Andrew Dunkley: Hi there. Thanks for joining us. This is a Q
and A edition of Space Nuts. Not only do
we talk astronomy and space science, we
pretend to answer questions from our
wonderful, uh, audience. We've got a bunch
today. Bill, uh, is asking about small
bodies in solar systems.
I don't think that's got anything to do with
weight loss, but we'll see. Uh, Peter is
asking about, uh, leaving something behind
that could survive the destruction of Earth.
Wow, that's a what if question. Tiny moons
and giant planets and issues with a
giant space station. Those are, uh,
questions we will endeavour to answer today
on this edition of Space Nuts.
Professor Fred Watson: 15 seconds. Guidance is internal.
10, 9. Ignition
sequence start. Space Nuts. 5, 4,
3, 2. 1. 2, 3, 4, 5, 5,
4, 3, 2, 1. Space Nuts
astronauts report it feels good.
Andrew Dunkley: And with us again is Professor Fred Watson
Watson, um, astronomer at large. Hello,
Fred Watson.
Professor Fred Watson: Hello, Andrew. Uh, fancy seeing you here.
Andrew Dunkley: Yes, unusual. Both wearing black.
Professor Fred Watson: Is it black? Uh,
it is, yeah.
It's the, um, this is the, the shirt
that if I have Jordy sitting on my lap, you,
you can't see him at all because it's exactly
the same colour as he is. Jet black.
Andrew Dunkley: Uh, that'll make a good Instagram photo.
Professor Fred Watson: Well, it might do. Just two eyes poking
out.
Andrew Dunkley: Now, um, we've got a lot to get through, so
we'll start straight away with, uh, a
question that comes from Bill
and he asks if small bodies in the solar
system formed by accretion of fine dust
and gas, why are they not all
fluffy, low gravity powder
puffs? Um, we're
all dense stony. Uh, or, uh, were
all dense stony or metallic objects
originally part of a larger body that could,
could differentiate under, uh, decent
gravity levels, then were smashed to small
pieces in collisions. Uh, thanks for the
great podcasts. Uh, that comes from Bill.
Um, so, yeah, why isn't everything
puffy?
Professor Fred Watson: Um, I think it was to start with. Well, there
you go. Yeah. So,
um, people often say,
people who should know better often say that
if you want to know how planet formation
starts, look under your bed because
the bits of fluff that you tend to find under
your bed are, uh, made of dust
sticking together, uh, usually by
electrostatic forces, which we think played a
part in the early, uh, evolution of planets.
Uh, these things stick together. You build
up bigger and bigger fluff balls. Um,
and eventually the
fluff balls, because. Exactly,
um, as Bill says, they do tend to collide
with one another. We're now talking about a
very, a very, very
dense, dusty environment. We're talking about
the protoplanetary disc that Surrounded the
sun. Uh, very dusty place
with lots of, um, basically
lots of capacity for uh, dust
fluff balls to build up to have bigger and
bigger sizes. Eventually
these various forces uh, will
cause the dust balls to sort of
collapse. Probably collisions will contribute
to, um, by that I mean that they
tend to lose their porosity. In other words,
they become more solid.
Um, now having said that, there are objects
in space that we know are very
porous.
Andrew Dunkley: Um, well we've found powder puff planets,
haven't we?
Professor Fred Watson: Yes, that's right. Uh, yes, almost exactly
a good description of them. I'm just thinking
more nearer to home though. Um, uh,
uh, Phobos, the larger moon of Mars,
is thought to have a composition a bit like
um. God, the word's gone.
Uh, stuff that forms when
eruptions, um, take place
underwater.
Andrew Dunkley: Like a honeycomb.
Professor Fred Watson: Yes, um, but it's got a word. Oh, that's
ridiculous. When you get to a certain age,
words just disappear. It'll come to me
in a minute. I know. Uh, but yeah,
the stuff that floats on the water
underground eruption. Pumice. The very word.
That's what I was looking for. Thank you.
Thank you, Andrew. So pumice is, you know,
it's porous, it's a stony
structure, uh, that's got a lot of gaps in it
and I guess that might well be an
intermediate structure of many of these
objects. Ah, as I said, Phobos is like
that, um, one of Saturn's moons and I
can't remember which one it is, it's the one
shaped like a potato. That'll probably come
to me in a minute as well. Uh, it's also
got that sort of structure. Um, so
maybe, you know, when you get things like
that colour colliding, uh, then
and building up in size, then you're
eventually going to get to this situation
where gravity takes over, uh, and it
pulls um, these low
density materials into something
more solid.
Andrew Dunkley: Um, is it a malthea, A mouth?
Professor Fred Watson: No, uh, it's one with a better known name.
Ah. It's very highly cratered and
potato shaped.
Andrew Dunkley: Ah, okay.
Professor Fred Watson: It's uh, yeah, it's one of the most cratered
objects in the, in the solar system. I'm
annoyed. I can't remember it. It's
ridiculous. I was getting too
old for this, Andrew.
Andrew Dunkley: Oh no you're not.
Professor Fred Watson: No, no, maybe I'm not.
Andrew Dunkley: No, it keeps your brain active.
Professor Fred Watson: Well, except it's demonstrating quite
clearly, uh, that the memory banks are
disappearing. Anyway, um, it'll come to me,
as I said, in a minute. It's not Enceladus,
but it's something like that. Uh, so, uh,
if you've got, you know, gravity taking over,
then you' to get basically solid rock
emerging from that. Uh, uh,
as Bill says, dense, stony or metallic
objects, that's basically what they turn
into. And then they collide. Uh,
um, the larger objects are differentiated.
That means the heavy stuff sinks to the
middle. Uh, exactly as Bill says, but they
collide. And that's how you can get stony
meteorites or metallic
meteorites because the metal tends to sink
mostly, uh, to the middle. So I
think, um, it is a natural process, but it's
one in a way it's counterintuitive to us. You
know, how do you get from a dust, a fluff
ball under your bed. How do you get from that
to a stone to a rock?
Andrew Dunkley: Gravity.
Professor Fred Watson: Yes, that's right, gravity. But over a long
period of time. Uh, and probably heat as
well. You know, you've got heat processes
coming into this too. So, um,
uh, I, uh,
think uh, what um,
Bill's saying is right. If the small bodies
in the solar system formed by accretion of
fine dust and gas, why are they not all
fluffy, low gravity powder puffs? Well, some
of them are and that's. Perhaps you could
describe them like um,
Phobos. Uh, perhaps he could describe them as
unevolved. They haven't evolved much.
I think it might be Hyperion, the one I'm
thinking of.
Andrew Dunkley: Okay.
Professor Fred Watson: I think it might be Hyperion. I'll have a
look, Have a look, See if it's um, shaped
like a potato and got lots of graters on it.
Andrew Dunkley: Yeah. Well, there's got to be a photo of it
somewhere. Yes, it is.
Professor Fred Watson: Yeah.
Andrew Dunkley: Yeah.
Professor Fred Watson: Okay, good.
Andrew Dunkley: It's got, it's got that big, um.
It's got a massive crater in it actually.
Professor Fred Watson: Yeah. Yes, yes. Yeah, yeah.
So, um, so these are you got
Andrew Dunkley: there in the end, Fred Watson.
Professor Fred Watson: In the end? Yeah, it's. Yes. It's just the
processing speeds down a bit. I must be
offline or something like that. Probably need
a reboot. God, don't say that. Might
never come back.
Andrew Dunkley: Well, that's happened. My car did that while
we were away. Uh, came home
and to, uh, to, to stop falling asleep. We
decided we'd go and do the groceries straight
after getting off a long haul flight.
Professor Fred Watson: Oh, yes. Yeah.
Andrew Dunkley: And the car wouldn't start.
Yeah, the battery died, so.
Professor Fred Watson: Oh, the battery died.
Andrew Dunkley: Yeah. That's another 315 bucks. Thank you
very much.
Professor Fred Watson: Yes.
Andrew Dunkley: Anyway, it happens. It was four years. It
lasted four years.
Professor Fred Watson: Oh, that's all right. That's about as long as
you get from a battery.
Andrew Dunkley: Yes, it is indeed. But thanks, Bill, for the
question. Uh, I think you answered it
yourself, but, um, y, uh, although
if you're, um, someone like me,
um, and you don't clean under the bed, uh,
you can watch planets evolve.
That's what's happening.
Professor Fred Watson: Yep, you can.
Andrew Dunkley: All right, uh, thanks, Bill.
Our next question, uh, is
coming from Peter.
Speaker C: Hello, this is Peter in Lamington
Spa. And I want to know
what would it take for
humans to
make something that will survive
the destruction of Earth
and then potentially be incorporated
into a new planet when all the bits of Earth
become a different planet and sometime in the
future, Is it possible?
Andrew Dunkley: Have a good evening. Thank you, Peter. That's
a what if question. Uh, yeah, I wonder.
That's a very. It's a long haul science
fiction situation. You build something
that will survive the destruction of Earth
and then somehow the planet reconstitutes
itself and billions of years later there's an
intelligent race living on the planet and
they go, oh, hello. What's all this then?
Professor Fred Watson: This is some leftover of humankind? Yeah,
whatever they were.
Andrew Dunkley: There was a TV series that I watched many
years ago called Childhood's End, and
it was about the destruction of Earth. And
before it, before it was destroyed,
um, the humans asked the aliens
that rescued the children, basically, um,
can we just leave something behind so they
know we were here? So they left. Music.
Professor Fred Watson: Lovely. I like that. Yes, I like that very
much.
Andrew Dunkley: I just spoiled the whole thing too, by the
way.
Professor Fred Watson: Um, I don't think you did really, uh,
because. Yes, that's a kind of concept, isn't
it, that you're leaving behind? Uh, and
my mind, when I read Peter's question or
heard Peter's question, went to
more concrete things, not necessarily made of
concrete.
Andrew Dunkley: I was about to say that.
Professor Fred Watson: But in a sense we've already done it, Andrew,
because there are five
little spacecraft which are,
ah, absolute, um,
uh, monuments to humanity leaving the
solar system, um, way, way beyond
the orbit of Earth. Voyager 1 is
probably beyond. Actually,
that's not quite true. I, uh, was going to
say beyond the limits of the sun. When it
turns into a red giant star, um,
Voyager 1 will probably survive,
um, the red giant phase of
our sun, uh, which will take place in
a few billion years, three or four
billion years. Um, it'll survive that,
but might not survive the formation of
a planetary nebula when you've got hot gas
coming off the, uh, being puffed off the
surface of the red Giant. It might actually
melt in that because it's because planetary
nebulae get to be light years in diameter.
Uh, our Voyager, uh, is only, well, it's
nearly a light day away. Um, on the other
hand, we've got 3 or 4 billion years to play
with because the sun's not going to do
anything really nasty, um, within that
time. So yes, Voyager 1 will be well out of
the way, probably will survive the
eventual evolution, um, and
uh, final evolutionary stages of the sun when
it actually turns into a white dwarf star. So
yes, uh, those spacecraft, Voyager
1, Voyager 2, Pioneer 10, Pioneer 11,
is that right? And New Horizons,
they're the five that are leaving the solar
system which will probably outlive humanity.
Andrew Dunkley: They probably won't. They probably won't be
the last.
Professor Fred Watson: They won't be the last. No, I think that's
right. Uh, but,
um, I mean
the idea of, um, the Earth, uh,
being destroyed, the
kinds of things that might destroy the Earth,
ah, are first of all, that eventual
evolution of the sun to a red giant star that
will almost certainly melt the Earth because
the Sun's, um, surface, put
it that way, will be, um, a quarter of a
mile from the Earth. And we might be on the
inside of, uh, could even,
uh, overtake the planet Mars. Uh,
so it's hard to imagine how you'd rebuild
the Earth, uh, from the debris that is really
just molecules, uh, because it'll have been
vaporised. Um, so I think, uh,
in addressing this question, you've really
got to think about things that uh, have
left the Earth. And that really basically
pushes your mind to spacecraft. There are
some spacecraft which are, uh, in orbit
around the sun, uh, which
are spacecraft that have been sent
exploring the inner solar system. Mostly
these days we try and get rid of them. We
plunge them, uh, into either, uh, Jupiter or
Saturn. Jupiter in the case of Galileo,
Saturn in the case of Cassini. Uh, those
spacecraft were destroyed purposely so that
they didn't accidentally land on one of the
moons of Jupiter or Saturn and leave microbes
behind. Um, so you're really talking about
something that's left the solar system. And
that leaves those five spacecraft
I've mentioned. And they definitely will
outlast humankind.
Andrew Dunkley: Okay, there you have it, Peter. Um, so we've
already done it kind of, uh, I don't think
you could probably build some kind
of monolith or something that would survive
the red giant phase of
the sun and, and overwhelm Earth. That would
all get destroyed, um, unless you did
it deep down inside. But I don't even Know,
if you could do that, I think a
red giant phase would be pretty cataclysmic,
wouldn't it?
Professor Fred Watson: Yeah. Uh, yes. If your planet's being
vaporised. Your planet's being vaporised. It
is, yeah.
Andrew Dunkley: Indeed. Thank you, Peter. Great to hear from
you. I love what if questions. So, um, thanks
for serving it up. This is Space Nuts with
Andrew Dunkley and Professor Fred Watson
Watson.
Professor Fred Watson: Swiften Tranquilly Base here.
The eagle has landed. Space Nuts.
Andrew Dunkley: Our next question, Fred Watson, comes from
Martin in Heswall. Heswall, is that right?
Professor Fred Watson: Yes, Heswall. Yeah.
Andrew Dunkley: Where's that? I'm going to guess it's the UK
somewhere.
Professor Fred Watson: It is indeed. It's, um, uh, on the Wirral
Peninsula. So if you think of Liverpool,
you've been to Liverpool? I have. And done
the Beatles experience.
Andrew Dunkley: Yes.
Professor Fred Watson: Is that right? Yes. Yeah. Well, across the
River Mersey from Liverpool is the Wirral
Peninsula and Heswall is
one of the towns on that, um, I've said
before, and in fact we've had listener
comments about this, I had a girlfriend once
who lived on the Wirral Peninsula and so I
used to be a very regular visitor there to a
village called Barnston, which was not that
far from Heswall. There you are. All right.
It's very pretty too. It's a pretty village.
Hmm.
Andrew Dunkley: Okay. Just wanted to know where you
were, Martin. So thank you for that. Uh, I
hope, uh, you'll answer this question.
According to Wikipedia, there are, uh, now
known to be 292 satellites,
uh, with confirmed orbits around Saturn.
Presumably, many of these moons are very
small. So is there a minimum size for
an object to be called a moon? And is
there a minimum size for an object to
maintain a stable orbit around a planet?
Uh, as all the giant plan have ring
systems, would the smaller particles just
be absorbed into the rings? Conversely, I
suppose that many objects could be knocked
out of the rings to form independent
satellites that may become permanently
separated from the rings. Will Saturn get,
uh, to 1,000 moons or more?
Also, uh, can you recommend a website
which has the latest data about, uh, the
solar system, as the numbers vary from one
site to the next, no doubt due to how recent
the information is. Keep up the work. Thanks,
Martin. Um, that's a good question because,
yeah, we know that the ring systems,
um, are full of dust and ice, but they've
also got larger objects that are referred to
regularly as moons.
Professor Fred Watson: Um, it's interesting, this was one of the
exact questions that came up in the Q and
A night, the science in the pub night that we
had on Lord Howe island at the Dark Sky
Festival.
Andrew Dunkley: That was Martin. He was, he was there.
No he probably wasn't.
Professor Fred Watson: Carry on. There was a Martin
there.
Andrew Dunkley: Did he have a British accent?
Professor Fred Watson: Uh no, he's quite Australian but
he's not from Haswell. But yeah, uh,
but interesting coincidence to get the two
and I don't think at the moment there is
a uh limiting size
to differentiate between a ring particle
and a moon. Um
so uh, it
is a great question um how
do you define a moon around a planet which is
festooned with objects orbiting around it
in the form of rings. So we think the rings
of Saturn are uh the debris
of uh probably a satellite
that came within the Roche limit
of the the planet. The Roche limit
being the point at which uh a solid
object can't actually survive within
that distance. In other words that close to
the planet. Um and so
it broke up into lots of small particles.
Probably the biggest ring sized
sorry the biggest ring particles
uh, uh in the region of 10 metres
because the rings themselves are only about
100 metres thick. Yeah it's quite
staggering.
Um and 250,000 kilometres
in diameter. So yes it's quite a contrast.
Um a sort of blade of material in space is
the way I've always described it. Uh
but some of the smaller satellites of
Saturn and some of them are actually embedded
in the ring system. Uh some of them
are measured in
single digit kilometres so they're not that
much more than
the biggest ring particles and I'm not sure
that there is a definition between the two.
Uh and in a sense you could say that every
solid object within Saturn's rings is a
satellite and so you're then talking about
millions or maybe even billions of
moons of Saturn. Uh it's a great
question and one that I don't have an answer
for and I perhaps ough who have checked it
out in the wake of the question that came uh
at the science in the pub science in the bolo
on Lord Howe island the week before last when
we did the Dark Sky Festival there.
Andrew Dunkley: Yeah, yeah he also
asked about uh website. Yeah ah
well I just did a quick cheque and top uh of
the tree is NASA for up
to date solar system information.
Uh yeah, uh but the other ones that you could
try uh the sky
mylive.com apparently is
very highly rated um says
it offers comprehensive information about the
most interesting celestial objects and sets
tools designed to support the exploration et
cetera. Uh the planets today
uh is also there and there's a specific uh
NASA page that you can look up
called Eyes on the Solar System. Um,
and it provides a 3D solar
scape if you like. So there's a few
ideas if you uh, want to um, chase them
up, Martin. But um, there'd be plenty more
out there. There's um, just to name a few
more, Global Solar Atlas,
um, the NOAA homepage,
the Space Weather Prediction homepage, um,
and Planet Labs just to name a few. So
um, and they're constantly being updated as
far as I'm aware, as things change or as
new things come to light. So might be worth
chasing all of those up because they do seem
to be, um, highly credentialed. Fred Watson?
Professor Fred Watson: Yeah, I was going to say I usually go
to NASA when I want the latest figures on
this sort of thing. M. So,
uh, you've confirmed that and also given a
few other options as well, which is good.
Andrew Dunkley: Yep, plenty to look at. There's lots of great
sites out there. Just don't go to the ones
that start with um, words
starting with F and e.
Professor Fred Watson: I was thought you were going to say don't go
to ones that start with space and have knots
in the.
Andrew Dunkley: That's, that's good advice too.
Professor Fred Watson: Yeah.
Andrew Dunkley: Okay, thank you, Martin.
Our final question today comes from
Finn.
Speaker C: Hello, Andrew and Fred Watson. It's Finn from
NAN in the Adelaide Hills in South
Australia. And a happy May 4th to you as
well. I was watching a 40 year old
documentary the other day about a space
station orbiting a planet. And this
space station, um, if it was to orbit
the Earth, I would like to know how that
would affect the orbit of our
moon and maybe the orbit of the Earth
around the sun. This space station being 150
kilometres diameter with a mass of about 10
to the 15 tonne. Um, I'd
like to know, and if for whatever reason this
space station happened to destroy our planet,
how would the rest of the planets in the
solar system be affected by that
destruction? One last question,
um, to you both is, um, what was the first
animal in space?
Andrew Dunkley: It was a dog.
Speaker C: Ah, ah, don't think it was that. It was
actually the cow because it jumped over the
moon. Thank you.
Professor Fred Watson: Dear, oh dear.
Andrew Dunkley: Finn. That was probably one of the worst dad
jokes I've ever heard. So,
but you know, most welcome on this show.
Professor Fred Watson: Yeah, it was good. It wasn't, wasn't even
adequate that one, was it?
Andrew Dunkley: No, it wasn't.
Professor Fred Watson: No, no.
Andrew Dunkley: We strive for adequacy and we didn't even
achieve that. Thank you, Finn.
Uh, so the substance of his question was,
uh, you got a space Station orbiting Earth
at 150 kilometres in
diameter or whatever. Uh what kind
of effect could that have on the orbit of the
Moon? And ye
get to the next part of the question after
that. Could it have, would that have any.
That's pretty big.
Professor Fred Watson: It's well the critical thing was the mass
which um Fin actually
mentioned as being 10 to
the 15 tonnes I think is what he said
uh which is 10 to the 18
kilogrammes. Um and
so the bottom line is that's not enough
uh the Earth just. Yeah, nah, ah forget it.
So the earth's uh six times 10 to the
24 kilogrammes. So it's
um, what is it? It's
six uh orders of magnitude bigger in
mass than uh this space station
and so the other. So
it's certainly not going to affect the orbit
of the ah Earth. It might perturb
the orbit of the Moon a bit.
Uh one of the considerations will be how far
away is it from the
Earth? And uh, our last question
actually pointed to an answer to that
and that is that if it's 150
kilometres in diameter it has to be a long
way away or else it's within the Roche limit
of the Earth uh and it would just break up
straight away. So I'm not going to guess how
far away it has to be but it'll be a long way
the earth if it's 150 kilometres in diameter.
Um so that again um, basically
mitigates any effects it might have
on the orbital dynamics of the Earth. It
certainly wouldn't affect the Earth's orbit
around the Sun. Might just tweak the
Moon's orbit around the Earth a bit.
Uh wouldn't cause the demolition of the
Earth. The orbits of the other planets
wouldn't even bother to, to take any
notice of it. Uh they are too
stable compared with uh, a
thing of that mass and that far away from the
Earth. So ah it's an interesting
thought uh and one that I
think um we can say yes you could have a
space station 150 kilometres in diameter
weighing 10 to the 15 tonnes uh and it
probably would not affect the status quo
terribly badly.
Andrew Dunkley: Okay, there you go. Uh and I just did a quick
cheque but um, there's not much information
about how fast space stations have to be away
to avoid the Roche limit. But a solid object
such as a Rocky body
with 150 kilometre diameter would have to be
at least 141,000 kilometres
away from Earth. Um probably better off being
over 200,000 kilometres away.
Professor Fred Watson: Yes that's the sort of distance I had in
mind. Something like that. Three times as
far away as the, the geostationary
satellites. Huh.
Andrew Dunkley: Are. There you are now.
He had a second question as to what would
happen to the other planets if Earth was
destroyed, no longer existed. I think we've
been down this road before and I can't
remember the answer.
Professor Fred Watson: Yeah, so, um, the other planets
would more or less stay in the present
orbits. Those orbits would be
perturbed, uh, differently from what they are
now. So perturbations are the gravitational
effects of other bodies in the solar system.
Uh, when you look at the way
things are in orbit, you start off with a two
body problem with the sun and your object in
orbit. But then you modify it by
taking into account the gravitational
attraction of other bodies and it becomes a
three body problem and then four body problem
and all the rest of it. Now,
that three body problem would change if the
Earth wasn't there. Um, or the N body
problem, I suppose it would be a solar system
with seven planets rather than eight. Uh,
that would change the dynamics of the planets
a little bit, but they would basically
remain in their present orbits, uh, with
just changes to the orbit rather than the
orbits being destroyed.
Andrew Dunkley: So, uh, in other words, if Earth disappeared,
no great loss.
Professor Fred Watson: No, not really. I mean, uh, you know, Douglas
Adams had it in one. Mostly harmless. Mostly
harmless, that's right.
Andrew Dunkley: Thanks to the white mice.
Professor Fred Watson: Yes, that's right.
Andrew Dunkley: I wonder how all the other mice felt about
that. You know, it was
musculus. Racism. That's what it was.
Professor Fred Watson: It is, yes. Mass racism. Exactly. So,
yeah, thanks.
Andrew Dunkley: Uh, Finn, great question. We always love
these what ifs. So, um, if you'd like to keep
sending in questions like that, or if you've
got something deadly serious to discuss with
us, like, uh, I don't know, exploding
rockets and whatever else, uh, you can send
them in to us. Uh, just go to spacenuts
IO or spacenutspodcast.com,
click on the Ask me anything button.
You won't be asking me, you'll be asking him.
But, uh, I'll read it out or you can send us
audio question. As long as you've got a
device with a microphone, you're all set. Uh,
and while you're there, have a look around.
Uh, that brings us to the end. Fred Watson,
thank you very much.
Professor Fred Watson: Great pleasure, Andrew. Always good to chew
the fat. And, uh, I, uh, hope we'll do it
again soon.
Andrew Dunkley: We will.
That's Professor Fred Watson Watson,
astronomer at large, part of the team here at
Space Nuts and thanks to Huw in the studio.
Uh, who couldn't be with us today. He was
seeing his dietitian after he reached 10 to
the 15 tonnes.
I'm surprised he survived. And from me,
Andrew Dunkley. Thanks for your company.
We'll be back again soon with another episode
of Space Nuts. See you then.
Professor Fred Watson: Bye. Bye.
Andrew Dunkley: You've been listening to the Space Nuts
podcast, available at
Apple Podcasts, Spotify,
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