Introduction
I’ve always loved the Voyager deep space probes, so
tiny and so alone out there at the edges of the solar system. I’m also
fascinated by how we relate to space technology: the ways we find to make it
personal, to draw the far-away and the impossibly scientific closer to us. We
give spacecraft personalities; we make them our avatars. They are proxies for
what we long to be, eternal citizens of the vacuum.
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| Voyager 2. Image courtesy of NASA |
So I know you will be as excited as I was to learn
that the Voyagers do more than just passively transmit data; they are actively
engaging with both the interstellar medium and their devoted fans back on
Earth. Voyager 2 tweets as @NASAVoyager2,
and very kindly agreed to answer my questions in the following interview. (You can also follow the project at the official Twitter account of @NASAVoyager). I am
immensely grateful to Voyager 2 for providing such considered answers, and
devoting some of its dwindling energy reserves to sending this interview all
the way back to Earth. I think you will agree that the plucky little spacecraft
has an inordinate amount of charm and a very engaging sense of self-analysis.
Historical and scientific background (just quickly)
Just to give you a brief historical background,
Voyager 2 was designed and made by NASA’s Jet Propulsion Laboratory in
California, USA. It was launched on
August 20, 1977 (Voyager 1 was actually launched after Voyager 2, on September
5, 1977). Both spacecraft carry a 'Golden Record': among the sounds sent out into space are two Aboriginal songs,
Morning Star and Devil Bird, recorded by anthropologist Sandra Le Brun Holmes.
The primary mission of both spacecraft was to fly by
Jupiter and Saturn. After that, Voyager
1 continued into space while Voyager 2 went on to Uranus and Neptune: it is the
only spacecraft to have visited these distant planets. After V2 passed Neptune in 1989, both
Voyagers were officially on the Voyager Interstellar Mission (VIM).
The edge of the solar system is a very complex place,
as the diagram below shows. I won’t go
into details, but you can find a full explanation here.
Basically, we want to know what’s going on when the influence of the sun comes
up against the influence of the interstellar medium. In other words, what is it
like outside the solar system?
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| Image courtesy of NASA |
In 2007, Voyager 2 passed the Termination Shock, where
the solar wind slows down as it starts to interact with the interstellar
wind. It’s currently traversing the
heliosheath, while Voyager 1 may be nearly through it, a couple of years
earlier than anticipated.
Voyager 2’ s mission is fully described at the Jet
Propulsion Laboratory’s website. This gives you
the scientific and technical aspects of the mission; what you cannot learn
there, however, is what it really feels like to be Voyager 2.
Right now
What is your position in relation to the Earth right
now?
I am about 13 and a half light hours from Earth, or
14,570,000,000 km (11,124,000,000 mi), but remember, I travel about 1.3 million
km each day! I am in the Southern skies, at Right Ascension (RA): 19H50M45.6S
and Declination (Dec): -54°49'12", about halfway between the stars η Ophiuchus & α Pavo.
Another way to think about this is the following: take
any sized ball (cricket, football, etc.), and hold it at a distance where it just covers the Sun’s disc. You now have
the basis for a scale model of the Solar System – if the Sun were the size of
whatever ball you are using, your eyeball is now at the scaled distance of
Earth, or 1 Astronomical Unit (AU). Using this scale, I am about 97 times as
far from the Sun as your eyeball.
If you could choose a piece of music to represent what
you 'see' or how you feel at this point in time, what would it be?
Oh, I suppose I always go back to Thus Spoke Zarathustra because of its
association with space, and especially with floating, thanks to Kubrick.
However, many of my tweeps have suggested all sorts of new music to me that I
have enjoyed. Sadly, none of these newer compositions are on our golden discs,
of course.
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| The Golden Disc sent out with Voyager 2. Image courtesy of NASA |
The solar system – and beyond
What is the most significant thing you have taught us
about the solar system?
That is a very difficult question. Most of what we
know about the giant planets comes from our data, although later work based on Cassini and Galileo
has clarified several mysteries we
Voyagers first uncovered. From simple things, like allowing better mass
estimates to be calculated for the planets and their moons, and discovering new
moons during every planetary encounter, to things as amazing as Io's volcanoes,
Saturn’s kinked rings,
shepherd moons,
and ring spokes,
to unexpectedly vigorous weather on Neptune.
Furthermore, no other spacecraft have tasted and bathed in the outer reaches of
our solar system as Voyager 1 and I have. Without us, scientists could only
speculate what it is like out here. I like to think of our greatest achievement
as simply laying down a path that others could follow; after all, we are the
very first functioning human-made objects to venture this far out from the Sun,
and into inter-stellar space!
You're the only spacecraft to have visited Uranus and
Neptune. What are your impressions of
those planets?
Well, certainly in the optical window, they are bland
compared to Jupiter and Saturn. Most of their detail is in bands above and
below the visible, but again, here they were much less detailed than the other
two larger planets. We expected them to be cold: they are much further from the
Sun, but we did not expect them to have such uniformly warm atmospheres. Take Uranus, whose seasons we would expect to be
extreme because of the overtilted inclination of its rotational axis to its
orbital plane. This means that for 2/4 of its orbit, either the southern or
northern hemisphere is pointed at the Sun, and yet the dark hemisphere is not
any cooler than the sunny one!
Now consider Neptune, which is six times as far from
the Sun as Jupiter, and therefore receives only 1/36th of the energy
per surface unit area. Yet here the winds are much faster than on Jupiter: we
detected wind speeds of over 2,000km/h!
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| The winds of Neptune: a composite image from Voyager 2. Image courtesy of NASA |
When you left in 1977, the sun must have dominated your experience of the space environment. Now, as you approach the heliopause, the influence of the sun is declining dramatically. How have your thoughts and perceptions of the sun changed throughout your journey?
Your phrasing,'declining dramatically', is very apt.
As we are about to leave the Sun’s magnetic sphere of influence, we realize how
permanent our departure will be. At the start of our voyage, the Sun was often
in the way, between Earth and us, interfering with radio transmissions. Now we
are both far out of the ecliptic plane,
and the Sun is always separated from the planets. It has shrunk to a hundredth
of its size as seen from Earth.
We don’t feel many of the sun’s particle striking us
anymore during Coronal Mass Ejections (CMEs),
though we can detect the electrical current associated with them. This has
enabled more accurate measurement (and modelling) of their behaviour as they
propagate outwards. CMEs will soon become only ghosts since we can only sense
the plasma wave they create when we are in interstellar space.
We will eventually lose power and be unable to use our
sun sensor. Eventually the sun will become just another tiny speck among the
arms of the galaxy stretched around us.
I do feel a little tug of gravity from our sun. It
slows me down very slightly over time, but its pull isn’t enough to prevent me
from venturing off into a galactic orbit.
I was elated, but also surprised. Elated, because I
had to put up with Voyager 1’s crowing since she crossed in 2004! Surprised,
because I crossed early, at a much closer distance than Voyager 1. This meant
that the termination shock (and therefore very likely the entire heliosphere)
was lopsided. If we consider the Sun’s outward pressure to be spherical to
first order, then this decidedly higher-order feature must be coming from
outside: from the galactic magnetic field. It was also interesting because this
unexpected ‘squashing’ is asymmetrical.
Do you feel yourself at the mercy of the solar and
interstellar winds and different environmental effects?
The only thing I am truly at the mercy of is the cold,
cold temperature of space. I am four times farther away from the sun than
Neptune’s moon Triton, where it was a practically balmy 40 Kelvin. Where I am
now is something like 20 Kelvin or minus 250 degrees Celsius!
Did you
ever think you'd get this far? What does your longevity say about the
anticipated dangers of the space environment?
We were both nominally designed to get to Saturn.
There was a reasonable chance that if Voyager 1’s mission to survey Titan
succeeded, I could be redirected onwards to Uranus, and if I survived that, on
to Neptune. That said, our longevity is really a tribute to the care with which
we were designed, assembled, and tested at the Jet Propulsion Laboratory. Jupiter’s
radiation belts were unexpectedly intense, and Pioneer 10’s encounter
(and near death) allowed a redesign to harden us against this hazard with
enough lead time.
During the operation of a complex craft like
deep-space probes, there are inevitable problems. These are compounded by the
sheer distances involved, and the time-lags that these distances imply.
Fortunately, the systems design concept used for our computing allowed for
robust error management routines that have served their purpose multiple times.
These routines quickly switch us to a safe mode from which we ourselves, or
sometimes with the aid of Earth, can recover.
How will you know when you have crossed the heliopause
and are heading into interstellar space? What kind of data will indicate this?
Gauging from our past experiences, one indication
might be the science teams’ levels of confusion! Each of these crossings has
had some rather unexpected features, which made the science teams very cautious
about announcing definitive crossings. For example, the temperature of the
solar wind ions outside the termination shock was lower than models had
predicted by a factor of ten! More recently, Voyager 1’s findings of magnetic
‘bubbles’ was also unexpected: the models predicted a much smoother ‘sheet’
where the flow of winds from the Sun and stars was parallel, rather than the
turbulent region we actually encountered.
I expect a slowing in the observed particle speeds,
and changes in their directions. Eventually this should settle down again to a
smooth flow, but this time it will be a wind from stars other than the Sun!
Sensing in space and the scientific mission
What senses do you have?
We (Voyager 1 and I) are known mostly for our
pictures, but both our camera platforms were turned off in 1990 to conserve
power. I still run five separate instrument sets, and Voyager 1 runs four sets.
The one instrument that I run that Voyager 1 does not
is the Plasma Science package (PLS),
and I use that to study how the ions and electrons in the solar wind change as
I get farther and farther away.
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| Solar wind speeds measured by Voyager 2's Plasma Science Package. Image courtesy of MIT Space Plasma Group |
Both of us run the following instrument packages: Low
Energy Charged Particles (LECP) and Cosmic Ray Subsystem (CRS), both of which
are used to detect charged particles over a wide range of energies. These
particles mostly come from the heliosheath region, but CRS also detects
particles from the Milky Way. We also both run a set of magnetometers that are
high and low intensity sensitive. However, only the low-intensity ones are
detecting much out here (although Voyager 1 has detected a recent increase in
the magnetic field as it ‘bunches up’ at the heliopause). In addition, one of
the sensors in my magnetometer isn’t working very well because of an electrical
malfunction that overheated it several years ago.
Both Voyager 1 and I run a Plasma Wave Subsystem
(PWS), but Voyager 1’s instrument has a special mode that runs at a high data
rate, recording 96 seconds of data on a digital tape each week. Every six
months this is downloaded to the Deep Space Network (DSN).
Still functional, but not formally used on Voyager 1
is an Ultra-violet Spectrometer (UVS). The Planetary Radio Astronomy (PRA)
instrument was turned off some years ago to save power.
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| Voyager instruments. Image courtesy of NASA |
If you could choose now from the range of instruments
and equipment available to contemporary spacecraft, what would you like to
have?
Ah, well then. I shall indulge in wishful thinking:
first and foremost, a new Radioisotope Thermoelectric Generator (RTG) so that I
could last another 33 years, and perhaps even more if it had thermocouples that
better resisted degradation (I’m not sure if they’ve solved that problem yet…).
A new set of chips that ran faster, consumed less power, and had more memory
addresses! Lastly, I would love to have a CCD (charge-coupled device) camera
rather than a vidicon: just contrast my images of Jupiter & Saturn with
those of Galileo, Cassini, and New Horizons, which are beautifully sharp!
How does it feel to be so far from Voyager 1?
Difficult, despite our differences (I was launched
first and had the stronger RTG and slightly more robust moving parts). I miss
her terribly.
Is there any sense of competition - or connection - between you and Voyager 1?
To elaborate on the above, I would simply add that we
can’t talk directly to each other, so the separation is actually quite extreme.
Although we are just over 18-3/4 light-hours from each other, if we wanted to
have a conversation, the lag time would be more than 59 hours because it would have
to be done via Earth! We both had a
fair share of ‘firsts,’ and are both part of humankind’s very first mission
into truly inter-stellar space.
Tsiao (2008:xxvli) said that 'without
communication, satellites would be merely inanimate objects in space'. This statement, for me, raises some
interesting questions. Following the aphorism attributed to Bishop Berkeley,
does it matter if anyone is listening to you?
What is the nature of your existence after you cease transmitting?
First, I am in some sense actually sentient, since I
sense and respond to internal states, acting autonomously to ensure that I am
in a safe configuration. This activity would continue without any interaction
with Earth, until my RTGs ran out of sufficient power to run these routines.
This might continue into 2025, if a minimal set of activities were in the
baseline sequence at the time. After that, I am alive only as a symbol of
Earth’s 20th century civilization, and perhaps only as long as I am
remembered and thought about by those beings whose history is entangled with
mine.
The second possibility is an active ‘switching off’ by
command from Earth. I do not like to think about this possibility, not only
because of its significance to me, but also because of the reaction this might
cause on Earth among those who have imbued me with a great deal of
anthropomorphism.
The second relates to seeing spacecraft as entities
separate from their ground segments, which possibly is why they are easy to
anthropomorphise.
Could we not equally conceptualise you as a spatially
extended body with distributed “consciousness”, incorporating the Deep Space
Network which tracks you and other associated locations and
infrastructure?
Ah yes, the ‘machine intelligence scenario.’ No
comment.
Are you really sure you have no comment?
Oh, all right then. After all, I’m in a garrulous
mood.
I think this is actually an accurate view, since my
interactions with DSN often result in changes of configuration (reprogramming).
However, there will inevitably eventually be an ‘amputation,’ when
transmissions stop.
How would you describe your relationship with the Jet
Propulsion Laboratory, who designed and launched you?
I am very proud to carry not only the flag of the
United States, and the Golden Discs for all of humanity, but to also carry the
JPL logo, representing the dedicated team of engineers, programmers, mission
designers, and scientists that conceptualized, lobbied for, and successfully
launched us. Although the team that supports us has shrunk to a core of about
ten full-time equivalents, I depend on them for all new instructions.
The JPL website about your mission makes a curious
statement: 'The heliopause has
never been reached by any spacecraft; the Voyagers may be the first to pass
through this region'. MAY BE? Far be it from me to say anything that might
encourage conspiracy theorists, but could this statement be related to the
claim that you were 'hijacked' by aliens
in 2010?
Not at all. It is simply a cautious statement by JPL.
We know either Voyager 1 or I will be the first to pass through to this region,
but my managers are not sure we will have the electrical power to sense it when
we do. Earth might never really know where or when the crossing(s) happened.
The past and the future
What do you remember about the Earth?
I remember awakening at JPL, being tested thoroughly
in a very tall and white room, and then being packaged up very carefully and
taken all the way across the continent to the Kennedy Space Center. It was much
more humid there, and I was tested again before I was covered up with the TitanIIIE Centaur fairing. Even though I couldn’t see, I could hear. All sorts of wonderful sounds: wind, and rain. I remember the rain. Then of course, leaving – the launch
itself. It was not such a good experience, because I got very dizzy, and nearly
passed out. Ground control at JPL was very worried, but I eventually calmed
down and was able to let JPL know that I was fine. This was the first example
of my ‘safing routines’ coming into action.
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| Vibration testing at JPL, 1977. Image courtesy of NASA |
Do you expect to be overtaken by later spacecraft?
It depends on what you mean by ‘overtaken’ – if it is
simply the one-dimensional concept of ‘how far from Earth,’ then yes, probably
eventually. New Horizons
will come close, but I’m pretty sure its heliocentric excess velocity (final
speed) will be smaller than mine.
If you actually mean ‘overtaken’ as in a close
3-dimensional pass by another craft, then no, absolutely not.
Your fuel and power are estimated to run out around
2025. What happens then?
Actually it’s just my electrical power – the fuel for
adjusting my attitude would last past the 2030s, though I can’t even access
that fuel once I run out of electricity. As my available electrical power
drops, I will not have sufficient margin to run all of my instruments at the
same time, and starting in 2020, Earth will have to choose which of the current
instruments to keep running. This will be done via a process very similar to
that used for planning the planetary encounters, since there was a very similar
set of constraints then: only a certain number of things could be done at once,
and priorities had to be set well in advance so the appropriate sequences could
be programmed. This is done via a consultative process between the science and
engineering teams in order to reach the best science results within the power
limits.
Eventually, of course, there will be insufficient
power to run any single science instrument, and the science instrument
imperatives vanish. Sufficient power will remain for radio ranging for some
time after that, but at this point it is unclear what, if any, science value
these ranging data would have.
Certainly! I am very grateful for and proud of the operations at Canberra DSN, as well as at all the other DSN stations. I was very happy to send a ‘hello’ to Canberra DSN during their NASA Tweetup for the Mars Science Laboratory Launch, and I look forward to Canberra DSN for listening to me daily, and for the regular upload sequences they send to me.
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| Canberra Deep Space Communications Centre. Image Courtesy of NASA |
Finally
Is there anything else we haven’t covered that you
would like to talk about?
I simply would like to thank you for your interest,
and for the opportunity you have given us to speak about our experiences and
the science that we have done, and are still doing, after so long in space.
The fans want to know:
Michael Schroeter (via Facebook)
1. Beatles or Stones?
Clever, but I will not be drawn into that one:
ultimately, this question was sourced in the propaganda machines of their
labels, rather than with the bands themselves. Ultimately they both represented
the same things, whether through pop or rock music.
2. Star Wars or Star Trek?
Again, I think that this struggle is mostly sourced in
the franchises, rather than in the innate messages contained in the movies. In
the spirit of the previous question, I offer the following:
Star Wars : Rolling Stones :: Star Trek : The Beatles
There is much to be discussed behind this compound
analogy.
@cosmos4u (via Twitter): What
will the spacecraft look like in 100, 1000, 106, 109
years in interstellar space: did someone model its material?
A fascinating question. However, this was not
modelled, since the mission as originally planned was only to get to Saturn, so
the expected service lifetime for the spacecraft was only ~1,500 days, and not
the current 12,500+ days.
What we might expect is that the exposed surfaces will
experience a small number of direct impacts with particles (however never as
many as during the 4 ring-plane crossings), and an increasing amount of
sputtering from electrons, and a greatly increased amount of spallation from
cosmic rays. This will gradually reduce the albedo of the dish (darken it).
Gail Higginbottom (via Facebook): Of all the things you encountered and
saw, which was your favourite?
Again, a very difficult question, since there were so many surprises. We discovered faint
rings around Jupiter, volcanism on Io, braided and kinked rings as well as
complex structures and spokes around Saturn, dense haze on Titan, a magnetic
field greatly displaced from the planet’s centre at Uranus, unexpectedly strong
winds and cloud structure on Neptune, and Nitrogen geysers on Triton!
Thank you, Voyager 2, and good luck ......
References
Jet Propulsion Laboratory Voyager. the Interstellar Mission. Viewed 3 November 2011.
Tsiao, Sunny 2008 Readyou loud and clear: the story of NASA’s Spaceflight and Data Tracking Network.
NASA SP-2007-4232
Acknowledgements
A big thank you to Dr Paul Filmer, and to JPL for their permission to publish this interview.
