Eduardo-Tarasca

LEFT: First release image of the binary asteroid (152830) Dinkinesh, photo-bombed by a small orbiting moon. (COURTESY: NASA/Goddard/SwRI/Johns Hopkins APL/NOAO) RIGHT: My own, pre-encounter impression of what Dinkinesh might have looked like.

Above: Animated image sequence of Lucy's approach showing Dinkinesh's moon passing behind it. (NASA/Goddard/SwRI/Johns Hopkins APL/NOAO) )

So clearly, once again, the universe has a few surprises up its sleeve! NASA and the Lucy mission team have just released the first images from the spacecraft's flyby of (152830) Dinkinesh yesterday, 1 November 2023. The images reveal an astonishing reality; that Dinkinesh, the smallest main belt asteroid we've visited so far, is the proud owner of it very own moon! That was something which did not even cross my mind when I was planning my pre-encounter render.

SIZE AND SHAPE:

While composing my pre-encounter render I assumed that Dinkinesh was a singular object, with rough dimensions of ~960 x 670 x 670 m, with an short-to-long axis ratio of abut 1.43, so modestly elongated. In reality, the Lucy team estimates that Dinkinesh itself is a bit smaller, at a maximum width of about 790 m while its moon is about 220 m across at its widest. If these preliminary estimates prove to be correct, then the diameter ratio between moon and parent is rather significant, at about 27.8%. By comparison two other binary asteroids we visited previously Ida/Dactyl and Didymos/Diamorphos have parent-moon ratios of 2.6% and 20.8% respectively.

As for shape, I had assumed a slow rotational period, thus excluding the top-shaped profile seen in other small rapidly-rotating asteroids. It looks to me that this assumption was wrong, as the images available thus far seem to suggest a hint of an equatorial ridge or bulge, similar to ones seen on Ryugu and Bennu, other top-shaped asteroids. Prior to the flyby, I had guessed that Dinkinesh might be bean-shaped, that has turned out to be incorrect. I do think I was somewhat correct in Dinkinesh being irregular in shape, with concavities and depressions being evident. One thing I can't quite tell from these early images is whether the ratio of Dinkinesh's equatorial axis lengths is closer to a value of 1 (roughly circular) or a more elongated ellipsoid.

ROTATIONAL PERIOD:

Prior to the encounter I had assumed that Dinkinesh had a rotational period of about 52 hours. Given the revelation that Dinkinesh is orbited by a moon, it seems likely that that 52-hour rotational period is actually represents the orbital period of the satellite around Dinkinesh. The parent asteroid itself likely rotates much more quickly perhaps quite rapidly, though I haven't seen any data on this yet.

COLOUR:

Based on the images released so far, and assuming that these are true colour images, then Dinkinesh's surface is far more monochromatic than I had anticipated, based on my choice to give Dinkinesh a slightly ruddy surface. I had based the reddish-grey colour of my pre-encounter render on the true-colour appearances of four S-class asteroids, Gaspra, Eros, Toutatis and Itokawa, all of which muted reddish-brown surfaces.

STRUCTURE:

Given its size and composition, my assumption previous to Lucy's flyby was that Dinkinesh would be a rubble-pile asteroid, that is, a body composed of a fairly loose collection (or aggregation) of small fragments held together by gravity. Its overall appearance and resemblance to Ryugu seems to be consistent with it being a rubble-pile asteroid, though this hasn't been officially confirmed by analysis of the data captured during the flyby.

Another question which came up during my analysis of other asteroids, was whether or not Dinkinesh was a contact binary. From a superficial glance at the first release images, it doesn't appear to be. I had guessed that Dinkinesh had a roughly one-in-three chance of being a contact binary, with my render ultimately excluding it entirely. While it's still early to full discount Dinkinesh being a contact binary, I don't think it very likely at the moment. If this is true, I can't say as I was right, as my analysis leaning me towards it being a non-contact binary ultimately relied on incorrect size and rotational period information.

BOULDERS:

The biggest elephant in the room concerning my pre-encounter render (at least in my mind) was the overall number, distribution and size of the boulders covering Dinkinesh's surface. While there do appear to be boulders and fragments scattered on its surface, they are far smaller and more subdued than I had anticipated. On the bright side, I was more correct about existence of dust and finer regolith covering it surface. I had speculated that it's surface would be richer in finer material than Dimorphos or Bennu, which seems to be bourne out by the images so far released.

With just a couple of hours to go before Lucy spacecraft makes its closest approach to asteroid (152830) Dinkinesh, I'd just like to take a moment to show my pre-encounter imaginings of what Dinkinesh might look like as well as discuss a bit why I made some of the choices I did in its rendering. For a more detailed analysis see my previous post.

Below is a sequence of eight images depiciting the approach and departure phase of the spacecraft's flyby of Dinkinesh. We start from a distance of 1,000 km just a few minutes before closest approach.

Guided by my analysis of other small asteroids previously visited by spacecraft, I have elected to portray Dinkinesh as a irregular, somewhat elongated, rubble pile asteroid. Its surface is strewn by hundreds of boulders and large rocky fragments, the largest approach 80 meters in length. The regions between these rocky outcrops (particularly lower-lying depressions) are covered by smoother areas of dust and finer regolith. Dinkinesh's shape is further disrupted by several large impact craters and significant concavities. The largest of these, seen along the centre of the body has given it an overall saddle or bean-shaped profile. If the panchromatic L'LORRI imager captures colour details of Dinkinesh's surface, if it is fairly dusty, it may have a brownish-reddiish-yellow cast to it, similar to what true colour images of other S-type asteroids such as Itokawa, Eros and Gaspra. Boulders and rocky fragments are probably less saturated in colour and probably brighter.

Below is a GIF animation of the image sequence above. Note that the size of the GIF (720 x 720 px) is a bit smaller than the image resolution captured by L'LORRI's image sensor. Assuming a closest approach distance of 425-km and Dinkinesh's overall dimensions being approximately 960 x 670 x 670 m, the target's apparent size within the camera's field-of-view is roughly representative. At closest approach, maximum image resolution is likely ~2m/px.

Note that the trajectory of the spacecraft relative to Dinkinesh as depicted in this sequence has been greatly simplified for the sake of reduced complexity. The path of the camera as it passes through closest approach is co-planar with Dinkinesh's orbit, running precisely parallel to the velocity vector of the asteroid. As such it passes directly above Dinkinesh's equator, with a Sun-Dinkinesh-Spacecraft phase angle of 0 degrees at closest approach. The trajectory of the real spacecraft is likely somewhat inclined to Dinkinesh's orbit.

Below is a tighter crop of the same above animation.

I am very certain that the upcoming flyby will prove wrong most if not all of my assumptions. While I think that rendering is at least grounded by the analysis of other similar objects thus far visited, nearly six-decades of space exploration has taught us repeatedly that the cosmos is capable of creating objects stranger and more wonderful than we could imagine. Regardless, if the flyby is successful, we will have one more object to add to our growing bestiary of objects in the solar system. Hopefully, this will help to create more accurate morphological predictions of unexplored objects in the future.

Thanks, and I look forward to comparing the results of the flyby to this rendering once the real images of Dinkinesh become available.

FOR THE TLDR VERISON INCKLUDING MY PRE-ENCOUNTER RENDER OF DINKINESH, PLEASE SEE MY NEXT POST

As of 2023, humanity's robotic emissaries have visited a total of sixteen asteroidal bodies. Hopefully on 1 November 2023, we will be able to add one more object to the list, the inner main belt asteroid (152830) Dinkinesh. With estimated dimensions of ~960 m x 670 m (McFadden et al, 2023), Dinkinesh will be the smallest main belt asteroid visited so far.

The majority of the encounters achieved so far, (10, 62.5%) were brief flybys or directed impacts, while 6 objects (Eros, Vesta, Ceres, Itokawa, Ryugu and Bennu) were subject to long-term, close-range observations of their surfaces. Two of these 16 objects (5535 Annefrank and 9969 Braille) were ultimately imaged at resolutions too low to make out clear surface features, though overall shape could be ascertained.

The Lucy spacecraft, launched just over a year ago, is on a nearly four-year trek to the L4 Jupiter Trojans (known as the "Greek camp") where it is to make flybys of four Trojan asteroids in just over a fifteen-month period before exploring L5 Trojan camp in 2033. During the latter exploration, Lucy will flyby 617 Patroclus, a fascinating close-in binary object.

Lucy's flyby of Dinkinesh on 1 November 2023 (12:54 EDT) is primarily a test of the spacecraft's terminal tracking system, which will allow Lucy to autonomously lock onto the tiny Dinkinesh and keep the asteroid within the field-of-view of its cameras and instruments. This capability will be a critical component of Lucy's future flybys. The terminal tracking system will begin actively monitoring the position of Dinkinesh roughly one hour before closest approach ( a distance of 16,000 km).

At closest approach, Lucy will be roughly 425-km from it's target. From this vantage point, Dinkinesh should have an apparent angular diameter of ~0.129 x 0.09 degrees, or in other words between a quarter and a sixth the apparent diameter of the full Moon. Due to the brevity of the flyby, and Dinkinesh' slow ~52-hour rotation, only about half of its surface is expected to be illuminated, and thus capable of being imaged. As of now, Dinkinesh's appearance is completely unknown, with few physical properties determined with any certainty.

Ahead of this flyby, I thought it might be an interesting exercise to not only speculate on what we might expect Dinkinesh to look like, but use this inquiry to compose a somewhat reasonable depiction. To inform this speculation, I think it may be helpful to look at a few well studied asteroids of similar size to Dinkinesh. These objects will hopefully provide some constraints on what might be possible. We'll think of these as potential Dinkinesh analogues.

ANALYSIS OF ASTEROID ANALOGUES:

To simplify this endeavour, I will focus on only five basic parameters to help narrow down the list of analogue candidates; size, spectral type, albedo, asteroid population, and rotational period. Of the sixteen asteroids we have visited with robotic spacecraft, eleven were larger than a kilometer in diameter, with five (Ceres, Vesta, Lutetia, Mathilde and Ida) having dimensions greater than 50-km across. Due to their massive sizes when compared to Dinkinesh, I have elected to exclude these objects from the candidates out-right. Two more asteroids, Braille and Annefrank, were excluded due to the low-resolution images captured of their surfaces.

SIZE ANALOGUES

As of this moment, we have collected detailed, close-up imagery of five objects with dimensions near-to or below 1-km. In order of ascending size, the smallest of these is the asteroid-satellite Dimorphos (0.177 x 0.174 x 0.116-km) followed by Itokawa (0.535 x 0.294 x 0.209 km), Bennu (0.565 x 0.535 x 0.508-km), Didymos (0.851 x 0.849 x 0.62-km) and finally Ryugu (1.004 x 0.876-km). Our assumed dimensions for Dinkinesh (0.960 x 0.670 x 0.670) fairly neatly covers the range of the larger objects Bennu, Didymos and Ryugu. Itokawa is roughly half the size of Dinkinesh.

If we loosen our 1-km size limit slightly we can include three additional asteroids for which up-close imagery is available: (4179) Toutatis (4.26 x 2.03 x 1.7-km), (2867) Steins (6.683 x 5.7 x 4.42-km) and (433) Eros (34 x 11.2 x 11.2-km). All three dwarf Dinkinesh in size, ranging from ~ 4.5 to 35-times larger. In terms of our knowledge of their surfaces, only Eros is very well characterised, while fybys of Toutatis and Steins yielded lower resolution imagery of both (3-m/px and 80-m/px respectively).

SPECTRAL ANALOGUES If we take spectral-type into account, only Itokawa and Didymos/Dimorphos share Dinkinesh's presumed S-type (stony) classification, with both Bennu and Ryugu belonging to the distinctly different carbonaceous family of asteroids. Papers describing spectral analyses thus far suggest that Dinkinesh is very likely a stony S-type asteroid, with some ambiguity concerning it's more precise classification, with some suggesting a strong S-type, while others suggest a Q-type or V-type surface, the later of which would put it in the same classification as the vastly larger (4) Vesta. Q-type asteroids are a relatively uncommon population of inner-belt asteroids which show strong olivine and pyroxene features in their spectra with some indication of metals as well. POPULATION ANALOGUES It should be noted that all of these objects belong to the Near-Earth population of asteroids, while Dinkinesh is a member of the Inner-Main Belt asteroids. Of the highlighted asteroids visited by spacecraft we have thus far considered, only Steins and Gaspra are inner main belt asteroid like Dinkinesh.

Of the more often visited near-earth asteroids, some groups are closer in proximity to the inner main-belt than others. Closest in my opinion are the Alinda asteroids (of which Toutatis is a member), whose semi-major axes of ~2.5 AU put them nearest to the vicinity of Dinkinesh' orbit, though most have far more eccentric orbits which allow it to cross Earth's orbit.

ALBEDO ANALOGUES:

Current estimates of Dinkinesh's albedo provide a value of ~0.27. Of our eight highlighted asteroids, Eros (0.27), Itokawa (0.32) and Steins (0.35) provide the closest matches, with Didymos (0.15), Dimorphos (0.15) and Toutatis (0.13) being poorer matches at roughly half as bright. The carbonaceous Bennu (0.04) and Ryugu (0.06) are the least analogous, possessing surfaces as dark as asphalt.

A summary and comparison of our primary group of eight asteroid analogues (GROUP A) are listed below and ranked by four key parameters (mean diameter, spectral type, geometric albedo, population, and rotational period). Bolded/underlined text indicates parameters with the closest match to what is known or suspected of Dinkinesh.

MEAN

SIZE (km): TYPE: ALB: POP: ROT:

DINKINESH (0.766) S/Sq (Sv) 0.27 1 - IMB 52.67

GROUP A CANDIDATES

Dimorphos 5 - (0.135) 2 - S 4 - 0.15 4 - Ap-NE 4 - 7.04

Itokawa 4 - (0.346) 1 - S/Sq 2 - 0.32 4 - Ap-NE 1 - 12.132

Bennu 3 - (0.536) 4 - B/F 7 - 0.04 4 - Ap-NE 7 - 4.29

Didymos 1 - (0.773) 2 - S 4 - 0.15 4 - Ap-NE 8 - 2.26

Ryugu 2 - (0.942) 4 - C 6 - 0.06 4 - Ap-NE 3 - 7.632

Toutatis 6 - (2.660) 2 - Sk 5 - 0.13 2 - ApAl-NE 1 - 176

Steins 7 - (5.601) 3 - E 3 - 0.35 1 - IMB 5 - 6.05

Eros 8 - (18.80) 2 - S 1 - 0.25 3 - Am-NE 6 - 5.27

Honorable Mention

Annefrank 7 - (5.000) 2 - S 2 - ~0.20 1 - IMB 1 - 15.12

2014 HQ124 4 - (~0.409 ) 2 - S? 2 - 0.29 3 - At-NE 1 - 16?

Gaspra 8 - (12.53) 2 - S? 2 - 0.24 1 - IMB 4 - 7.042?

Ap-NE = Apollo - Near-Earth Asteroid

ApAl0NE = Apollo/Alinda - Near Earth Asteroid

IMB = Inner Main Belt Asteroid

Am-NE = Amor Near-Earth Asteroid

At-NE = Aten Near-Earth asteroid

(25143) Itokawa (Hayabusa, JAXA)

(65803) Didymos (bottom) and Diamorphos (upper-right) (DART, NASA)

Dimorphos (DART, NASA)

(101955) Bennu (OSIRIS-REX, NASA)

(162173) Ryugu (Hayabusa 2, JAXA)

(4179) Toutatis (Chang'e 2, CNSA)

(2867) Steins (Rosetta, ESA)

(433) Eros (NEAR-Shoemaker, NASA)

Using the unweighted rankings above, I have assigned scores to each candidate object. Like in golf, the lower the score the better. A brief summary is given in the section below withe score for each candidate given in the parentheses. For reference, Dinkinesh would have a score of five. SCORECARDS

Dimorphos (15) -- Is of the right spectral type, but is smaller, less reflective, rotates more quickly. Overall not the best analogue

Itokawa (12) -- Overall, probably one of the best analogues. A close match in terms of spectral class and only slightly more reflective than Dinkinesh. It's rotational period is the longest of all of the Near-Earth asteroids, though only about a quarter of Dinkinesh's assumed rotational period. It is also smaller than Dinkinesh, about half its size.

Bennu (25) -- Not a very good match in any of the parameters listed above. Bennu is a bit smaller than Dinkinesh, about 70% it's mean diameter. It is also much darker than Dinkinesh and belongs to the wrong spectral class, being a carbonaceous instead of stony asteroid. It also rotates much faster than Dinkinesh

Didymos (19) -- In some respects Didymos is a very good match. Like Dinkinesh it is an S-type asteroid and it is closest to Dinkinesh in terms of mean diameter. Unfortunately, Didymos rotates very rapidly, so much so that it has likely distored its shape into a somewaht elongated ellipsoid. Didymos is also darker than Dinkinesh.

Ryugu (19) -- In terms of size, Ryugu is a fairly close match to Dinkinesh, however its spectral class (carbonaceous) and thus albedo would appear to make it a very poor analogue.

Toutatis (16) -- Despite much larger than Dinkinesh (~3.5 times larger), Toutatis is a fairly good match to Dinkinesh in terms of its spectral type and slow rotation. Its semi-major axis and asteroid population , is also fairly close to Dinkinesh, with Toutatis being a Near-Earth asteroid whose orbit carries it out into the main-belt.

Steins (19) -- Steins appears to be a fairly good analogue candidate for Dinkinesh, though its score may not fully reflect that. It is located in the same region of the asteroid belt as Dinkinesh (the inner main belt). Steins is about eight times larger then Dinkinesh, a bit more reflective, spins faster and is a member of the related, bust somewhat distinct E-class asteroids, compared to Dinkinesh's apparent S/Sk classification.

Eros (20) -- Eros is by far the largest of the eight objects we looked at in Group A. It is a fairly good match in terms of its spectral class, albedo/reflectivity and general location in the Solar System, being largely between the otrbits of the Earh and the inner main belt.

Annefrank (13) -- While asteroid 5535 Annefrank scored second-best among all of the asteroid analogues we looked at, it was ultimately excluded due to the low-resolution with which it's surface was imaged.

Gaspra (17) -- Aside from being much larger than Dinkinesh, Gaspra is a fairly good potential analogue for Dinkinesh in terms of spectral type, albedo and population (both being inner belt asteroids).

Overall, as indicated by its score, I think that of all of the small asteroid (0.1-5 km) we have good, up-close imagery of, Itokawa is probably the best match to Dinkinesh in terms of spectral type, albedo and size.

PREDICTIONS:

SHAPE:

Light-curve data suggests that Dinkinesh has a some-what elongated shape with a ratio of equatorial axes being at least 1.43. If correct, this would appear to exclude an overall spherical or top-shaped profile (like Bennu and Ryugu). This makes sense, given Dinkinesh's slow rotation compared to Bennu and Ryugu, with both objects likely having been distorted by a period of even more rapid rotation in their pasts. Dinkinesh's slow rotation also, in my mind, reduces the likelihood of a fairly smooth, ellipsoid or egg-shaped body (like the rapidly rotating Didymos and its moon Dimorphos).

Due to it's fairly slow rotation compared to Didymos, Ryugu and Bennu, Dinkinesh likely hasn't experienced the same strong centrifugal forces which tend to shift away from the poles and towards the equator, possibly via intermittent landslides. This movement of material across the surface would likely erase any local depressions or concavities, creating a fairly uniform topology, with only slight variations due to impact cratering.

Thus, I suspect that Dinkinesh has a fairly rough, irregular surface, it's semi-elongated shape sculpted by moderately deep depressions, concavities and impacts. That's not to say however, that some on-going modification and in-filling has effected these features. However gravity, electrostatic transport and impacts are likely the primary drivers contributing to Dinkinesh's overall surface topology and shape.

If Dinkinesh's equatorial dimensions are indeed close to the 1.43 aspect ratio, it would be far less elongated than Toutatis (~2.09), Eros (3.03), and Itokawa (1.81).

I have also elected to portray Dinkinesh with a saddle or bean-shape, similar to other moderately elongated objects such as Itokawa and Eros.

Contact Binary?

Another factor to consider when concerning the overall shape of Dinkinesh is whether or not it is a contact binary. Contact binaries are not uncommon among minor celestial bodies such as asteroids, comets, Trojans and Kuiper belt objects. Famous examples include the KBO 486958 Arrokoth, 67P/Churyumov–Gerasimenko, 4769 Castalia, 216 Kleopatra and the Jupiter Torjan 624 Hektor. These candidate objects are typically highly elongated or possess a distinctive bilobate shape (in some ways similar to an hourglass or dog-bone).

Of the eight asteroids we have surveyed above, only Itokawa and Toutatis are confirmed or strongly presumed contact binaries. Of the eighteen candidate contact binaries listed on the following link https://en.wikipedia.org/wiki/Contact_binary_(small_Solar_System_body), there are some trends to consider. All these objects (with the exception of Arrokoth) are small in size and belonging to the Near-Earth populations of asteroids. This is likely due in no small part to the relative close proximity to the Earth, allowing higher resolution RADAR and light-curves to determine estimations of overall shape. They have mean diameters ranging in size from 4.5-km (8/P Tuttle) to 0.163-km (2013 US3), averaging ~1.19-km in diameter (median of 0.6-km). Next we will look at rotation periods for these objects, which range from as little as 4 hours to as much as 450 hours, with an average rotational period of 51.2 hours (median of 16 hours). Given an estimated rotational period of ~52 hours, Dinkinesh falls very comfortably within above ranges for both size and rotational period, making a pre-encounter determination of whether or not it's a contact binary from these paramaters alone (size and rotational period) difficult.

Another important factor to consider is equatorial aspect ratio. Generally speaking larger aspect ratios (well above one and preferably close to or exceeding a value of 2) can be a good indicator of whether an object is possibly a contact binary, though it can also result from redistribution of material on a rapidly rotating body. Since Dinkinesh is a fairly slow rotator, we can disregard that for the most part. Unfortunately we only have reasonable dimension estimates for one of these eighteen objects (2063 Bacchus, 2.6 x 1.1 x 1.1-km, aspect ratio of 2.36), thus we will have to look at a few other objects for more data. Itokawa (0.535 x 0.294 x 0.209 km, aspect ratio of 1.819), Toutatis (4.26 x 2.03 x 1.7-km, aspect ratio of 2.099), 216 Kleopatra (276 × 94 × 78, aspect ratio of 2.94), Hektor (370 × 195 × 195 km, aspect ratio of 1.897), Arrokoth (35.95 × 19.90 × 9.75 km, aspect ratio: 1.81). Together these objects possess aspect-ratios averaging 2.154 (median value of 1.998).

All things considered, if I had to guess (which I do in this case) I would say there is a roughly one-in-three chance of Dinkinesh being a contact binary, versus a two-in-three chance of it being a somewhat elongated, non-contact binary. The latter is the the shape I have chosen for this rendering. That being said, my limited and likely overly naive analysis did not yield a clear answer on this aspect of Dinkinesh's shape, I could easily see Lucy imagery prove otherwise.

CRATERS:

Given it's location in the Solar System, the inner asteroid belt, Dinkinesh has a good chance of being a fairly well-cratered surface. Impact velocities in the vicinity of Dinkinesh may be ~4.3 km/s while impact probabilities of 11.98x10^-18 collisions per km per year (P.S. Zain et al, 2020), roughly three and a half times the global average of the asteroid belt itself. This might suggest a fairly heavily cratered surface.

Looking to our list of potential analogues, we can seek out some trends which may help us tease out how many evident craters an object Dinkinesh's size might have and even, how large a crater on Dinkinesh is likely to get. The total number of observed craters and diameter of the largest crater in km and as a ratio of mean diameter for each of the most imaged bodies are as follows: Itokawa (37, 90m, 0.26), Ryugu (77, 290m, 0.31), Bennu (1500, ~220m, 0.37). Among these three sub-kilometer asteroids, the largest evident crater on their surfaces have diameters equal to a quarter to a third their mean diameters (0.25-0.37).

If these observed trends carry over to Dinkinesh, and assuming the estimated dimensions of ~960 x 670 x 670 meters is correct, the largest of Dinkinesh's craters should be around 200-250 meters across, in line with what has been observed on Itokawa, Ryugu and Bennu.

BOULDERS AND DUST:

A common feature of the sub-kilometer asteroids we have thus far visited with spacecraft and imaged extensively, are rubbly, fragmented and boulder-strewn surfaces. A possible trend among small sub-kilometer rubble-pile asteroids is the relationship between the longest dimension of the largest surface boulder and the mean diameter of the parent body. Itokawa has ~200 boulders larger than 10-m covering its surface, with the largest being about 50 meters across. Likewise Ryugu is fairly well-covered by boulders with roughly 820 larger than 5-m and the largest being ~116 meters in length. Bennu has 200 boulders larger than 5-m across, with the largest being about 58-m across. If we look at the ratio of largest boulder length vs mean parent body diameter, we find the following values for the three asteroids Itokawa (0.145), Ryugu (0.123) and Bennu (0.108).

The size-frequency distribution of boulders observed on these small asteroids have been observed to follow relatively simple power laws. The slopes of these power laws have been determined for the following objects; Ryugu (-2.65), Bennu (-2.9), Itokawa (-3.9), Dimorphos (-2.45) and Toutatis (-4.4). For this rendering I used an average slope value of -3.025 to depict the size-frequency distribution of the hypothetical boulders on Dinkinesh's surface. While I feel the existence of relatively large, discrete boulders on Dinkinesh's surface is a fairly conservative guess, it is possible that such features may be buried beneath layers of dust or fine regolith, or may be otherwise absent on a monolithic body, should Dinkinesh prove not to be a rubble-pile.

With a maximum expected resolution of ~2m/px, Lucy's flyby images will probably be insufficient to individually resolve the smallest assumed boulders and rocky fragments on its surface. That being said, it should be sufficient to at least reveal the overall texture of its surface, whether Dinkinesh has a rough, boulder-strewn surface or a smooth surface, mantled by a layer of dust or fine-regolith. I think it likely that Dinkinesh's surface will be some some combination of these two possibilities. Perhaps there are regions or outcrops of shattered boulders and stony fragments surrounded by areas of smooth, dusty plains. In this way, I think the surface of Itokawa, may, once again, be a good analogue of what I might expect Dinkinesh to look like. I must admit that I consider this more of a hunch than an expectation I can back with a great deal of a priori reasoning.

Of the sub-kilometer asteroid we've seen up-close, most had surfaces which were largely devoid of fine dust (Bennu, Ryugu and Dimorphos's surfaces were covered by shattered rock larger than a few centimetres in size.) Larger asteroids such as Toutatis and Eros where almost globally convered by thick mantles of fine dust and regolith. Itokawa and Didymos on the other hand had somewhat intermediate surfaces, existing somewhere between the two extremes. Rocky, boulder covered patches surrounded by regions of smooth, fine dust. I think Dinkinesh will more resemble the surfaces of Itokawa and Didymos than Ryugu or Bennu, but again that's just my hunch.

STRUCTURE: Given its size and slow-rotation I think there is a very good chance that Dinkinesh will be a rubble-pile, similar in some ways to Bennu, Ryugu, Dimorphos and Itokawa. Jourdan et al (2023) has suggested that analysis of Itokawa dust particles retrieved by the Hayabusa 2 spacecraft indicated that Itokawa may be far older than was assumed for rubble-pile asteroids, 4.2 billion years. The shock-absorbent nature of a highly porous and fragmented body, may make them very resilient to disruption by large collisions. By contrast, a monolithic kilometer-scale asteroid composed of a single solid fragment might have a lifespan of only 100 million years, with many of these, once fragmented, form rubble-piles of their own. As such, over the entire history of the Solar System, it seems likely that most small asteroids would be rubble-piles by this time.

AXIAL TILT:

Of all the parameters which could effect how Dinkinesh will appear during the day of the flyby, one I have not touched upon this far is the object's obliquity or axial tilt. In all my research, I have failed to find any real indication of what this value may be. If Dinkinesh has a high obliquity then, depending on where it is in its orbit around the Sun when Lucy conducts its flyby, only one hemisphere of the body may be visible.

Research by C. Lhotka et al, 2013, which sampled the spin orientations of 100 out of the 550,000 known asteroids at the time, suggested that the common range of obliquities may lie in a wide range of values from 9 degrees (low obliquity) to 170 degrees (nearly flipped over and with a retrograde rotation, like Venus). Their analysis suggests that the most representative value among the 100 asteroids sampled is ~75 degrees. There analysis showed that values between -20 to 20 degrees were uncommon among those asteroids sampled and values between -8 and 8 degrees were not represented. If we can confidently extrapolated from this small sample to the greater asteroid population, then it is probable that Dinkinesh has a prograde rotation, with an axial tilt of roughly 75 degrees (perhaps between 20 to <90 degrees.

At the moment I don't have any real good way of determining this value. For simplicity sake I have elected to portray Dinkinesh with either a near-zero obliquity/axial tilt or conversely around equinox. To showcase its predicted shape, I have also chosen to depict it largely broadside to the Sun, with its long-axis parallel to its direction of travel.

SUMMARY OF PREDICTED VALUES

Structure: Rubble-pile

Overall shape: Somewhat elongated, saddle-shaped, irregular

Equatorial aspect ratio: ~1.43

Contact binary: No

Surface: Bouldery, boulder-strewn, with large fragments and some to ubiquitous dust/fine regolith cover

Largest crater diameter: ~25-60% of long-axis diameter

Long-axis diameter of largest boulder: ~10-15% of Dinkinesh's long-axis diameter (~80-90 m)

Axial Tilt: Between 20-75 degrees

In conclusion, I think of the images of visited asteroids I have seen, I think this one of Itokawa in true colour, is likely the best representation of what I think Dinkinesh might look like.

ABOVE: Colour-composite image of Itokawa as captured by the Hayabusa spacecraft on 19 September 2005 (Courtesy: JAXA / ISAS / colour composite by Ricardo Nunes)

Thanks

Hello All,

Happy (Belated) New Year's! Tomorrow, January 24th, is the 37th anniversary of Voyager 2's historic flyby of the enigmatic ice giant, Uranus. Having exceeded its planned four to five year mission to explore Jupiter (1979) and Saturn (1981), Voyager 2 flew past Uranus on 24 January, 1986, with closest approach occurring around 17:59 UTC at a distance of 81,500 km (50,640 miles). To date, Voyager 2's encounter marks the first and only mission to conduct in situ observations and measurements of Uranus and its system of moons.

I'm currently working on a piece to commemorate this historic event, stay tuned...

Cheers

UPDATE: 01/25/2023:

I have uploaded a new digital painting to commemorate Voyager 2's 1986 flyby, you can see it here...

UPDATE: If you would like to see the revised, 10th anniversary version of this piece, you can check it out here.

Today, March 10th, marks the 10th Anniversary of "The Seventh Planet as viewed from Miranda", the first piece of Solar System-themed space art I posted to DeviantArt in 2012. While, overall, I am very happy with the way the piece turned out and the reception it has received over the past decade, there are a few aspects that I always wanted to change. To commemorate this 10th anniversary I have elected to upload a remastered version which will reflect all the corrections I wanted to make over the years.

I hope that the revised piece will better reflect a sense of realism and scientific accuracy while staying true to the original composition. The 10th anniversary remaster will be uploaded today, March 10th 2022 around 8:00 pm Eastern Standard Time.

The Original Piece

Hope to see you then...