Planetary Systems

Asteroid Belt

Asteroids are rocky and metallic objects that orbit the Sun but are too small to be considered planets. They are known as minor planets. Although it seems to be a very dense belt in the schematic diagram of Figure 07-10a, spacecraft that have flown through this zone have found that it is really quite empty and that asteroids are separated by very large distances. Asteroids range in size from Ceres, which has a diameter of about 1000 km, down to the size of pebbles. Sixteen asteroids have a diameter of 240 km or greater. They have been found inside Earth's orbit to beyond Saturn's orbit. Most, however, are contained within a main belt that exists between the orbits of Mars and Jupiter. Some have orbits that cross Earth's path and some have

Figure 07-10a Asteroid Belt
[view large image]

even hit the Earth in times past. One of the best preserved examples is the Barringer Meteor Crater near Winslow, Arizona (Figure 07-10b).

Asteroids are material left over from the formation of the solar system. One theory suggests that they are the remains of a planet that

		was destroyed in a massive collision long ago. More likely, asteroids are material that never coalesced into a planet. In fact, if the estimated total mass of all asteroids was gathered into a single object, the object would be less than 1,500 kilometers across -- less than half the diameter of our Moon. Figure 07-10c shows some asteroid orbits, all of which are close to the planetary plane, in the same direction as the planets. Asteroids in the Main Belt take about 3 - 6 years to complete a revolution. They spin as they revolve in just hours.
Figure 07-10b Meteor Crater [view large image]	Figure 07-10c Orbits [view large image]

Much of our understanding about asteroids comes from examining pieces of space debris that fall to the surface of the Earth. Asteroids that are on a collision course with the Earth are called meteoroids. When a meteoroid strikes our atmosphere at high velocity, friction causes this chunk of space matter to incinerate in a streak of light known as a meteor. If the meteoroid does not burn up completely, what's left strikes Earth's surface and is called a meteorite. Of all the meteorites examined, 92.8 percent are composed of silicate (stone), and 5.7 percent are composed of iron and nickel; the rest are a mixture of the three materials (Figure 07-10a). Stony meteorites are the hardest to identify since they look very much like terrestrial rocks.

On September 2005, the Japanese spacecraft Hayabusa arrived at asteroid Itokawa and stationed itself only 20 kilometers away (Figure 07-10d, not in proportion). Although a long term goal is to find out how much ice, rock and trace elements reside on the asteroid's surface, a shorter term goal is to determine the mass of the asteroid by measuring the attraction of the drifting spacecraft. In November, a small coffee-can sized robot is scheduled for release and is expected to hop around the asteroid taking pictures. Also in November, the spacecraft will fire pellets into asteroid Itokawa and collect some of the debris in a capsule. In December, the spacecraft will make its journey back to Earth and will deliver the capsule in 2007 June. The return trip has been delayed because of communication problem (resolved in January 2006), and other issues.

Figure 07-10d Asteroid Itokawa

The spacecraft was seriously damaged. Nevertheless, the project team is trying its best to bring it back by 2010. Eventually, JAXA managed to have it retrieved on June 13, 2010. Although the sampling mechanism did not work, thousands of 10-100

m particles were found in one of the sample containers, apparently

introduced during the spacecraft impact into the surface of the asteroid. Many of these particles are shown to be asteroidal grains by their chemistry and mineralogy, but they are mixed with contaminant particles from the spacecraft. Thus, instead of returning several grams of sample, Hayabusa has returned less than a milligram of sample (see "Hayabusa Asteroid Itokawa Samples")

In March, 2015, NASA's Dawn spacecraft went into orbit about Ceres. With a diameter of 950 kilometres it is the largest dwarf planet in the middle of the asteroid belt. Excitement was first directed at a mysterious bright spot inside the crater Occator, believed to be water outgassing into space, possibly from a pocket of ice on the surface that releases gas as it warms when exposed to the sun during Ceres's 9-hour day. (Figure 07-10e, NewScientist 2015, 28 March, 12 Dec) The spacecraft will analyse the planetoid with three instruments: a camera to look for surface ice deposits, an infrared spectrometer and a neutron detector to look for ice features beneath the surface. Measurements suggest the bright regions are mostly made of icy salt deposits that originated beyond the solar system's "snow line". In the early solar system this would have been several hundred million kilometres farther from the sun than Ceres's current location, suggests it was born in the Kiuper belt and has migrated inwards at some later date.

Figure 07-10e Ceres
[view large image]

Meanwhile, Hayabusa 2 (launched on 2014) is due to arrive at the C-type asteroid Ryugu in 2018 and will attempt to collect a sample.

It returned to Earth on December 6, 2020 and unload the sample back to a landing site in Australia. Analysis of the returned samples suggest that Ryugu is likely quite uniform on a macroscopic scale and resembles chondrites, which represents some of the most primordial material available for study about the formation and evolution of the solar system. They are in the form of hydrated matrix such as clay, along with a number of organic materials. (see "Study of Hayabusa2 Asteroid Samples", and the "Hayabusa2 Mission" ).

Figure 07-10f SRC [view large image]

Figure 07-10f shows the actual SRC (Sample Return Capsule) retrieved at the landing site without any thought of having it quarantined. There is no pre-caution for any untoward contaminant. A pair of white gloves seems to be sufficient for a clean retrieval .

Then, there is NASA's OSIRIS-REx. The primary mission is to obtain a sample of at least 60 g from Bennu (Figure 07-10g) -

a carbonaceous near-Earth asteroid, and return the sample to Earth for a detailed analysis. The material returned is expected to enable scientists to learn more about the formation and evolution of the Solar System, its initial stages of planet formation, and the source of organic compounds that led to the formation of life on Earth. It was launched on 8 September 2016, and rendezvoused with Bennu on 3 December 2018. On 20 October 2020, OSIRIS- touched down on Bennu and successfully collected a sample weighting between 400 g and over 1 kg of sample material. OSIRIS-REx is expected to return with the sample to Earth on 24 September 2023 (see an invitation to witness the beginning of its homeward bound journey on May 10, 2021.).

Figure 07-10g OSIRIS-REx Sample Collection [view large image]

Note that : there is no mention of checking for any sign of non-terrestrial replicating entity. The returned sample will be available for distribution to laboratories worldwide without going through any sterilization procedure.

It never occurs to them there could be harmful substance within the sample or on the capsule, probably because there is no water in the asteroid and therefore it could not breed life. The situation is quite different in the Mars Sample Return mission (see "A Summary of the Mars Sample Return (MSR) Concept").

Anyway, China will launch in 2024 another "sample return" mission to the HO3 asteroid which loops around Earth as a quasi-satellite (see "Asteroids, Hubble rival and Moon base: China sets out space agenda").