The science return from OSIRIS-REx has been surprisingly swift as the spacecraft returns data on near-Earth asteroid 101955 Bennu. We’re aided here by the timing, as early results are being discussed at the ongoing conference of the American Geophysical Union (AGU) in Washington, DC. The imagery we’ve received of Bennu’s surface has scientists buzzing. Thus Humberto Campins (University of Central Florida) a member of the OSIRIS-REx Science Team, who notes the comparison between what we see now and the Arecibo radar imagery in the late 1990s:
“The images are spectacular and spot on, what we expected thanks to predictions made with the instrumentation at the Arecibo Observatory in the late 90s and early 2000s. We will spend a year and a half mapping Bennu and have to wait until mid 2020 [when] we collect the sample, but it is pretty amazing to actually see it now. Christmas came early.”
The Arecibo work began shortly after the asteroid’s discovery in 1999, when both the Puerto Rico observatory and the Goldstone planetary radar system were used to examine Bennu. A second Arecibo investigation led to a shape model and pole orientation study published in 2013 by the OSIRIS-REx science team chief, Michael Nolan (University of Arizona). Arecibo’s radar data also firmed up Bennu’s size and rotation period, while even detecting a boulder on the asteroid’s surface. The 2013 model predicted Bennu’s shape, rotation rate, inclination and diameter, all of which have been confirmed by the OSIRIS-REx OCAMS camera suite.
“Radar observations don’t give us any information about colors or brightness of the object, so it is really interesting to see the asteroid up close through the eyes of OSIRIS-REx,” Nolan said. “As we are getting more details, we are figuring out where the craters and boulders are, and we were very pleasantly surprised that virtually every little bump we saw in our radar image back then is actually really there.”
Image: This mosaic image of asteroid Bennu is composed of 12 PolyCam images collected on Dec. 2 by the OSIRIS-REx spacecraft from a range of 24 km (15 miles). Credit: NASA/Goddard/University of Arizona.
But science observations were, of course, occurring long before the arrival at Bennu. During the approach phase, which began in August, OSIRIS-REx turned its two spectrometers, the OSIRIS-REx Visible and Infrared Spectrometer (OVIRS) and the OSIRIS-REx Thermal Emission Spectrometer (OTES), on the target. We learn that the resulting data show the presence of oxygen and hydrogen atoms bonded together as hydroxyls, which researchers believe exist across the entire asteroid in water-bearing clay minerals.
To be sure, Bennu is too small an object to have had water on its surface at any point in its evolution, but at some point, its rocky components must have interacted with water. The implication is that liquid water was present on Bennu’s parent body, a much larger asteroid. What a find for the OSIRIS-REx team, given that the mission was designed to study the volatiles and organics found in the early Solar System through the lens of this asteroid. Rolling the dice on a sample mission here is paying off, as by 2023 we’ll have surface materials in a lab right here on Earth..
“This finding may provide an important link between what we think happened in space with asteroids like Bennu and what we see in the meteorites that scientists study in the lab,” said Ellen Howell, senior research scientist at the UA’s Lunar and Planetary Laboratory, or LPL, and a member of the mission’s spectral analysis group. “It is very exciting to see these hydrated minerals distributed across Bennu’s surface, because it suggests they are an intrinsic part of Bennu’s composition, not just sprinkled on its surface by an impactor.”
The number and variety of boulders on the surface is said to be unexpected, as is the size of the large boulder near the asteroid’s south pole. Observations from the ground pegged its height at 10 meters, but OSIRIS-REx is showing us through OCAMS imagery that it is more like 50 meters tall, with a width of approximately 55 meters. Given the scarcity of smooth surfaces, the search for a suitable sample site may be a complicated one.
With orbital insertion planned for December 31, we have an interesting conjunction for deep space aficionados, as New Horizons will be making its flyby of Kuiper Belt object Ultima Thule later that evening. After the OSIRIS-REx orbit is established, the spacecraft will remain in orbit until February, when it will begin another series of survey flybys. The upcoming orbit, by the way, is interesting in its own right. It will take the spacecraft between 1.4 and 2 kilometers from Bennu’s center, making this the tightest orbit of a space object by any spacecraft.
But before the orbit can be established, mission controllers are working on Bennu’s mass, a vital issue given its effects on the gravitational field of the object. The current preliminary survey passes within 7 kilometers of the north pole, equator and south pole, retrieving data that will also be useful in understanding the internal structure and composition of the asteroid. This mission has plenty of work ahead, but the early results could not be more interesting.
Voyager 2 has now gone interstellar, making it not only NASA’s single longest-running mission but one of only two spacecraft that have crossed over from the heliosphere to true interstellar space, what scientists call the Local Interstellar Medium (LISM). On that note, it’s interesting to put the Solar System in context. Depending on how you define the term, the Solar System takes in a great deal of interstellar space. Many astronomers put its outer edge at the outer Oort Cloud, perhaps 100,000 AU away, and both Voyagers have yet to reach the inner Oort.
At an estimated 1,000 AU, the inner boundary of the Oort Cloud is where the vast cometary cloud around our star becomes apparent, housing in its entirety trillions of comets and extending about 40 percent of the way to the Alpha Centauri stars. The Voyagers will keep going, of course, and will reach the inner Oort in perhaps 300 years, though without working instrumentation. The steady diminishment of power from the crafts’ radioisotope thermal generators (about 4 watts per year) means we have ten years or less to power instruments.
What a splendid run this has been, and we’re still performing good science.
Image: Artist’s concept of Voyager 2 with 9 facts listed around it. Credit: NASA.
Speaking of instruments, it is the robust health of one in particular that has made the Voyager 2 crossover so apparent. Both Voyagers carry a Plasma Science Experiment (PLS), but the one on Voyager 1 stopped working in 1980. But Voyager 2’s PLS told the tale: Measuring the plasma outflow from the Sun — commonly called the solar wind — Voyager 2 could chart the speed, density, temperature, pressure and flux of the plasma. The steep decline in the speed of the solar wind particles on November 5, and the fact that since that time, the PLS has seen no solar wind flow around Voyager 2, leaves little doubt the craft has departed the heliosphere.
“Working on Voyager makes me feel like an explorer, because everything we’re seeing is new,” said John Richardson, principal investigator for the PLS instrument and a principal research scientist at the Massachusetts Institute of Technology in Cambridge. “Even though Voyager 1 crossed the heliopause in 2012, it did so at a different place and a different time, and without the PLS data. So we’re still seeing things that no one has seen before.”
Cameras aboard the Voyagers were turned off long ago to conserve power, but beside the PLS, three other instruments continue to function: The low-energy charged particle instrument, the magnetometer, and the cosmic ray subsystem. All of these show data consistent with Voyager 2’s having crossed the heliopause. We can now compare results from both Voyagers as we investigate the interstellar medium, learning how the heliosphere itself interacts with the plasma flow JPL calls the ‘interstellar wind.’
Image: Animated gif showing the plasma data. Credit: NASA/JPL-Caltech.
Bear in mind we also have the Interstellar Boundary Explorer (IBEX) in operation, making observations of the boundary from within the heliosphere. Also in the cards is the Interstellar Mapping and Acceleration Probe (IMAP), which will operate at the L1 Lagrange point about 1.5 million kilometers from Earth to monitor solar wind interactions at the edge of the heliopause by collecting and analyzing particles that make it through the boundary from the LISM. The latter mission is scheduled for launch in 2024, and I’ll have more to say about it soon.
Long-haul missions to deep space demand payloads that can function for decades and perhaps centuries, a fact that has concerned mission designers contemplating component lifetimes in this harsh environment. It’s heartening to think of the two Voyagers, then, for both were built to last five years, enough to make their flybys of Jupiter and Saturn. Uploaded programming helped with the Uranus and Neptune flybys, the latter occurring 12 years after launch. Who would have thought that 41 years into the mission we would still be taking data?
We’re learning numerous lessons about spacecraft longevity by their example, and can contemplate future missions specifically built for interstellar medium exploration. The challenges of getting to, say, the 550 AU gravity lens of the Sun are immense, but if spacecraft built so long ago can leave the heliosphere, next-generation missions are well within our capability. What kind of interstellar precursor will follow the Voyagers and New Horizons out toward the Oort?
You wouldn’t think the Yarkovsky effect would have any real significance on a half-kilometer wide pile of rubble like the asteroid 101955 Bennu. With a currently estimated mass somewhere between 60 and 80 billion kilograms, Bennu seems unlikely to receive much of a nudge from differences in heat on the object’s surface. But the people who specialize in these things say otherwise. Sunlight warms one side of the asteroid while the other experiences the cold of space. Rotation keeps the dark side radiating heat, accounting for a tiny thrust.
We call it the Yarkovsky effect after Ivan Osipovich Yarkovsky, a Polish engineer who came up with it in 1901, though if we want to give credit across the board, we might refer to the Yarkovsky-O’Keefe-Radzievskii-Paddack (YORP) effect. Here we honor, in addition to Yarkovsky, an American scientist, a Russian astronomer and a NASA aerospace engineer, all of whom played a role in our understanding of the phenomenon as it relates to asteroids.
Image: Ivan Osipovich Yarkovsky (1844-1902). Credit: Wikimedia Commons.
The YORP effect turns up in interesting places, such as the near-Earth asteroid 2000 PH5, whose rotation rate has been spun up about as fast as any asteroid known, an effect traced over a four-year period by a team led by Stephen Lowry at Queens University Belfast (citation below). When it comes to Bennu, where we now have OSIRIS-REx in active investigation, researchers have calculated that the effect has shifted its orbit about 284 meters per year toward the Sun since 1999. Remember that Bennu originally came our way from the main asteroid belt, a movement inward that was presumably assisted by the same YORP effect.
On a scale of billions of years, then, YORP can create serious movement within the Solar System. But one reason for having OSIRIS-REx on the case is that we need to learn more about how such effects work so we can make better predictions about the future position of asteroids. Will a given asteroid present problems, with a potential trajectory that could intersect with the Earth? The calculation is by no means easy. With YORP alone, so much depends on the nature of the object, and how it absorbs and releases heat. We’d better learn as much as we can about such objects, a need that plays a role in asteroid missions that also investigate the evolution of the Solar System and the ancient debris that circulates among the planets.
Image: This artist’s concept shows the Origins Spectral Interpretation Resource Identification Security – Regolith Explorer (OSIRIS-REx) spacecraft contacting the asteroid Bennu with the Touch-And-Go Sample Arm Mechanism or TAGSAM. The mission aims to return a sample of Bennu’s surface coating to Earth for study as well as return detailed information about the asteroid and its trajectory. Credit: NASA’s Goddard Space Flight Center.
If the YORP effect makes our orbital calculations problematic, so too do the gravitational forces imparted by the Sun, nearby planets and other asteroids. As this JPL news release points out, astronomers can predict the exact dates of the next four passes Bennu will make near our planet (defined here as within 7.5 million kilometers, or .05 AU). The years in question are 2054, 2060, 2080 and 2135. But things get increasingly tricky as we look further out. For each time Bennu comes near the Earth, our planet gives its trajectory another slight twitch.
If you’re trying to figure out where Bennu will be in coming decades, then, you have to take into account the increasingly fuzzy effects that occur with each pass by the Earth, so that by 2060, when another such passage is predicted, we can only say that the asteroid will pass the Earth at about twice the distance from Earth to the Moon. But it could pass any point in a 30 kilometer window of space. Keep magnifying these numbers with future orbits and you can see why firm predictions become so difficult.
By 2080, according to calculations performed by Steven Chesley at the Center for Near-Earth Object Studies (CNEOS) at JPL, the best window we can derive for Bennu’s passage is 14,000 kilometers wide. Switch ahead to 2135, a time when Bennu’s orbit is thought to take it closer than the Moon, and the flyby window reaches 160,000 kilometers. This is, by the way, a projection for the single near-Earth asteroid for which we have the best orbital assessment in our database.
We’ve been studying Bennu through optical, infrared and radio telescopes every six years since its discovery in 1999 to measure factors like shape, rotation rate and trajectory. Given all that, CNEOS can say that looking ahead over the next century, the asteroid has a 99.963 percent change of missing the Earth. That’s heartening, but it’s clear that tightening up our numbers will help. And we can do a lot by way of studying how the YORP effect nudges the asteroid.
“There are a lot of factors that might affect the predictability of Bennu’s trajectory in the future, but most of them are relatively small,” says William Bottke, an asteroid expert at the Southwest Research Institute in Boulder, Colorado, and a participating scientist on the OSIRIS-REx mission. “The one that’s most sizeable is Yarkvovsky.”
Optical images from OSIRIS-REx will help determine Bennu’s precise location and its exact orbital path as of now, giving us a read on how its trajectory is changing with time. With the spacecraft tracking Bennu over a two-year period, the variance from the projected trajectory will help to determine the size of the YORP effect’s changes. We’ll also learn a great deal about the amount of solar heat radiating from the asteroid from what type of surfaces, which will help us refine the YORP numbers, a huge help in tightening the trajectories of other asteroids.
OSIRIS-REx should eventually be able to tell us how craters and boulders change photon scattering and momentum transfer. Says Chesley:
“We know surface roughness is going to affect the Yarkovsky effect; we have models. But the models are speculative. No one has been able to test them.”
Refining models through on the spot observation is a major reason for doing OSIRIS-REx. When the mission is over, the team believes our projections of Bennu’s orbit will be 60 times better than what we now have. If only Ivan Osipovich Yarkovsky could be here to see this.
The paper on 2005 PH5 is Lowry et al., “Direct Detection of the Asteroidal YORP Effect,” Science Vol. 316, Issue 5822 (13 April 2007), pp. 272-274 (abstract).
Almost all the exoplanets we know have been detected in evolved stellar systems, places where the protoplanetary disk has dissipated and the planets around the star can be observed. Seeing inside a disk in formation is tricky business, though prominent studies at stars like Beta Pictoris have told us much about the evolution of these disks as planets do begin to emerge. But just how common are disks with ring and gap structures? Do all such disks produce planets?
We’re beginning to learn more as instruments like the Atacama Large Millimeter Array (ALMA) continue to be used to examine infant systems. Many of these show disks that are uniform in appearance, lacking discernible features like rings or gaps. Others are brighter, marked by concentric rings with separations that imply planet formation. It’s natural enough that early efforts have been devoted to brighter disks with their suggestion of planetary activity.
Image: Until recently, protoplanetary disks were believed to be smooth, pancake-like objects. The results from this study show that some disks are more like doughnuts with holes, but even more often appear as a series of rings. The rings are likely carved by planets that are otherwise invisible to us. Credit: Feng Long.
A new effort led by Feng Long (Kavli Institute for Astronomy and Astrophysics at Peking University in Beijing, China) has now appeared that gives us a valuable statistical look at possible planets in protoplanetary disks. Where this study stands out is in its choice of targets. Rather than looking at disks based on the brightness suggestive of ring structures, the researchers chose 32 stars with disks of varying brightness to get a sense of general disk properties.
This is an important point, so let me quote the paper on it:
…the small number of systems observed at high-spatial resolution (? 0.1”) to date limits our knowledge about the origins of disk substructures. Moreover, the set of disks imaged at high resolution is biased to brighter disks, many with near/mid-IR signatures of dust evolution, and collected from different star-forming regions and thus environments. These biases frustrate attempts to determine the frequency of different types of substructures, how these substructures depend on properties of the star and disk, and any evolution of substructures with time.
So dim disks as well as bright ones are in the mix here. Feng Long’s survey of young stars targeted disks in the Taurus star-forming region, a vast cloud of dust and gas some 450 light years from Earth. 12 of the stars with protoplanetary disks showed clear indications of rings and gaps within them suggestive of planet formation. Out of the analysis we learn that super-Earths and Neptunes are probably the most common kind of planets forming in these disks, a finding that reinforces exoplanet statistics gathered from fully formed planetary systems.
Of the 12 stars with protoplanetary disks surveyed, only two show disks consistent with the development of a gas giant like Jupiter. All these possible planets emerge out of the team’s calculations applied to the ALMA data, but have not yet been observed. From the paper:
The presence of wider gaps at larger radii hints for planet-disk interaction. The low intensity contrast in most ring and gap pairs suggests the possible link to low mass planets. We follow the diagnostic used in planet-disk interaction simulations (the separation of ring and gap normalized to gap location) to infer planet mass, and find that super-Earths and Neptunes are good candidates if disk turbulence is low (? = 10?4), in line with the most common type of planets discovered so far.
Image: The Taurus Molecular Cloud, pictured here by ESA’s Herschel Space Observatory, is a star-forming region about 450 light-years away. The image frame covers roughly 14 by 16 light-years and shows the glow of cosmic dust in the interstellar material that pervades the cloud, revealing an intricate pattern of filaments dotted with a few compact, bright cores — the seeds of future stars. Credit: ESA/Herschel/PACS, SPIRE/Gould Belt survey Key Programme/Palmeirim et al. 2013).
Key to this study is the need to determine what the rings and gaps in some young systems really indicate. Are they planets in formation, or are they structures formed through other mechanisms? An alternative explanation in the literature involves variations in the chemistry across the disk depending on distance from the star. These presumed variations in pressure are called ‘ice lines.’ They are condensation fronts that occur when disk temperatures drop at larger distances from the priimary, causing various volatiles to freeze out onto dust grains.
However, the new study finds no correlations between stellar properties and gap or ring structures in the surrounding disks, noting no concentration of gap radii around major ice line locations. The conclusion: The rings and gaps do indeed flag nascent planets as the most likely cause of their formation, although other processes may also contribute to the result. The researchers will now adjust the spacing of the ALMA antennae to increase the array’s resolution, while probing at other frequencies sensitive to different sizes of dust grains.
The paper is Long et al., “Gaps and Rings in an ALMA Survey of Disks in the Taurus Star-forming Region,” Astrophysical Journal Vol. 869, No. 1 (6 December 2018). Abstract / Preprint.
You would think that helium, being the second most common element in the universe, would have been detected in exoplanet atmospheres long ago. A major constituent of the atmosphere at both Jupiter and Saturn, helium seems a natural because planets form from dust and gas from previous stellar generations, but it turns out that the first helium detection on an exoplanet occurred only this year, in a study led by Jessica Spake (University of Exeter).
The planet in question, WASP-107b, yielded its helium signature in data gathered by the Hubble Space Telescope, a detection that showed clear signs of a comet-like tail forming as the planet’s atmosphere escaped. Note the space-based detection: It’s significant because Earth’s atmosphere is opaque to the ultraviolet light the atoms in such an eroding atmosphere absorb.
Could we make this kind of fine-grained study from the surface of the Earth? It turns out there’s a way: Helium in its long-lived metastable state (as compared to its ground state) can be traced in the near-infrared as well as the ultraviolet. If you’re interested in the details on how metastable helium emerges under conditions like these, Kevin Anderton does a nice job explaining things at the atomic level in this Forbes article.
The point is that researchers are beginning to make these detections using ground-based instruments. Thus today’s paper, recounting new work from the University of Geneva (UNIGE), a research effort that includes Spake. The paper’s account of a helium detection from the ground appears in the journal Science for December 6.
The planet in this study is HAT-P-11b, a transiting ‘warm Neptune’ in Cygnus some 124 light years away, where it orbits 20 times closer to its star than the Earth does to the Sun. Central to the work is the Carmenes spectrograph installed on the 4-meter telescope at Calar Alto, Spain, which allowed the researchers to identify helium escaping from an exoplanet whose overheated upper atmosphere is streaming the gas into space in the form of an extended cloud.
Moreover, the instrument’s high spectral resolution allowed the scientists to detect the position and speed of the helium atoms in the atmosphere. According to Spake:
“This is a really exciting discovery, particularly as helium was only detected in exoplanet atmospheres for the first time earlier this year. The observations show helium being blasted away from the planet by radiation from its host star. Hopefully we can use this new study to learn what types of planets have large envelopes of hydrogen and helium, and how long they can hold the gases in their atmospheres.”
Image: Artist’s impression of the exoplanet HAT-P- 11b with its extended helium atmosphere blown away by the star, an orange dwarf star smaller, but more active, than the Sun. Credit: Denis Bajram.
Vincent Bourrier is a co-author of the study and member of the European project FOUR ACES, an acronym standing for Future of Upper Atmospheric Characterization of Exoplanets with Spectroscopy. Bourrier’s numerical simulations of the movement of the planet’s helium support the spectroscopic observations:
“Helium is blown away from the dayside of the planet to its nightside at over 10,000 km/h,” Bourrier explains. “Because it is such a light gas, it escapes easily from the attraction of the planet and forms an extended cloud all around it.”
First author Romain Allart (UNIGE) adds that proximity to the host star led the team to suspect a high impact on the atmosphere, including the shedding of helium into nearby space. We should expect, in other words, to find numerous other exoplanet atmospheres in this configuration, worlds inflated like a helium balloon. But we’re only now beginning to analyze them. While losing atmosphere is not uncommon in a giant planet close to its star, the predominant element identified in eroding exoplanet atmospheres to this point has been hydrogen.
The Carmenes work makes it clear that ground-based observations can retrieve data on the most extreme atmospheric conditions in exoplanet atmospheres. This is good news for future atmospheric studies, as is the fact that two new high-resolution spectrographs similar to Carmenes are in the works. The Near Infrared Planet Searcher (NIRPS) is undergoing testing at the University of Geneva and will be installed in Chile at the end of 2019. The other, called SPIRou (SpectroPolarimétre Infra-Rouge), is beginning an observational campaign in Hawaii after achieving first light in April. The advent of next-generation extremely large telescopes will further the field yet again by allowing study of escaping atmospheres at smaller planets.
Will we begin to find escaping atmospheres laden with heavier elements? Carbon and oxygen would be slow to escape while hydrogen is the easiest element lost, meaning a giant planet should see a greater amount of helium relative to hydrogen over time. The relative mix we find as we study these escaping atmospheres will help in the analysis of planetary evolution.
The Allart et al. paper is “Spectrally resolved helium absorption from the extended atmosphere of a warm Neptune-mass exoplanet,” Science 6 December 2019 (abstract). The WASP-107b work with the first detection of exoplanet helium is Spake et al., “Helium in the eroding atmosphere of an exoplanet,” Nature 557 (02 May 2018), 68-70 (abstract).