Cheap Astronomy - Dear CA #141: Sorting The Universe

Sorting the asteroid belt

0:00It's the 365 Days of Astronomy podcast, coming in 3, 2, 1.

0:20Hi, this is Steve Nerlick. Why, why, why, why, why, why, why, why Cheap Astronomy? Yeah, why? And this is Dear Cheap Astronomy, episode 141, Sorting the Universe. In astronomy, it is often useful to put names to things, and to group things together, so we know which is this, and which is that.

0:52Such categorization could work at very large scales, like how we sort all the galaxies into the Hubble sequence, but we might also do something closer to home.

1:06Dear Cheap Astronomy, How are asteroids organized? The large majority of asteroids in the solar system are in the asteroid belt between Mars and Jupiter. There are maybe 1 to 2 million that are more than a kilometer in diameter, and probably millions more that are smaller than a kilometer in diameter. Nonetheless, there is a huge amount of space between Mars and Jupiter, so the asteroid belt is nothing like that bit out of the Empire Strikes Back.

1:39If you stood on one asteroid, it's unlikely you'd be able to see another one, and the odds of successfully navigating the asteroid belt are about 1 to 1. Inwards from the asteroid belt, there is one known asteroid that orbits within Venus's orbit, and 22 within Earth's orbit. There's also some with a perihelion, that is, the point of their orbit closest to the Sun, that's within Mercury's orbit, which are hence called Mercury crosses,

2:11and then there's Venus crosses, which include all the Mercury crosses, as well as yet more with perihelions that are within Venus's orbit, but not Mercury's orbit. And then, following the same logic, you have an even larger number of Earth crosses. All these, and some close-by Mars crosses, fit within the general category of near-Earth asteroids, of which there are just over 2,400, also including some Earth-Trojan asteroids,

2:43which follow the Earth's orbit at the Sun-Earth L4 and L5 Lagrange points. And please refer to a whole bunch of other Cheap Astronomy episodes to find out what the heck that means. But once again, there are millions of asteroids in the asteroid belt, between Mars and Jupiter, and about 2,000, that get any further in. Out beyond the asteroid belt, you do find a few more asteroids, that are still mostly within the orbit of Jupiter,

3:14but beyond that, you still get lots of objects, but they tend to be more ice than rock, and so are arguably comets rather than asteroids, even though they won't develop a tail unless their orbit takes them closer to the Sun. Further out than that, you have to start talking about the Kuiper belt, where we start calling things Kuiper belt objects rather than asteroids. But once again, all the asteroid groupings we've mentioned so far

3:45are mostly afterthoughts to the asteroid belt. Within the belt, we can define inner, middle, and outer main belt asteroids, which have been largely organised by the influence of Jupiter's gravity, which creates what are known as Kirkwood gaps between the inner, middle, and outer asteroid belts. And at a lower level of detail in the asteroid belt, we've defined asteroid families, which are essentially clusters of asteroids with similar compositions,

4:19and which are probably the products of earlier collisions between predecessor asteroids. One of the biggest named asteroid families is in the inner asteroid belt, called the Flora family, of which the biggest member is Flora, at over 140 kilometres in diameter, which is thought to be a 60% remnant of one of the predecessors that underwent a collision, while the smaller members of the Flora family are smaller fragments of that collision.

4:49One of those is 965 Gaspra, which is about 18 kilometres in diameter, and we know what it looks like up close, since the Galileo spacecraft flew by it in 1991. Much of the total population of meteorites that land on Earth are thought to originate from the region of the inner belt that the Flora family lies in, and it's thought that some of the debris from the impact that created the Flora family

5:20has made its way to Earth in the form of L-chondrite meteorites. There's also some thinking that the collision that created the Flora family was fairly recent, say in the last half billion years. That prompted some thinking that the Flora-related collision might actually be the origin of the dinosaur-ending Chixcalaub impact on Earth 65 million years ago. And yes, OK, we mean all the dinosaurs apart from the birds.

5:50When we say there's some thinking, that's code for someone published a paper on it, and so far it hasn't been widely embraced by others. Everyone is pretty settled on the idea that the Chixcalaub impactor was definitely an asteroid, based upon the high iridium, and also ruthenium, that's associated with both the impact debris that's spread around Earth, as well as drill cores from the crater itself at the Yucatan Peninsula in Mexico.

6:23But, who knows, perhaps we'll figure it all out one day. And we'll be able to say with confidence that that rock came from there, and not from there.

6:35This is the middle bit. Organising things into categories does help us to speculate on how particular phenomena came to be, as well as planning for the future, where we might need to decide which kind of asteroids we should crash on the moon, for example. And even in the more speculative area of whole-of-universe cosmology, it turns out that categorisation can also be useful.

Primordial black holes and dark matter

7:07Dear Cheap Astronomy, A Primordial Black Hole's Dark Matter Candidates Well, yes, although they certainly aren't the only candidates. To recap, a primordial black hole is a hypothetical object which originated in very primordial times, probably within the first second of the universe. It is somewhat puzzling that when the Big Bang apparently brought lots of

7:38mass-generating particles into existence within a very small volume and at an extremely high density, the whole universe didn't just form a black hole there and then, which would have been a quick end to the story. But not only did things with mass suddenly come into being, they were all flung outwards as well. Although that primarily involved them moving with, rapidly expanding space-time, rather than actually being flung

8:09relative to space-time. But perhaps within that initial sub-second inflation of the early universe, there might have been a few ripples and glitches that did result in a few massive particles being sufficiently clumped together to form black holes. Although after that first second, everything had expanded too much for that to remain possible. potentially those primordial black holes ranged from the very small to the very large.

8:40However, really small ones would have all since evaporated through Hawking radiation over the subsequent 14 billion years of the universe's lifetime. So, if there are still primordial black holes around now, there's probably none smaller than a tennis ball, a tennis ball with the mass of a large-ish asteroid. It has been hypothesized that primordial black holes may have been the seeds of what are now supermassive black holes

9:10found at the center of most galaxies. But apart from that, all other black holes we can observe seem to be clearly the result of big stars collapsing under their own mass. So, if primordial black holes are actually dark matter, it would need to be the case that there are an awful lot of tennis ball-sized ones floating around undetected, remembering that there's apparently around six times as much dark matter as there is visible matter.

9:42Of course, primordial black holes could be bigger than tennis balls, and hence wouldn't need to be quite as numerous, but then you have to start wondering why those bigger black holes aren't occasionally visible insofar as they should block out the background star field. So, if primordial black holes are to be a dark matter candidate, then most of them pretty much have to be tennis ball-sized. A recent paper outlines how some primordial black holes might have

10:12colour charge, which is one of those quantum attributes assigned to subatomic particles, where colour apparently relates to how quarks interact with the strong force, and colour charge indicates some imbalance between quarks and gluons. The key point is that if primordial black holes do have colour charge, then that might make them detectable. This is the primary job of theoreticians. The current standard model

10:43of the universe says that 26% of it is dark matter, even though no one has a clue what dark matter is. But if the theoreticians say it might have colour charge, then the boffins who look through telescopes can start looking for colour charge signatures. After all, there could be a Nobel Prize in it if they do find those signatures, although there may be a lot less in it if they don't find them. The paper also suggests that most primordial black holes

11:14that might possess a colour charge were probably very small, that is, sub-tennis ball size, and hence would have mostly evaporated over the last 14 billion years. But, as is always the case in our universe, if you look far enough away, then you are looking back in time. So we could still pick up evidence that those colour charge primordial black holes were there back then, which might then substantiate that A,

11:45there really have been primordial black holes, and B, that they're out there in sufficient quantity to account for dark matter, or otherwise to account for a fair proportion of it. Not sure this is an outcome you want to hold your breath for, but that's the nature of science. We check out all the possibilities, and steadily rule most of them out, until we're left with just one, hopefully correct, option. This is the end bit.

Categorizing the universe

12:17So, there you go. We can even sort hypothetical objects into hypothetical categories, and in doing so, get a bit closer to a solid understanding of what's actually going on out there, well, at least a bit. But that's it for another episode of Dear Cheap Astronomy. If you've got a space science question, or you just want to sort something out, why not write to

12:48cheapastro at gmail dot com, and we'll find the right spot for it. Thanks for listening. Steve Nellick, Cheap Astronomy. You are listening to the 365 Days of Astronomy podcast. Cool.

13:19The 365 Days of Astronomy podcast is produced by the Planetary Science Institute. Audio post-production is by me, Richard Drumm. Project management is by Aviva Yamani, and hosting is donated by LibSyn.com. This content is released under a Creative Commons attribution, non-commercial 4.0 international license. Please share what you love, but don't sell what's free. This show is made possible thanks to the generous donations

13:50of people like you. Please consider supporting our show on patreon.com forward slash CosmoQuestX and get access to bonus content. Without your passion and contribution, we won't be able to share the stories and inspire the worlds. We invite you to join our community of storytellers and share your voice with listeners worldwide. As we wrap up today's episode, we're looking forward to unraveling more stories from the universe. With every new discovery

14:20from ground-based and space-based observatories and each milestone in space exploration, we come closer to understanding the cosmos and our place within it. Until next time, let the stars guide your curiosity. intéreme up into a Following day! ee o o o o o o o o o ee o o o

14:51o o o o o o o o o o