloud surrounding the Earth. This is consistent with the models of Pahlevan and Stevenson (2007) who demonstrated a viable mechanism for equilibrating the oxygen isotopic compositions of the Earth-Moon system via exchange reactions mediated by a moderately long lived disk. Such a disk and its isotopic exchange efficiency would be greatly enhanced by increased water content in the proto-Earth. Depending on the accretional loss of water during the initial accretion of both bodies, there could be several tenths of an Earth mass of water vapor in this cloud. Although most of this water would be lost from the system due to the high temperature of the debris disk, this same high temperature would ensure the equilibration of the silicates in the cloud. If the oceans of the larger proto-Earth dominate the contribution to this disk, then the silicate particles and SiO vapor in the debris cloud will be equilibrated with the Earth before the cloud coalesces into the Moon.

## **6. Conclusions**

54 Space Science

mantle due to impact induced heating. Meanwhile the planetesimals impacting the Earth at this late stage would have had 500 million years since their accretion to lose a large fraction

If runaway growth morphed into oligarchic growth and only formed proto-planets in size ranges somewhere between Ceres and the Moon, then it may be much easier to explain volatile loss during the growth of the Earth. Because the initial proto-planets would be small, they would more easily lose volatiles that reached the surface than would a 90% finished Earth. In addition, a large number of collisional aggregation events are required to increase the mass of the Earth to its present value (~6 x 1027g) starting from a population of much smaller objects (~1026g). Each of these events would strip away any nascent hydrosphere on the colliding proto-planets, and could partially dehydrate the interior of the resultant body. However, if the interior of the growing proto-Earth remained near saturation (~3% water by mass) throughout the accretion process, the resulting planet would have many times more water than is needed to form the current hydrosphere (<0.025% of the Earth by mass). In fact, the above statement would still be true even if 95% of the water contained in all of the

of their own volatile content, probably resulting in a net loss of water from the Earth.

saturated proto-planets that accreted to form the Earth were lost in the growth process.

Comets still impact the Earth today (e.g., Tunguska) and the frequency of such impacts was higher in the first billion years of Solar System history [Chyba, et al., 1994]. Comets therefore must have contributed some fraction of the water currently on the modern Earth. But we submit that a significant hydrosphere and a very wet atmosphere already existed on the proto-Earth prior to the Moon forming event, due to the accretion of ice laden planetesimals. Since up to 3% water can be dissolved in the modern terrestrial mantle at equilibrium [Righter, 2007] and even more water could have been present "in transit" through the mantle and crust as water worked its way up to the surface from the deeper planetary interior, we see no possible alternative but to accept that the early Earth had quite a large complement of water and may have been a bit more massive than expected when the moon forming collision occurred.

One might ask what the modern Earth would have been like if the Moon-forming event and the Late Heavy Bombardment had never occurred. The Earth's surface is already 75% ocean even though water comprises much less than 0.1% of the Earth's total mass. Had the early Earth not lost its original wet atmosphere, hydrosphere and some very large fraction of the water dissolved in its upper mantle, the entire surface of the Earth might today be covered by water to depths of at least several hundred miles, assuming that natural atmospheric erosion would have eliminated a substantial fraction of the initially accreted ice. Even with the moon forming event and Late Heavy Bombardment, the interior of the planet should still be rich in dissolved water and hydrogen: dissolved hydrogen could certainly be plentiful in the Earth's core, and most mantle magmas should be fully saturated in water.

If the proto-Earth and the "Mars-size impactor" were both equilibrated bodies containing significant reservoirs of liquid water prior to impact, the energy of the collision would vaporize any oceans on the surfaces of the bodies while simultaneously equilibrating the isotopic composition of both bodies in the debris cloud surrounding the Earth. This is consistent with the models of Pahlevan and Stevenson (2007) who demonstrated a viable mechanism for equilibrating the oxygen isotopic compositions of the Earth-Moon system via

**Water equilibrates oxygen isotopes in the Earth-Moon system** 

**Water on the newborn Earth** 

Some planetesimals in the early solar system accreted from wide feeding zones and took a long time to heat to sufficiently high temperatures to lose the volatiles and ices they initially contained. Models for the formation of the terrestrial planets suggest that planets grew by the aggregation and growth of proto-planets that themselves grew quickly via runaway accretion from planetesimals in narrow feeding zones. Because these planetesimals had not yet had time to warm to the stage where they would lose a large fraction of their water and volatiles prior to their aggregation into proto-planets, many of those bodies that accreted should have contained reasonably large quantities of ice. The initial composition of the Earth contained more than enough water to form the modern hydrosphere depending on the position of the snowline, the fraction of planetesimals formed via gravitational instabilities or turbulent aggregation and the overall mass of the solar nebula. Even with substantial accretional loses and atmospheric erosion during the series of giant collisions between proto-planets that formed the terrestrial planets, more than enough water should have been available on and within the Earth to account for several modern oceans, especially when one includes the contributions from the decreasing, but continuous infall of comets to modern times.

We do not need to wonder where the Earth's water came from; it clearly arrived with the planetesimals accumulated by the proto-Earth during the accretion process. Instead we should be asking how all of the initial water was lost from the Earth and what the consequences of these possible loss mechanisms are. Modeling the formation of the Earth and the other terrestrial planets from modern (dry) meteoritic matter, even carbonaceous meteorites, is not appropriate, and is certainly inconsistent with results one gets by following a planetesimal from its origin beyond the snowline, into the accreting planet. What effect might such large quantities of water have had on the geochemical differentiation of proto-planets, or of the Earth prior to the Moon forming event? Could a more massive but water filled proto-Earth better account for the properties of the Earth-Moon system during and after the giant collision with a Mars sized body (e.g., Pahlevan and Stevenson, 2007)? No one has yet investigated such possibilities: There is much more work still to do.

#### **7. References**

Blum, J. 1990, in Formation of the Stars and Planets, and the Evolution of the Solar System, ed. B. Battrick, (Noordwijk: ESTEC), 87.

Blum, J., and Wurm, G. 2000, Icarus, 143, 138 – 146. Cameron, A.G.W. and Benz, W. 1991 Icarus, 92, 204–216. Canup, R.M. and Asphaug, E. 2001 Nature. 412,708–712. Canup, R.M. 2004 Icarus. 168. 433–456.

**Part 3** 

**Planetary Science** 


**Part 3** 

**Planetary Science** 

56 Space Science

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Chyba, C.F., Owen, T.C., Ip, W.-H. 1994 In Hazards due to comets and asteroids (T. Gehrels,

Ciesla, F.J. and Charnley, S.B. 2006 In Meteorites and the Early Solar System II (D.S. Lauretta

Cuzzi J. N. and Weidenschilling S. J., 2006, in Lauretta D. S., McSween H. Y. Jr, eds, Meteorites and the Early Solar System II. Univ. Arizona press, Tucson, p. 353

Hayashi, C., Nakazawa, K., & Nakagawa, Y. 1985, Protostars and Planets II, 1100-1153;

Jacobsen S. B. \* Remo J. L. Petaev M. I. Sasselov D. D., 2009, LPSC 40 Abstr. # 2054.

Huss, G. R., Rubin, A. E., Grossman, J.N. 2006 In Meteorites and the Early Solar System II (D.S. Lauretta & H.Y. McSween, eds.), Univ. Arizona Press, Tucson 567-586.

Johansen, A., Oishi, J.S., Mac Low, M.M., Klahr, H., Henning, T. and Youdin, A., 2007

Lunine, J. 2006 In Meteorites and the Early Solar System II (D.S. Lauretta & H.Y. McSween,

Nichols, R. 2006 In Meteorites and the Early Solar System II (D.S. Lauretta & H.Y. McSween,

Righter,K., Drake, M.J., Scott, E.,. 2006 In Meteorites and the Early Solar System II (D.S. Lauretta & H.Y. McSween, eds.), Univ. Arizona Press, Tucson 803-828.

Wadhwa, M., Amelin, Y., Davis, A.M., Lugmair, G.W., Meyer, B., Gounelle, M., Desch, S.J.

2006 In Protostars and Planets V (B. Reipurth, D. Jewett and K. Keil, eds.), 835-848,

Ormel, C.W., Cuzzi, J.N. and Tielens, A.G.G.M., 2008, Astrophys. J. 679, 1588 – 1610.

Ringwood, A.E., 1979, Origin of the Earth and Moon. Springer-Verlag, New York.

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Das, A. & Srinivasan, G. 2007 LPSC 38 Abstr. #2370.

Drake, M. 2005 Meteoritics & Planet. Sci., 40, 519-527. Drake, M.J. and Righter, K. 2002 Nature, 416, 39-44.

Hayashi, C 1981 Prog. Theoret. Phys. Suppl. 70, 35-53.

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Wänke, H., 1981, *Phil. Trans. Roy. Soc. Lond.* A 303, 287–302. Wetherill, G. and Stewart, G. 1989 Icarus, 77, 330-357. Wetherill, G. and Stewart, G. 1993 Icarus, 106, 190 - 209.

Wurm, G., Blum, J., and Colwell, J. E. 2001, Icarus, 151, 318 – 321. Youdin, A. N. & Shu, F. H., 2002, Astrophys. J. 580, 494–505. Youdin, A. N. & Goodman, J., 2005, Astrophys. J. 620, 459–469.

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**4** 

*Poland* 

Wlodarczyk Ireneusz

**OrbFit Impact Solutions for Asteroids (99942)** 

The best systems with the exact impact solutions for dangerous asteroids are presented by the JPL Sentry System: http://neo.jpl.nasa.gov/risk/ and by the NEODyS CLOMON2:

From many years on the top of these lists were two asteroids: (99942) Apophis (is still up now, October 2011) and (144898) 2004 VD17 – now is removed from the list of the dangerous asteroids. Thanks to the courtesy of those who made free available OrbFit software and its

It is now possible to compute individually dates of possible impacts of selected dangerous asteroids or the energy of impact and others impact factors. In this respect we investigated the motion of these recently discovered minor planets: (99942) Apophis and (144898) 2004 VD17 - the most dangerous for the Earth, according to the Impact Risk Page of NASA:

To compute exact impact solutions of asteroids it is necessary to include some additional small effects on the asteroid's motion. The inluence of relativistic effects, the perturbing massive asteroids, the Yarkovsky/YORP effects, solar radiation pressure, different ephemeris of the Solar System were investigated. To compute gravitational forces perturbing the motion of (99942) Apophis and (144898) 2004 VD17 from different massive

SOLEX computes positions of the Solar System bodies by a method which is entirely based on the numerical integration of the Newton equation of motions (Vitagliano, A. 1997). With the use of Solex it was possible to compute all close approaches between (99942) Apophis and (144898) 2004 VD17 with all nearly 140000 numbering asteroids. Similar work with (15)

Selected orbit solutions for (99942) Apophis and (144898) 2004 VD17 were presented during Meeting on Asteroids and Comets in Europe - May 12-14, 2006 in Vienna, Austria. At that time the new version of OrbFit (3.3.2) was released and gave better results of computations of impact probability mainly with the use of non linear monitoring and multi ple solutions method (Milani et al., 2002, Milani et al., 2005a and Milani et al.,

http://newton.dm.unipi.it/neodys/index.php?pc=4.1

source code at: http://adams.dm.unipi.it/~orbmaint/orbfit/

asteroids, the free software Solex from A. Vitagliano was used:

Eunomia using Solex was done by Vitagliano and Stoss (2006).

http://chemistry.unina.it/~alvitagl/solex/.

**1. Introduction**

http://neo.jpl.nasa.gov/risk/.

*Chorzow Astronomical Observatory; Rozdrazew Astronomical Observatory* 

**Apophis and (144898) 2004 VD17** 
