What was the pioneer anomaly?

Daniel and Jorge dig into the mystery of why the Pioneer spacecraft's path surprised scientists.

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Episode Transcript

Speaker 1 (00:08):
Hey, Daniel, what's the first thing you think about when
you see something in space you don't understand. Well, my
first hope is always aliens. But my first thought is
that it's probably a boring Mistand so as a scientist,
your mind goes straight to the most dramatic explanation. Yeah,
we're all hoping that somebody out there hits the science jackpot.

(00:31):
Everybody wants to find the thing that frocks the world. Yeah,
you know, people might not realize that science is a
little bit like playing the lottery. It's mostly wrong numbers,
but occasionally you get lucky and you win the alien jackpot.
What's the price you might need to get wiped out? Yeah? Maybe,
but think about what we might learn along the way
in our dying moments. I am more hammy cartoonists and

(01:09):
the creator of PhD comments. Hi, I'm Daniel. I'm a
particle physicist, and I'm constantly on the hunt for something
weird in science. Not in your department, in science in general.
There's plenty of weird things in your department, weird stuff
growing where it shouldn't be. But no, I'm looking for
weird stuff in the data, hoping to win that jackpot

(01:30):
is your data like it's a scratch of kind or
like you make up the numbers. No, we are just
looking at particles smashing together and hoping that something that
comes out of it is unexplained. It's weird, it's something
that our current theories of physics do not predict. Well,
congratulations to all of you because you've won the jackpot.
You are listening to our podcast Daniel and Jorge Explain
the Universe, a production of I Heart Radio. And your

(01:51):
prize is that you get to hear all about the incredible,
amazing mysteries of our universe, all the weird stuff that's
going on out there, all the little hints we have
about weird stuff that might be going on that could
be clues that reveal deep secrets about the nature of reality. Yeah,
and just like a real lottery, this episode might be
a winner or a dud. We'll find out. We'll have

(02:13):
to scratch off the surface. The price is the same
either way. That's right. The price is right meaning nothing,
You get what you pay for. But yes, science is
full of interesting mysteries out there, and it does happen
a lot, doesn't it. Daniel, did the scientists look out
into the world, or or try an experiment or do
something and things don't happen the way you expect them

(02:33):
to happen. Yep, everybody out there in graduate school knows
that no experiment goes right the first time. But usually
the reason you've got a weird result is that you
messed it up a little bit. You didn't calibrate things,
or you plug something in backwards, or you misread your
instrument or something. Usually there's a conventional explanation. But what
we're all looking for are those moments when you've done

(02:54):
everything correctly but the answer is not what you expected. Yeah,
don't they say that the real moments in science or
not the eureka moments, but them that's interesting moments exactly
wait a minute moment. There are sometimes those moments. There's
a fantastic audio tape that's floating out there of recording
of two astronomers listening to one of the first pulsars

(03:16):
ever seen. They happen to be recording themselves for reasons
I don't know, while they were observing, and you can
hear them going, wait a second, look at this, Oh
my gosh, is that what we think it is? WHOA
that's interesting. Really, it's for real. Wasn't acted out so
those especially they were being recorded. It's for real, it's
for real. I encourage you to go out there and
check it out. In fact, maybe we'll dig it out

(03:37):
for our episode that's coming up on how pulsars were discovered.
But yeah, most moments of scientific discovery like, dang it,
my experiment didn't work. Well, maybe there's a better explanation,
you know, after you check off all the boring explanations.
Sometimes it's an exciting, fascinating explanation. Something new might be
going on, or sometimes it's something routine. It could be

(03:57):
either one absolutely and people might remember few years ago
we thought we had discovered new trinos going faster than
the speed of light. I won't say we there because
I wasn't among the group of people I thought that
was real. And humanity does something good, it's the royal
we including when they mess it up, it's them, it's
the Italians, them Italians. I won't touch that. But as

(04:19):
soon as that paper came out and I saw that
it had been written in Microsoft Word, I was like, no,
there's something wrong with this paper. What do you mean?
I wrote all my papers in Microsoft word. Yeah, point made.
Actually I wrote them in Adobie FrameMaker, which tell you
how old I am A little bit. Wow. That really
does teach you. But usually, as in the case of
the Fast and the Lighten New Trinos, is just a mistake.

(04:42):
In that case, they forgot to plug a cable incorrectly
and so their calibration was wrong and they mismeasured the speed.
But sometimes these are famous experiments in the history of physics.
We see something we don't understand and it's actually a clue.
It's the first hint of a dramatic realization of uncovering
something we had no idea about. And a great example
is the photoelectric effect, which was an experiment around a

(05:06):
hundred and twenty years ago that was the first clue
that the universe was quantum mechanical. Yes, it was very
illuminating and electrifying at the same time for humanity and
very effective. Yeah. So today we'll be talking about one
such mystery in science, and in particular in space science
that happened a few years ago. Daniel right, how long
ago was this or is it still ongoing? It was

(05:28):
resolved a few years ago, but it went on for
several decades. This is an outstanding mystery that puzzled people,
that caused them to dig deep into their attic of
ideas for possible boring and exciting explanations for what could
be happening to this spacecraft. Yes, it involves spacecrafts and
planets and basically just need toward space and mystery. And

(05:49):
it's an interesting journey right here. Amazingly, there's almost no
talk of aliens in this whole topic. Not yet. It
spressed to surface Daniel and see if we can win
the library here so to be on the program, we'll
be talking about what is the Pioneer anomaly. Now, it's anomaly, right,

(06:13):
not animone, Just get confused. It's not an underwater sea creature.
It's not something you want deep fried and served before
your dinner. It's an anomaly. It's something that's not understood,
that's anomalous, and it's something that happened in space around
what time. Well, it has to do with the Pioneer spacecraft,
which was launched in the seventies and has one of
these incredible careers. You know, they expected to go on

(06:35):
for several years and then it operated for decades and
decades and decades and of course it's still out there, right,
Pioneer ten and Pioneer eleven are still out there flying
out into the depths of space being pioneers. Of course,
now I have to admit I did not know what
the Pioneer spacecraft is or the anomaly. I was a
little puzzled by this, but as usually, we were wondering

(06:57):
how many of you out there no or new this
Pioneer anomaly is. So thank you to everybody who volunteered
to answer these questions. If you'd like to participate for
a future episode, please don't be shy. Right to me
two questions at Daniel and Jorge dot com. So think
about it for a second. If someone asked you what
the Pioneer anomaly is or was, what would you say.

(07:20):
Here's what peopled to say. I don't know, but I
imagine it's an abnormality're pretending to spacecraft or something. I'm
not totally sure what the Pioneer a normally is. Um
I can end the guest that has something to do
with the Pioneer spacecraft that were launched in the early seventies.
And I think, and now a very long why, I

(07:45):
have no idea. I don't know I have no idea
what the Pioneer anomaly was, but I assume it has
something to do with the Pioneer spacecraft and obviously something
that you would not normally expect to have of help them.
I think that the Pioneer anomally was maybe like this

(08:05):
first irregularity found about our universe, and since it's the
Pioneer anomally, I think that this maybe marked like a
turning point for scientists and made them more curious about
what other anomalies can be found in our universe. Al Right,
not a lot of name recognition here, No, I was

(08:27):
a little surprised. I thought this was a little more famous.
I remember hearing about this in high school and thinking, woh,
that's cool. I bet that's something real. Whoa you heard
about it in high school? Like, oh yeah, like in
physics class, or how did you hear about it from
your physics parents. I was a nerd, big shocker, so
I just kind of interested in this stuff, and you know,

(08:48):
anything out there in space that was unexplained, anything that
might be a clue as to how something is working
in the universe. I gobbled that stuff up to what
I heard that we didn't understand where this spacecraft was
and what it was doing, and why I was doing
these weird things I thought. I think the real anomaly
is that you used the past tense when you said
you were a nerd. It's not up to me to

(09:12):
evaluate that. All right, Well, there is something called the
Pioneer anomaly, and let's break it down for people. Daniel,
what is the Pioneer anomaly? So, as expected, it's an
anomoly that has to do with the Pioneer spacecraft and
specifically is that we didn't understand where it was going
and why it seemed to be going off course for
reasons we did not understand. Okay, so it was a spacecraft,

(09:36):
meaning like a satellite. I always get confusible to say
spacecraft because I imagine you know, Battlestar Galactica or you know,
people in uh in a cockpit or something. But really
would just mean, like, you know, any device we launched
into space is called the spacecraft. Yeah, it's not a
satellite because it's not in orbit around Earth or any

(09:57):
other sort of body. It's just sort of sent out
there to explore. And it's called pioneer because it really
was a pioneering mission. It was the first thing from Earth,
first scientific object sent out to explore the outer Solar System.
It was the first man made object to go through
the asteroid belt. For example, it was the first close
flyby of Jupiter, Like the first close up pictures of

(10:17):
Jupiter came from Pioneer. So when was it launched. It
was a while ago, right, maybe older than us. It's
been a nerd since longer than I have. Pioneer ten
was launched in March of seventy two, and Pioneer eleven
in April of seventy three. And you know, these were
iconic missions. Before this, we didn't really know what things
looked like out there. We had telescopes from Earth that
were pretty good, I guess, nothing compared to what we

(10:40):
have now. But if you call up in your mind
an image of Jupiter, you probably have a pretty detailed
image thanks to Hubble and thanks to all the spacecraft
we sent to visit close up. But in the seventies,
if you tried to do that, all you could have
was like a blurry smear. We just didn't know in
detail what these planets looked like until we went to
visit them. So this was the first spacecraft to go

(11:00):
and do that. Wow, even our telescopes couldn't get us
a good picture. I mean they were all right, but
nothing compared to what Pioneer could do. And it was
an exciting moment even for the public. When these pictures
started to come back. They had like a prime time
television show to release these to the public, and people
were glued to their screens. That TV show actually won
an Emmy. Really, they had a whole show where they
just revealed the photographs. Yeah, exactly. It was exciting. People

(11:23):
wanted to know, so it was really popular. It was
you know, back in the day, every sort of launch
was a special moment, and these kind of pictures they
were priceless. They still are now. Was it a daytime
Emmy or a prime time Emmy? Those are two very
different things. I think it was a prime time Emmy.
That's the good one. Right, Well, it depends on you
know what you're going for. So they launched a spacecraft

(11:44):
in seventy two, and they launched like a brother, like
a sibling sister spacecraft in the next year. Yeah, Pioneer
ten went to visit Jupiter and then Pioneer eleven did
Jupiter and Saturn, and this is the end of the series,
like these are ten and eleven. There was Pioneer six,
Pioneers seven, Pioneer eight, Pioneer line. Those are in sort
of solar orbit there like experimental spacecraft to develop the technology.

(12:05):
But these have all lasted for decades, Like we heard
from Pioneer ten last in two thousand and three, which
is like thirty years after we launched it. Wow. Now
it's kind of weird because if you're like Pioneer number thirteen,
are you still a pioneering the thirteen first want to
do something? That's true? But Pioneer tend you know, it

(12:29):
was the furthest object from humanity for a long time,
and it was like the farthest humans have had an
impact on the universe was Pioneer ten. More recently, we
sent Voyager one, which was launched in seventy seven and
it was going faster. So it's now the furthest object
from humanity, even though it left later. Now paint the
picture for us, what what do these spacecraft look like?

(12:51):
They basically look like a big satellite dish, which is
how they communicate back to us, and then there's like,
you know, a cube of electronics strap to the back
and that's where you have like instruments and you know,
things that make measurements. This one has like a cosmic
rate telescope on it, something to analyze plasma, something to
analyze radiation. And then they've got a few things sticking

(13:13):
off of them, like these big arms that stick off,
and that's where the power sources are. For example, these
things are powered by radiation. There's a radio isotope thermoelectric
generator's plutonium in it, and it's basically just plutonium is
decaying and that the energy from the decay turns into heat,
which then they turn into electricity, and that powers this thing.

(13:34):
It's like a plutonium battery. Well, it's a nuclear powered
spacecraft basically, right, Yeah, how else could you power this thing?
Solar panels wouldn't be effective after decades when you're so
far from the Sun that it just looks like another star,
and we couldn't charge up our batteries and send them
out there. So really nuclear power is the only way
to power these very very long lived spacecraft. All right.

(13:56):
So we sent it out, and we sent it out
to Jupiter and Saturn, and as something happened along the way,
something weird happened. Yeah, the weird thing is that it
wasn't flying the way that we thought it should. A
Pioneer turns out to be sort of unexpectedly a really
really sensitive instrument to measure gravitational pull of everything in
the Solar System, and that's because it almost never fires

(14:18):
its thrusters. It's just sort of like a ball we
threw out into space. A lot of the other satellites
that we send out there, like Voyager, it's got a
bunch of thrusters and it's constantly firing them to change
this direction. So it's hard to predict exactly where it
should be. A Pioneer, it's just spin stabilized, and we
just launched it out there, so we can use sort
of Newton's theory of gravity and then Einstein's modification to

(14:39):
predict exactly where it should be at any given moment.
And when they measure to see where it actually was,
it turns out it was kind of off course. Wait
a minute. First of all, we just threw it out there.
It doesn't have like, you know, a thrusters to steer
it or to change its direction. It does have some
small thrusters and a little bit of fuel but these
are just in case it needs, of course correction. But yeah,

(15:01):
unlike later, spacecraft is primarily just like a rock that
we throw out into space. Now we keep it spinning
a right that helps stabilize it to go in the
right direction. But essentially we just toss it out there
and then we just measure to see where it went,
and that tells us something about like the gravity that
experiences along the way. That's wow. And how do we

(15:21):
know where it is? If we just throw it out
on into space. It sends us messages and it answers messages,
and we can tell how far away it is by
how long those messages take to come back. And we
can also tell how fast it's going by the Doppler
shift of its messages. The faster it's going, the more
the wavelength is change of the messages that return. So
we have those two pieces of information, how far away

(15:43):
it is and how fast it's going. And I guess
we triangulate like if we've talked it one day and
then we talked it the next month, and we can
sort of as the Earth move, we can is that
how we can tell where it is? Yeah, Also because
we can tell where the signal is coming from right
and located in the sky, and so we can tell
where it is, how far way it is, and how
fast it's moving, and then we compare that to like

(16:03):
our model of where we think it should be. Are
you want to keep up to date and like is
this thing going in the right direction and isn't it
a crash into Jupiter or just fly by? Right? All right?
So then we threw this spacecraft into space and it
didn't go where we thought it would, right, that's the anomaly.
That's the anomaly. It was hundreds of kilometers off course.
Something was pulling on this thing. It was slowing it

(16:26):
down in a way that we did not understand. There's
some small acceleration of this thing towards the Sun that
didn't have an explanation from any known physics. Weird, hundreds
of kilometers off course, that's that's not a little. It's
not a little like if I if I missed an
address by kilometers of being another state. Yeah, it's actually

(16:47):
a really really tiny effect, but it adds up over
the thousands and thousands kilometers of its journey. The effect
of this force, this unexplained anomalous force, is ten billion
times smaller then the acceleration we feel here on Earth
from the gravitational pull of the Earth. So that's small
effect really adds up over the lifetime the decades of

(17:08):
this craft's flight to get it hundreds of kilometers off course,
all right, And so and we knew one thing about it,
which is that this mystery acceleration was directed towards the Sun.
That's one clue. That's one clue. The other clue was
that the same thing happened for Pioneer eleven wasn't just
like some weird thing like, oh, maybe we shot Pioneer
tan off in the wrong direction or make a mismeasurement

(17:30):
early on, like the same thing happened to another spacecraft.
I see, did it happen for Pioneer one through nine? Uh?
Those guys didn't go out into the outer Solar System,
so they didn't get a chance to probe that. I see.
They weren't true pioneers. Yeah, And the Voyager missions which
went out sort of in the same area, they had
lots of complicated thrusters and were always doing these adjustments,

(17:52):
and so they weren't nearly as precise probes of gravity.
They weren't just like pure flying rocks. All right, so
that's the Pioneer anomaly, the big mr. What was happening
to the spacecraft that was steering them off course. So
let's get into what could be possible explanations for this
effect and also how it was possibly fixed. But first

(18:12):
let's take a quick break. All right, we're talking about
the Pioneer anomaly. These are spacecraft we launched in the
seventies out towards Tupiter and Saturn, and they sort of

(18:34):
got off course for some mysterious reason. Daniel, Um. These
are spacecrafts that are still out there, right. They're gonna
be out there for a long time. They're gonna be
out there forever. They just keep going, right. There's nothing
we can do to stop them or pull them back.
They're just sort of headed out into the galaxy. They
won't run out of batteries. They will run out of
batteries eventually, you know, the half life of their plutonium

(18:56):
batteries like eighty seven years, So it's gonna go for
a while. But yeah, eventually it will run out of battery.
But that's not going to stop it, right, It doesn't
need battery to go. It just has its velocity and
it just keeps flying. Oh wow, like basically too trash
out into space. He'll be there forever. But we labeled
our trash, right, we put our names on it. We
said exactly who we are and what we're like found,

(19:18):
come conquer us and us. This is reperiment on us. Yeah,
it's commediently pointing towards us, right, yeah, exactly. And we
put a plaque on it. We said exactly how to
get to Earth. We put like a pulsar map, so
if you knew where the pulsars were in the galaxy,
you could exactly triangulate where our solar system was. We
put a picture of humans on it. We even recorded
some sounds from Earth and put it on there, as

(19:41):
if like they would know how to play a record
and know how to interpret that. So we threw these
spacecraft out into the outer Solar system and they started
veering off course for some mysterious reason. Now that people
freak out or were they just sort of puzzled about
this At first, they were just puzzled and they figured, well,
we must have made a mistake somewhere, or there's tiny
little effect that we hadn't accounted for, because you know,

(20:03):
it's not just as simple as you toss a rock
out into space and then you do gravitational calculations. There
are lots of really small effects. You know. One set
of small effects is like, well, there's a lot of
different sources of gravity. It's not just the Sun that's
pulling on these things, and it's all the planets and
all the moons and all the rocks in the asteroid
belt and all the tiny little things in the Kuiper Belt,

(20:26):
and you know, even other stars provide tugs. So at
first people thought, oh, we just haven't been as careful
as we needed to to sort of tie up all
the loose ends I see, like there could have been
like it may be passed by a big asteroid that
maybe pulled it off course or something. Yeah, or even
simple things like maybe we're analyzing the data wrong, you know,
we mismeasured it or misinterpreted this result or something like that. First,

(20:49):
when you see something weird in your data, that's your
assumption is that we messed up or we just weren't
thorough enough. And the cool thing about pioneers that's very
very precise data, and so it allows for really detailed tests,
and so you can like go through your whole list
of ideas for what could be affecting the flight of
this thing, add them all up, and then compare them
to the number. It's a really valuable way to check

(21:09):
your understanding, right because I imagine you know, like you said,
your first wish is that it's aliens. But I imagine
you're also sort of afraid a little bit, like, oh
my god, what if I made a mistake, Like what
if I I am totally doing this where I have
a typo in my formula or something. Right now, does
that emotion also pass through you? I'm sure it does,
you know. For those guys, you remember there was one

(21:30):
Martian probe that crashed because somebody typed the number in
in the wrong units, Like there was a European group,
an American group, and somebody put something in in pounds
and somebody else interpreted in kilograms and oops, and then
the whole thing crashed and burned. So yeah, mistakes do happen, absolutely, Yeah,
just don't work with those Americans and they're non international system.

(21:53):
In my personal research, there's very little opportunity for aliens
to affect our data. I mean, that would be pretty
awesome if aliens change the collisions at the LHC and
cause new weird stuff to happen as a way to communicate. Actually,
that would be a pretty cool science fiction story, right,
discovering aliens through particle collisions. Alien pranksters, alien physics pranksters.

(22:15):
Exactly exactly, Aliens, if you're listening, I welcome that. Please
mess with our data. It's the large age on coliders.
Send us a message. The first physics from camm Alright, so,
so what were some of the things that they thought
could be happening here with the Pioneer anomaly. Well, the
first thing that comes to mind is just the location
of everything in the Solar System. So they went back,

(22:35):
they did a really detailed check and they thought, for example, like,
how well do we actually know the location of all
the planets, you know, Earth and Jupiter and Saturn. If
those are off by a little bit, could that explain
you know, a little gravitational tug. And you know, we
know these things really well because we've been watching these
planets for a long time and they mostly just obayue gravity.

(22:56):
So we have really really detailed models. We know the
location these planets down to the meter, and these effects
are just too small. So, like we think we know
the gravity from all the planets and all the little
things in the Solar system, there is uncertainly there, but
it can't explain an effect of this size, all right,
So it wasn't our measurement of the planets could be

(23:16):
the measurement of you know, asteroids, because there is a
lot of dark asteroids out there. There were some asteroids
out there, but those are all really small. You know. Remember,
gravity is really weak, and so to have any sort
of effect, you either have to be large or you
have to be close. So because the same thing happened
to Pioneer eleven, we didn't think it was like a
one off event that just like some object happened to

(23:37):
get near one of these things. So it seemed like
it had to be some sort of more systemic thing.
Another thing people thought about is like, well, what about
the solar wind? Right, the Sun is pushing on things,
It's not just tugging on them with gravity is actually
pushing on everything. You know, it's sending out streams of particles.
You could use that as a solar sales and navigate
the Solar system. So if you're like hunting down for

(23:58):
tiny little effects. You might want to consider the effects
of the photons and the protons and the electrons that
the Sun is streaming out right, but the solar wind
is pushing things up. But here something was kind of
pulling the spacecraft towards the Sun right exactly, So it's
the wrong direction. If anything, you would expect the solar
wind to accelerate it, to push it further out into

(24:18):
the Solar system, making go faster. So we needed some
other effects, something that was tugging it back into the
Solar system. And so this long list of basic checks
were done, and none of them could explain what was
going on. None of them are sort of strong enough
to account for the deviations of hundreds of kilometers, So
it wasn't the basic stuff. Then that's when people started
to get creative. I imagine, Yeah, they started to get creative.

(24:40):
They thought, well, what if there's something else on the
spacecraft that's basically giving it a little push. You know
what if there's like effectively a thruster, Because think about
the batteries on these spacecraft. These things generate heat, and
things that are hot radiate photons, and when you radiate
a photon, you're basically getting pushed. You know, you shoot
off a photon to the left, then by conservation momentum,

(25:03):
you're going to the right, and everybody, everything that is
hot is giving off photons. Like me and you, we
glow in the infrared. Right, you put on night vision
goggles like the Predator, you can see a human body
because it's giving off in for a red photon. I
knew you would work in aliens somehow. Wait does that mean?
Like if I put a flashlight out into space and

(25:25):
turn it on, it would start to go like it
would basically act like a rocket. Yes, a flashlight is
a rocket, absolutely. It throws particles out the back, and
so it has to go the other direction. Now, if
you put two flashlights shining in the opposite directions, it
won't go anywhere. They'll balance each other. So the pioneer
people thought, well, it's got these things on it, but
they glow in every direction, right, and so if heat

(25:48):
is the same everywhere in the spacecraft, that's not going
to effectively give it a push. When did? It depends
on the shape, Like a hot sphere would give up
photons in all directions, But maybe, like I don't know,
like it's something it looks like a dish might not. No,
you're right, and it's a really interesting clue. And people
thought for a while about this, like maybe the complicated
shape of the spacecraft is not giving off heat in
the same direction everywhere. But there was another important clue,

(26:11):
which was that this effect wasn't dropping as a function
of time, like it wasn't fading. And we know the
heat from these batteries should be fading, like this is
a radioactive thing. It decays over the half life of
eighty seven years. These things eventually cool and then just
become dead. And so if it's due to the heat
of the batteries, you would expect this thing to fade
with time. But the data we had showed that it

(26:33):
was constant. So people thought, well, can't be the batteries.
I see, it wasn't cool enough. It wasn't getting cool alright,
So then what else did did they think it could be?
So that's when he got exciting. They thought, well, we
can't explain this using any sort of known physics, any
solar wind or any gravity or any heating of the spacecraft,
so let's get creative. And people thought, well, maybe we've

(26:55):
accidentally created something which measures like the expansion of the universe,
because they thought maybe space is expanding inside the Solar
System and it creates this weird gravitational potential. And we
know that gravity and time are connected because, for example,
if you go near a black hole, time slows down

(27:16):
for you. So over vast stretches of space, potentially time
is getting slowed down as you move through these like
expanded space. Whereas our clocks, the ones that we're using
to sort of predict where this thing goes, these really
precise atomic clocks basically assume that space is flat. So
if we're measuring time differently than something that's flying out

(27:39):
into the Solar System, then maybe that could explain it,
meaning like maybe the clock on the spacecraft is wrong
or do you mean like it's actually in a different time,
it's actually in a different time. Yeah, this falls under
this whole set of ideas like the non uniformity of time, right,
And we've talked a lot in the podcast about how
my clock and your clock can disagree but both be

(28:01):
correct because there is no universal sense of time. So
this is an idea sort of along that direction, like
maybe we're measuring the expansion of space and those gravitational
effects are distorting the clock on the spacecraft, not that
it's wrong, but you know it's different. It's just differently timed.
I see. So maybe the idea is that maybe it
is it was supposed to be, but our measurement of

(28:22):
where it is is wrong because the timing in the
clock on board is different than it we think it
should be. And that was so the general concept like
maybe something in this direction will help us because we
know gravity and time are connected. But they couldn't actually
make it work. They try to get the map all
work out and say, is this consistent with general relativity?

(28:42):
What we know about the expansion in the universe? Would
that explain that? And it could never be made consistent,
and so it would have to be like some weird
deviation have to be some special case of how times
affected by gravity for some reason only in our solar system.
And so you know, when you start to develop like
a new idea of for how to explain some weird science,
you want as simple a new idea as possible. You

(29:04):
don't want to have to like add all sorts of
weird bells and whistles and exceptions and stuff. And so
this started off sort of promising and then ended up
being like, it doesn't really fit. It wasn't just that
they it was in the wrong time zone or daylight savings.
That couldn't explain it either. No, I mean I make
that mistake all the time, and they usually blame it
on aliens, which is why I miss a meeting. But no,

(29:26):
they couldn't explain this using any sort of general relativistic
effects on the clock or even simple modifications to general
relativity to allow for effects on the clock of the
expanding space in the Solar System. So that didn't work out.
You can't blame the engineers, and you can't blame Einstein
or general relativity. What else could it have been. Well,

(29:47):
people got excited for a while when they thought maybe
it was dark matter. Right, we've accounted for the gravitational
sources that we know about the planets and the rocks
and the Sun, But we also know that most of
the gravity in the Unit verse isn't from the stuff
that we can see. It's actually from this weird invisible matter.
We still won't understand that's everywhere, and we think that

(30:08):
it fills the galaxy. In fact extends beyond the galaxy
and it's five times as much of it as there
is normal matters. So if you're studying details of gravitational effects,
you might expect to have to take into account the
dark matter. Interesting. Yeah, like our solar system could be
or is probably bated in dark matter, right, that could
potentially affect the gravity of things we send out into space. Absolutely,

(30:31):
dark matter definitely is here. It's everywhere. It's all around us.
There's dark matter with us in this room, the dark
matter all around the Earth. We can't see it or
detective because we think it only has a gravitational interaction.
And remember gravity is super duper weak, so usually we're
only sensitive to dark matter if there are huge amounts
of it, you know, like galaxy sized amounts to affect

(30:52):
how the galaxy spins. It's really hard to detect like
local blobs of dark matter. So people got excited when
they thought maybe Pioneer ten and eleven are like a
local prob they're telling us how much dark matter there
is here. Interesting, But wouldn't if there was some anomaly
in dark matter, wouldn't it affect all the other planets too? Yeah, exactly,
it would. And we also don't think that there is

(31:13):
that much dark matter. We know that there's five times
as much dark matter in general in the universe, but
we don't think in our Solar System there's that much
because doesn't clump up we think the way normal matter does.
Like if you took the normal matter in the galaxy
and just spread it out throughout the whole volume of
the galaxy, it would be pretty thin. That's the way
dark matter is. There's five times as much of it,

(31:34):
but it's much more spread out than normal matter is.
So sort of in the volume of the Solar System,
it's like one million of the mass of the Sun
of dark matter, right, see, So it couldn't be that either.
So then they couldn't blame it on a technical issue
or sort of a theoretical issue, And so it was
a big mystery, I mean for decades, right, Like they've

(31:56):
seen this anomaly for you know, a thirty almost forty years. Yeah,
people really worked on it for a long time, and
it was in the nineties of people that is really
detailed investigation went through all these possible explanations and couldn't
explain it, And then people had a lot of fun
coming up with theories of new physics to try to
explain it potentially, and it's sort of like a standing

(32:17):
question in science for quite a while. People even try
to explain it using like weird modified gravity, like maybe
gravity doesn't work the way Einstein and Newton thought we did,
and somehow gravity changes when you get out into the
outer Solar system. But you know, that doesn't really make
sense because we understand how Pluto and Neptune and Neurinus operate.
So there are a lot of questions about what could

(32:39):
explain this on a lot of crazy ideas thought up
to explain it, none of which were ever really very
compelling or which worked. So it's sort of a question like,
are we gonna find some boring explanation for what this is?
Or is somebody going to come up with a new
idea that actually, you know, makes it all click together
and tells us something new and deep about the universe? Right?
Are you going to get a try again when you
scratch out the start of it? Or are you gonna

(33:00):
win a million aliens in the process. All right, Well,
let's get into how this mystery was finally resolved, if
it was resolved at all. But first let's take a
quick break. All right, So the Pioneer anomaly was a

(33:28):
big mystery for decades back in the seventies, eighties, and nineties,
spacecraft we throughout their into space. We're veering off course, Daniel,
How was it all resolved in the end. If it
was so, we actually do think we have a pretty
good understanding of what happened by now, and it's due
to some sort of like data archaeology by some researchers

(33:49):
really dedicated, really interested in understanding how this worked. They
went and found some old data in the mid two
thousand's and did a really detailed study and they think
they've cracked it. Oh wow, in the mid to of thousands.
So all this time, since the seventies, these space crafts
were out there off course and people just sort of
shrugged it off for a while, or that they just
gave up, or or what. Well, you know, when there

(34:10):
are these puzzles in science, it's not always easy to
know how to crack them, and so sometimes they'll be
out there for a long time, decades even people like,
well that's not understood, but nobody really knows what to
do about it. So finally somebody said, well, I'm gonna
try to go back and find some more data, dig
out some mold data nobody's ever seen before, and make
a more detailed study than anybody's ever done before, and

(34:32):
maybe we'll figure it out. Wow, that's wild to me, Like,
how do you sleep at night? Like, let's say you're
the scientists or the engineer who worked on this, Like,
how do you ever be at peace? You know, It's
like if somebody told you, hey, I know it's your
car mysteriously moves an inch every night, and you're like, well,
I don't know, how could you get to sleep? I
think that's why smart scientists have great ideas in the

(34:53):
middle of the night, because their brains are always working
on these things, are always chewing away on these puzzles
and these questions. All right. So then in the two thousands, UM,
a group of people said, hey, we want to figure
this out. So they say, sort of dug through all
the old data. But that was kind of hard, right,
because this is data from the seventies. Yeah, they literally
dug through old data and they were specifically focused on

(35:16):
this question of the heat from the batteries and whether
or not it really was decaying over time. And what
they discovered is that we didn't actually have all the data.
Some of the earliest data from the spacecraft was like
stored on magnetic tapes and not really included in most
later analyzes because it was kind of a pain to
go and find it and recover it and process it

(35:36):
and stuff. So they did a bit of like archaeology,
and they found more than a dozen boxes of magnetic
tapes stored under a staircase at JPL in Pasadena, and
they worked with like an old school programmer to create
software that could read these tapes and clean it up.
So they got sort of a longer time series on
the data than anybody else had, like data on what

(35:58):
the position of it, or like the signals we were
getting from it, or what on the position of it is,
specifically the earlier positions. Remember we talked about how we
didn't think it was just heat coming off the spacecraft
unevenly because it would have faded with time, and it
didn't look like it was fading with time. So to
trying to answer that question in more detail, they said, well,
let's look at earlier data data when it had just

(36:20):
left the Earth to see if we can spot this
effect earlier on. I see, you're looking for a weird
effect on where you think it should be different from
when then we're where it is, right, yeah, exactly. So
you're you're basically comparing curves. Right, you have the curve
of what you expect and the curve of where it
actually was. And now you're trying to come up with
a new expectation, like can we tweak how we understand

(36:42):
this thing flies so that what we expect matches what
we observe. And you also have to sort of look
back in time in the Solar system, right, like in
this back then, what where were all the planets and
asteroids and all that in order to think about where
it should have been exactly. It's a complicated calculation, and
to do this right you need to know not just
where it was, but you also need to know, as

(37:02):
you were saying, where it's giving off heat. Like the
shape of this thing really affects how it glows, because,
as we're talking about before, it's not just a sphere, right.
Physicists always like to assume everything is a spear at
first order, but the details matter when you're making really
really precise measurements. And this thing has two big hot
batteries on one side, and then like a cool dish

(37:25):
on the other side. But it also has a bunch
of instruments that use that electricity, and so as they
draw current, they get hot, and as the heat up,
they glow, and these kind of effects. Is just the
glowing heat from these things is big enough to explain
the effect that we're seeing. Yeah, and I think you know,
you say batteries and that makes me think of, you know,
like as a battery or a car battery. But really

(37:46):
these are like nuclear reactors right there their generators, Yeah, exactly,
they're not fission or fusion. They're just sort of like
slow rolling radioactive to k that generates the electricity. But yeah,
they are nuclear reactors. But also everything on the space
scrapt that he uses that electricity eventually leaks some of
that energy. Nothing is perfect, right, and it leaks that
energy into heat, just the same way everything that you

(38:08):
use as electronics in it will eventually heat up. Your
computer heats up as you use it. Right, there's no
like fire burning inside your computer. It's just inefficiency from
the use of electricity. So if you have an object
on a spacecraft that's drawing electricity, not even just the batteries,
but the equipment on the spacecraft gets hot as it
uses the electricity. To understand the effect of all this stuff,

(38:29):
you need to know like exactly where everything was on
the spacecraft and how hot it got. These days, we
can do that pretty well. We have like really fancy
software to model this kind of stuff, but you know,
we don't have the records that match that software from
a really really old instrument, Like this thing was built
forty years ago. You know, we have like blue plants

(38:50):
drawn by hand by the original designers. So these researchers
had to go in and build a model of the
sort of heat flow of Pioneer spacecraft by hand like later.
And it's involved like fifteen thousand individual pieces like exactly
where this cable goes and exactly how thick that piece
of aluminum it was, and like this is a huge effort. Wow.

(39:12):
So yeah, they had to basically create a virtual model
of the spacecraft and then put it out into space
with the heat source and see if it would deviate
the way that we they were seeing in the data.
It's really incredible precision. You know. Think about like when
you drive your car, do you expect your car to
slow down when you turn the headlights on right. You
don't expect it to, but actually it does. Right. Turning

(39:34):
on the headlights slows down your car because you're basically
shooting photons away from your car. So this is the
kind of effect that we're looking for. It's amazing that
it actually happens and they were able to figure it out.
So now they're calculations, which include more time information and
a much more detailed model of the spacecraft actually match
really well what we see. So they think they solved

(39:57):
the mystery. They think they've solved the mystery. Turns out
it was the plutonium in the power source next to
Saturn exactly. It's just uneven heating right eventually over many years.
The fact that one side of this thing is hotter
than the other means that it gave off more photons,
which gave it a little bit of a push. It's
like if you had two flashlights out in space and
one of them was a teensy bit brighter than the

(40:19):
other one, you would get pushed away from that flashlight,
tiny little bit. And that explains why it was being
pulled towards the Sun. Because you know that the way
this basecraft is designed, you know you always want to
point the dish back towards Earth kind of right, And
so that means that the hot stuff is sort of
in front of you, which was slowing it down. M Yeah,

(40:42):
we're pointing the hot bits of this thing out towards
the Aliens and the cold side back towards Earth. So yeah,
that effectively slows it down a tiny little bit. I
guess the lesson is, you know, it's always better to
have a hot back end. I'm not going to touch
that at all, but Pioneer eleven apparently had the same
hot back end, and that's why the same thing happened
to Pioneer eleven. The story all sort of fits together

(41:04):
and makes sense. And I love these sort of science
stories when it all this clicks together and the explanation
matches the data and it all just sort of works.
It's like, man, math is correct, physics works, the universe
actually makes sense. It's incredible. I guess you're simultaneously impressed
by yourselves but also disappointed that you're so good at
figuring out the universe. Kind of yeah, it's not as

(41:27):
exciting an explanation is like we discovered a new way
that time flows, or gravity is broken, or something like that,
or we have a clump of hidden dark matter in
the Solar System that would be more exciting, but you
never know, Like science is about exploration, which means you
never know what you're gonna find. Usually it's boring dust
and rubble, but sometimes sometimes it's a real diamond, it's

(41:49):
a gem, something that gives you a clue about the
nature of reality, and you never know, which is why
you just gotta keep scratching, all right. So then this
is sort of in the community, this is the most
accepted explain aation of what happened. Like everyone feels like,
all right, these researchers totally nailed it, they totally explains it,
or is there still some sort of uncertainty about it both.
I think people accept this as an explanation for what

(42:12):
happened to Pioneer ten and Pioneer eleven, So that sort
of anomaly has been solved, but there are still plenty
of anomalies and how spacecraft moved, not just Pioneer ten
and Pioneer eleven, but every spacecraft that does one of
these sort of gravitational slingshots ends up going off in
the direction that we don't quite understand. This is called
the fly by anomally. Basically, every time one of these

(42:34):
things happens, it doesn't quite work out the way we
expect and nobody really understands it. So there's a long
list of things we don't understand about how things move
into Solar system. Even though as basic as just like
gravitational mechanics about things moving in the Solar System, still
have questions that need to be answered, right, which is
all the more impressive that they can. I'm gonna say

(42:55):
we because this is a good thing, So I'm gonna
play the royal we here. We can like land spacecraft
in Mars right and like know exactly where it's going
to land on the planet. That's that's so amazing. It's
really incredible what we the scientists and engineers have accomplished.
It's too bad what they messed up on. All right, Well,
that's the pioneer anomaly, and I guess we can reclassify

(43:17):
it now, Daniel as a pioneer nomally, or the pioneer
story or the high precision test of science, that is
the pioneer spacecraft. That's not as catchy, but maybe more accurate. Again,
another reminder of how you know vast space is I mean,
this thing is still within our backyard, just the solar SYSM.

(43:40):
And even even then, these distances are huge and there's
a lot that can happen out there when you go
out into deep space. Yeah, Pioneer ten is more than
twelve billion kilometers from Earth. It's tens of light hours
from Earth and moving away really really fast. It's destined
to reach another star system in about two million years.
That's sixty seven light years from here. So maybe in

(44:03):
two million and sixty seven years we'll get a message
back from the aliens saying thanks for your garbage, Thanks
for your garbage. With the hot front end, back end,
it's a hot back end. Let leave, we turn it
around and let's see if we can turn this around,
and we'll back into this cultural misunderstanding with the aliens.

(44:23):
All right, Well, we hope you enjoyed that. Thanks for
joining us, see you next time. Thanks for listening, and
remember that Daniel and Jorge explained. The Universe is a
production of I Heart Radio or more podcast from my
Heart Radio, visit the I heart Radio app, Apple Podcasts,

(44:46):
or wherever you listen to your favorite shows. Yeah,