We have two stories with good news on the asteroid impact front this morning. The first, out of the University of Hawaii’s Institute for Astronomy, is the announcement of the detection of a small asteroid prior to its entering the Earth’s atmosphere. That many not sound unusual, but this is the first time an object could be detected in time to move people away from a impact site, even though asteroid 2019 MO was only about 4 meters across and burned up in the atmosphere. The key is warning time, and here that time would have been half a day.
An impactor like the 20-meter object that exploded over Chelyabinsk, Russia in 2013 could, with these same methods, be detected by the ATLAS facility at Maunaloa (Hawaii) several days before impact. ATLAS is made up of two telescopes, one on Hawai?i Island, the other 160 kilometers away at Haleakal?, Maui, providing whole-sky scans every two nights. About 100 asteroids larger than 30 meters in diameter are discovered by the facility every year.
In the case of 2019 MO, the ATLAS Maunaloa site picked up the object on the morning of June 22 when it was about 500,000 kilometers from Earth. Meanwhile, the Pan-STARRS 2 survey telescope at Haleakal? had imaged the same part of the sky about two hours earlier than ATLAS, revealing the incoming asteroid. The combination of data from the two sites allowed the object’s entry path to be refined, showing a likely impact that matched later Nexrad weather data in Puerto Rico, showing an entry over the ocean about 380 kilometers south of San Juan.
Image: A map of the predicted trajectory and final impact location for asteroid 2019MO. The predicted path is based on observations from the University of Hawai?i’s ATLAS and Pan-STARRS survey telescopes. Credit: Larry Denneau (IfA/ATLAS), Brooks Bays (SOEST).
So we’re getting some lead time on impactors even at this small scale, an indication that surprises from larger objects will be less likely in the future. Likewise cheering is news from research at NASA Ames which has just appeared in a special issue of Icarus, growing out of a workshop sponsored at Ames by the NASA Planetary Defense Coordination Office. Its theme: A new look at Tunguska, the 1908 impact that wreaked havoc in Siberia.
500,000 acres of uninhabited forest were flattened by the event, which was seen by few but left enough damage to alert scientists that this had not been a volcanic explosion or a mining accident, even though the first serious investigations didn’t occur until the 1920s. Today we look back at Tunguska as a kind of signature event, says Ames scientist David Morrison:
“Tunguska is the largest cosmic impact witnessed by modern humans. It also is characteristic of the sort of impact we are likely to have to protect against in the future.”
And the good news is that computer modeling discussed in the new papers shows that the interval between impacts like Tunguska is not, as has been previously estimated, on a timescale of centuries but rather millennia. 50 million combinations of asteroid and entry properties were analyzed by the computer models deployed here, some of them trying to match atmospheric pressure waves with the seismic signals recorded on the ground at the time, while others homed in on the kind of event that could produce the Tunguska treefall pattern and soil burn distribution. Four different computer modeling codes arrived at similar conclusions.
Thus we tighten our understanding of how rocks break apart in the atmosphere. The results show a stony Tunguska impactor between 50 and 80 meters in diameter that entered the atmosphere at about 15 kilometers per second, producing the equivalent of a 10 to 30 megaton explosion at between 9.5 and 14.5 kilometers altitude. The researchers used the latest estimates of the asteroid population to make the calculation of millennia between such impacts, correcting the earlier estimate that had been based on a smaller impactor.
Image: Trees flattened by the intense shock wave created in the atmosphere as the space rock exploded above Tunguska on June 30, 1908. The photograph was taken by the Soviet Academy of Science 1929 expedition led by Leonid Kulik. 500,000 acres, the size of a large metropolitan city, were flattened. Flattening trees requires an immense shock wave. Credit: Wikimedia Commons.
The new Tunguska work also draws upon extensive video observations and maps of the Chelyabinsk event. The advances in computer models that result show that the Chelyabinsk impactor was likely a stony asteroid that broke up about 24 kilometers above the ground, producing a shock wave like that of a 550 kiloton explosion. Chelyabinsk-class objects are thought to impact the Earth every 10 to 100 years, so learning how to provide sufficient warning, as the ATLAS and Pan-STARRS work shows us is now possible, can potentially save lives.
“Because there are so few observed cases, a lot of uncertainty remains about how large asteroids break up in the atmosphere and how much damage they could cause on the ground,” says Lorien Wheeler, a researcher from Ames, working on NASA’s Asteroid Threat Assessment Project. “However, recent advancements in computational models, along with analyses of the Chelyabinsk and other meteor events, are helping to improve our understanding of these factors so that we can better evaluate potential asteroid threats in the future.”
The seven papers on Tunguska and computational modeling of impact events growing out of the Ames workshop appear in a special issue of Icarus that can be found here.
Tuesday’s post on asteroids and what it would take to deflect or destroy one has been usefully reinforced by a new paper from Mike Nolan (Lunar and Planetary Laboratory, University of Arizona) and colleagues, who discuss their findings in Geophysical Research Letters. Here we’re looking at observations of the near-Earth asteroid (101955) Bennu, both archival (extending back to 1999) and current, drawing on the OSIRIS-REx mission.
You’ll recall that OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer) is in operation around the asteroid, its observations helping us understand the object’s rotation, structure and composition, with a sample return planned for 2023. The Nolan paper fills us in on observed changes in rotation, which are apparent on the order of about 1 second per century. The asteroid’s rotation is speeding up.
Exactly what’s going on here is something we can hope OSIRIS-REx can help nail down. One possibility is a process like the Yarkovsky?O’Keefe?Radzievskii?Paddack effect (YORP), by which asteroids are known to be affected because of the uneven distribution of solar heating across their surfaces. The effects of YORP depend on the shape and orientation of the individual asteroid and can cause either a slowdown or uptick in the object’s spin rate.
Or are there other processes at work here? Even boulders on the surface and their relative positions can play into changes in asteroid spin. It’s important to find out because over astronomical time periods, a faster spinning asteroid could eventually shed some of its mass. One thing, then, that OSIRIS-REx will be looking for is the presence of landslides or other surface evidence of such changes. Nolan points to the possibilities:
“As it speeds up, things ought to change, and so we’re going to be looking for those things and detecting this speed up gives us some clues as to the kinds of things we should be looking for. We should be looking for evidence that something was different in the fairly recent past and it’s conceivable things may be changing as we go.”
Image: This series of MapCam images was taken over the course of about four hours and 19 minutes on Dec. 4, 2018, as OSIRIS-REx made its first pass over Bennu’s north pole. The images were captured as the spacecraft was inbound toward Bennu, shortly before its closest approach of the asteroid’s pole. As the asteroid rotates and grows larger in the field of view, the range to the center of Bennu shrinks from about 11.4 to 9.3 km (7.1 to 5.8 miles). This first pass was one of five flyovers of Bennu’s poles and equator that OSIRIS-REx conducted during its Preliminary Survey of the asteroid. Credit: NASA/Goddard/University of Arizona.
We’re fortunate in having data from ground-based telescopes as well as Hubble to study the object over time. 110 million kilometers from Earth, the spinning Bennu completes a full rotation every 4.3 hours. The Hubble data on rotation rate showed a slight mismatch with the predictions of the earlier observations. And while Nolan and team point out that a change in the asteroid’s shape could account for its change in rotation, they clearly favor the YORP hypothesis. Having OSIRIS-REx at Bennu offers the opportunity to put YORP ideas to a close-up test.
The increase in Bennu’s rotation over the past two decades does not fit some earlier analyses of the YORP effect, making the spacecraft’s work all the more important. As the paper notes:
The OSIRIS-REx science team will independently measure the rotational acceleration during its 2-years of proximity operations. The precise shape determination, surface boulder distribution, gravity measurements, and thermal property determinations will allow for a better connection between the dominant drivers of the YORP effect (if confirmed) and their relative importance. The OSIRIS-REx team can measure the stability of the rotation state, to confirm whether this acceleration is a steady increase due to the YORP effect, or some other (likely episodic) process such as mass movement. Thus, our observations form a critical baseline for future work.
Within two years, we should have the OSIRIS-REx data independently providing Bennu’s rotation rate, which should help to identify the cause. We’ll also be looking at Bennu with other instruments for the next several decades to see whether further changes in rotation rate, consistent with YORP or not, emerge. Usefully, work like this allows us to compare and contrast in situ measurements with ground-based observations, giving us the chance to hone our skills at asteroid analysis for application to the larger population.
The paper is Nolan et al., “Detection of Rotational Acceleration of Bennu Using HST Light Curve Observations,” Geophysical Research Letters 31 January 2019 (abstract).
If we were to find an asteroid on a trajectory to impact the Earth, what strategies would we use to stop it? Recent work from Johns Hopkins University shows that there is a wide range in our thinking on what happens to asteroids under various mitigation scenarios. Much depends, of course, on the asteroid’s composition, which we must account for in our models. A good thing, then, that we are supplementing those models with sampling missions like OSIRIS-REx and Hayabusa-2.
Let’s look at the JHU work, though, which updates earlier results from Patrick Michel and colleagues, reported in a 2013 paper; the latter had considered the 5 km/s head-on impact of a 1.21 km diameter basalt impactor on a 25 km diameter target asteroid, with a model varying mass, temperature and material brittleness. Michel’s work showed evidence that the asteroid being targeted would be completely destroyed by the impactor. What Charles El Mir and colleagues at Johns Hopkins have been able to show is that other outcomes are likely.
“We used to believe that the larger the object, the more easily it would break, because bigger objects are more likely to have flaws,” says El Mir. “Our findings, however, show that asteroids are stronger than we used to think and require more energy to be completely shattered.”
Using essentially the same scenario as Michel, El Mir, K. T. Ramesh (JHU) and Derek Richardson (University of Maryland) have created a new model that offers a more detailed look at the smaller-scale processes that take place during such a collision. “Our question was, how much energy does it take to actually destroy an asteroid and break it into pieces?” adds El Mir.
Discussing methods, the authors note their model’s calculation of the first tens of seconds following impact, with transition to computer code integrating longer-term effects. From the paper:
The multi-physics material model is centered around the growth mechanism of an initial distribution of subscale flaws. Rate effects in the model are a natural outcome of the limited crack growth speed, which is explicitly computed based on the local stress state. In addition, porosity growth, pore compaction, and granular flow of highly damaged materials are captured at the material-point level. We validated the model’s predictive capability by comparing the dynamic tensile strength with high-strain-rate Brazilian disk experiments performed on basalt samples.
Image: A frame-by-frame showing how gravity causes asteroid fragments to reaccumulate in the hours following impact. Credit: Charles El Mir/Johns Hopkins University.
The first phase of the scenario is shown in the video below, available at https://www.youtube.com/watch?time_continue=1&v=Vt_xwQYafOY for email subscribers who would like to follow it up in their browsers. What emerges is that millions of cracks form in the asteroid as the crater is created, with the asteroid surviving the hit rather than being shattered, while the surviving core then has sufficient gravitational pull to act on the fragments swirling around it.
In the second phase, available at https://www.youtube.com/watch?time_continue=1&v=ZjBgljnCtWk, what we have left is not a rubble pile held loosely by gravity, but rather a surviving core whose fragments have been redistributed. Can we, then, hope to break an asteroid into small pieces, or is it best to find ways to nudge the entire object onto a new trajectory? The latter still involves the question of asteroid survivability, as we need to move it without breaking it into smaller impactors.
The paper shows the capability of the parent asteroid to withstand huge shock:
The collision imparted substantial damage onto the target, with most of the damage localized under the impact site, resulting in a heavily fractured but not fully damaged “core”. The material points were then converted into soft spheres and handed over to pkdgrav [the modeling software] in a self-consistent manner to calculate the gravitational interaction of the ejected material. We observed substantial ejecta fallback onto the largest remnant of the parent body, with a recovered mass of the largest remnant being 0.85 that of the parent body, indicating that the disruption thresholds for such targets may be higher than previously thought.
Thinking ahead to asteroid mitigation strategies is simple prudence, and requires continuing study of how asteroids respond to the various methods now under consideration to destroy them or change their trajectory. This paper gives us a glimpse of the changing parameters of research on the matter, a window into the ongoing analysis that will refine our planning.
The paper is El Mir et al., “A new hybrid framework for simulating hypervelocity asteroid impacts and gravitational reaccumulation,” Icarus Vol. 321 (15 March 2019), pp. 1013-1025 (abstract). The Michel paper is “Collision and gravitational reaccumulation: Possible formation mechanism of the asteroid Itokawa,” Astronomy & Astrophysics 554, L1 (abstract).
That small spacecraft can become game-changers, our topic last Friday, is nowhere more evident than in the success of Rover 1A and 1B, diminutive robot explorers that separated from the Hayabusa2 spacecraft at 0406 UTC on September 21 and landed soon after. Their target, the asteroid Ryugu, will be the site of detailed investigation not only by these two rovers, but also by two other landers, the German-built Mobile Asteroid Surface Scout (MASCOT) and Rover 2, the first of which is to begin operations early in October. Congratulations to JAXA, Japan’s space agency, for these early successes delivered by its Hayabusa2 mission.
Surface operations will be interesting indeed. Both rovers were released at an altitude of 55 meters above the surface, their successful deployment marking an advance over the original Hayabusa mission, which was unable to land its rover on the asteroid Itokawa in 2005. Assuming all goes well, the mission should gather three different samples of surface material for return to Earth in 2020. The third sample collection is to take advantage of Hayabusa2’s Small Carry-on Impactor (SCI), which will create a crater to retrieve subsurface material.
Why Ryugu? The object is a carbonaceous asteroid that has likely changed little since the Solar System’s early days, rich in organic material and offering us insight into the kind of objects that would have struck the Earth in the era when life’s raw materials, along with water, could have been delivered. It has also proven, as the JAXA team knew it would, a difficult landing site, with an uneven distribution of mass that produces variations in the gravitational pull over the surface.
On that score, it’s interesting to note that the Hayabusa2 controllers are sharing data with NASA’s OSIRIS-REx mission to asteroid Bennu. Likewise a sample return effort, OSIRIS-REx will face the same gravitational issues inherent in such small, irregular objects, which can be ameliorated by producing maps of each asteroid’s gravity. The three-dimensional models produced for the Dawn spacecraft at Ceres are the kind of software tools that will help both mission teams understand their targets better and ensure successful operations on the surface.
But back to Rover 1A and 1B, which have landed successfully and are both taking photographs and sending data, the first time we have landed and moved a probe autonomously on an asteroid surface. Although the first image was blurred because of the rover’s spin, it did display the receding Hayabusa spacecraft and the bright swath of the asteroid just below. Here’s JAXA’s mission tweet of that first image.
This is a picture from MINERVA-II1. The color photo was captured by Rover-1A on September 21 around 13:08 JST, immediately after separation from the spacecraft. Hayabusa2 is top and Ryugu's surface is below. The image is blurred because the rover is spinning. #asteroidlandingpic.twitter.com/CeeI5ZjgmM
Says Tetsuo Yoshimitsu, who leads the MINERVA-II1 rover team:
Although I was disappointed with the blurred image that first came from the rover, it was good to be able to capture this shot as it was recorded by the rover as the Hayabusa2 spacecraft is shown. Moreover, with the image taken during the hop on the asteroid surface, I was able to confirm the effectiveness of this movement mechanism on the small celestial body and see the result of many years of research.
The ‘hop’ Yoshimitsu refers to is a reference to the means of locomotion the rovers will use on the surface. Remember that these vehicles are no more than 18 centimeters wide and 7 centimeters high, weighing on the order of 1 kilogram. In Ryugu’s light gravity, the rovers will make small jumps across the surface, a motion carefully constrained so as not to reach the object’s escape velocity. Below is the first Rover-1A image taken during a hop.
Image: Captured by Rover-1A on September 22 at around 11:44 JST. Color image captured while moving (during a hop) on the surface of Ryugu. The left-half of the image is the asteroid surface. The bright white region is due to sunlight. Credit: JAXA.
And have a look at an image taken during landing operations before Rover-1B reached the surface. Here the asteroid terrain is clearly defined.
Image: Captured by Rover-1B on September 21 at around 13:07 JST. This color image was taken immediately after separation from the spacecraft. The surface of Ryugu is in the lower right. The coloured blur in the top left is due to the reflection of sunlight when the image was taken. Credit: JAXA.
Yuichi Tsuda is Hayabusa2 project manager:
I cannot find words to express how happy I am that we were able to realize mobile exploration on the surface of an asteroid. I am proud that Hayabusa2 was able to contribute to the creation of this technology for a new method of space exploration by surface movement on small bodies.
I would say Tsuda’s pride in his team and his hardware is more than justified. As we go forward with surface operations, let me commend Elizabeth Tasker’s fine work in spreading JAXA news in English. Even as JAXA offers live updates from Hayabusa2 in English and the official Hayabusa2 site offers its own coverage, Tasker, a British astrophysicist working at JAXA, has provided useful mission backgrounders like this one, as well as running the English-language Hayabusa2 Twitter account @haya2e_jaxa, and keeping up with her own Twitter account @girlandkat. There will be no shortage of Ryugu news in days ahead.
On the matter of interstellar visitors, bear in mind that our friend ‘Oumuamua, the subject of yesterday’s post, was discovered at the University of Hawaii’s Institute for Astronomy, using the Pan-STARRS telescope. The Panoramic Survey Telescope and Rapid Response System is located at Haleakala Observatory on Maui, where it has proven adept at finding new asteroids, comets and variable stars. Consider ‘Oumuamua a bonus, and according to a new paper from Greg Laughlin and Darryl Seligman (Yale University), a type of object we’ll be seeing again.
Pan-STARRS may find objects like this every few years, but we’ll get a bigger payoff in terms of interstellar wanderers with the Large Synoptic Survey Telescope (LSST), now under construction at Cerro Pachón (Chile). Laughlin and Seligman think that this instrument will up the discovery rate as high as several per year, allowing us to see ‘Oumuamua in context, and also, perhaps, setting up the possibility of an intercept mission with a kinetic impactor.
More on that in a moment. But first, it’s interesting to see theories about its place of origin springing up in the brief interval since ‘Oumuamua’s passage. One of the stars of the Carina/Columba association (165-275 light years from Earth) is suggested, as is the double star system HD 200325. One recent survey of more than 200,000 nearby stars could find no conclusive evidence, but does suggest that 820,000 years ago, ‘Oumuamua encountered Gliese 876. There is even a possibility, which Laughlin and Seligman dismiss, that the object originally was ejected from our own Solar System and has now had a new encounter with it.
But back to a potential mission to ‘Oumuamua. What the authors have in mind is a kinetic impactor, which has the great advantage of producing a debris plume that we could look at with a spectroscope. We have a history of comet exploration dating back to Comet Giacobini Zinner in 1985 (International Sun-Earth Explorer), the flurry of missions — Giotto, Vega 1, Vega 2, Sakigake, Suisei — that investigated Comet Halley in 1986, the Deep Space 1 mission at Comet Borrelly (2001), and Stardust at Comet Wild 2. And the, of course, there is Deep Impact, a kinetic impactor that struck Comet Tempel I, and the European Space Agency’s highly successful Rosetta at Comet 67/Churyumov-Gerasimenko.
Even now we have the Osiris-REx mission enroute to the asteroid Bennu on a sample return mission. Thus a mission to an object from outside the Solar System seems feasible, though the challenges are obvious. As the paper notes:
Such a mission would face a number of challenges, including (1) the large heliocentric velocities of objects on hyperbolic trajectories, and (2) the lack of substantial time following the discovery of the target object for mission planning and execution, and (3) uncertainty in targeting during final approach. It is worth noting that when ‘Oumuamua was detected and announced in late October 2017, it had already passed its periastron location (which occurred on 9 September, 2017), and indeed, was already more than 1 AU from the Sun.
You may recall Andreas Hein and a team from the Initiative for Interstellar Studies, who have explored potential rendezvous missions to ’Oumuamua (see Project Lyra: Sending a Spacecraft to 1I/’Oumuamua). Laughlin and Seligman consider their mission as complementary to Hein and team, assessing how to investigate an interstellar object using chemical propulsion. Like Deep Impact, the actual impactor would be accompanied by a companion flyby probe that would examine the results spectroscopically. The feasibility of such a mission depends on having sufficient lead-time to launch the interceptor to the incoming object on a hyperbolic orbit.
Lead time is considered here in terms of the expected arrival directions and speeds of such objects. The authors do this by assuming a kinematic distribution similar to Population I stars, bearing in mind that the number of interstellar asteroids may be as much as 1016 higher than the number of stars. The paper samples the distribution of such asteroids in a cube of 10 AU around the Sun, pinpointing where they would be detectable and for how long. Such knowledge would allow us to determine optimal interception trajectories.
Image: This is Figure 3 from the paper. Caption: A sky map showing the probability that a future interstellar asteroid will approach the Solar System on a trajectory parallel to that direction. The darker colors indicate a higher probability. The axes denote degrees from a heliocentric point of view and the ecliptic is plotted in black. The sky positions of the constellations Serpens and Lepus, which are close in proximity to the Solar apex and anti-apex respectively, are plotted for context. The black circle indicates the sky location that ‘Oumuamua entered our Solar System, consistent with the prediction that the majority of these objects will approach with velocities parallel to the galactic apex. Credit: Laughlin & Seligman.
I send you to the paper for the specifics, but do note this with regard to ‘Oumuamua. When it was discovered, the object was three weeks beyond periastron passage, making reaching it problematic. Putting their trajectory analysis methods to work on ‘Oumuamua, the authors find that with an earlier detection, interception of ‘Oumuamua would not have been out of the question. Future interstellar asteroids could be reached given early detection and favorable trajectories — in fact, the authors conclude that wait times for mission opportunities should be in the range of 10 years, once we have the LSST (scheduled for first light in 2021) available.
And these interesting specifics on a potential mission:
The SpaceX Falcon Heavy quotes a payload capability to Mars of 16,800 kg, which we conservatively use for the payload constraint to L1. The Deep Impact mission to Tempel I had an impactor weighing ? 400 kg and a scientific package weighing ? 600 kg (A’Hearn et al. 2005). Due to the uncertainty of the position of the ISO [interstellar object], it seems appropriate to use ? 16 impactors, with a total weight of 400 kg. The mission program is greatly assisted by the expected 40 km/s velocity of impact with the hyperbolic ISO. Assuming that the remainder of the payload consists of fuel and oxidants, to account for the oxidants and efficiency of the rocket, we allow ? 1200 kg of fuel (with specific energy similar to compressed hydrogen) to produce the ?V. Equating the kinetic energy to the energy produced by the fuel, we calculate that a maximum ?V ? 15 km/s should be attainable, to impart the same amount of kinetic energy (per impact) as the Deep Impact Tempel I interception did.
Image: This is Figure 7 from the paper. Caption: Trajectory of the minimum-?V mission interception mission sent on July 25th 2017, which had a flight time of 83.38 days. The trajectories for ‘Oumuamua, the Earth, and the rocket are plotted in red, blue and grey respectively in four day intervals in the smaller circles, while the larger circles are plotted in 28 day intervals. The arrows indicate the positions in space of ‘Oumuamua and the rocket on the launch and interception date, 7/25/2017 and 10/16/2016. Projections in the X-Y, Y-Z and X-Z planes are shown in the left, right upper, and right lower panel respectively. Credit: Laughlin & Seligman.
16 impactors to an interstellar asteroid on a manageable trajectory, with the promise of spectroscopic analysis to equal those we have achieved with previous cometary missions. With a discovery rate ramping up to several per year once LSST is available, we should have targets to work with in the 2020s, helping us learn whether what we know of ‘Oumuamua is indicative of the population of these objects. The close-up study of remnants of planetary formation around other stars is now becoming possible provided we know where to look and when to launch.
The Laughlin & Seligman paper is “The Feasibility and Benefits of In Situ Exploration of ‘Oumuamua-like Objects,” accepted at the Astronomical Journal and available as a preprint.
In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For many years this site coordinated its efforts with the Tau Zero Foundation. It now serves as an independent forum for deep space news and ideas. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image courtesy of Marco Lorenzi).
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