# Are there any known asteroids with average density similar to that of Earth's?

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In this answer I mention that for very low orbits around spherical bodies, the period tends to scale only as the inverse square root of the density, and not the diameter.

For a low orbits where the semi-major axis is close to the radius of the central body, the period is related to the average density of the body and unrelated to it's size.

So a low orbit around a spherical asteroid (which there usually aren't) made of a mixture of rock and iron (which there usually aren't) will be roughly 90 minutes just like LEO (low Earth orbit), even if it's only 1 km in diameter.

I'd like to use an example, but I don't know where to find an asteroid density table, nor even a histogram, that includes something close to 5 g/cm^3.

Question: Are there any known asteroids with average density similar to that of Earth's?

Some example mean densities from Wikipedia (some rocky planets included for good measure):

body mean density (g/cm^3) Earth 5.5 Mercury 5.4 Venus 5.2 Mars 3.9 Io 3.5 Vesta 3.4 Moon 3.3 Hygiea 2.8 Pallas 2.7 Ceres 2.1

Apparently 21 Lutetia (~80x120 km) is one of the most dense asteroids known at about 3.4 g/cm$$^3$$. Unfortunately that is barely over 60% of Earth's density. 21 Lutetia is unique in that scientists believe that it has a primordial core, that is, it has a core from the formation of the solar system (and it hasn't been destroyed after all these years). We believe it is hidden under the surface. It would be rich in metals, explaining the density, and because it's hidden under the surface, 21 Lutetia would look no different from any other asteroid, which is the case. This asteroid is a prime target for mining because of its metal core. I suppose the answer to your question would be no, unless 3.4 is sufficient for your definition of "close".

## Chapter 9

-Most are unchanged since their formation in the solar nebula.

-They have similar densities.

-They have a similar range of orbital inclinations.

-Asteroids and comets formed at different times.

-Asteroids are much larger than comets.

-Comets are much larger than asteroids.

-Asteroids formed inside the frost line, while comets formed outside.

-Asteroids cluster together due to their mutual gravity and this creates gaps in their distribution.

-There is a large population of asteroids too faint to see called the "gap" asteroids.

-Jupiter's gravity causes orbital resonances that nudge asteroids out of these areas.

-There are either pure metal or pure rock asteroids, but no mixtures.

-Thus there is a "gap" in the composition of asteroids.

-along Jupiter's orbit, 60° ahead of and behind Jupiter

-in orbits that cross Earth's orbit

-the density of asteroids is high enough for a large collision to pulverize a number of asteroids.

-the period of an orbiting asteroid would be a simple fraction (like 1/3 or 1/4) of Jupiter's orbital period.

-the period of an orbiting asteroid would be the same as Jupiter's orbital period.

-the period of an orbiting asteroid would be the same as Mars's orbital period.

Large asteroids became spherical because many small collisions chipped off pieces until only a sphere was left this did not occur with small asteroids.

Large asteroids were once molten and therefore became spherical, but small asteroids were never molten.

Small asteroids have odd shapes because they were all chipped off larger objects.

-a streak of light caused by a star moving across the sky

-a comet that burns up in Earth's atmosphere

-a small moon that orbits one of the giant planets

-a fragment of material from the solar system that has fallen to Earth's surface

the crusts and mantles of asteroids.
Mars.

-about half the diameter of Earth's Moon.

-about the size of Earth's Moon.

-with enough mass to gather hydrogen and helium and become a gas giant.

-It is made of planetesimals that formed beyond Neptune's orbit and never accreted to form a planet.

-It is made of planetesimals between the orbits of Mars and Jupiter that never formed into a planet.

-It is made of planetesimals formed in the outer solar system that were flung into distant orbits by encounters with the jovian planets.

-It consists of objects that fragmented from the protosun during a catastrophic collision early in the formation of the solar system.

## Major milestones in asteroid research

The first asteroid was discovered on January 1, 1801, by the astronomer Giuseppe Piazzi at Palermo, Italy. At first Piazzi thought he had discovered a comet however, after the orbital elements of the object had been computed, it became clear that the object moved in a planetlike orbit between the orbits of Mars and Jupiter. Because of illness, Piazzi was able to observe the object only until February 11. Although the discovery was reported in the press, Piazzi only shared details of his observations with a few astronomers and did not publish a complete set of his observations until months later. With the mathematics then available, the short arc of observations did not allow computation of an orbit of sufficient accuracy to predict where the object would reappear when it moved back into the night sky, so some astronomers did not believe in the discovery at all.

There matters might have stood had it not been for the fact that that object was located at the heliocentric distance predicted by Bode’s law of planetary distances, proposed in 1766 by the German astronomer Johann D. Titius and popularized by his compatriot Johann E. Bode, who used the scheme to advance the notion of a “missing” planet between Mars and Jupiter. The discovery of the planet Uranus in 1781 by the British astronomer William Herschel at a distance that closely fit the distance predicted by Bode’s law was taken as strong evidence of its correctness. Some astronomers were so convinced that they agreed during an astronomical conference in 1800 to undertake a systematic search. Ironically, Piazzi was not a party to that attempt to locate the missing planet. Nonetheless, Bode and others, on the basis of the preliminary orbit, believed that Piazzi had found and then lost it. That led German mathematician Carl Friedrich Gauss to develop in 1801 a method for computing the orbit of minor planets from only a few observations, a technique that has not been significantly improved since. The orbital elements computed by Gauss showed that, indeed, the object moved in a planetlike orbit between the orbits of Mars and Jupiter. Using Gauss’s predictions, German Hungarian astronomer Franz von Zach (ironically, the one who had proposed making a systematic search for the “missing” planet) rediscovered Piazzi’s object on December 7, 1801. (It was also rediscovered independently by German astronomer Wilhelm Olbers on January 2, 1802.) Piazzi named that object Ceres after the ancient Roman grain goddess and patron goddess of Sicily, thereby initiating a tradition that continues to the present day: asteroids are named by their discoverers (in contrast to comets, which are named for their discoverers).

The discovery of three more faint objects in similar orbits over the next six years— Pallas, Juno, and Vesta—complicated that elegant solution to the missing-planet problem and gave rise to the surprisingly long-lived though no longer accepted idea that the asteroids were remnants of a planet that had exploded.

Following that flurry of activity, the search for the planet appears to have been abandoned until 1830, when Karl L. Hencke renewed it. In 1845 he discovered a fifth asteroid, which he named Astraea.

The name asteroid (Greek for “starlike”) had been suggested to Herschel by classicist Charles Burney, Jr., via his father, music historian Charles Burney, Sr., who was a close friend of Herschel’s. Herschel proposed the term in 1802 at a meeting of the Royal Society. However, it was not accepted until the mid-19th century, when it became clear that Ceres and the other asteroids were not planets.

There were 88 known asteroids by 1866, when the next major discovery was made: Daniel Kirkwood, an American astronomer, noted that there were gaps (now known as Kirkwood gaps) in the distribution of asteroid distances from the Sun (see below Distribution and Kirkwood gaps). The introduction of photography to the search for new asteroids in 1891, by which time 322 asteroids had been identified, accelerated the discovery rate. The asteroid designated (323) Brucia, detected in 1891, was the first to be discovered by means of photography. By the end of the 19th century, 464 had been found, and that number grew to 108,066 by the end of the 20th century and was almost 1,000,000 in the third decade of the 21st century. The explosive growth was a spin-off of a survey designed to find 90 percent of asteroids with diameters greater than one kilometre that can cross Earth’s orbit and thus have the potential to collide with the planet (see below Near-Earth asteroids).

Asteroids are small, rocky objects that orbit the Sun. Although asteroids orbit the Sun like planets, they are much smaller than planets.

There are lots of asteroids in our solar system. Most of them live in the main asteroid belt &mdasha region between the orbits of Mars and Jupiter.

Some asteroids go in front of and behind Jupiter. They are called Trojans. Asteroids that come close to Earth are called Near Earth Objects, NEOs for short. NASA keeps close watch on these asteroids.

## Contents

In 1772, Italian-born mathematician Joseph-Louis Lagrange, in studying the restricted three-body problem, predicted that a small body sharing an orbit with a planet but lying 60° ahead or behind it will be trapped near these points. [2] The trapped body will librate slowly around the point of equilibrium in a tadpole or horseshoe orbit. [9] These leading and trailing points are called the L4 and L5 Lagrange points. [10] [Note 1] The first asteroids trapped in Lagrange points were observed more than a century after Lagrange's hypothesis. Those associated with Jupiter were the first to be discovered. [2]

E. E. Barnard made the first recorded observation of a trojan, (12126) 1999 RM 11 (identified as A904 RD at the time), in 1904, but neither he nor others appreciated its significance at the time. [11] Barnard believed he had seen the recently discovered Saturnian satellite Phoebe, which was only two arc-minutes away in the sky at the time, or possibly an asteroid. The object's identity was not understood until its orbit was calculated in 1999. [11]

The first accepted discovery of a trojan occurred in February 1906, when astronomer Max Wolf of Heidelberg-Königstuhl State Observatory discovered an asteroid at the L4 Lagrangian point of the Sun–Jupiter system, later named 588 Achilles. [2] In 1906–1907 two more Jupiter trojans were found by fellow German astronomer August Kopff (624 Hektor and 617 Patroclus). [2] Hektor, like Achilles, belonged to the L4 swarm ("ahead" of the planet in its orbit), whereas Patroclus was the first asteroid known to reside at the L5 Lagrangian point ("behind" the planet). [12] By 1938, 11 Jupiter trojans had been detected. [13] This number increased to 14 only in 1961. [2] As instruments improved, the rate of discovery grew rapidly: by January 2000, a total of 257 had been discovered [10] by May 2003, the number had grown to 1,600. [14] As of October 2018 [update] there are 4,601 known Jupiter trojans at L4 and 2,439 at L5. [15]

The custom of naming all asteroids in Jupiter's L4 and L5 points after famous heroes of the Trojan War was suggested by Johann Palisa of Vienna, who was the first to accurately calculate their orbits. [2]

Asteroids in the leading (L4) orbit are named after Greek heroes (the "Greek node or camp" or "Achilles group"), and those at the trailing (L5) orbit are named after the heroes of Troy (the "Trojan node or camp"). [2] The asteroids 617 Patroclus and 624 Hektor were named before the Greece/Troy rule was devised, resulting in a Greek spy in the Trojan node and a Trojan spy in the Greek node. [13] [16]

Estimates of the total number of Jupiter trojans are based on deep surveys of limited areas of the sky. [1] The L4 swarm is believed to hold between 160,000–240,000 asteroids with diameters larger than 2 km and about 600,000 with diameters larger than 1 km. [1] [10] If the L5 swarm contains a comparable number of objects, there are more than 1 million Jupiter trojans 1 km in size or larger. For the objects brighter than absolute magnitude 9.0 the population is probably complete. [14] These numbers are similar to that of comparable asteroids in the asteroid belt. [1] The total mass of the Jupiter trojans is estimated at 0.0001 of the mass of Earth or one-fifth of the mass of the asteroid belt. [10]

Two more recent studies indicate that the above numbers may overestimate the number of Jupiter trojans by several-fold. This overestimate is caused by (1) the assumption that all Jupiter trojans have a low albedo of about 0.04, whereas small bodies may have an average albedo as high as 0.12 [17] (2) an incorrect assumption about the distribution of Jupiter trojans in the sky. [18] According to the new estimates, the total number of Jupiter trojans with a diameter larger than 2 km is 6,300 ± 1,000 and 3,400 ± 500 in the L4 and L5 swarms, respectively. [18] These numbers would be reduced by a factor of 2 if small Jupiter trojans are more reflective than large ones. [17]

The number of Jupiter trojans observed in the L4 swarm is slightly larger than that observed in L5. Because the brightest Jupiter trojans show little variation in numbers between the two populations, this disparity is probably due to observational bias. [4] Some models indicate that the L4 swarm may be slightly more stable than the L5 swarm. [9]

The largest Jupiter trojan is 624 Hektor, which has a mean diameter of 203 ± 3.6 km. [14] There are few large Jupiter trojans in comparison to the overall population. With decreasing size, the number of Jupiter trojans grows very quickly down to 84 km, much more so than in the asteroid belt. A diameter of 84 km corresponds to an absolute magnitude of 9.5, assuming an albedo of 0.04. Within the 4.4–40 km range the Jupiter trojans' size distribution resembles that of the main-belt asteroids. Nothing is known about the masses of the smaller Jupiter trojans. [9] The size distribution suggests that the smaller Trojans may be the products of collisions by larger Jupiter trojans. [4]

The largest Jupiter trojans
Trojan Diameter (km)
624 Hektor 225
617 Patroclus 140
911 Agamemnon 131
588 Achilles 130
3451 Mentor 126
3317 Paris 119
1867 Deiphobus 118
1172 Äneas 118
1437 Diomedes 118
1143 Odysseus 115
Source: JPL Small-Body Database, NEOWISE data

Jupiter trojans have orbits with radii between 5.05 and 5.35 AU (the mean semi-major axis is 5.2 ± 0.15 AU), and are distributed throughout elongated, curved regions around the two Lagrangian points [1] each swarm stretches for about 26° along the orbit of Jupiter, amounting to a total distance of about 2.5 AU. [10] The width of the swarms approximately equals two Hill's radii, which in the case of Jupiter amounts to about 0.6 AU. [9] Many of Jupiter trojans have large orbital inclinations relative to Jupiter's orbital plane—up to 40°. [10]

Jupiter trojans do not maintain a fixed separation from Jupiter. They slowly librate around their respective equilibrium points, periodically moving closer to Jupiter or farther from it. [9] Jupiter trojans generally follow paths called tadpole orbits around the Lagrangian points the average period of their libration is about 150 years. [10] The amplitude of the libration (along the Jovian orbit) varies from 0.6° to 88°, with the average being about 33°. [9] Simulations show that Jupiter trojans can follow even more complicated trajectories when moving from one Lagrangian point to another—these are called horseshoe orbits (currently no Jupiter Trojan with such an orbit is known). [9]

### Dynamical families and binaries Edit

Discerning dynamical families within the Jupiter trojan population is more difficult than it is in the asteroid belt, because the Jupiter trojans are locked within a far narrower range of possible positions. This means that clusters tend to overlap and merge with the overall swarm. By 2003 roughly a dozen dynamical families were identified. Jupiter-trojan families are much smaller in size than families in the asteroid belt the largest identified family, the Menelaus group, consists of only eight members. [4]

In 2001, 617 Patroclus was the first Jupiter trojan to be identified as a binary asteroid. [19] The binary's orbit is extremely close, at 650 km, compared to 35,000 km for the primary's Hill sphere. [20] The largest Jupiter trojan—624 Hektor— is probably a contact binary with a moonlet. [4] [21] [22]

Jupiter trojans are dark bodies of irregular shape. Their geometric albedos generally vary between 3 and 10%. [14] The average value is 0.056 ± 0.003 for the objects larger than 57 km, [4] and 0.121 ± 0.003 (R-band) for those smaller than 25 km. [17] The asteroid 4709 Ennomos has the highest albedo (0.18) of all known Jupiter trojans. [14] Little is known about the masses, chemical composition, rotation or other physical properties of the Jupiter trojans. [4]

### Rotation Edit

The rotational properties of Jupiter trojans are not well known. Analysis of the rotational light curves of 72 Jupiter trojans gave an average rotational period of about 11.2 hours, whereas the average period of the control sample of asteroids in the asteroid belt was 10.6 hours. [23] The distribution of the rotational periods of Jupiter trojans appeared to be well approximated by a Maxwellian function, [Note 2] whereas the distribution for main-belt asteroids was found to be non-Maxwellian, with a deficit of periods in the range 8–10 hours. [23] The Maxwellian distribution of the rotational periods of Jupiter trojans may indicate that they have undergone a stronger collisional evolution compared to the asteroid belt. [23]

In 2008 a team from Calvin College examined the light curves of a debiased sample of ten Jupiter trojans, and found a median spin period of 18.9 hours. This value was significantly higher than that for main-belt asteroids of similar size (11.5 hours). The difference could mean that the Jupiter trojans possess a lower average density, which may imply that they formed in the Kuiper belt (see below). [24]

### Composition Edit

Spectroscopically, the Jupiter trojans mostly are D-type asteroids, which predominate in the outer regions of the asteroid belt. [4] A small number are classified as P or C-type asteroids. [23] Their spectra are red (meaning that they reflect more light at longer wavelengths) or neutral and featureless. [14] No firm evidence of water, organics or other chemical compounds has been obtained as of 2007 [update] . 4709 Ennomos has an albedo slightly higher than the Jupiter-trojan average, which may indicate the presence of water ice. Some other Jupiter Trojans, such as 911 Agamemnon and 617 Patroclus, have shown very weak absorptions at 1.7 and 2.3 μm, which might indicate the presence of organics. [25] The Jupiter trojans' spectra are similar to those of the irregular moons of Jupiter and, to a certain extent, comet nuclei, though Jupiter trojans are spectrally very different from the redder Kuiper belt objects. [1] [4] A Jupiter trojan's spectrum can be matched to a mixture of water ice, a large amount of carbon-rich material (charcoal), [4] and possibly magnesium-rich silicates. [23] The composition of the Jupiter trojan population appears to be markedly uniform, with little or no differentiation between the two swarms. [26]

A team from the Keck Observatory in Hawaii announced in 2006 that it had measured the density of the binary Jupiter trojan 617 Patroclus as being less than that of water ice (0.8 g/cm 3 ), suggesting that the pair, and possibly many other Trojan objects, more closely resemble comets or Kuiper belt objects in composition—water ice with a layer of dust—than they do the main-belt asteroids. [20] Countering this argument, the density of Hektor as determined from its rotational lightcurve (2.480 g/cm 3 ) is significantly higher than that of 617 Patroclus. [22] Such a difference in densities suggests that density may not be a good indicator of asteroid origin. [22]

Two main theories have emerged to explain the formation and evolution of the Jupiter trojans. The first suggests that the Jupiter trojans formed in the same part of the Solar System as Jupiter and entered their orbits while it was forming. [9] The last stage of Jupiter's formation involved runaway growth of its mass through the accretion of large amounts of hydrogen and helium from the protoplanetary disk during this growth, which lasted for only about 10,000 years, the mass of Jupiter increased by a factor of ten. The planetesimals that had approximately the same orbits as Jupiter were caught by the increased gravity of the planet. [9] The capture mechanism was very efficient—about 50% of all remaining planetesimals were trapped. This hypothesis has two major problems: the number of trapped bodies exceeds the observed population of Jupiter trojans by four orders of magnitude, and the present Jupiter trojan asteroids have larger orbital inclinations than are predicted by the capture model. [9] Simulations of this scenario show that such a mode of formation also would inhibit the creation of similar trojans for Saturn, and this has been borne out by observation: to date no trojans have been found near Saturn. [27] In a variation of this theory Jupiter captures trojans during its initial growth then migrates as it continues to grow. During Jupiter's migration the orbits of objects in horseshoe orbits are distorted causing the L4 side of these orbits to be over occupied. As a result, an excess of trojans is trapped on the L4 side when the horseshoe orbits shift to tadpole orbits as Jupiter grows. This model also leaves the Jupiter trojan population 3–4 orders of magnitude too large. [28]

The second theory proposes that the Jupiter trojans were captured during the migration of the giant planets described in the Nice model. In the Nice model the orbits of the giant planets became unstable 500–600 million years after the Solar System's formation when Jupiter and Saturn crossed their 1:2 mean-motion resonance. Encounters between planets resulted in Uranus and Neptune being scattered outward into the primordial Kuiper belt, disrupting it and throwing millions of objects inward. [29] When Jupiter and Saturn were near their 1:2 resonance the orbits of pre-existing Jupiter trojans became unstable during a secondary resonance with Jupiter and Saturn. This occurred when the period of the trojans' libration about their Lagrangian point had a 3:1 ratio to the period at which the position where Jupiter passes Saturn circulated relative to its perihelion. This process was also reversible allowing a fraction of the numerous objects scattered inward by Uranus and Neptune to enter this region and be captured as Jupiter's and Saturn's orbits separated. These new trojans had a wide range of inclinations, the result of multiple encounters with the giant planets before being captured. [30] This process can also occur later when Jupiter and Saturn cross weaker resonances. [31]

In a revised version of the Nice model Jupiter trojans are captured when Jupiter encounters an ice giant during the instability. In this version of the Nice model one of the ice giants (Uranus, Neptune, or a lost fifth planet) is scattered inward onto a Jupiter-crossing orbit and is scattered outward by Jupiter causing the orbits of Jupiter and Saturn to quickly separate. When Jupiter's semi-major axis jumps during these encounters existing Jupiter trojans can escape and new objects with semi-major axes similar to Jupiter's new semi-major axis are captured. Following its last encounter the ice giant can pass through one of the libration points and perturb their orbits leaving this libration point depleted relative to the other. After the encounters end some of these Jupiter trojans are lost and others captured when Jupiter and Saturn are near weak mean motion resonances such as the 3:7 resonance via the mechanism of the original Nice model. [31]

The long-term future of the Jupiter trojans is open to question, because multiple weak resonances with Jupiter and Saturn cause them to behave chaotically over time. [32] Collisional shattering slowly depletes the Jupiter trojan population as fragments are ejected. Ejected Jupiter trojans could become temporary satellites of Jupiter or Jupiter-family comets. [4] Simulations show that the orbits of up to 17% of Jupiter trojans are unstable over the age of the Solar System. [33] Levison et al. believe that roughly 200 ejected Jupiter trojans greater than 1 km in diameter might be travelling the Solar System, with a few possibly on Earth-crossing orbits. [34] Some of the escaped Jupiter trojans may become Jupiter-family comets as they approach the Sun and their surface ice begins evaporating. [34]

On 4 January 2017 NASA announced that Lucy was selected as one of their next two Discovery Program missions. [35] Lucy is set to explore six Jupiter trojans. It is scheduled for launch in 2021 and will arrive at the L4 Trojan cloud in 2027 after a fly-by of a main-belt asteroid. It will then return to the vicinity of Earth for a gravity assist to take it to Jupiter's L5 Trojan cloud where it will visit 617 Patroclus. [36]

The Japanese space agency has proposed the OKEANOS solar sail for the late 2020s to either analyse a Trojan asteroid in situ or to perform a sample-return mission.

## Largest Asteroid Might Contain More Fresh Water than Earth

The largest known asteroid could contain more fresh water thanEarth and looks like our planet in other ways, according to a new study thatfurther blurs the line between planets and large space rocks.

Astronomers took 267 imagesof asteroid Ceres using the Hubble Space Telescope. From these images andsubsequent computer simulations, they suggest Ceres may have a rocky inner coreand a thin, dusty outer crust.

A team led by Peter Thomas of Cornell University said todaythat Ceres is nearly spherical, which suggests that gravity controls its shape.Also, the asteroid's non-uniform shape indicates that material is not evenlydistributed throughout the inside.

These and other new clues, including Ceres' low density, pointto an interior loaded with frozen water, the astronomers said.

The results are detailed in the Sept. 8 issue of the journal Nature.

Ceres has long been considered one of the tens of thousands ofasteroids that make up the asteroid belt between Mars and Jupiter. At 580 miles(930 km) in diameter &ndash about the size of Texas &ndash it's the largestasteroid in the belt, accounting for about 25 percent of the belt's totalmass.

Astronomers had thought Ceres might never have been heatedenough to create layers of material.

But computer models now suggest Ceres has a differentiatedinterior &ndash dense material in the core and lighter stuff near thesurface. Possible configurations include a mantle rich in water ice around arocky core.

If this mantle is composed of at least 25 percent water, Cereswould have more fresh water than Earth, according to a statement released bythe Space Telescope Science Institute, which operates Hubble for NASA and theEuropean Space Agency.

"The most likely scenario from the knowledge we have onhow other objects form, it probably has a rocky core and a mantle. That mantleis probably some watery, icy mix, with other dirt and constituents. That mantlecould be as much as ? of the whole object," study coauthor Joel Parker ofthe Southwest Research Institute told SPACE.com."Even though it's a small object compared to Earth, there could be a lotof water."

On Earth, fresh water makes up only a thin layer just a fewmiles deep in some places, less in others. The water layer proposed for Ceres,while smaller in circumference, is many miles thicker.

The total volume of water on Earth is about 1.4 billion cubickilometers, around 41 million of which is fresh water. If Ceres' mantleaccounts for 25 percent of the asteroid's mass, that wouldtranslate to an upper limit of 200 million cubic kilometers of water,Parker said.

Since all the nine "regular" planets havedifferentiated interiors, this new view of Ceres has some astronomers callingCeres a "mini-planet," adding fuel to an ongoingdebate over exactly what qualifies as a planet.

Other researchers recently announced the discovery of 2003 UB313,a round object in our solar system 1-1/2 times larger than Pluto and aboutthree times further away from the Sun. But even an object of this size &ndashat 2,100 miles in diameter roughly four times the size of Ceres &ndash doesn'treceive universal endorsement as being a planet.

One astronomer, Brian Marsden, whoruns the Minor Planet Center where data on small bodies is collected, says thatif Pluto is considered a planet, then any other round worlds should also beconsidered planets. Under this definition, which some other astronomerssubscribe to, Ceres 2003 UB313 and a handful of other large objects would benamed planets. The alternative, Marsden and otherssay, is to stop calling Pluto a planet.

Another explanation is that Ceres is a sort of 'baby' planet&ndash an underdeveloped version of Earth and other rocky planets. Looked atthis way, Ceres appears as other fledgling planets might have looked more than4 billion years ago.

The leading theory for planet formation holds that small rockscollided, stuck and gradually grew. Depending on location and orbit, adeveloping world may or may not have encountered enough raw materialto become as large as the four traditional rocky planets.

"Ceres is an embryonic planet," said observation teammember Lucy McFadden of the Department of Astronomy at the University ofMaryland. "Gravitational perturbations from Jupiter billions of years agoprevented Ceres from accreting more material to become a full-fledgedplanet."

In 2015 scientists will get a close up look at Ceres when theNASA Dawn mission orbitsthe asteroid. A closer look should provide more clues about the asteroid'scomposition.

## SOLAR SYSTEM | Asteroids, Comets and Space Dust

### Introduction

Asteroids and comets must be regarded as minor members of the Solar System. Compared with planets they are of very low mass, and they have even been referred to as cosmic debris. The asteroids, dwarf worlds most of which are well below 1000 km in diameter, are found mainly between the orbits of Mars and Jupiter, though some stray from this ‘main belt’ comets have been described as ‘dirty snowballs’, and though they may become very conspicuous in the sky they are very insubstantial. This article reviews the asteroids and comets, together with the large amount of thinly-spread material lying in the Solar System.

## Asteroid Deflection

The Asteroid Deflection Research Center will develop ways to neutralize threatening asteroids.

### Transcript

Practicing asteroid deflection. I'm Bob Hirshon and this is Science Update.

On any given day, the odds of a large asteroid hitting the Earth are, well, astronomically low. But if it happened, it could wipe out an entire city, or worse. That's why Iowa State University has created an Asteroid Deflection Research Center, led by aerospace engineer Bong Wie.

Wie:
We are not going to propose some science-fictional scheme. We will be very realistic in selecting many options already available to us.

Possible strategies include blowing up the asteroid in space with a nuclear weapon, smacking it with a projectile, or using a spacecraft's gravity to slowly drag it off course. Wie will be working with scientists around the world they'll not only model these approaches on computers, but also practice on small, non-threatening asteroids in space. I'm Bob Hirshon for AAAS, the Science Society.

### Making Sense of the Research

You probably haven't been up nights worrying about an asteroid hitting the earth, but that's okay. A handful of scientists like Dr. Wie are doing the job for you&mdashand they actually might be able to do something to prevent it.

Although there are no known asteroids on a collision course with earth right now, in the (very) long run, it's only a matter of time before one hits. It's happened many times in the past. The most famous impact happened about 65 million years ago, when an asteroid 6 to 10 kilometers in diameter slammed into Mexico's Yucatan Peninsula, setting off an explosion equivalent to 100 trillion tons of TNT&mdashover 6 billion times as powerful as the nuclear bomb that leveled Hiroshima at the end of World War II. Among other things, the impact incinerated everything within hundreds of miles, set off gigantic tidal waves, earthquakes, and volcanic eruptions, and ejected dust and debris into the sky that blocked the sun for years. The majority of species on earth at the time, including the dinosaurs, were killed off by this catastrophic event. Luckily, humans weren't around yet.

However, the human race dodged a bullet in the form of another impact just a century ago. On June 30, 1908, a massive explosion in the sky obliterated some 80 million trees in the remote forests of Siberia, Russia. Scientists believe the explosion was a meteor or comet, much smaller than the asteroid that killed the dinosaurs, which burned up several miles above the earth's surface. Because the earth rotates, had the object struck about five hours later, it may have completely destroyed St. Petersburg, Russia's capital at the time.

Since then, scientists have learned that meteors and comets burn up in the atmosphere surprisingly often, but most of the time the fireworks are too small and distant to notice. Recent estimates suggest that the object that exploded over Siberia may have been as little as 40 meters (131 feet) in diameter, and that objects that size may strike the earth, on average, once every few hundred years. In the past, similar impacts may have gone unnoticed because they happened over the oceans or unpopulated areas. However, just one impact on a populated area could be devastating, and the spread of the human population across the earth's land mass grows with each passing year.

As you heard, there are several possible strategies for deflecting an asteroid. One is to blow it up in space with a nuclear weapon. This may be the most effective approach, since it could completely vaporize the asteroid&mdashbut it might merely shatter the asteroid into hundreds or thousands of fragments, some of which could still be large enough to cause damage. Another possible strategy is to strike the asteroid with an object like an unmanned satellite, and knock it off course like a billiard ball. Finally, a large, heavy spacecraft could be sent to use its own gravitational pull to tow the asteroid slowly out of earth's path. This strategy would require a long time to execute, but it could potentially work on clusters of space rubble that couldn't be deflected by other means.

Of course, nobody wants to test these ideas for the first time on a potential doomsday asteroid. Computer models provide the easiest means to test hypotheses, but practical experiments are in the works as well: the European Space Agency, for example, is preparing an experimental asteroid deflector mission called Don Quijote. The mission will involve two spacecraft: one to fly up to an asteroid and assess its mass, shape, and gravity field, and another to strike the rock at just the right place and time to set it off course.

Now try and answer these questions:

1. What are possible consequences of an asteroid impact?
2. What are some possible strategies for asteroid deflection? What are their pros and cons?
3. What challenges do scientists face in trying to prepare for something this rare and large-scale?

You may want to check out the July 11, 2008, Science Update Podcast to hear further information about this Science Update and the other programs for that week. This podcast's topics include: news from space (deflecting asteroids, catching a supernova in the act, capturing the sun in 3-D, and new names for old planets).

### Going Further

Read more about the Don Quijote mission on the official European Space Agency site.

The National Geographic News article Are Asteroids History's Greatest Killers? links mass extinctions to asteroid impacts throughout Earth's history, while Undetectable Asteroids Could Destroy Cities describes a potential future risk from small, less obvious asteroids.

The National Geographic Interactive feature Asteroids: Deadly Impact explores asteroids, meteors, and their impact on Earth.

## Magnetosphere

Earth’s outer core is made from liquid metal which conducts electricity. The liquid convects, and this motion generates magnetic fields. Earth’s rotation helps organize this motion into huge cylindrical roles that align with the Earth’s axis.

This generates a magnetic field similar to a bar magnet, with a magnetic north pole and south pole. This field surrounds the Earth and deflects most of the charged particles from the solar wind. Without this geomagnetic field, the solar wind would directly hit Earth’s atmosphere eroding the air away.

Mars doesn’t have a strong magnetic field, and it is believed that because of this its atmosphere is mostly vanquished.

Some solar wind particles are trapped by Earth’s magnetic field and are channeled down into the atmosphere. There, they are slammed into air molecules about 150 km / 93.2 mi up. This energizes the molecules, which respond by emitting light in different colors. This glow is called aurora.

The auroras happen near the geomagnetic poles, far north and south. They form ribbons and sheets, depending on the shape of the magnetic field.

Analysis revealed that a magnetic reversal takes place once every 40.000 years on average. When it will happen, compass needles would likely point in many different directions for a few centuries while the switch is being made. After this, the south will become north and vice versa.

## The Dangers Of Asteroids To Earth

Since the early days of the Earth, asteroids have been hitting our planet, and they continue to be a major threat to life on Earth. A section of scientists claims that asteroids wiped out the dinosaurs around 66 million years ago and are a constant yet distant threat to humans. Most asteroids are too small to do any damage to the planet or do not have an orbit that crosses paths with Earth. However, the asteroids that are big enough could be able to do damage that ranges from wiping out a city to causing a global mass extinction events. Scientists and government programs have been set up around the world that are dedicated to detecting and tracking near-Earth asteroids that would be large enough to cause major damage to the planet. While the odds of a major asteroid impacting Earth are low, we must be vigilant in finding them because if a major asteroid did hit Earth is would be devastating for humanity and the planet.