In December 2020 the Hayabusa2 spacecraft brought back samples from the asteroid Ryugu for scientists to study on Earth. What exactly is an asteroid? How did Hayabusa2 get there? What can we learn from Ryugu?
Science communicator at the Natural History Museum in London, Khalil Thirlaway, was in conversation with curator Helena Bates of the Natural History Museum in London, to answer questions like those mentioned above.
Dr Thirlaway obtained a PhD in Immunology from Nottingham University. His first degree was in Biology from Bristol University.
Dr Bates has just finished a four-year PhD at Oxford University. Her first degree was in geophysics at Imperial College London.
She works in the planetary science division at the Natural History Museum and she is the interim curator of meteorites. This means she looks after the meteorite collection at the museum and she decided whether other scientists can do research on the samples
The Natural History Museum in London is a natural history museum that exhibits a vast range of specimens from various segments of natural history. It is one of three major museums on Exhibition Road in South Kensington, the others being the Science Museum and the Victoria and Albert Museum. The Natural History Museum’s main frontage, however, is on Cromwell Road.
The museum is home to life and earth science specimens comprising some 80 million items within five main collections: botany, entomology, mineralogy, palaeontology and zoology. The museum is a centre of research specialising in taxonomy, identification and conservation.
The conversation began with a general discussion about meteorites
The Natural History Museum houses one of the world’s largest and finest collections of meteorites.
The collection contains approximately 2,000 individual meteorites in about 5,000 registered pieces.
The meteorite collection started when the British Museum acquired three meteorites in 1802, just as people were beginning to accept the idea that meteorites were natural phenomena.
The images below are of a meteorite known as ‘Imilac’ (stony-iron; pallasite), It was found in Atacama, Chile in 1822
The Imilac meteorite formed in the asteroid belt in the early solar system.
This extra-terrestrial piece is part of an ancient pallasite meteorite. It is a slice from one of the world’s largest specimens of its kind.
It’s thought to have been part of a much larger meteor that weighed up to 1,000 kilogrammes and exploded over the Atacama Desert in northern Chile, possibly in the fourteenth century.
Following the explosion, fragments of various sizes were scattered across a vast area of barren desert.
Not only is the meteorite beautiful, it contains information about our own planet’s early history, from the very beginning of the solar system.
Pallasites are a class of stony–iron meteorite.
They consist of centimetre-sized olivine crystals of peridot quality in an iron-nickel matrix.
The conversation continued by explaining the difference between asteroids, comets, meteors and meteorites.
Asteroids are minor planets, especially of the inner Solar System. Larger asteroids have also been called planetoids. These terms have historically been applied to any astronomical object orbiting the Sun that did not resolve into a disc in a telescope and was not observed to have characteristics of an active comet such as a tail. As minor planets in the outer Solar System were discovered that were found to have volatile-rich surfaces similar to comets, these came to be distinguished from the objects found in the main asteroid belt.
Diagram of the Solar System’s asteroid belt, which is between Mars and Jupiter. Asteroids come in all shapes and sizes.
Bits of asteroids can be knocked off and some of these bits can be drawn towards Earth.
A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process called outgassing. This produces a visible atmosphere or coma, and sometimes also a tail. These phenomena are due to the effects of solar radiation and the solar wind acting upon the nucleus of the comet. Comet nuclei range from a few hundred meters to tens of kilometres across and are composed of loose collections of ice, dust, and small rocky particles. The coma may be up to 15 times Earth’s diameter, while the tail may stretch beyond one astronomical unit. If sufficiently bright, a comet may be seen from Earth without the aid of a telescope and may subtend an arc of 30° (60 Moons) across the sky. Comets have been observed and recorded since ancient times by many cultures.
Comets usually have highly eccentric elliptical orbits, and they have a wide range of orbital periods, ranging from several years to potentially several millions of years. Short-period comets originate in the Kuiper belt or its associated scattered disc, which lie beyond the orbit of Neptune. Long-period comets are thought to originate in the Oort cloud, a spherical cloud of icy bodies extending from outside the Kuiper belt to halfway to the nearest star. Long-period comets are set in motion towards the Sun from the Oort cloud by gravitational perturbations caused by passing stars and the galactic tide. Hyperbolic comets may pass once through the inner Solar System before being flung to interstellar space. The appearance of a comet is called an apparition.
Comets are distinguished from asteroids by the presence of an extended, gravitationally unbound atmosphere surrounding their central nucleus. This atmosphere has parts termed the coma (the central part immediately surrounding the nucleus) and the tail (a typically linear section consisting of dust or gas blown out from the coma by the Sun’s light pressure or outstreaming solar wind plasma).
Comet Hale–Bopp seen from Earth in 1997, and C/2011 W3 (Lovejoy) imaged from Earth orbit
Hale-Bopp is a long-period comet that was discovered in 1995 and that reached perihelion in Spring, 1997. Because it was much brighter than comets normally are when it was first discovered outside the orbit of Jupiter, there was considerable speculation that in the Spring of 1997 Hale-Bopp would be a spectacular sight.
Comets and asteroids are similar except that comets come from outside the Solar System but their exaggerated elliptical orbit bring them close to the Sun on occasions.
Meteoroids are significantly smaller than asteroids, and range in size from small grains to one-meter-wide objects. Objects smaller than this are classified as micrometeoroids or space dust. Most are fragments from comets or asteroids, whereas others are collision impact debris ejected from bodies such as the Moon or Mars.
A meteorite is a solid piece of debris from an object, such as a comet, asteroid, or meteoroid, that originates in outer space and survives its passage through the atmosphere to reach the surface of a planet or moon. When the original object enters the atmosphere, various factors such as friction, pressure, and chemical interactions with the atmospheric gases cause it to heat up and radiate energy. It then becomes a meteor and forms a fireball, also known as a shooting star or falling star; astronomers call the brightest examples “bolides”. Once it settles on the larger body’s surface, the meteor becomes a meteorite. Meteorites vary greatly in size. For geologists, a bolide is a meteorite large enough to create an impact crater.
The conversation continued with the main subject. The Hayabusa 2 mission to investigate the asteroid Ryugu.
162173 Ryugu, provisional designation 1999 JU3, is a near-Earth object and a potentially hazardous asteroid of the Apollo group. It measures approximately 1 kilometre in diameter.
Why study Ryugu?
This asteroid had already been investigated on Earth. The sunlight reflected off it has been examined by telescopes on the Earth and in space. The spectrum of this light gives information about the material that makes up the surface.
Close examination of the visible-light spectrum reflected off Ryugu reveals a pattern of dark lines—called absorption lines. These patterns can provide important scientific clues that reveal hidden properties of the asteroid. These patterns of lines within spectra act like fingerprints for atoms and molecules.
The spectra provided evidence of clay minerals on the asteroid. These minerals are very common on Earth. They are only really found in things like rivers or areas that have been exposed to water. Finding clay minerals on Ryugu suggests that it was once exposed to water. This is exciting because water was important for the evolution of the Earth. Finding evidence of water outside of the Earth is why scientists wanted to send a mission there. They are hoping that Ryugu can provide information about how the Earth and life on it evolved.
Humans can’t visit the asteroid because it is so small and its mass isn’t big enough to produce enough gravity to stand on it.
Gravitational field strength, g, = GM/r2 where G is the gravitational constant, which has a value of 6.67 x 10−11Nm2kg−2, M is the mass of the asteroid (4.5 x 1011) and r is the distance from the surface to the centre of the asteroid (500m). g is about 1.2 x 10-4N/kg (this is about 1/80000th of the value of g on Earth).
As mentioned above Ryugu has been examined by telescopes but it is better to send probes to visit it,
Ryugu was discovered on 10 May 1999 by astronomers with the Lincoln Near-Earth Asteroid Research at the Lincoln Lab’s ETS near Socorro, New Mexico, in the United States. It was given the provisional designation 1999 JU3. The asteroid was officially named “Ryugu” by the Minor Planet Center on 28 September 2015 (M.P.C. 95804). The name refers to Ryūgū-jō (Dragon Palace), a magical underwater palace in a Japanese folktale. In the story, the fisherman Urashima Tarō travels to the palace on the back of a turtle, and when he returns, he carries with him a mysterious box, much like Hayabusa2 returning with samples.
To gain more information about Ryugu the Japanese set up an asteroid sample-return mission. The Japanese space agency (JAXA) is very good at sample-return missions.
Hayabusa 1, launched on 9 May 2003, successfully rendezvoused with a near-Earth asteroid called Itokawa in mid-September 2005. After arriving at Itokawa, Hayabusa studied the asteroid’s shape, spin, topography, colour, composition, density, and history. In November 2005, it landed on the asteroid and collected samples in the form of tiny grains of asteroidal material, which were returned to Earth aboard the spacecraft on 13 June 2010.
Apparently, Itokawa isn’t as interesting, science-wise, as Ryugu, but because the mission was so successful the Japanese decided to go ahead with a second mission to Ryugu.
Hayabusa2 (Japanese: はやぶさ2, “Peregrine falcon 2”) is an asteroid sample-return mission operated by the Japanese state space agency JAXA. It is a successor to the Hayabusa mission, which returned asteroid samples (19mg) for the first time in June 2010. Hayabusa2 was launched on 3 December 2014 and rendezvoused in space with near-Earth asteroid 162173 Ryugu on 27 June 2018. It surveyed the asteroid for a year and a half and took samples. It left the asteroid in November 2019 and returned the samples to Earth on 5 December 2020 UTC. Its mission has now been extended through at least 2031, when it will rendezvous with the 1998 KY26 asteroid.
Hayabusa2 carried multiple science payloads for remote sensing and sampling, and four small rovers to investigate the asteroid surface and analyse the environmental and geological context of the samples collected.
A Japanese H-2A rocket on Dec. 3 launched the nation’s Hayabusa 2 asteroid sample-return mission from the Tanegashima Space Center at 1:22 p.m. local time. Credit: JAXA
Artist’s impression of Hayabusa2 firing its ion thrusters.
European researchers were involved with the mission. The German space agency built some of the instruments. It was an international collaboration
The mission was launched in 2014 and it took four years for Hayabusa2 to get to the asteroid.
The orbit of Ryugu is similar to Earth’s, but it is more squashed.
Animation of Hayabusa2 orbit from 3 December 2014.
Hayabusa2 used the Earth’s gravity to reach its destination.
In orbital mechanics and aerospace engineering, a gravitational slingshot, gravity assist manoeuvre, or swing-by is the use of the relative movement (e.g. orbit around the Sun) and gravity of a planet or other astronomical object to alter the path and speed of a spacecraft, typically to save propellant and reduce expense.
Gravity assistance can be used to accelerate a spacecraft, that is, to increase or decrease its speed or redirect its path. The “assist” is provided by the motion of the gravitating body as it pulls on the spacecraft.
It took quite a while for Hayabusa2’s orbit to match Ryugu’s orbit. The difficulty of the mission has been likened to throwing a dart from Japan and having it land on a 6cm diameter target in Brazil.
Lots of work had to be done before the launch, mainly to provide solutions for things that could go wrong. How things could be avoided or how problems could be overcome. Luckily Hayabusa1 provided lots of information to help prepare Hayabusa2 for approaching the asteroid.
The primary goal of Hayabusa 2 was to bring back samples from Ryugu.
Emily Lakdawalla, Charles H. Braden, and Loren A. Roberts for The Planetary Society – https://www.planetary.org/multimedia/space-images/spacecraft/hayabusa2-instrument.html
Hayabusa2 carried some camaras which took images throughout the journey.
Flypast the Earth
Approaching the asteroid, which has rather a bizarre shape. The shape is described as a “spinning-top” shape.
Hayabusa2 carried four small rovers to explore the asteroid surface in situ, and provide context information for the returned samples. Due to the minimal gravity of the asteroid, all four rovers were designed to move around by short hops instead of using normal wheels. They were deployed at different dates from about 60 m altitude and fell freely to the surface under the asteroid’s weak gravity.
One of the rovers, MASCOT (Mobile Asteroid Surface Scout), was developed by the German Aerospace Center (DLR) in cooperation with the French space agency CNES. Its power supply lasted about 17 hours. It contained a camera, thermometer and devices to measure any magnetic fields.
Image taken by a rover
The mission used target markers on the surface of the asteroid to help Hayabusa 2 make just the right contact.
Shooting target markers to help with contact.
One of the camera’s on board kept the target marker in its field of view. This enabled the craft to “boop” (like a fist bump) onto the surface at the desired spot to collect samples. Touch and go manoeuvres.
The video shows the scooping up of the sample, with some of the surface flying about. The instruments were protected from flying debris.
Like a fist bump
Hayabusa2 touched down briefly on February 22, 2019 on Ryugu, fired a small tantalum projectile into the surface to collect the cloud of surface debris within the sampling horn, and then it moved back to its holding position The second sampling was from the sub-surface, and it involved firing a large copper projectile from 500 m altitude to expose pristine material, and after several weeks, it touched down on 11 July 2019 to sample the sub-surface material, using its sampler horn and tantalum bullet.
Above left: Before the “bullet”. Above right: After the “bullet”
The surface of the asteroid had been impacted by micro meteorites, solar radiation and “space weather” (dust raining down). This means the surface will not give the whole story so you need to get underneath the surface. This is why the mission fired the “bullet” into the surface to clear some of it away so the scooping up process collected some of the sub-surface materials. Hopefully the sub-surface is pristine.
The spacecraft collected and stored the samples in separate sealed containers inside the sample-return capsule (SRC), which was equipped with thermal insulation. The container was 40 cm external diameter, 20 cm in height, and a mass of about 16 kg
On 13 November 2019, commands were sent to Hayabusa2 to leave Ryugu and begin its journey back to Earth. This journey home only took six months, because at that time the asteroid and Earth were reasonably close to each other,
Hayabusa2 used its ion engines for changing its orbit slightly to allow it to return to Earth more easily.
An ion thruster or ion drive is a form of electric propulsion used for spacecraft propulsion. It creates thrust by accelerating ions using electricity.
The Hayabusa2 mission cost 900 million dollars, which is quite cheap for a sample return mission. Hayabusa 1 reduced the cost because its techniques were re-used for the Hayabusa 2 mission.
The impact of the “bullet” and “boop” would have changed the orbit of the asteroid a little. However, the impact was miniscule in comparison to other objects impacting on it. Even sunlight can cause the orbit to change.
During the day, the surface of the asteroid is illuminated by the Sun, so it absorbs heat and grows warmer. During the night, however, the surface cools down, emitting the heat it absorbed as radiation. This radiation exerts a force on the asteroid, acting as a sort of mini-thruster that can slowly change the asteroid’s direction over time. The force is called the Yarkovsky effect.
Ivan Osipovich Yarkovsky (24 May 1844, Asveya, Vitebsk Governorate – 22 January 1902, Heidelberg) was a Polish Russian civil engineer.
Hours before Hayabusa2 flew past Earth in late 2020, it released the capsule, on 5 December 2020 at 05:30 UTC. The capsule was released spinning at one revolution per three seconds. The capsule re-entered the Earth’s atmosphere at 12 km/s and it deployed a radar-reflective parachute at an altitude of about 10 km, and ejected its heat-shield, while transmitting a position beacon signal. The sample capsule landed at the Woomera Test Range in Australia. The total flight distance was 5.24 x 109 km (35.0 AU).
With the successful return and retrieval of the sample capsule on 6 December 2020 (JST), Hayabusa2 is using its remaining 30 kg of xenon propellant (from the initial 66 kg to extend its service life and fly out to explore new targets. It came close to Earth to eject the sample container but never entered the Earth’s orbit.
The Japanese scientists quickly retrieved the container on the 6th of December. This was good as it limited any contamination from the Earth.
Replica of Hayabusa’s sample-return capsule (SRC) used for re-entry. Hayabusa2’s capsule is of the same size, measuring 40 cm in diameter and using a parachute for touchdown.
The container in the desert
Retrieving the sample container
The volatile substances were collected and tested before the sealed containers were opened. The good news was that none of the Earth’s atmosphere leaked in.
The samples are being curated and analysed at JAXA’s Extraterrestrial Sample Curation Center, where international scientists can request a small portion of the samples. The spacecraft brought back a capsule containing carbon-rich asteroid fragments that scientists believe could provide clues about the ancient delivery of water and organic molecules to Earth.
The mission collected 5.4g of material (more than anticipated). It was dark and like rubble.
As of December 2020, description of overall bulk sample is planned to be done by JAXA in the first six months. 5 wt% of the sample will be allocated for the detailed analysis by JAXA. 15 wt% will be allocated for initial analysis, and 10 wt% for “phase 2” analysis among Japanese research groups. Within a year, NASA (10 wt%) and international “phase 2” research groups (5 wt%) will receive their allotment. 15 wt% will be allocated for research proposals by international Announcement of Opportunity. 40 wt% of the sample will be stored unused for future analysis.
Very happy Jaxa scientists, who had to be quarantined in their lab after their visit to Australia, due to the Covid-19 pandemic.
Image taken by the German instruments on the rover. Little white flecks are calcium aluminium inclusions.
So far examination of the sample has shown it is very similar to meteorites already on Earth with confirmation that clay minerals are present. There are also calcium aluminium inclusions. These inclusions are like dust bunnies.
Dust bunnies (or dustbunnies), in American English, are small clumps of dust that form under furniture and in corners that are not cleaned regularly (there are a lot in my house as I don’t like cleaning). Dust bunnies have been used as an analogy for the accretion of cosmic matter in planetoids, such as asteroids. This accretion took place at the beginning of the Solar System.
A calcium–aluminium-rich inclusion or Ca–Al-rich inclusion (CAI) is a submillimetre- to centimetre-sized light-coloured calcium- and aluminium-rich inclusion found in carbonaceous chondrite meteorites. The four CAIs that have been dated using the Pb-Pb chronometer yield a weighted mean age of 4567.30 ± 0.16 Myr. As CAIs are the oldest dated solids, this age is commonly used to define the age of the Solar System.
CAIs consist of minerals that are among the first solids condensed from the cooling protoplanetary disk. They are thought to have formed as fine-grained condensates from a high temperature (>1300 K) gas that existed in the protoplanetary disk at early stages of Solar System formations. Some of them were probably remelted later resulting in distinct coarser textures. The most common and characteristic minerals in CAIs include anorthite, melilite, perovskite, aluminous spinel, hibonite, calcic pyroxene, and forsterite-rich olivine.
Using the lead-lead isotope chronometer (‘Pb–Pb dating’), the absolute age of four CAIs have been calculated. They yield a weighted mean age of 4567.30 ± 0.16 Myr, which is often interpreted as representing the beginning of the formation of the planetary system (so-called ‘CAI time-zero). It is of note that all four Pb-Pb dated CAIs come from the same group of meteorites (CV chondrites).
Lead–lead dating is a method for dating geological samples, normally based on ‘whole-rock’ samples of material such as granite. For most dating requirements it has been superseded by uranium–lead dating (U–Pb dating), but in certain specialized situations (such as dating meteorites and the age of the Earth) it is more important than U–Pb dating.
So, the presence of these inclusion gives a rough age of the Solar System, generally about 4.5 billion years, which is older than the Earth.
The clay minerals show the asteroid was exposed to water. There was also some evidence of impacts on the asteroid.
The sample contained iridium.
Iridium is a chemical element with the symbol Ir and atomic number 77. A very hard, brittle, silvery-white transition metal of the platinum group, iridium is considered to be the second-densest metal (after osmium) with a density of 22.56 g/cm3 as defined by experimental X-ray crystallography. It is the most corrosion-resistant metal, even at temperatures as high as 2000 °C.
X-ray crystallography (XRC) is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles and intensities of these diffracted beams, a crystallographer can produce a three-dimensional picture of the density of electrons within the crystal. From this electron density, the mean positions of the atoms in the crystal can be determined, as well as their chemical bonds, their crystallographic disorder, and various other information.
Iridium is found in meteorites in much higher abundance than in the Earth’s crust. For this reason, the unusually high abundance of iridium in the clay layer at the Cretaceous–Paleogene boundary gave rise to the Alvarez hypothesis that the impact of a massive extraterrestrial object caused the extinction of dinosaurs and many other species 66 million years ago. Similarly, an iridium anomaly in core samples from the Pacific Ocean suggested the Eltanin impact of about 2.5 million years ago.
The Willamette Meteorite, the sixth-largest meteorite found in the world, has 4.7 ppm iridium.
The Cretaceous–Paleogene (K–Pg) boundary, formerly known as the Cretaceous–Tertiary (K-T) boundary, is a geological signature, usually a thin band of rock. The K–Pg boundary marks the end of the Cretaceous Period, the last period of the Mesozoic Era, and marks the beginning of the Paleogene Period, the first period of the Cenozoic Era. Its age is usually estimated at around 66 Ma (million years ago), with radiometric dating yielding a more precise age of 66.043 ± 0.011 Ma.
The Alvarez hypothesis posits that the mass extinction of the non-avian dinosaurs and many other living things during the Cretaceous–Paleogene extinction event was caused by the impact of a large asteroid on the Earth.
The Eltanin impact is thought to be an asteroid impact in the eastern part of the South Pacific Ocean during the Pliocene-Pleistocene boundary around 2.51 ± 0.07 million years ago.
The scientists working on the Ryugu samples would like to find out how much water the asteroid was exposed to and how much water it might have contained. They are also interested in seeing how it is like other asteroids. Conclusion so far is that Ryugu is unusual due to the water exposure and impact heating.
NASA is also carrying out a sample-return mission. It will be interesting to see how the samples differ. Hopefully the research will explain how the Earth was formed.
As mentioned earlier Hayabusa2 will visit other asteroids. It still has working cameras and other examination methods.