Hayabusa2: Multiple Paths for Analyzing an Asteroid

Ryugu is classified as a carbonaceous, or C-type asteroid, a class of objects thought to incorporate water-bearing minerals and organic compounds. Carbonaceous chondrites, the dark carbon-bearing meteorites found on Earth, are thought to originate in such asteroids, but it has been difficult if not impossible to determine the source of most individual meteorites.

Hence the significance of the Hayabusa2 mission. JAXA’s successful foray to Ryugu represents the first time we’ve been able to examine a sample of a C-type asteroid through direct collection at the site. Ralph Milliken is a planetary scientist at Brown University, where NASA maintains its Reflectance Experiment Laboratory (RELAB). The laboratory expects samples collected at Ryugu to arrive in short order. Milliken is interested in the history of water in the object:

“One of the things we’re trying to understand is the distribution of water in the early solar system, and how that water may have been delivered to Earth. Water-bearing asteroids are thought to have played a role in that, so by studying Ryugu up close and returning samples from it, we can better understand the abundance and history of water-bearing minerals on these kinds of asteroids.”

Milliken is also one of many co-authors on a new paper in Nature Astronomy looking at the thermal history of subsurface materials exposed on Ryugu. The paper examines data the spacecraft collected during its operations there, which can now be compared to the sample collection. We learn that the asteroid may not be as water-rich as originally thought, leading to various scenarios about how it might have lost its water. High on the list of possibilities is that this ‘rubble pile’ asteroid — essentially loose rock maintaining its shape because of gravity — dried after a collision or other disruption and subsequent reformation.

Image: Japan’s Hayabusa2 spacecraft snapped pictures of the asteroid Ryugu while flying alongside it two years ago. The spacecraft later returned rock samples from the asteroid to Earth. Credit: JAXA.

Bear in mind how Hayabusa2 proceeded with its sampling at Ryugu. During the 2019 rendezvous, the spacecraft fired a projectile into the asteroid’s surface that exposed the subsurface rock examined here. A near-infrared spectrometer was used to compare the water content of the surface with the material below, showing the two to be similar in water content. The authors see that as a clue that Ryugu’s parent body dried out, rather than the surface of Ryugu being dried out by the Sun, perhaps in a close solar pass earlier in its history.

In other words, heating by the Sun in one or more close solar passes would be likely to occur at the surface, without penetrating deep into the asteroid. What Hayabusa2’s spectrometer shows is that surface and sub-surface are both comparatively dry, which is an indication that it was the parent body of Ryugu, rather than an event happening to Ryugu itself, that produced this result.

Ahead for the Ryugu analysis is the need to study the size of the particles excavated from below the surface, which could play a role in how the spectrometer measurements are interpreted.

“The excavated material may have had a smaller grain size than what’s on the surface,” says Takahiro Hiroi, a senior research associate at Brown and another study co-author. “That grain size effect could make it appear darker and redder than its coarser counterpart on the surface. It’s hard to rule out that grain-size effect with remote sensing.”

The beauty of the successful sample return is that hypotheses about Ryugu’s past can now be evaluated through comparison of the remote sensing data and actual laboratory work. These are exciting times indeed for the scientists studying the extensive collection of asteroid debris Hayabusa 2 brought back. We can expect significant papers on all this in 2021.

The paper is Kitazato et al., “Thermally altered subsurface material of asteroid (162173) Ryugu,” Nature Astronomy 4 January 2021 (abstract).


DESTINY+: Mission to 3200 Phaethon

With successful operations at Ryugu (Hayabusa2) and Bennu (OSIRIS-REx), asteroid exploration seems to be moving full tilt, with the prospect of surface samples on the way. We can also look ahead to 16 Psyche, the object of interest for a NASA mission planned to launch in 2022, and the Lucy mission to Jupiter’s trojan asteroids, with launch now scheduled for 2021. The latest asteroid entry comes in the form of an interesting collaboration between the Japan Aerospace Exploration Agency (JAXA) and the German Aerospace Center (DLR) targeting asteroid 3200 Phaethon, a flyby mission designed to launch no earlier than 2024.

DESTINY+ is its name, the acronym standing for Demonstration and Experiment of Space Technology for INterplanetary voYage with Phaethon fLyby and dUst Science (try to say all that quickly before you’ve had your morning coffee). The agreement for the bilateral mission was signed on November 11 as part of a joint strategy dialogue between the two space agencies.

The new mission continues German and Japanese cooperation in space. You may recall that DLR and the French space agency CNES (National Center for Space Studies) developed the MASCOT rover that was deployed on Ryugu in 2018, with several papers already published on its findings. The BepiColombo mission to Mercury grows out of ESA and Japanese collaboration, while the proposed Martian Moons eXploration (MMX) mission, developed by JAXA, will include a rover developed by DLR and CNES, along with scientific instrumentation from NASA.

At Phaethon, we’re looking at an interesting object, an Apollo asteroid that appears to be the parent body of the Geminids meteor shower. The Apollo asteroids are the largest group of near-Earth objects, numbering over 10,000, but among them, Phaethon has a special claim to fame. Carsten Henselowsky is DESTINY+ project manager at DLR:

“With a minimum approach distance of approximately 21 million kilometres, Phaethon gets closer to the Sun than the planet Mercury. In the process, its surface heats up to a temperature of over 700 degrees Celsius, causing the celestial body to release more dust particles. The aim of the DESTINY+ mission is to investigate such cosmic dust particles and to determine whether the arrival of extraterrestrial dust particles on Earth may have played a role in the creation of life on our planet.”

Image: Animation of Phaethon’s orbit. Credit: Phoenix7777. Data source: JPL HORIZONS. CC BY-SA 4.0.

The flyby will take the spacecraft to within 500 kilometers of Phaethon at a time in its orbit when the asteroid is approximately 150 million kilometers from the Sun. Key to the success of the mission is the DDA dust analyzer built by DLR (DESTINY+ Dust Analyzer). The plan is to study the dust environment through the entire four-year cruise to Phaethon, with projected flyby in 2028. The DDA is, according to this recent presentation at the Europlanet Science Congress, “the technological successor to the Cosmic Dust Analyzer (CDA) aboard Cassini-Huygens, which prominently investigated the dust environment of the Saturnian system.”

With the goal of determining the origin of each dust particle, mission scientists will be focusing on the proportion of organic matter given the possible delivery of organics to the early Earth by such particles. The asteroid is on a highly eccentric orbit resembling that of a comet more than an asteroid, crossing the orbits of Mercury, Venus, Earth and Mars. Dust tails have been observed coming from the object, likely the result of solar heat creating surface fractures.

Launch will be aboard an Epsilon S vehicle from the Uchinoura Space Center in Japan. The spacecraft will use solar-electric propulsion, with a 1.5-year period after launch in which it will raise its orbit. A series of lunar flybys will then accelerate the probe into an interplanetary trajectory. The scientific payload will, in addition to the dust analyzer, include two cameras: The Telescopic Camera for Phaethon (TCAP) and the Multi-band Camera for Phaethon, MCAP.

By chance, I ran across a passage this morning in Oliver Morton’s book The Moon: A History for the Future (The Economist, 2019) that explains why it takes so long to get to objects that, like NEAs, sound as though they should be nearby. Such serendipity coaxes me into quoting it:

…what matters in space is delta-v, not distance. To get from the Moon to the Earth requires only about a fifth of the delta-v that is needed to make the same journey in the opposite direction. Another demonstration of this proposition is that the amount of delta-v it takes to get a spacecraft to the surface of the Moon can also get it to destinations much farther afield. “Near-Earth asteroids” (NEA) are only near inasmuch as they have orbits that very occasionally bring them moderately close to the Earth. At any given time a typical NEA will be 100 or 200 times as far away as the Moon. But in terms of delta-v, quite a lot of these asteroids are just as easy to reach as the Moon is; it is just a matter of getting on to the right trajectory and waiting. Indeed, it takes no more delta-v to reach the little moons of Mars than it does the great Moon of Earth (the surface of Mars is another matter).

Image: Asteroid explorer DESTINY+. Credit: JAXA/Kashikagaku.


OSIRIS-REx: Sample Collection at Asteroid Bennu

A spacecraft about the size of an SUV continues operations at an asteroid the size of a mountain. The spacecraft is OSIRIS-REx, the asteroid Bennu, and yesterday’s successful touchdown and sample collection attempt elicits nothing but admiration for the science team that offered up the SUV comparison. They’re collecting materials with a robotic device 321 million kilometers from home. Yesterday’s operations seem to have gone off without a hitch, the only lingering question being whether the sample is sufficient, or whether further sampling in January will be needed.

If all goes well, we will acquire the largest surface sample from another world since Apollo. TAGSAM is the Touch-And-Go Sample Acquisition Mechanism aboard the craft, a 3.35-meter sampling arm extended from the spacecraft as OSIRIS-REx was descending roughly 800 meters to the surface. The ‘Checkpoint’ burn occurred at 125 meters as the craft maneuvered to reach the sample collection site, dubbed ‘Nightingale.’ The ‘Matchpoint’ burn followed ‘Checkpoint’ by 10 minutes to match Bennu’s rotation at point of contact. A coast past the ‘Mount Doom’ boulder was followed by touchdown in a crater relatively free of rocks.

This is dramatic stuff. The image below is actually from August during a rehearsal for the sample collection (images of yesterday’s touchdown are to be downlinked to Earth later today), but it’s an animated view that gets across the excitement of the event. Mission principal investigator Dante Lauretta (University of Arizona. Tucson) had plenty of good things to say about the result:

“After over a decade of planning, the team is overjoyed at the success of today’s sampling attempt. Even though we have some work ahead of us to determine the outcome of the event – the successful contact, the TAGSAM gas firing, and back-away from Bennu are major accomplishments for the team. I look forward to analyzing the data to determine the mass of sample collected.”

Image: Captured on Aug. 11, 2020 during the second rehearsal of the OSIRIS-REx mission’s sample collection event, this series of images shows the SamCam imager’s field of view as the NASA spacecraft approaches asteroid Bennu’s surface. The rehearsal brought the spacecraft through the first three maneuvers of the sampling sequence to a point approximately 40 meters above the surface, after which the spacecraft performed a back-away burn. Credit: NASA/Goddard/University of Arizona.

The goal is 60 grams of material, with the first indication of sample size being new images of the surface to see how much material was disturbed by the TAGSAM activities. Michael Moreau (NASA GSFC) is OSIRIS-REx deputy project manager:

“Our first indication of whether we were successful in collecting a sample will come on October 21 when we downlink the back-away movie from the spacecraft. If TAG made a significant disturbance of the surface, we likely collected a lot of material.”

Images of the TAGSAM head, taken with the camera known as SamCam, should provide evidence of dust and rock in the collector, with some possibility of seeing inside the head to look for evidence of the sample within. Beyond imagery, controllers will try to determine the spacecraft’s moment of inertia by extending the TAGSAM arm and spinning the spacecraft about an axis perpendicular to the arm. Comparison to data from a similar maneuver before the sampling should allow engineers to measure the change in the mass of the collection head.

Between the imagery and the mass measurement, we should learn whether at least 60 grams of surface material have been collected. Once this has been verified, the sample collector head can be placed into the Sample Return Capsule (SRC) and the sample arm retracted as controllers look to a departure from Bennu in March of 2021. If necessary, a second maneuver, at the landing backup site called ‘Osprey,’ could take place on January 12, 2021.

Image: These images show the OSIRIS-REx Touch-and-Go Sample Acquisition Mechanism (TAGSAM) sampling head extended from the spacecraft at the end of the TAGSAM arm. The spacecraft’s SamCam camera captured the images on Nov. 14, 2018 as part of a visual checkout of the TAGSAM system, which was developed by Lockheed Martin Space to acquire a sample of asteroid material in a low-gravity environment. The imaging was a rehearsal for a series of observations that will be taken at Bennu directly after sample collection. Credit: NASA/Goddard/University of Arizona.

Sample return is scheduled for September 24, 2023, with the Sample Return Capsule descending by parachute into the western desert of Utah. So far so good, and congratulations all around to the OSIRIS-REx team!


OSIRIS-REx: Tracking Bennu’s Unusual Activity

OSIRIS-REx, the little spacecraft with the big acronym (standing for Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer) has been on station for a year and a half at asteroid Bennu, monitoring the unexpected activity that distinguishes the object. Particle ejection from the surface is the subject of a revised special issue of the Journal of Geophysical Research: Planets collecting 10 papers on the matter.

Specifically, the spacecraft has found that particles of rock mostly of pebble-size are being ejected repeatedly — one or two per day — from the asteroid’s surface, some of them escaping into space, some moving into a temporary orbit, with the rest falling back onto the surface. Just days after entering orbit on December 31 of 2018, OSIRIS-REx began to spot the activity, which the introduction to the special issue refers to as “ongoing mass shedding” involving millimeter- to centimeter-scale particles. What we have on our hands here is an asteroid that is active.

Dante Lauretta (University of Arizona) is OSIRIS-REx principal investigator:

“We thought that Bennu’s boulder-covered surface was the wild card discovery at the asteroid, but these particle events definitely surprised us. We’ve spent the last year investigating Bennu’s active surface, and it’s provided us with a remarkable opportunity to expand our knowledge of how active asteroids behave.”

Image: This mosaic image of asteroid Bennu is composed of 12 images collected on Dec. 2, 2018, by the OSIRIS-REx spacecraft’s PolyCam instrument from a range of 24 kilometers. Credit: NASA/Goddard/University of Arizona.

Bennu is another example of the blurring of the distinction between comets and asteroids, the former composed of ice, rock and dust and subject to heating by the Sun during the approach to perihelion that causes the formation of the familiar tail as vapor and dust is carried into space. The rock and dust characteristic of asteroids wouldn’t seem to lend itself to such activity, but the papers here point to thermal fracturing that is the result of repeated heating and cooling as the object rotates as the likely driver. Also implicated: Impacts from meteoroids hitting the surface. From a paper on thermal fatigue by Jamie Molaro (Planetary Science Institute) and colleagues:

If thermal fatigue indeed plays a role in Bennu’s activity, this has broad implications for our understanding of active asteroids and the asteroid population as a whole. Previous works have hypothesized that thermal fracture processes may generate activity on active asteroids with small perihelion distances, such as (3200) Phaethon (Jewitt & Li, 2010). Our results support this hypothesis. With a diurnal temperature variation of hundreds of degrees, Phaethon’s surface is likely to be subject to thermal shock processes, with fatigue operating at depth to weaken and prepare the rock for disruption. However, the fact that thermal fatigue alone may be capable of generating activity suggests that there may be many more active asteroids than are currently known, likely including many in near?Earth space. With less energetic activity, a lack of tails or comae would make such bodies hard to identify from ground?based observations, and previous missions to visit asteroids up close lacked the capability to detect ejection events like those observed on Bennu.

Image: Occurrence of particle ejection events (circles) in the orbit of Bennu (orange). Ejection events of 2 to 19 particles are denoted by small gray circles; events of 20 or more particles are denoted by large black circles. The phases of the OSIRIS?REx mission that included dedicated particle monitoring (Orbitals A–C) are indicated with blue hatches. The Sun (yellow) and the orbits of Mercury (green), Venus (light blue), Earth (dark blue), and Mars (red) are also shown for reference. Credit: Hergenrother et al. (2020).

Mission scientists have been able to use the unexpected stream of particles as a probe of the asteroid’s gravitational field, for many of them were found to be orbiting much closer than the spacecraft itself, their trajectories sensitive to the irregular gravity of the object. The precision of the measurement is actually higher than would have been possible solely through the instruments aboard OSIRIS-REx. Chesley calls them “an unexpected gift for gravity science at Bennu since they allowed us to see tiny variations in the asteroid’s gravity field that we would not have known about otherwise.”

Remember that this is a sample return mission, with a touchdown planned for October 20 to gather surface material, some of which may contain particles that were ejected and have fallen back to the surface. We can look forward to the spacecraft’s return in September of 2023.

The papers on Bennu are gathered in “Exploration of the Activity of Asteroid (101955) Bennu,” a special edition of the Journal of Geophysical Research: Planets first published in April of 2020 and now updated as of 19 August. The paper quoted above is Molaro et al., “Thermal Fatigue as a Driving Mechanism for Activity on Asteroid Bennu,” JGR Planets 21 July 2020 (abstract).


Ryugu: An Asteroid’s Interactions with the Sun

The near-Earth asteroid Ryugu is only about a kilometer wide, but it’s telling us a good deal about its own history and that of the Solar System itself thanks to the two touchdowns of Hayabusa2, in February and July of 2019. The geological changes so clear on Earth, the bombardments from objects creating craters here and elsewhere, all mark the evolution of large bodies, but the asteroids take us back to the system’s earliest days with little change. They’re bundles out of the deep freeze of time.

Now we wait for the sample return, currently on its way back to Earth, with arrival in December of this year. Aboard will be surface materials collected during both touchdowns, which will complement the data on the chemical and physical composition of the asteroid already gathered. A team led by Tomokatsu Morota (University of Tokyo) has been using Hayabusa2’s onboard ONC-W1 and ONC-T imaging instruments to analyze the dusty matter kicked up by the spacecraft’s engines during the two touchdowns.

Fine grains of dark-red colored minerals turned up in this analysis. The team believes these were produced by solar heating, indicating close passage by the Sun at some point in Ryugu’s past. This view is reinforced by the distribution of the dark red-matter. Spectral examination shows the material can be found at specific latitudes on the asteroid, corresponding to areas that would have received the most solar radiation in the asteroid’s past. Adds Morota:

“From previous studies we know Ryugu is carbon-rich and contains hydrated minerals and organic molecules. We wanted to know how solar heating chemically changed these molecules. Our theories about solar heating could change what we know of orbital dynamics of asteroids in the solar system. This in turn alters our knowledge of broader solar system history, including factors that may have affected the early Earth.”

Image: False color map of the surface of Ryugu with craters marked by circles. Credit: © 2020 Morota et al.

The distribution of surface materials suggests a history of disruption by impacts and what the paper describes as thermal fatigue and mass wasting. Let me quote from this section:

The thickness of the mixed layer of redder and bluer materials is estimated to be a few meters, derived from the minimum crater size (~10 m in diameter) that penetrates to the underlying blue materials. The presence of ejecta rays with a length of a few tens of meters that consist primarily of redder materials implies that the redder material layer originally had a minimum thickness of a few tens of centimeters. Solar heating is more likely than space weathering to be the source of the reddening of Ryugu’s surface, because space weathering typically affects only a thin layer of ~100 nm, whereas the diurnal and annual thermal skin depths (the depth at which temperature variations decay to 1/e of their value at the surface) are ≲10 cm and ~1.5 m, respectively.

It’s encouraging to hear from Morota’s group that their spectral studies and examination of Ryugu’s albedo indicate that both the dark-red material once heated by the Sun as well as blue unheated materials would have been collected in Hayabusa2’s two forays to the surface. Also on the agenda in coming months is a close look at the distribution of Ryugu’s craters and boulders. The surface craters hold information about the characteristics of the asteroid’s rocks and the history of small impacts. They also greatly complicated the search for a safe landing site.

What produced the differences between the two types of materials remains unclear:

Two distinct types of material are present on the surface with different colors: bluer material distributed at the equatorial ridge and in the polar regions and redder material in the mid-latitude regions. However, the cause of these spectral variations is not understood.

The paper is Morota et al., “Sample collection from asteroid (162173) Ryugu by Hayabusa2: Implications for surface evolution,” Science Vol. 368, Issue 6491 (8 May 2020), pp. 654-659 (abstract).


Ryugu’s Clues to the Early Solar System

Asteroid 162173 Ryugu, recently explored in depth by the Hayabusa2 spacecraft, is a C-type asteroid, rich in carbon. About a kilometer in diameter, it is evidently composed of highly porous material, and seems to have been formed by the agglomeration of fragments from a larger parent body that was broken apart by impacts. We learn this from a new paper in Nature that examines the object’s high porosity and the significantly low mechanical strength of its rock fragments, which affect how it would act if hitting an atmosphere.

Matthias Grott, of the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) is the principal investigator for MARA, the DLR-built radiometer that flew on board Hayabusa2 and landed aboard the Mobile Asteroid Surface Scout (MASCOT). Says Grott:

“The published results are a confirmation of the results from the studies by the DLR radiometer MARA… It has now been shown that the rock analysed by MARA is typical for the entire surface of the asteroid. This also confirms that fragments of the common C-type asteroids like Ryugu probably break up easily due to low internal strength when entering Earth’s atmosphere.”

Image: An artist’s conception of the MARA radiometer at work within MASCOT on the surface of Ryugu. Credit: DLR.

Breaking up into fragments upon entering Earth’s atmosphere would help to explain why carbon-rich meteorites are relatively uncommon on Earth, as most would have burned up on the way to the ground. But the new paper goes on to make the case that fragile, porous asteroids like this may link us directly to the evolution of primordial dust into planetary bodies. Hence the importance of missions like Hayabusa2 and OSIRIS-REx, which actually visit and sample the minor objects that would have formed during the earliest era of the Solar System’s development.

The paper in Nature looks at the global properties of Ryugu, an analysis that complements and confirms the findings of the MASCOT lander. The asteroid’s surface temperature has been examined through the Thermal Infrared Imager (TIR) aboard the spacecraft by the team working with first author Tatsuaki Okada of the Japanese space agency JAXA. Measuring in the 8 to 12 micrometer wavelength range during day and night cycles, they find a surface that heats quickly when exposed to sunlight. Grott adds:

“The rapid warming after sunrise, from approximately minus 43 degrees Celsius to plus 27 degrees Celsius suggests that the constituent pieces of the asteroid have both low density and high porosity.”

Image: Formation scenario for Ryugu. Credit: Okada et al.

We should be learning a lot more when the surface samples from two landing sites, currently traveling back to Earth with the spacecraft, land in Australia at the end of this year. The paper draws not only on already acquired surface data but high resolution mapping from orbit. The authors argue that C-type asteroids are most likely formed from ‘fluffy dust’ or pebbles in the early Solar System, leading to large asteroids in the main belt with high porosity. Further thoughts re planet formation from the paper:

These C-type asteroids might share their highly porous nature with planetesimals that formed from fluffy dust in the early Solar System and could have strongly affected planetary formation processes such as cratering and collisional fragmentation by attenuating shock propagation . The possibility still cannot be ruled out that Ryugu’s low thermal inertia and low density arise from surface materials different from carbonaceous chondrites, such as the organic- rich material discovered on comet 67P/Churymov-Gerasimenko . This question will, however, be resolved upon sample return.

What the samples will reveal, then, is whether Ryugu is made up of material similar to chondritic meteorites — chondrules are millimeter-sized spheres of rock considered to be building blocks of planet formation — or organic-rich material like that found on comet 67P/ Churyumov-Gerasimenko. We’ll have that answer comparatively soon, but there is something else to look forward to when it comes to probing exotic materials: The sample returns from the OSIRIS-REx mission at asteroid Bennu, expected in 2023, and samples of Phobos and Deimos. The latter should reach us in 2029 as JAXA explores the asteroid-like Martian moons in the ‘Martian Moons eXploration’ (MMX) mission, scheduled for launch in 2024.

Image: Artist’s conception of Hayabusa2 nearing the Earth for sample return. Credit: DLR.

The paper is Okada et al., “Highly porous nature of a primitive asteroid revealed by thermal imaging,” published online by Nature 16 March 2020 (abstract).