Almost exclusively, the only way in which we can determine an asteroid’s composition is by looking at its spectra and comparing this to meteorites we have on the Earth. However, we have managed to collect an in-situ sample of an asteroid and 2 more sample return missions are due to deliver over the next 3 years.
On détermine la composition d’un astéroïde en regardant son spectre et en comparant celui-ci aux météorites que nous avons sur la Terre. Toutefois, nous avons réussi à ramasser un échantillon in-situ d’un astéroïde, et il y aura deux autres missions de retours d’échantillons dans les trois prochains ans.
Des missions de retours d’échantillons
L’agence spatiale japonaise (JAXA) a lancé deux missions: Hayabusa et Hayabusa 2. Hayabusa, lancée en mai 2003 a atteri sur l’astéroïde Apollo 25143 Itokawa (1998 SF36) en novembre 2005. Le petit échantillon ramassé (environ 1 gramme) est revenu sur Terre en juin 2010. Hayabusa 2, lancee en 2014 a atteri sur l’astéroïde Apollo 162273 Ryuga (1999 JU3) en février 2019, et aussi en juin 2019. Elle rapportera deux petits échantillons sur Terre le 6 décembre 2020 http://www.hayabusa2.jaxa.jp/en/
The Japanese space agency (JAXA) has launched 2 missions; Hayabusa and Hayabusa 2. Hayabusa was launched in May 2003 and landed on the Apollo class asteroid 25143 Itokawa (1998 SF36) in November 2005. It collected a very small sample of the surface (about 1 gram) and this was returned to Earth in June 2010. Hayabusa 2 launched in 2014 and landed on the Apollo class asteroid 162173 Ryugu (1999 JU3) in Feb 2019, and again in June 2019. It will return two small samples to Earth on 6th December this year (2020). http://www.hayabusa2.jaxa.jp/en/
NASA have also launched, in Sept. 2016, an asteroid sample return mission; Osiris Rex. This mission has now rendezvoused with it’s target, the Apollo class asteroid 101955 Bennu (1999 RQ36), and will return a sample of this object to Earth in 2023. Bennu is a well-studied object as its current orbital data show it to be on a predicted collision path with the Earth on 25th Sept 2175. A key part of the mission to this asteroid is to determine the nature of the object, its rotation, the consequential Yarkovsky effects, tensile strength and more detailed orbital and physical characteristics such as would be needed for a deflection mitigation. https://www.nasa.gov/osiris-rex
Image credit and coutersy to: JAXA, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba Institute of Technology, Meiji University, University of Aizu, AIST.
The asteroids are a diverse set of objects and several taxonomic definition schemes have historically been proposed. Current schemes relate albedo and spectra and define groupings along these lines. As we have seen, the albedos of asteroids can vary widely. The distribution of the albedo of the population however shows two distinct peaks, broadly centred around ~0.05 and ~0.15. The spectra of asteroids are reflection spectra; asteroids of course generate no energy or light themselves and we see them because they reflect sunlight (and other wavelengths of solar emission).
Spectroscopic observation and analysis of asteroids has developed in scope, scale and accuracy since initial observations. In 1874, the German astrophysicist Hermann Vogel (b.1841 d.1907) detected Hβ absorption lines in Vesta and Flora. Spectrographic techniques and observations of asteroids substantively began in 1929 when the Russian astronomer N Bobrovnikoff first reported colour variations in his photographic spectral observations of the larger minor planets. The use of photo-electric detectors in the 1960s provided greater insights but the advent and use in the 1980s, and onwards to today, of long-slit spectrographs based on CCD (Charge Coupled Devices) provided the ability for astronomers to gain high resolution results over a wide part of the spectrum.
One of the challenges facing astronomers in analysing and interpreting asteroid reflectance spectra is the effects of ‘space weathering’. This is the changing of the asteroid’s primordial surface caused by exposure to the Sun’s radiation and solar wind, and also due to cosmic rays. These change the surface both atomically and chemically and lead to a reddening and darkening of the surface over long (in human terms) periods of time.
At a ‘top’ level, there are three main classifications of asteroid; the carbonaceous (C), the stony / silicaceous (S); and ‘others’, a rather large group of smaller classes. The most recent comprehensive definition of groups – the Small Main Belt Asteroid Spectroscopic Survey (SMASS-II) defines 26 groups; with ‘S’ having six sub-groups and ‘C’ two. Any asteroid which does not fit into any of the current classifications is given the designation U. We will take a brief look here at the C and S types, the D type (the Jovian Trojans), and the high albedo X class.
The carbonaceous asteroids (numbering about 21% of all known asteroids) are very dark (typically 0.04 albedo) and grey and exhibit a few absorption lines. Whilst very similar to the carbonaceous chondrite meteorites found on the Earth, they also show an absorption line at 3µm which indicates the presence of silica hydrates. They show no signs of thermal heating change.
The most common class of asteroid are the stony / silicaceous asteroids. These, amounting to ~44% of the population, are much brighter (typical albedos are > 0.15) than the carbonaceous and they are reddish in colour. With strong absorption lines at wavelengths corresponding to iron oxide, silicates of iron and magnesium, they are found predominately within the mid to inner main belt and their spectra most closely matches that of the ordinary chondrite meteorites. The match to ordinary chondrites is not particularly good, although this is considered to be due to surface space weathering.
The Trojan asteroids are predominately D class (which contains ~1% of the total asteroid population). These are dark and red, but show very few spectral features and there are no matches or analogies to the meteorites found on Earth. This is not surprising as most meteors, and thus meteorites, have their origins within either the main belt (due to collisions) or from comets (when the Earth’s orbit intersects that of the comet’s). Meteorites originating from Trojan asteroids are to be expected as very rare.
The X class of asteroids (~17% of the population and comprising four subgroups) have higher albedos and show distinct signs of a high temperature / igneous nature and history. Their spectra are very comparable to the iron/nickel and enstatite (magnesium silicate pyroxene) meteorites. These classes of asteroid are mostly found in the inner part of the main belt.
We will look at the mass of the asteroids, both individually and in total. As a bit of fun, before next month’s blog, have a guess at the total amount of mass of the asteroids, compared to the mass of the Earth. I suspect the actual answer may surprise you!