I’m glad to see the widespread coverage of the DART mission results, both in terms of demonstrating to the public what is possible in terms of asteroid threat mitigation, and also of calming overblown fears that we have too little knowledge of where these objects are located. DART (Double Asteroid Redirection Test) was a surprisingly demonstrative success, shortening the orbit of the satellite asteroid Dimorphos by an unexpectedly large value of 33 minutes. The recoil effect from the ejection of asteroid material, perhaps as high as 0.5% of its total mass, accounts for the result.
Watching the ejecta evolve has been fascinating in its own right, as the interactions between the two elements of the binary asteroid come into play along with solar radiation pressure. Asteroids have previously been observed that displayed a sustained tail, as Dimorphos did after impact, and the DART results suggest that the hypothesis of similar impacts on these objects is correct. Thus we learn valuable lessons about how asteroids behave when impacted either by technologies or by natural objects. We can expect the study of ‘active asteroids’ to get a boost from the success of this mission.
The two images below are from the Hubble instrument, which observed the development of Dimorphos’ tail. Jian-Yang Li (Planetary Science Institute) is lead author of a recent paper in Nature on the evolution of the ejecta. Li comments on the interplay between the gravity of Dimorphos and parent asteroid Didymos as well as the pressure of sunlight in the first two and a half weeks after the impact. Bear in mind that an impact on a single as opposed to a binary asteroid would not display such complex effects. The presence of Didymos was indeed useful:
“A simple way to visualize the evolution of the ejecta is to imagine a cone-shaped ejecta curtain coming out from Dimorphos, which is orbiting Didymos. After about a day, the base of the cone is slowly distorted by the gravity of Didymos first, forming a curved or twisted funnel in two to three days. In the meantime, the pressure from sunlight constantly pushes the dust in the ejecta towards the opposite direction of the Sun, and slowly modifies and finally destroys the cone shape. This effect becomes apparent after about three days. Because small particles are pushed faster than large particles, the ejecta was stretched towards the anti-solar direction, forming streaks in the ejecta.”
Image: Ejecta from Dimorphos 4.7 days (above) and 8.8 days (below) after impact, taken on October 1 and October 5, 2022, respectively. The Sun is at the 8 o’clock direction. The ejecta is pushed by the sunlight towards the 2 o’clock direction and increasingly stretched to form streaks. Credit: NASA, ESA, STScI, Jian-Yang Li (PSI), Image Processing: Joseph DePasquale.
So we’ve learned that slamming a 570 kilogram spacecraft into this type of asteroid at something over 22,000 kilometers per hour can alter its orbital speed. Data from the Light Italian CubeSat for Imaging of Asteroids (LICIACube) is part of the current analysis, while we’ll learn yet more about the effects of the impact from the European Space Agency’s Hera mission, which will survey both Didymos and Diomorphos, focusing on the crater left by DART and the changes to the mass of the impacted asteroid.
The matter of locating those hazardous asteroids that have yet to be identified is now highlighted by what we can consider the next planetary defense mission, the Near-Earth Object Surveyor, planned for a 2028 launch. The mission will carry a 50 centimeter diameter telescope operating at two infrared wavelengths, conducting a multi-year survey in search of near-Earth objects larger than 140 meters. The goal is to find 90 percent of such objects coming within 48 million kilometers of Earth’s orbit. The observation strategy employed should allow accurate enough determination of asteroid orbits to allow them to be found again and their trajectories tracked.
Image: Near-Earth asteroids and the possibilities of impact. Credit: NASA.
The paper on DART, one of five papers recently published in Nature on the mission, is Jian-Yang Li et al., “Ejecta from the DART-produced active asteroid Dimorphos,” Nature 01 March 2023 (abstract).
I have further thoughts on ‘Stapledon thinking,’ as discussed in the last post, but my second piece on the topic isn’t ready just yet, and in any case I want to give a quick nod to a topic we looked at a few months back, the discovery and analysis of Near Earth Objects that orbit between the Sun and the orbit of Earth. So far we haven’t found many of these ‘twilight objects,’ but the attempts to find them continue.
As witness current work with an exceptional instrument. The Dark Energy Camera is a wide-field CCD imager, mounted on a 4-meter telescope at Cerro Tololo (Chile), that was designed for the Dark Energy Survey. The latter mapped hundreds of millions of galaxies to look for insights into the structure of the cosmos. The DES ended in 2019, but DECam continues to produce data that have helped us find fascinating objects like 2015 TG387, a dwarf planet on an extreme orbit that takes it to aphelion at 1000 AU, with a closest solar approach of 65 AU. DECam has also found 12 new moons of Jupiter and a number of ‘stellar streams’ produced when small galaxies interact with the Milky Way. Now we learn that it has flagged three Near Earth Objects on inner system orbits.
These NEOs stand out beyond the fact that they are part of that small population of asteroids found inside the orbits of Earth and Venus. One of them, the 1.5-kilometer-wide 2022 AP7, merits more than a passing glance, as there is a faint chance it could one day intersect with Earth’s orbit. That would be problematic because the size of this asteroid takes it into the category that Scott S. Sheppard (Carnegie Institution for Science) calls ‘planet killers.’ 2022 AP7 is the second asteroid of this size found in the ongoing survey. Another, designated 2021 PH27, is the closest known asteroid to the Sun, and has the largest effects from general relativity in the system.
So should we worry about the 1.5-kilometer 2022 AP7? Only in the sense that we should keep an eye on it. Unfortunately, its designation as a Potentially Hazardous Asteroid (PHA) is going to generate scare stories, so let’s untangle the definition of a PHA.
What we know about 2022 AP7 is that it crosses Earth’s orbit with a perihelion near 0.83 AU and an aphelion near the orbit of Jupiter. Asteroids that cross Earth’s orbit are known as Apollos, and this one is the largest asteroid designated as a PHA that has been found in the last eight years. According to the paper on these observations, the Minimum Orbit Intersection Distance (MOID) with Earth is 0.0475 AU. That’s close enough to declare it a PHA, which basically tells us that this is an object that merits continued observation rather than one posing imminent danger.
The Center for Near Earth Object Studies, which computes asteroid and comet orbits, defines a Potentially Hazardous Asteroid this way:
Potentially Hazardous Asteroids (PHAs) are currently defined based on parameters that measure the asteroid’s potential to make threatening close approaches to the Earth. Specifically, all asteroids with a minimum orbit intersection distance (MOID) of 0.05 au or less and an absolute magnitude (H) of 22.0 or less are considered PHAs.
So asteroids that fail to approach closer to the Earth than 0.05 au (this works out to about 7,480,000 km) – or are smaller than 140 meters in diameter – do not fit the definition of PHAs. The concern is that over timespans of hundreds of years or more, such an object’s orbit may take it closer still to Earth, making identification of PHAs a necessary part of our planetary defense. Much can happen over extended periods of time to nudge the orbit of an NEO. Consider another PHA, the now well-known and visited Bennu, studied up close by the OSIRIS-REx mission.
The CNEOS monitors PHA orbits and updates them as new data become available. Let me quote NASA’s Planetary Defense Coordination Office about Bennu:
CNEOS predicts that the next time Bennu will pass Earth within the Moon’s orbit will be in 2135. This particularly close approach will change Bennu’s orbit by a small amount, which is uncertain at this time and which may lead to a potential impact on Earth sometime between 2175 and 2199. CNEOS has calculated that the cumulative risk of impact by Bennu during this 24-year period is 0.037 percent or a 1 in 2,700 chance. That means there is a 99.963 percent probability that Bennu will not impact Earth during this quarter-century period.
Note that “1 in 2,700” statement. CNEOS has been clear that this is not an impact probability for a single year, but the cumulative probability of impact over all years between 2175 and 2199. The risk of an impact in 2175, for example, is listed as 1 in 24,000. And the figures adjust as observations accumulate. A close approach to the Moon and Earth will itself tweak Bennu’s orbit by an amount that we can only estimate. Thus new data are constantly sought to tighten the window of uncertainty. One of the many good reasons for the OSIRIS-REx mission was to do just this as we revise the future possibilities for an impact. Remember, we’re looking a century out.
I feel comfortable with this arrangement. We’re doing an excellent job of identifying NEOs, we’re learning about asteroids through observation and sample return missions, and we’re investigating means of altering the trajectories of any objects that one day may threaten us. Which is to say I’m not losing any sleep about NEOs, as long as this level of vigilance and investigation continues, as it surely must in the name of planetary defense.
For more on CNEOS, see its Sentry: Earth Impact Monitoring page, which provides extensive background information.
Image: Artist’s impression of an asteroid that orbits closer to the Sun than Earth’s orbit. Credit: NSF NOIRLab.
The aforementioned Scott Sheppard is lead author of the paper on the three new NEAs. He, notes that the survey has found one other PHA thus far:
“Our twilight survey is scouring the area within the orbits of Earth and Venus for asteroids. So far we have found two large near-Earth asteroids that are about 1 kilometer across, a size that we call planet killers. There are likely only a few NEAs with similar sizes left to find, and these large undiscovered asteroids likely have orbits that keep them interior to the orbits of Earth and Venus most of the time. Only about 25 asteroids with orbits completely within Earth’s orbit have been discovered to date because of the difficulty of observing near the glare of the Sun.”
We looked at asteroids on orbits interior to Earth’s last summer (see The Challenge of ‘Twilight Asteroids’), so I’ll just remind readers that asteroids on orbits entirely within the orbit of Earth are known as Atiras, while those within the orbit of Venus are Vatiras (only one of these is currency known). If there are any asteroids entirely within the orbit of Mercury, they would be known as Vulcanoids, but none have been found yet. I turn to the paper for a note about the frequency of the Atira asteroids:
There are likely several more 1 km sized Atira-type asteroids left to find, which probably have low semimajor axes and high inclinations, like 2021 PH27, making them hard to find for most asteroid surveys. The DECam twilight survey is covering sky geometries and areas that most other surveys do not cover to depths not usually obtained, filling an important niche in the survey for the last few remaining relatively large unknown NEOs.
Noteworthy is the apparent lack of smaller asteroids in the survey:
…the twilight survey has discovered more larger asteroids (?1 km) than smaller ones even though the survey is sensitive to smaller asteroids. This might suggest the smaller asteroids are dynamically less stable and/or more susceptible to break-up from the extreme thermal and gravitational environment near the Sun, though additional discoveries of asteroids with orbits near the Sun must be made to determine statistically if the smaller asteroids are under-abundant since in general they are also harder to detect.
The paper is Sheppard et al., “A Deep and Wide Twilight Survey for Asteroids Interior to Earth and Venus,” Astronomical Journal Vol. 164, No. 4 (29 September 2022), 168 (abstract). Note as well Shappard’s “In the Glare of the Sun,” Science Vol. 377, Issue 6604 (21 July 2022), 366-367 (abstract).
Although I had Europa on my mind yesterday, I hadn’t thought to find a connection between the icy Jovian moon and the DART mission. Yet it turns out the Double Asteroid Redirection Test imaged Jupiter and Europa in July and August as the spacecraft moved toward yesterday’s encounter with the binary asteroid Didymos. Controllers used the spacecraft’s DRACO imager (Didymos Reconnaissance and Asteroid Camera for Optical navigation) to examine the visual separation between moon and planet, homing in on variations in the pixel count and intensity as the targets moved across the detector. All this in anticipation of the spacing that would soon be detected between the larger asteroid Didymos and its tiny companion Dimorphos.
Says Peter Ericksen, SMART Nav software engineer at APL:
“Every time we do one of these tests, we tweak the displays, make them a little bit better and a little bit more responsive to what we will actually be looking for during the real terminal event.”
Image: This is a cropped composite of a DART Didymos Reconnaissance and Asteroid Camera for Optical navigation (DRACO) image centered on Jupiter taken during tests of DART’s SMART Nav system. DART was about 435 million miles (700 million kilometers) from Jupiter, and about 16 million miles (26 million kilometers) from Earth, when the image was taken. Two brightness and contrast stretches, made to optimize Jupiter and its moons, respectively, were combined to form this view. From left to right are Ganymede, Jupiter, Europa, Io and Callisto. Credit: NASA/Johns Hopkins APL.
Jupiter and Europa were only part of the extensive testing before last night’s event, involving thousands of pictures of stars. A successful impact was the result. Nice work by the DART team!
It will take time to determine how well the experiment worked, which means measuring the impact’s effect on the tiny asteroid, but the data will help enormously as we continue to assess strategies for adjusting the trajectory of any future objects that may pose a danger to the Earth. We’ll be getting imagery from the Italian LiciaCube spacecraft within days, and further information from ESA’s Hera mission, which will make follow-up studies at Didymos and Dimorphos in four years.
I’ve long believed that efforts like these, necessary to ensure planetary security, will be a powerful driver for space technologies going forward. The threat of a catastrophic collision with an asteroid is small, but the image below, likewise from JHU/APL, gives us a sense of the possibilities. I think of Arthur C. Clarke’s Rendezvous with Rama (1972), where an impact in 2077 causes catastrophic damage to parts of Europe, leading to the development of the protective system of technologies that eventually spots Rama, the enigmatic alien vessel entering our Solar System.
Let’s hope we’re far enough ahead in the game to have the technologies in place to avoid that kind of impact in the first place. DART is an early step in that direction.
We have the Zwicky Transient Facility at Palomar Observatory to thank for the detection of the strikingly named ’Ayló’chaxnim (2020 AV2). This is a large near-Earth asteroid with a claim to distinction, being the first NEO found to orbit inside the orbit of Venus. I love to explore the naming of things, and now that we have ’Ayló’chaxnim (2020 AV2), we have to name the category, at least provisionally. The chosen name is Vatira, which in turn is a nod to Atira, a class of asteroids that orbit entirely inside Earth’s orbit. Thus Vatira refers to an Atira NEO with orbit interior to Venus.
As to the ’Ayló’chaxnim, it’s a word from indigenous peoples whose ancestral lands took in the mountainous region where the Palomar Observatory is located. I’m told by the good people at Caltech that the word means something like ‘Venus Girl.’ On June 7, people of Pauma descent gathered for a ceremony at the observatory, having been asked by the team manning the Zwicky Transient Facility to choose a local name.
I couldn’t tell you how ’Ayló’chaxnim is pronounced, but with the ZTF on watch, it’s possible we’ll find more Vatiras, or at least Atiras, which seem to be more numerous, so we may have more Pauma names to come and perhaps we’ll learn. 2020 AV2 is 1 to 3 kilometers in size and has an orbit tilted about 15 degrees from the plane of the Solar System. On its 151 day orbit, it stays interior to Venus and comes close to the orbit of Mercury. Postdoc Bryce Bolan at Caltech flagged it as a candidate in early 2020.
The ZTF itself is a survey camera mounted on the Samuel Oschin Telescope at Palomar, conducting a wide-field survey making rapid scans of the sky. 2020 AV2, says Caltech’s George Helou, who is a ZTF co-investigator, is on an interesting orbit, surely the result of migration from further out in the system:
“Getting past the orbit of Venus must have been challenging. The only way it will ever get out of its orbit is if it gets flung out via a gravitational encounter with Mercury or Venus, but more likely it will end up crashing on one of those two planets.”
Image: The Zwicky Transient Facility field of view. The ZTF Observing System delivers efficient, high-cadence, wide-field-of-view, multi-band optical imagery for time-domain astrophysics analysis. The camera utilizes the entire focal plane of 47 square degree of the 48-inch Samuel Oschin Schmidt telescope, providing the largest instantaneous field-of-view of any camera on a telescope of aperture greater than 0.5 m: each image will cover 235 times the area of the full moon. Credit: Zwicky Transient Facility.
This close to the Sun, Vatiras are only going to be visible at dusk or dawn. As the University of Hawaii’s Scott Sheppard points out in a recent issue of Science, our asteroid surveys mostly take place with a dark night sky, which implies that small objects orbiting between the Earth and the Sun are not likely to be found. Modeling of the NEO population predicts that objects as large as 2020 AV2 are unlikely among Vatiras but smaller objects could be plentiful. Asteroid surveys interior to Venus’ orbit are few, so there is work here for facilities like the ZTF, or the NSF’s Blanco 4-meter telescope in Chile with the Dark Energy Camera (DECam) to fill out this population. Both have fields of view sufficient to carry out this kind of survey.
So let’s get down to the asteroid mitigation question. Sheppard points out that what with current NEO surveys coupled with formation models for these objects, more than 90 percent of what he calls ‘planet killer’ NEOs have probably already been found – these would be objects larger than 1 kilometer, and he’s talking here about the entire range of NEOs, not just those interior to the orbits of Earth or Venus. He writes:
The last few unknown 1-km NEOs likely have orbits close to the Sun or high inclinations, which keep them away from the fields of the main NEO surveys. The 48-inch Zwicky Transient Facility telescope has found one Vatira and several Atira asteroids, making it one of the most prolific asteroid hunters interior to Earth. To combat twilight to find smaller asteroids, one can use a bigger telescope. Large telescopes usually do not have big fields of view to efficiently survey. The National Science Foundation’s Blanco 4-meter telescope in Chile with the Dark Energy Camera (DECam) is an exception. A new search for asteroids hidden in plain twilight with DECam has found a few Atira asteroids, including 2021 PH27.
Sheppard’s also describes a category he calls ‘city killers,’ which takes in NEOs larger than 140 meters; of these, he believes we have found about half. The progress in tracking NEOs has been heartening as we learn about potentially dangerous trajectories, and turning to twilight surveys like these will help us learn more about NEOs hidden in the glare of the Sun.
It turns out that the Zwicky team recently found the asteroid with the smallest known semimajor axis (0.46 AU). This is 2021 PH27, an object with high eccentricity whose orbit crosses the orbit of Mercury as well as Venus. Thus, given our categorization, PH27 is an Atira rather than a Vatira. With a perihelion of 0.13 AU, this NEO shows 1 arc minute of precession per century, the highest of any object in the Solar System including Mercury. This is another large NEO at about 1 kilometer in size. Although as Sheppard notes:
…because the diameter of these interior asteroids is calculated with an assumed albedo and solar phase function, the actual diameters for both of these discoveries could be under 1 km. This would put them in a more-expected population and make them less of a statistical fluke.
Image: 2020 AV2 orbits entirely within the orbit of Venus. Credit: Bryce Bolin/Caltech
Clearly we have much to do to build our catalog of objects close to the Sun. We can extend the catalog of exotic names as well. Asteroids called Amors are those that approach the Earth but do not cross its orbit. Apollos do cross the orbit of the Earth but have semimajor axes greater than Earth’s. Atens, in turn, cross Earth’s orbit but have semimajor axes less than that of the Earth. Sheppard points out that NEOs have dynamically unstable orbits, and speculates that a reservoir that replenishes their numbers must exist because the overall count seems to be in a steady state.
Among possible reservoirs are those that may exist in long-term resonances with Venus or Mercury, and there may conceivably be a population of asteroids not yet observed, the so-called Vulcanoids, that could have orbits entirely within the orbit of Mercury. Sheppard’s excellent article makes the point that Vulcanoids would be at the mercy of many factors, including Yarkovsky drift, collisions and thermal fracturing from proximity to the sun, so they’re likely uncommon. We do know that spacecraft observations of the region near the Sun seem to rule out Vulcanoids larger than 5 kilometers, but stable reservoirs for smaller objects may exist. Remember, too, that we have found numerous exoplanets closer to their host stars than the Vulcanoid region in our Solar System.
Overall, NEOs in the Sun’s glare should not be too prolific:
Fewer Atiras should exist than the more-distant NEOs, and even fewer Vatiras, because it becomes harder and harder for an object to move inward past Earth’s and then Venus’ orbit. Random walks of a NEO’s orbit through planetary gravitational interactions can make an Aten into an Atira and/or Vatira orbit and vice versa. Atiras should make up some 1.2% and Vatiras only 0.3% of the total NEO population coming from the main belt of asteroids (4). 2020 AV2 itself will spend only a few million years in a Vatira orbit before crossing Venus’ orbit. Eventually, 2020 AV2 will either collide with or be tidally disrupted by one of the planets, disintegrate near the Sun, or be ejected from the inner Solar System.
Scott Sheppard’s article is “In the Glare of the Sun,” Vol. 377 Issue 6604 (21 July 2022), pp. 366-367 (full text). For more on the Zwicky Transient Facility, see Graham et al., “The Zwicky Transient Facility: Science Objectives,” Publications of the Astronomical Society of the Pacific Vol. 131, No. 1001 (22 May 2019). Full text.
It pleases me to learn that Dutch astronomer Jan Oort was among the select group of people who have seen Halley’s Comet twice. At the age of 10, he saw it with his father on the shore at Noordwijk, Netherlands. In 1986, he saw it again from an aircraft. What a fine experience that would have been for a man who brought so much to the study of comets, including the idea that the Solar System is surrounded by a massive cloud of such objects in orbits far beyond those of the outer planets.
Image: Dutch astronomer Jan Oort, a pioneer in the study of radio astronomy and a major figure in mid-20th Century science. Credit: Wikimedia Commons CC BY-SA 3.0.
Halley’s Comet is a short-period object, roughly defined as a comet with an orbit of 200 years or less, and thus not a member of the Oort Cloud. But let’s linger on it for just a moment. The most famous person associated with two appearances of Halley’s Comet is Mark Twain, who was born in 1835 with the comet in the sky, and who sensed that its approach in 1910 would also mark his demise. As Twain put it:
I came in with Halley’s Comet… It is coming again … and I expect to go out with it… The Almighty has said, no doubt: ‘Now here are these two unaccountable freaks; they came in together, they must go out together.’
And so they did.
Edging in from the Oort
The Oort Cloud is an intriguing concept because by some accounts, it may extend halfway to the nearest star, meaning that it’s conceivable that the cometary cloud around the Sun nudges into a similar cloud around Centauri A/B, assuming there is one there. We use the Oort to explain the appearance of long-period comets, assuming that among these trillions of objects, a few are occasionally nudged out of their orbits and fall toward the Sun. The concept makes sense but observational data is sparse, as these dark objects are not directly observable until one of them moves inward.
Image: The presumed distance of the Oort cloud compared to the rest of the Solar System. Credit: NASA / JPL-Caltech / R. Hurt.
We’ve recently learned about a long-period comet with interesting properties indeed. C/2014 UN271 (Bernardinelli–Bernstein) is the object in question, named after the two astronomers who discovered it in Dark Energy Survey (DES) data at a heliocentric distance of 29 au. Recent work with the Hubble Space Telescope has determined that the object may be as much as 130 kilometers across, making it the largest nucleus ever seen in a comet. Moreover, we can assume that it’s not an aberration.
David Jewitt (UCLA) is a co-author of the paper on this work:
This comet is the tip of the iceberg for many thousands of comets that are too faint to see in the more distant parts of the solar system. We’ve always suspected this comet had to be big because it is so bright at such a large distance. Now we confirm it is.”
Getting an accurate read on an object like this was no easy matter. At this distance from the Sun, the nucleus is too faint to be resolved even by the Hubble instrument, so Jewitt and team had to rely on data showing the spike of light where the nucleus was thought to be. Lead author Man-To Hui (Macau University of Science and Technology) led the development of a computer model of the surrounding coma, adjusting it to the Hubble data and then subtracting its glow, leaving behind the nucleus. Observations from the Atacama Large Millimeter/submillimeter Array (ALMA) confirmed its size and also made it clear that the nucleus is, as Jewitt puts it, “blacker than coal.”
Image: Sequence showing how the nucleus of Comet C/2014 UN271 (Bernardinelli-Bernstein) was isolated from a vast shell of dust and gas surrounding the solid icy nucleus. Credit: NASA, ESA, Man-To Hui (Macau University of Science and Technology), David Jewitt (UCLA). Image processing: Alyssa Pagan (STScI).
Intercepting a Comet
If long period comets are difficult objects to study from Earth orbit, we may need to get up close with a spacecraft. It’s good to hear that the European Space Agency has approved the mission known as Comet Interceptor for construction, slotting it to fly in 2029 in the same launch that will carry the Ariel exoplanet finder into space. We’ve studied comets before, of course, including Halley’s, with notable success. But it’s obvious that short-period comets like the former and Rosetta target 67/P Churyumov–Gerasimenko would have been changed by their long proximity to the inner Solar System. What will we find when we study a newly arriving Oort object?
Michael Küppers is an ESA scientist working on the Comet Interceptor mission:
“A comet on its first orbit around the Sun would contain unprocessed material from the dawn of the Solar System. Studying such an object and sampling this material will help us understand not only more about comets, but also how the Solar System formed and evolved over time.”
Both Ariel and Comet Interceptor will proceed to the L2 Lagrangian point 1.5 million kilometers from the Earth, where the latter will wait for a target, presumably an Oort object jostled inward by gravitational interactions. Here we rely on the fact that comets are often detected more than a year before they reach perihelion, a time too short to allow for the construction of a dedicated space mission. The plan is to make Comet Interceptor ready to move when the time comes, performing a flyby of the incoming object and releasing twin probes to build up a 3D profile of the comet.
Image: An illustration of the L2 point showing the distance between the L2 and the Sun, compared to the distance between Earth and the Sun. Credit: ESA.
ESA will build the spacecraft and one of the two probes, the other being developed by the Japanese space agency JAXA. Given that over 100 comets are known to come close to Earth in their orbit around the Sun, along with the 29,000 asteroids cataloged so far, it will likewise be useful to have a better understanding of the composition of a pristine comet in case it ever becomes necessary to take action to avert an impact on Earth.
And if the target turns out to be an interstellar new arrival like ‘Oumuamua? So much the better. We should be finding more such newcomers shortly, given the success of the Pan-STARRS observatory and the development of the Large Synoptic Survey Telescope, now known as the Vera C. Rubin Observatory, under construction in Chile. Waiting in space for an Oort object or an interstellar comet means we won’t need to know the target in advance, but can adjust the mission as data become available. In any case, ESA is optimistic, saying Comet Interceptor “is expected to complete its mission within six years of launch.”
An ESA factsheet on Comet Interceptor can be found here. The paper on C/2014 UN271 (Bernardinelli–Bernstein) is Man-To Hui et al, “Hubble Space Telescope Detection of the Nucleus of Comet C/2014 UN271 (Bernardinelli–Bernstein),” Astrophysical Journal Letters Vol. 929, No. 1 (12 April 2022) L12 (abstract).
What makes the asteroid 16 Psyche interesting is that it may well be the exposed core of a planet from the early days of Solar System formation, a nickel-iron conglomeration that normally would lie well below a surface mantle and crust. It’s also an M-class asteroid, a category of which it is the largest known sample. These are mostly made of nickel-iron and thought to be fragmented cores, though many have a composition that has not yet been determined.
Image: Deep within the terrestrial planets, including Earth, scientists infer the presence of metallic cores, but these lie unreachably far below the planets’ rocky mantles and crusts. Because we cannot see or measure Earth’s core directly, asteroid Psyche offers a unique window into the violent history of collisions and accretion that created the terrestrial planets. Credit: University of Arizona.
M-class asteroids have been imaged before — the Rosetta spacecraft imaged the non-metallic 21 Lutetia in 2010, and 216 Kleopatra has been imaged by ES0’s 3.6 meter telescope at La Silla as well as Arecibo — but now we’ll see one from close orbit.
For Psyche will be visited by a spacecraft of the same name, slated for launch in 2022 via Falcon Heavy, with arrival in 2026 after a 3.5-year cruise under solar electric propulsion. A single Mars flyby will occur along the way. The plan is to spend 21 months in orbit around the asteroid. You’ll recall that this mission will be testing NASA’s DSOC package (Deep Space Optical Communication), using laser techniques to communicate with Earth, See Deep Space Network: A Laser Communications Future for more on the DSOC and its capabilities.
The Psyche spacecraft’s Hall thrusters, using solar arrays as the power source, will be used beyond lunar orbit for the first time in this mission. The propellant is xenon, a neutral gas that will be accelerated and expelled from the spacecraft by electromagnetic fields after being ionized, producing a now familiar blue beam. The spacecraft will carry 922 kilograms of xenon, enough to run the Hall thrusters for years without exhausting available fuel supplies. NASA engineers estimate that chemical methods would require about five times that amount of propellant to achieve the same mission.
Image: The photo on the left captures an operating electric Hall thruster identical to those that will propel NASA’s Psyche spacecraft, which is set to launch in August 2022 and travel to the main asteroid belt between Mars and Jupiter. The xenon plasma emits a blue glow as the thruster operates. The photo on the right shows a similar non-operating Hall thruster. The photo on the left was taken at NASA’s Jet Propulsion Laboratory in Southern California; the photo on the right was taken at NASA’s Glenn Research Center.
We have fairly scant information about Psyche, as witnessed by the image below, which comes out of an asteroid-imaging project nicknamed HARISSA that is being run by the European Southern Observatory, using adaptive optics on the Very Large Telescope. The survey data along with ground-based radar imaging has determined that Pyche is roughly 226 kilometers wide and contains two interesting surface features, the first of which is a bright area christened Panthia. The second is a huge crater about half the size of the asteroid itself, which the ESO team calls Meroe. No moons have turned up in this work, at least none larger than one kilometer, eliminating one marker for determining the asteroid’s mass.
Image: Views of Psyche from the HARISSA survey, with Meroe and Panthia highlighted. Credit: ESO/LAM.
Orbiting between 378 and 497 million kilometers from the Sun, between the orbits of Mars and Jupiter, Psyche takes five years to complete an orbit, with a rotation period of a little over four hours. If the asteroid is the core of what would have been a planet-sized object, the mission’s science instruments should be able to make the call. They include a multi-spectral imager, a magnetometer, a radio instrument for gravity measurement (tricky at this potato-shaped object) and a gamma-ray and neutron spectrometer. We can hope this will be sufficient to untangle a past likely marked by violent collisions in the era in which the planets were forming.
To keep up with Psyche mission developments from the inside, check the Psyche blog at Arizona State. Here’s a snip from the most recent entry (June 14), from Paige Arthur, who captures some of the excitement of being involved in a deep space mission at building 179, the famed home of JPL’s Spacecraft Assembly Facility:
Engineers and technicians adorned in lab coats, gloves, masks, and hair nets meander around on the floor below, taking measurements with multimeters and mating connectors not too different from the ones Will and I had been handling in the testbed moments before. Along the opposite wall are plaques commemorating all of the spacecraft that have been assembled there — Mariner, Ranger, Voyager, Galileo, Cassini, Curiosity, Opportunity, and, most recently, Perseverance. But in this moment I don’t notice any of it, because my attention is totally captured by the massive spacecraft suspended in the middle.
The Psyche Chassis is a huge black, silver, and gold box the size of a car with an antenna dish fixed to one end. Long struts surround the dish like the spindly legs of a massive aluminum spider, and red electronic boards connected by thick, snaking cords populate the sides. I’ve spent the last two and a half years at JPL working with software simulations and engineering models, with bits of pieces of Psyche’s brain, eyes, and heart, but seeing the physical manifestation of all that work suspended before me in the form of such a tremendous machine still brings me chills.
I’ve never worked on a spacecraft, but I’ve felt the chills, particularly at JPL one day many years ago when I watched Spirit and Opportunity being readied for shipment to Florida. Go Psyche.