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Black Holes, Navigation, The Big Crunch & Re-Entry Speeds: Your Cosmic Questions Answered |... · Space News Today

0:00
Hi there. Thanks for joining us once
0:02
again. This is a Q&A edition of Space
0:05
Nuts. My name is Andrew Dunley. Great to
0:06
have your company. Uh we will be
0:08
answering audience questions exclusively
0:10
on this episode. We'll never do it
0:13
again. Yes, we will. Uh we're going to
0:15
be um looking at uh a concept that Ross
0:19
has put up about black holes equaling
0:22
dark matter. We'll uh we'll explain that
0:24
or he will and we'll try and tear it
0:26
apart. Uh Sandy is asking about
0:29
navigation in space. Uh John is talking
0:33
relativity, time, black holes, and the
0:36
big crunch. I knew we'd get a question
0:39
about the big crunch because we talked
0:40
about it so recently. And the speed of
0:44
re-entry is a question from Andy. We'll
0:46
deal with all of that on this episode of
0:49
Space Nuts.
0:50
>> 15 seconds. Guidance is internal. 10 9
0:55
ignition sequence start.
0:57
>> Space nuts.
0:58
>> 5 [music] 4 3 2
1:00
>> 1 2 3 4 5 5 4 3 2 1
1:02
>> Space Nuts
1:04
>> astronauts report. It feels good.
1:06
>> And with us once again is Professor Fred
1:08
Watson, astronomer at large. Hello Fred.
1:11
>> Hi Andrew. Good to talk again. Uh seems
1:14
like only a few minutes ago that we were
1:16
talking.
1:17
>> It does, doesn't it? Yes. Extra.
1:19
>> That's called relativity I think. Time
1:20
dilation.
1:21
>> Yeah. Um, I I'll tell you something
1:23
funny. We we had our granddaughters
1:25
around last night. They were supposed to
1:26
stay the night, but they both
1:27
chickenened out, so dad had to come and
1:29
pick them up at 9:00, but um uh they
1:32
were they were um having a bit of fun
1:34
and um they like doing craft. And one of
1:36
them built a telescope with a piece of
1:39
paper out of and and was looking through
1:41
it and Judy said, "What have you made?"
1:42
And it was the four-year-old. She said,
1:45
"I I I made a a looking through thing."
1:50
That's what a telescope is. It's a
1:52
looking through thing. [laughter] Um,
1:54
which is nearly what they were
1:56
originally called before the telescope
1:58
was in 10. Yeah,
2:00
>> she's ahead of her time.
2:01
>> Device for seeing a device for seeing a
2:04
far. [sighs]
2:05
>> Yeah. Well, a looking through things.
2:08
Yeah.
2:08
>> Very cute.
2:09
>> I love that. Yeah.
2:11
>> I um [snorts]
2:12
>> Yeah. You know, you know, you don't know
2:14
what you might have, you know, released
2:16
in that child's brain. She might become
2:19
the next great astronomer using a
2:22
looking through thing to make
2:24
discoveries about the universe.
2:26
>> Yeah. Yeah. Well, that's why we called
2:27
it Vera. No, her name's uh her name's
2:30
Felicity. [laughter]
2:32
>> That's a good name as well. I like
2:33
Felicity.
2:34
>> It [snorts] is nice.
2:36
>> Uh shall we get to our first question?
2:39
>> Oh, all right.
2:40
>> All right. Uh if black holes are the
2:43
center of most galaxies uh and have been
2:46
eating up matter almost from the
2:48
beginning of the universe, can this be a
2:50
possible explanation of dark matter? The
2:53
black holes have eaten it. Now this uh
2:57
comes from Ross Simon. I had to smile
3:00
when I read his name because Ross Simon
3:02
used to be a famous newsreader on the
3:04
Australian Broadcasting Corporation's TV
3:06
news service. I remember Ross, he was he
3:09
was brilliant.
3:11
might be the same one. You never know. I
3:13
>> was going to say it's not the same Ross,
3:14
is it? [laughter]
3:15
>> I hope so. It would be lovely. But, uh,
3:18
if it's not.
3:19
>> Yeah. So, um, whether or not you are the
3:21
famous Ross, Simon, Ross, lovely to hear
3:24
from you. Uh, and, um, I mean, it's it's
3:28
it is tempting, uh, to lump black holes
3:32
and dark matter together. And indeed,
3:34
um, that was looked at as being one of
3:37
the first explanations of dark matter,
3:40
uh, that we've got space full of black
3:43
holes that we don't see because they're
3:45
black holes, um, and that they might
3:48
account for the dark matter. was the
3:50
so-called macho theory, massive compact
3:52
halo objects
3:55
uh which was popular in the 80s
3:59
um because it was only in the late '7s
4:02
that people started taking the idea of
4:04
dark matter seriously when we realized
4:07
that um something like 80% of the matter
4:12
in the universe is invisible to us. Uh
4:16
now
4:18
that's perhaps slightly different from
4:20
what Ross is asking about because he's
4:22
talking about material being sucked into
4:24
black holes. Uh and that is certainly
4:28
something that happens. But that's not
4:31
matter that's missing. That's just gone.
4:34
Uh the the bottom line is that the
4:36
universe as we see it today has this
4:39
mystery in that we know that there is
4:41
stuff there that has a gravitational
4:43
effect. It holds galaxies together. It
4:45
holds galaxy clusters together, causes
4:48
uh gravitational lensing all over the
4:50
place. Um but we have no way of
4:54
detecting what it is other than through
4:55
its gravity. So it's it's some people
4:58
used to call it missing matter. It's not
5:00
missing. It's definitely there. uh this
5:03
dark matter is around and it it's
5:05
probably in the rooms that you and I are
5:07
sitting in um at the moment uh because
5:10
it it tends to be where normal matter is
5:13
and we think
5:13
>> Judy and I were actually talking about a
5:15
dark matter the other day but you know I
5:17
won't elaborate.
5:19
[gasps]
5:20
>> Well what you do in your spare time
5:22
Andrew is [laughter]
5:23
entirely up to you especially with your
5:26
wife. Um um so yeah so so but um but the
5:30
the the black hole thing did come in
5:34
because of this theory back in the 80s
5:36
that match massive compact halo objects
5:38
objects that are kind of dead stars or
5:42
orphan planets or more especially black
5:44
holes might be uh the source of dark
5:47
matter the source of this gravitation
5:49
that we we see present in large on large
5:52
scales like galaxy clusters and
5:54
galaxies. What ruled that out uh was
5:57
work carried out at a number of
5:58
observatories including here uh in
6:01
Australia uh in fact in a survey which
6:03
was called Macho uh looking for these
6:06
things uh and it was that if you if you
6:10
had a universe full of black holes that
6:12
you can't see you would still be able to
6:14
detect them by what's called
6:16
gravitational microlensing because
6:18
occasionally one of these black holes
6:20
would pass in front of a distant star
6:22
and because black holes distort the
6:24
space around that has behaves like a
6:27
lens and you magnify the light of the
6:30
distant star. So you you get a microl
6:32
lensing event has a very characteristic
6:34
shape. It's a star getting brighter uh
6:37
to a sharp cusp and then fading away
6:40
again quite symmetrically. Uh and they
6:43
we do see them. They're caused by normal
6:45
stars and and their planets. But in the
6:48
numbers that you would have to have for
6:50
black holes to be dark matter, they are
6:54
not there. That they weren't enough. The
6:56
numbers were far too low. And that's
6:58
when the emphasis shifted to WIMPs, the
7:00
weakly interacting massive particles,
7:02
which is just one class of uh subatomic
7:05
particles that we think dark matter
7:07
might be. So that's where the theory
7:08
stands at the moment. So black holes uh
7:12
you know in the and and I think Ross is
7:14
talking about super massive black holes
7:16
at the centers of galaxies. Yes, they've
7:17
been swallowing stuff up for 13.8
7:19
billion years as far as we can tell. Um
7:22
but they don't explain why today in
7:25
today's universe um something like
7:28
four-fifths of the m matter in the
7:30
universe is invisible to us.
7:32
>> Yeah. Um well there's so many things we
7:35
don't understand and and and as yet uh
7:38
which was brought up in a question
7:40
recently, we have not been able to
7:43
capture or identify a black uh a dark
7:46
matter particle. So um yeah, until we
7:50
can find some absolute proof and and
7:53
study it, we we're probably going to
7:56
>> just keep working with theory, I would
7:58
imagine. Yeah, there's there are um
8:01
techniques that can be brought to bear.
8:03
Um one of the theories about dark matter
8:07
is that if that while dark matter
8:11
particles don't interact with normal
8:13
matter particles, they may interact with
8:15
each other. In other words, if you bring
8:16
two dark matter particles together, it's
8:18
thought they might annihilate and
8:20
produce a signal in gamma radiation. So
8:23
you get this flash of gamma rays which
8:25
might have a characteristic spectrum.
8:27
And people are looking for that
8:30
phenomenon in the centers of galaxies
8:34
because that's where you would expect
8:36
the dark matter to be at its densest. So
8:38
it's where you would expect the dark
8:39
matter particles to interact with with
8:41
each other. Um so far the results have
8:44
been a bit mixed on that. But it's one
8:46
possible way that we might eventually uh
8:49
discover what dark matter is. May maybe
8:52
maybe dark matter is like a negative
8:55
photograph. Remember in the days of uh
8:57
manual photography, you you'd take the
9:00
film and it would be negative and then
9:01
you turn it into the photograph.
9:04
>> Maybe dark matter is the negative of the
9:06
universe.
9:07
>> Well, yeah. I mean, um you know, you
9:11
there might well be a way that there is
9:14
a sort of dark what can I call it? A
9:17
dark particle physics. um um a whole uh
9:22
sweep of subatomic particles which fall
9:27
under what we lump together as dark
9:29
matter. But it's not just a single
9:30
particle. It's many different ones just
9:32
like the particles of normal matter. The
9:34
16
9:35
>> uh normal matter part sorry 16 subatomic
9:38
particles. They include forces as well
9:39
as matter when you count the 16. But you
9:41
know what I mean? You've got this this
9:43
suite of different particles that make
9:45
up normal matter. Maybe there's a suite
9:47
of different particles that in some ways
9:49
are a negative uh that make up um that
9:52
make up dark matter. So I think that's
9:56
we've been assuming it's just the one
9:57
thing. It could be all sorts of things.
10:00
>> Could be there could be, you know, they
10:01
could be atoms and molecules made out of
10:03
dark matter because they they interact
10:08
with each other. I don't know it. Look,
10:09
I'm not a particle physicist, but um the
10:12
possibilities seem not exactly endless
10:15
because particle physics has certain
10:16
rules that you've got to follow. Uh but
10:19
yeah, I I'm still pretty optimistic that
10:21
we're going to get to the bottom of dark
10:22
matter hopefully while I'm still alive
10:25
cuz I want to know. Yeah, we all do. We
10:28
all do. Uh thanks for the question,
10:30
Ross. Uh really good discussion point.
10:32
We get a lot of questions about dark
10:34
matter and and black holes. Uh while
10:36
we're on the subject of black holes,
10:37
there was a an article released on the
10:40
BBC uh early this year. Uh I know it's
10:43
still early this year, but right, you
10:45
know, I'm talking the 3rd of January,
10:47
where scientists captured the first ever
10:48
visual proof of two super massive black
10:51
holes in a death spiral. So, we we're
10:54
really starting to be able to find out
10:56
more and more uh through our increased
10:59
technology and the capacity to observe
11:02
and and create images of these things.
11:04
So, uh that was pretty exciting story. I
11:06
read that one the other day. I thought
11:07
I'd um give it a mention. But um yeah,
11:10
they they uh of course uh the popular
11:14
press uh created their own photo which
11:17
has absolutely got nothing to do with
11:19
it. [laughter] That's all right.
11:20
>> But uh yeah, it sells the story, doesn't
11:24
it? But uh yeah, they've got the image
11:26
of these two black holes um basically
11:30
getting ready to devour each other. I
11:32
think the big one will win.
11:35
>> Yes,
11:36
>> probably. Well,
11:37
>> thanks Ross.
11:38
>> Yeah, probably.
11:39
>> Yeah, thanks Ross. Good to hear from
11:41
you.
11:44
>> Okay, we checked all four systems and
11:47
>> Space Nuts.
11:48
>> Our next question comes from Sandy.
11:53
>> Good day, Fred and Andrew. It's Sandy
11:54
here from Melbourne again. Thanks for a
11:57
cracking show as usual. Um, my question
12:00
today is about navigation in the solar
12:02
system for the various spacecraft that
12:04
we've sent into deep space. Um, being a
12:07
sci-fi nerd, my mind naturally goes to
12:10
fancy graphics of star charts and orbit
12:12
parts on a giant screen. However, wanted
12:16
to ask how mission planners of various
12:18
space agencies plot orbits. Do they take
12:21
into account objects like asteroids for
12:23
any close calls or is the space so vast
12:26
it's not really necessary? Thanks,
12:28
Heaps. Sandy, cheers.
12:31
>> Thank you, Sandy. Uh, good to hear from
12:33
you. I don't think we've heard from
12:34
Sandy in a little while, but um yeah,
12:36
he's um he he does a lot of great um
12:41
astrophotography and he's got a a pretty
12:44
amazing setup that he's he's shown me in
12:46
the past about how he does it. Computers
12:48
all plugged into telescopes and yeah,
12:51
all this great software. It's a bit out
12:53
of my league. Um I might get there one
12:55
day. Uh navigation in space, plotting
12:58
orbits, all that kind of jazz. Um I I
13:02
must confess I struggle to get my head
13:05
around it. Like it's not like driving a
13:07
car. You've got to do um you know when
13:10
it comes to space you've got um much
13:12
less resistance, much more much more
13:15
reaction to minute
13:18
um thrust and micro thrust and all all
13:22
sorts of other things. But you've got to
13:23
be looking in all directions, not just,
13:25
you know, on the plane of the planet
13:28
when you're driving a car type of thing.
13:30
I don't know what I'm trying to say, but
13:32
um yeah, how how does it all work, Fred?
13:35
>> Uh it's it's sort of the equivalent of
13:39
plotting things on a screen, but not
13:40
quite [clears throat] the same. Um but
13:43
yeah, you know, the the uh idea that um
13:48
[laughter] excuse me minute that I'm
13:51
sorry, I've got this cough. Sandy's idea
13:53
[clears throat] that we
13:55
need to take into account I'm all right.
13:57
I'm all right.
14:00
Yeah, we need to take into account the
14:02
positions of asteroids and things of
14:04
that sort. That's exactly right. Um, so
14:06
when you um chart uh a pathway
14:11
through space, which is all done
14:14
numerically, you know, it doesn't we can
14:17
make displays of them and and I think
14:19
people do as well, but the reality is
14:22
that the real hardcore is locked up in
14:24
the numbers and the equations. Um what
14:27
you have to do is to uh at any instant
14:30
along the orbit of the uh of the
14:32
spacecraft because it is always an
14:34
orbit. Uh usually for something like you
14:37
know going between the planets it will
14:38
be an orbit around the sun. Uh that's
14:41
the way orbital mechanics work. Uh soon
14:44
as you switch on your thrusters then you
14:47
change that orbit. Uh but when all the
14:50
thrusters are off and your main engines
14:52
are off, you are following a trajectory
14:54
which is essentially an orbit. Um not
14:58
always a closed one. It could be an open
15:00
orbit. Uh which is what's happening to
15:01
the five spacecraft that are leaving the
15:03
solar system.
15:05
So uh but that orbit it's uh the future
15:09
position of your spacecraft is dictated
15:12
by the gravitational influence not just
15:14
of the sun and the earth and probably
15:17
the moon but all the planets. All of
15:19
them exert a gravitational pull. Uh and
15:23
um um that goes down to the asteroids as
15:25
well. When you get if you're passing
15:26
through the asteroid belt, you need to
15:28
know where they all are. They would all
15:30
all the ones which are known and there's
15:32
more than a million known asteroids now.
15:35
Uh you would have them kind of built
15:37
into your software that's looking uh at
15:40
the uh at the direction that your
15:42
spacecraft is going in. If there was any
15:43
risk of a collision, it would flag that
15:46
and [clears throat] um it would also
15:47
take into account the gravitational
15:49
influence of any close encounters of
15:51
asteroids. So it's it's a very precise
15:54
science um as you know because we you
15:58
know we know when for example the New
16:01
Horizons uh flyby of Pluto a decade ago
16:05
uh in 2015 um that the precision with
16:08
which that was executed was unbelievable
16:10
and it's because of orbital mechanics
16:12
and how well we understand these
16:13
gravitational influences uh that that
16:16
let you do that. Um so uh yes space
16:19
navigation in some ways it's easier uh
16:21
than navigation on uh you know than
16:25
driving a car because with driving a car
16:26
you've always got to the predict
16:28
unpredictability of the other road
16:31
users. The great thing about orbital
16:33
mechanics is you know what the other
16:35
planets the other asteroids and all the
16:36
rest of it are going to do. And ju just
16:39
[clears throat] one other adjunct to
16:41
this if I may. Um uh some years ago
16:44
there was several papers which talked
16:47
about the interplanetary superighway.
16:50
Uh and these are effectively low energy
16:52
trajectories between the planets and
16:54
it's based on exactly what I've just
16:56
been saying. You can map where um the
16:59
gravitational pull of all the objects
17:02
will take you. And it turned out that
17:04
you can you if you can put um a
17:06
spacecraft at one of your lrangee
17:08
points, these gravitationally stable
17:11
points, then leading from that are these
17:14
various low energy pathways that take
17:16
you to the the ground points of other
17:17
planets. Uh and that's the
17:19
interplanetary superighway. It might
17:21
take you decades to get from uh from um
17:25
you know from the Earth Lrange points to
17:28
something like Mars's or Jupiter's
17:30
Lrange points. It's a very slow process,
17:32
but it it it does exist. This this
17:36
almost like an imaginary highway which
17:37
is changing all the time as the planets
17:39
go around in their orbits. Uh just an
17:42
interesting aspect of the of the
17:43
navigation in space.
17:45
>> Yeah, I imagine that a lot of this uh
17:49
would be pre-programmed into the
17:52
computers of these these vessels. Um
17:55
they they do everything ahead of time
17:58
cuz these these things are on autopilot.
18:00
the the long haul spacecraft that are
18:02
going out to do these missions.
18:04
>> Uh so it would because and I've been in
18:07
the cockpit of a a commercial uh
18:10
airliner um long before you can't do
18:13
that anymore, long before we had any
18:15
issues like that. But um
18:17
>> and and watching the process like the
18:19
plane flies itself and the pilots sit
18:23
back and tell dad jokes to the the
18:25
tower. Um no, that's what happened.
18:27
[laughter]
18:28
But I I would imagine it's the same in
18:30
space. All these things are
18:32
pre-programmed, pre-calculated. Uh and
18:35
then contingencies built in just in case
18:38
something gets in the way that you
18:39
didn't anticipate. Um they they modify
18:42
the spacecraft to sense a problem and go
18:45
around it. I I would imagine.
18:49
>> Yeah. And in fact the you know the
18:53
likelihood of something it's so
18:55
predictable and our knowledge of of the
18:59
of the sort of congestion in space if I
19:02
can put it that way is so deep that um
19:06
it's unlikely that something's going to
19:08
come along to surprise you. You know you
19:10
suddenly see something ahead that you've
19:11
got to avoid. Uh because that avoidance
19:14
might actually be very difficult. Uh you
19:16
can you can do things. So perhaps the
19:19
the the best example I can give you
19:21
again it goes back to New Horizons and
19:24
that is that once the Jupiter encounter
19:26
sorry the Pluto encounter had happened
19:29
uh back in July 2015
19:32
um they looked for other potential
19:35
targets and eventually found the object
19:39
Arakov that was discovered as part of
19:41
surveys looking for future targets and
19:44
they worked out at what point they had
19:46
to apply a thrust to the spacecraft to
19:49
change its trajectory so that it would
19:52
intersect with Araoth. And it and it all
19:54
happened, you know, perfectly smoothly.
19:58
Um I think it was a couple of years
20:00
later when the Araoth uh flyby took
20:03
place. I can't remember when it was,
20:04
maybe even a bit later than that, maybe
20:07
five years later. [clears throat] But um
20:09
but that yeah, that that all happened.
20:11
that was the the nearest thing to oh
20:14
there's something ahead we need to
20:15
change course to either interact with it
20:18
or avoid it. Um and it was a very
20:20
leisurely process and
20:22
>> and you're right you're right about uh
20:24
navigation on the planet on roads being
20:26
much more dangerous. We were walking
20:28
along the street the other day and
20:29
somebody turned right off the main road
20:31
into our uh into our part of town and
20:35
>> uh went to the right hand side of the
20:37
traffic island instead of the left right
20:39
where we were walking.
20:41
So
20:42
>> yeah, I don't think she noticed to be
20:44
honest. Honest, she just went up the
20:46
wrong side of the road.
20:48
>> Anyway, it happens. But we we always
20:50
keep an eye out for that kind of thing.
20:53
There you go, Sandy. Thanks for the
20:54
question. The answer is easy peasy.
20:56
Really [laughter]
20:58
>> with modern computers.
20:59
>> It's a lot harder if you're doing it by
21:01
hand.
21:02
>> Yes.
21:03
>> All right. This is a Q&A edition of
21:05
Space Nuts with Andrew Dunley and
21:06
Professor Fred Watson.
21:11
Space nuts. Okay, our next question
21:14
comes from John in 27 parts. Hey guys,
21:18
love the show. Every time I listen to a
21:20
new episode, my mind goes crazy thinking
21:22
about new possibilities and questions. I
21:24
have two questions about time dilation.
21:28
Uh in general relativity, uh if general
21:31
re relativity causes time to be observed
21:33
at different rates,
21:35
would that mean someone orbiting very
21:38
close to one of the first black holes in
21:40
existence would experience a universe
21:42
that has existed for a much shorter
21:45
period of time?
21:47
Uh, as a follow-up to this, if the new
21:50
theory about the big crunch turns out to
21:52
be true, would the finite time of the
21:56
universe form the big bang to uh uh from
22:00
the big bang to the big crunch be
22:02
considerably shorter if again you were
22:05
orbiting close to a black hole? Uh we
22:08
see the universe as being 13.79
22:11
billion years old and uh new estimates
22:14
put the big crunch at 20 billion years
22:17
into the future. My brain hurts thinking
22:20
that these time scales could be
22:21
considerably different due to time
22:23
dilation. All the best uh John from
22:26
Suffach in the UK. [gasps]
22:29
>> Oh, there's a lot in there.
22:31
Um yeah and and so it's it's quite
22:35
complicated because time dilation uh
22:39
depends on your vantage point.
22:41
>> So yeah, if you're in orbit around a
22:43
black hole uh you're in an intense
22:46
gravitational field.
22:48
>> You're also stuffed, but we'll just deal
22:50
with that another time.
22:52
[sighs and gasps]
22:52
>> Um yeah, you're you experience time just
22:57
at the normal rate.
22:59
>> Yeah. um what uh what um an outside
23:04
observer looking at you would see would
23:06
be your time going very slowly. The time
23:08
would be dilated.
23:10
>> So um I I suppose what we're talking
23:14
about here is that in terms of the what
23:18
you might call the frame of rest of the
23:19
universe itself.
23:22
uh that's what we what we see when we
23:26
look at the universe in general and
23:28
that's what gives us the 13.79 or 13.8
23:31
billion year age of the universe um your
23:35
perception of that so that time
23:37
effectively wouldn't change uh but your
23:40
perception of it if you were in orbit
23:41
around the black hole would it would
23:43
probably appear to look as though it was
23:45
going very quickly uh but that's cuz
23:48
your time's slower. uh and likewise um
23:53
uh with the you know the density of the
23:56
universe being higher uh at earlier
23:59
stages. Yes, we we do know time dilation
24:01
takes place. You can you can actually
24:03
see that uh because um when uh
24:07
scientists look at the light curves of
24:11
supernovi exploding stars, they have a
24:13
light curve, their light increases and
24:15
then decreases uh in a more gradual way
24:18
with a very characteristic shape. Uh
24:21
those light curves are dilated. They're
24:24
stretched when we look at ones in the
24:26
early universe. So the phenomenon does
24:28
happen but um it doesn't happen at a
24:32
level that's going to significantly
24:34
shorten our um perception you know the
24:37
the universe's perception of its of its
24:40
own history if I put it that way. I
24:41
think I'm talking a little bit in
24:43
riddles here but I hope John follows me
24:45
that it it really is all about your um
24:49
your frame of rest as we call it your
24:50
vantage point uh on the universe because
24:53
that's what time dilation is all about.
24:55
It's about people seeing time going
24:57
differently depending on their
24:58
viewpoint. Our viewpoint here uh from
25:01
earth is probably that of the universe
25:04
of a as a whole effectively uh because
25:07
we are not in an intense gravitational
25:09
field. It the gravitational field of the
25:12
sun is the strongest thing we feel that
25:14
keeps the the earth in orbit but it's
25:16
it's nothing like what you would find
25:18
around a black hole. And so we've got
25:20
probably a fairly unbiased view of the
25:24
universe and its history. Now,
25:26
>> are we in a gravity well?
25:30
>> Yeah, we are. I mean, the the earth
25:31
itself creates a gravity well. Uh, and
25:33
that's what keeps us stuck to the earth
25:36
because the gravitational potential at
25:39
your head is a little bit different from
25:41
what it is at your feet. And that's
25:43
what's pulling you down, the changing
25:45
shape of space.
25:46
>> There you go. All right. Um,
25:48
>> I'm not spaghettifying you.
25:50
>> No. No. So, um, did we unpack everything
25:54
there? [laughter]
25:58
>> Yeah, I think so. I think we I think we
25:59
covered most of it.
26:00
>> Okay. Um,
26:02
>> look, I know.
26:04
>> Sorry. Go ahead.
26:05
>> No, I was going to say that he also
26:07
asked if the big crunch would happen
26:10
faster than the expansion.
26:12
>> So, yes. So that I kind of you know lump
26:15
that into the the fact that the the
26:17
times that we observe from our
26:19
>> location in the universe probably yes 20
26:21
billion years down the track seems about
26:23
right for the big crunch if if the
26:25
recent desi results yeah it's what
26:27
happens
26:28
>> um um uh I was going to say that like
26:33
you John uh these things make my brain
26:36
hurt so um don't think it's it's
26:38
peculiar to uh to um to a few people we
26:42
I think I think most physicists, you
26:45
know, they get their they really have to
26:47
get their heads around the things like
26:50
seeing pe seeing things from different
26:52
vantage points. It's not entire it's not
26:54
intuitive at all.
26:56
>> No. No. All right, John. Thank you. And
26:59
I hope all is well in Suffach. That's uh
27:01
that's basically what one of one of the
27:04
eastern most points of England, isn't
27:06
it?
27:06
>> I used to live in Suffach. But uh yeah,
27:09
not not far from Cambridge, which is in
27:12
Cambridge High, but SuffK I was over the
27:14
border in Suffach. You're right. SuffK,
27:17
Norfolk and Suffach are the two counties
27:18
in East Anglia, that sort of
27:21
semic-ircular bit that sticks out not
27:23
very much north of the Temp's Estie.
27:26
>> There you go. All right.
27:27
>> Very pretty country. Yeah.
27:28
>> Yes. Good to hear from you. Our final
27:31
question today comes from Andy.
27:35
>> Hi guys, this is Andy from London. I'm a
27:38
new listener to your podcast and quite
27:40
new to science, so forgive me if this is
27:42
a stupid question, but I was just
27:45
wondering um when craft re-enter
27:48
atmosphere and they have to come in at a
27:50
certain angle to stop from burning up um
27:54
why do the not just come through slowly
27:56
to to get away with the friction effect
28:00
which uh causes the heat? Thanks.
28:04
>> Thank you, Andy. Um, and thanks for
28:06
finding Space Nuts and being a new
28:08
listener. Um, you've only got 590 odd
28:12
episodes to catch up now. So, um, yeah,
28:15
have [clears throat] fun with that. Uh,
28:16
we've certainly had fun with it. Um, I
28:19
think we've had this question before,
28:21
maybe asked a different way. Um,
28:27
and and when it comes to a spacecraft,
28:29
we're orbiting the planet, which is
28:31
essentially it's just constantly
28:32
falling. you're just maintaining a
28:34
velocity that stops you falling in. Um,
28:38
you do have to slow down to re-enter so
28:41
that that arc is is, you know, reduced
28:46
or increased. Can't remember which. Uh,
28:47
and and then you fall through the
28:49
atmosphere. Um, you can't stop and just
28:53
sort of ease your way back in as against
28:56
a space elevator which would be able to
28:58
do that if we ever build one. But that's
29:02
a different set of but a space elevator
29:04
is essentially not orbiting. It is
29:07
stationary to a point on the planet
29:09
which means it goes up and down. Is that
29:12
making sense?
29:13
>> Yeah. Um yes it is. [laughter]
29:16
That's good. That's the first time ever.
29:18
>> Everything you've said is correct. Um
29:20
space elevators uh hypothesized Buzz
29:24
Aldrin told me uh that night I had
29:26
dinner with him it's never going to
29:28
happen. And he he was quite right
29:29
because um the space elevator has to sit
29:32
on the equator.
29:33
>> Uh every spacecraft in the sky crosses
29:35
the equator. So you're always going to
29:37
get things banging into it. Uh it would
29:39
be very difficult to build one. Uh you
29:42
know apart from the structural thing. Uh
29:44
so neglecting the space elevator for a
29:46
minute, what you said is absolutely
29:48
right. In order to stay in orbit, you
29:50
have to achieve basically a horizontal
29:53
velocity of about 8 kilometers/s.
29:56
>> [gasps]
29:56
>> uh and that's otherwise you just fall
29:59
back to earth. So that's what all the
30:03
you know the the the huge amount of fuel
30:05
that is carried by a rocket being
30:07
launched that's what it's all about.
30:10
It's about getting up to a height of two
30:12
or 300 kilometers and getting that
30:15
orbital velocity getting that horizontal
30:17
velocity of 8 kilometers/s.
30:20
So, um, what you could do, uh, is, and
30:25
and you know, going, this is hopefully
30:28
help helping Andy, uh, if you had
30:32
unlimited amounts of fuel, you could
30:35
turn the rocket round, uh, from its
30:37
orbital position and fire uh, your
30:41
rockets to act as a braking system to
30:44
slow the thing down. And then you could
30:46
gently tiptoe down through the
30:47
atmosphere, constantly firing your
30:49
rockets. It's actually what um Musk does
30:53
with his with his uh Falcon rockets. He
30:56
he he's got enough fuel left that he can
30:59
bring the empty spacecraft back down
31:01
intact uh and and use it again uh
31:05
without needing a heat shield. Um so you
31:08
could do that. Uh and he's demonstrated
31:11
that we can. Uh but it turns out that uh
31:15
it's much more effective to use this
31:19
process called aerobreing where you
31:21
actually use the atmosphere itself to
31:24
slow the spacecraft down. Uh because you
31:27
don't need any fuel for that. You just
31:28
need something that's going to stop it
31:30
burning up. Uh so getting from this 8
31:32
kilometers/s
31:34
down to a few, you know, a meter or two
31:37
per second uh for a splashdown or a
31:39
touchdown uh is the tricky bit. And
31:42
you've got to uh you know you you you
31:45
you've got to use whatever means are at
31:47
your disposal and the easiest one is
31:49
arrow braingreaking using the atmosphere
31:51
itself to slow you down. I should point
31:53
out that um I think the first stage
31:56
Falcon rockets they're not at orbital
31:57
velocity uh when they turn around and
32:00
come back. They haven't got up to that 8
32:02
kilometers/s because there's a second
32:04
stage that lets them do that and they
32:06
still burn up basically coming back into
32:09
the atmosphere. the second stages.
32:13
>> Good question. Not a stupid question at
32:14
all.
32:15
>> No. No. And and you know,
32:19
it's rocket science. I mean, we we often
32:22
say when something's not difficult, it's
32:24
not rocket science. This is rocket
32:25
science. Um orbital speeds are
32:29
significant. They're they're to stay out
32:32
there. They've got to um do 17,000 mph,
32:36
>> which is 8 kilometers/ second.
32:38
>> Yeah. Mark 25. Um, to slow down so that
32:42
you can return to Earth safely, you've
32:44
got to go from that speed to subsonic
32:47
speed.
32:48
>> Yeah.
32:49
>> And using fuel to do that would be
32:51
exhaustive.
32:52
>> Yeah, that's right. That's exactly
32:54
right. You know, you basically you're
32:57
looking at the same amount of hardware
33:00
in terms of a rocket and its fuel to to
33:03
get you up there. you'd need the same
33:05
amount to bring you back just to do a
33:07
gentle touchdown on the earth.
33:10
>> Um uh notwithstanding the first stage
33:13
recovery that we're starting to see with
33:15
well not starting they've been doing it
33:17
for 10 years with the Falcon 9 rockets.
33:21
>> Yeah. Uh I'm sure the technology will
33:23
improve and we'll find better ways but
33:25
at the moment using the atmosphere as a
33:27
free braking system.
33:28
>> Yeah. Yeah.
33:29
>> Yeah. Works really well.
33:31
>> It does.
33:31
>> Except when it doesn't. But that's only
33:33
happened a couple of times.
33:34
>> Yeah, it is.
33:35
>> Um, yeah. [sighs]
33:36
Thank you, Andy. Great question, though.
33:38
Uh, really appreciate it. And don't
33:40
forget, if you've got questions for us,
33:41
you can send them through via our
33:44
website, spacenutspodcast.com,
33:46
spacenuts.io. You click on the little
33:48
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33:51
me anything. I finally figured that out.
33:54
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33:57
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33:58
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34:03
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34:10
Uh, while you're there, you might want
34:12
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34:14
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34:22
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appreciate it greatly. Uh, I think we're
34:48
all done. Fred, thank you so much.
34:51
Uh, a pleasure, Andrew. Great to hear
34:53
from the listeners as always. Great
34:55
questions. Uh, get my mind thinking
34:58
again. [sighs] Uh, so uh, thank you
35:01
everybody and thanks to you Andrew too
35:03
for keeping on the rails.
35:06
I do my best and sometimes I don't. Uh,
35:09
but yes, thank you Fred. We'll see you
35:11
next time. Professor Fred Watson,
35:13
astronomer at large. And thanks to Hugh
35:15
in the studio who definitely does keep
35:17
it all together. Although he couldn't be
35:18
with us today because um, he re-entered
35:21
his garage at excess velocity. couldn't
35:23
get down to subsonic speed and uh well
35:27
he'll be in traction for a couple of
35:28
weeks. Uh from me, Andrew Duckley,
35:30
thanks for your company. We'll see you
35:31
on the next episode of Space Nuts.
35:33
Bye-bye.
35:34
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