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Mysterious force acting on deep space probes

By Tatarigami in News
Thu May 17, 2001 at 10:02:00 AM EST
Tags: Science (all tags)

Launched in 1972, Pioneer 10 is the NASA spacecraft which has travelled furthest from Earth -- over 7 billion US miles. But scientists from NASA's Jet Propulsion Laboratory say it should have gone even further in that time. An unexplainable force is causing its progress out of the solar system to slow.

The same effect has been observed in the other Pioneer series probes, headed out of the solar system in the opposite direction to Pioneer 10, and scientists still collecting data from these vehicles are at a loss to explain it. Whatever this force is, it is exerting a constant deceleration on P10 of about 0.0000000098 Newtons, or one ten-billionth of the force the Earth's gravity exerts to keep us in our seats.

It has been proposed that currently-accepted theories on the way gravity works need to be re-examined, but this suggestion was ruled out early on, based on the premise that if our ideas about gravity are wrong, it should be observable in the motion of the planets around the sun.

Other possibilities which have been examined and marked as improbably are the gravitational attraction of planets and smaller objects in the solar system, the minute pressure of light from distant stars on the vehicles, inaccuracies in the methods used to measure the crafts' progress, interaction with the sun's magnetic field, outgassing or thermal radiation from the vehicles themselves, and even an unnoticed interaction with dark matter.

The conclusion? Nothing yet. The Mysterious Force isn't going to lose its capital letters any time soon.


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What's doing this?
o Aliens 4%
o Freemasons 16%
o Dark matter 8%
o Someone left the handbrake on 27%
o I am and there's nothing you can do about it 33%
o Inoshiro 9%

Votes: 86
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Related Links
o Jet Propulsion Laboratory
o Pioneer series probes
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o Also by Tatarigami

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Mysterious force acting on deep space probes | 46 comments (21 topical, 25 editorial, 0 hidden)
My guess is... (3.00 / 2) (#2)
by John Milton on Wed May 16, 2001 at 07:58:13 PM EST

perhaps excess dust in deep space. Smacking into a space probe moving at that speed would have a noticeable effect.

"When we consider that woman are treated as property, it is degrading to women that we should Treat our children as property to be disposed of as we see fit." -Elizabeth Cady Stanton

Space dust (3.33 / 3) (#3)
by Tatarigami on Wed May 16, 2001 at 08:19:38 PM EST

Well, IANASA (I Am Not A Space Alien), but I'd tend to think if there was a lot of dust out there, it would obscure our view of other stars more.

Then again, I may be talking through my hat. If there was space dust, I think the site I took my info from would lump it under the 'dark matter' heading, since Pioneer doesn't have any instrumentation that could pick it up.

[ Parent ]
Dust (3.33 / 3) (#44)
by Bios_Hakr on Thu May 17, 2001 at 09:47:08 PM EST

When scientists first began to study the stars in earnest (at least in modern recorded times), they noticed "holes" in space. It was almost as if the night sky was like swiss cheese. Later, with the invention of raido astronomy, it was found that these areas of blackness were heave concentrations of dust. Scientists know that there is A HELLA LOT OF DUST in space. It is generally accepted that most, if not all, dust surrounding a planetary system has been sucked into the parent star, or the planets that make up the system.

[ Parent ]
ISM density (4.00 / 1) (#45)
by SEWilco on Sun May 20, 2001 at 11:54:07 AM EST

The interstellar medium (interstellar gas & dust) is much less dense than normal around our solar system due to the Scorpius-Centaurus Association superbubble and the Geminga supernova bubble. Perhaps we're seeing a slight increase in the ISM density -- of course these researchers should know all about this, so it's still a puzzle...

[ Parent ]
Amazing figure (4.00 / 3) (#6)
by SIGFPE on Wed May 16, 2001 at 08:52:53 PM EST

That suggests a very high accuracy in all of the figures involved - the mass of the craft, the rate at which dust hits the craft, the rate at which matter may be escaping from the craft and so on. Surely when the craft was originally built it wasn't designed as a device for measuring position, mass, velocity and acceleration this accurately. So I'm pretty amazed that this number isn't within experimental error.

What should give me confidence that their tolerances are good enough to report such a figure with any degree of confidence?

Fudged (3.00 / 2) (#8)
by Tatarigami on Wed May 16, 2001 at 09:02:48 PM EST

The article I pulled my info from said that the force was one ten billionth as powerful as Earth's gravity, so I came up with that figure myself by dividing an acceleration of 9.8 m/s/s by ten billion. So it's not flawlessly accurate. It didn't occur to me while I was writing it up, but I should have mentioned that.

Since one of the proposed reasons for the discrepancy was the Earth itself being slightly out of position, it seems to follow that the spacecraft are being tracked from here, which wouldn't place the burden of measuring distances on Pioneer itself.

As for what should give you confidence -- they're wearing labcoats, man! How can you doubt them?


[ Parent ]
Pioneer can't be completely passive (3.50 / 2) (#12)
by SIGFPE on Wed May 16, 2001 at 09:13:59 PM EST

which wouldn't place the burden of measuring distances on Pioneer itself.
I'd love to know what the distance measuring technique is as this can't be a passive process. A mere rock the size of Pioneer would be undetectable at that distance. Whatever the process is it will require active participation of Pioneer even if the observations are made from Earth. I'll have to do some research and find out what they do.
[ Parent ]
Two Ways (4.00 / 2) (#22)
by Devil Ducky on Wed May 16, 2001 at 11:51:07 PM EST

Pioneer has a homing radio signal. It may not be radio, but considering the decade it came from it probably is, but that doesn't matter. The exact radio homing frequency is known (not by me but ny NASA), and yes it would be quite accurate at least 10^10, probably more since it is NASA. They are rocket scientists!

The radio signal would be sent in a spherical shape (or conical depends on the anntenna, either way) so the signal will have a curve when it reaches Earth. The signal will be pretty flat, but not perfectly so. Measuring this curvature, combined with the beginning frequency through a medium with a known loss, put into a differnetial equation I wish I never had to learn and you get a distance.

It's actually considered to be more precise than the old fashioned way of measuring objects in space. Which is to look draw a direct line from a point on the earth to the object in the sky, wait one day (month, year, some period of time) and draw a new line from the object to the earth. The angle of change * the distance the earth traveled in that time period combined with some geometry I never bothered to learn would give you the answer.

This is one of the ways to use a sextant. The other involves measuring the angle between you and two known points, north star, middle star of Orion's belt. Another is to measure your angle from one point, wait some time, measure again; this would tell you your speed or your location. Accoring to Heidenberg(?) you can never know both at the same time...

IIRC, a parsec (Han Solo's measure of time) is how far an object is away from the earth to make an angle of change = 1 degree in one second, it may be in radians or the time may be different I can't really remember.

Devil Ducky

Immune to the Forces of Duct Tape
Day trading at it's Funnest
[ Parent ]
the parsec (3.00 / 2) (#36)
by krlynch on Thu May 17, 2001 at 12:18:34 PM EST

A parsec (or PARallax SECond) is a distance (and hence time) unit of about 3.26 light years. It is actualy defined as the height of the isosceles triangle whose base is one AU with an included angle is one arc second.

I didn't follow what you were trying to say with the rest of your post, unfortunately, so I can't comment/correct.

[ Parent ]

Yes (3.00 / 1) (#38)
by Devil Ducky on Thu May 17, 2001 at 12:36:02 PM EST

How do you say that it's a distance and hence time? Sure, time is a distance but a distance is not neccessarily time.

In the case of Hans Solo, he wished to imply speed. But using a parsec is like:
"Is your car fast?"
"It made it to the Mall in 2 miles."
"Wow, it usually takes me 4 miles."

The way Star Wars fans have explained this indiscretian is quite elaborate and (under current theories from the likes of Stephen Hawking, but not Albert Einstein) true to Physics.

I just realized, no one cares about this, not even me...

Devil Ducky

Immune to the Forces of Duct Tape
Day trading at it's Funnest
[ Parent ]
You're forgetting a constant... (3.00 / 2) (#39)
by darthaggie on Thu May 17, 2001 at 01:08:47 PM EST

How do you say that it's a distance and hence time? Sure, time is a distance but a distance is not neccessarily time.

Sure it is, if your name is Howard J. Photon, and you're traveling at the speed of light.

I am BOFH. Resistance is futile. Your network will be assimilated.
[ Parent ]

Time == Distance (4.00 / 1) (#40)
by krlynch on Thu May 17, 2001 at 01:31:01 PM EST

How do you say that it's a distance and hence time?

I should have been more clear...but I'm a theoretical physicist, so I have degrees granting me the right to be obtuse :-)

I said that its a distance and hence a time because modern theory (namely special and general relativity, or other closely related possibilities) and experiment strongly show that the speed of light is the same constant in all locally inertial reference frames during all epochs of the Universe's history since the so called "decoupling" time (when the universe had cooled enough that photons no longer had the energy to split into electron-positron pairs) and the universe became "transparent". Because of this, the "speed of light" becomes nothing more than a unit conversion constant that relates the distance, length, time, velocity, momentum, mass, and energy units. In fact, in high energy physics, we set c==1 (with no units!), and measure everything else in energy units. So when I say "distance==time" I mean that you could just as correctly use the same units to measure them both.

This is not as wierd as it sounds: think "light-year". This is usually used as a distance measure, but it is defined by reference to a time measure, so can just as correctly be used as a time measure.

[ Parent ]

Some quick calculations (3.50 / 2) (#42)
by SIGFPE on Thu May 17, 2001 at 02:24:02 PM EST

so the signal will have a curve when it reaches Earth
Nice technique! A quick application of Pythagoras shows the bulge at the centre of a wavefront the width of the earth would be around 1-2m. Much larger than I expected so it seems this is a practical method!

The error is potentially very large - every mm error in measuring the bulge would result in about 6 million km of error. Bulges in wavefronts sound like hard things to measure to me and I'd love to know what the setup would be.

OTOH I suspect that the distance doesn't need to be measured all that accurately for these results because the velocity can be measured using doppler shift.
[ Parent ]

Indeed (4.00 / 1) (#43)
by krlynch on Thu May 17, 2001 at 04:40:42 PM EST

OTOH I suspect that the distance doesn't need to be measured all that accurately for these results because the velocity can be measured using doppler shift.

You are correct. They measure the velocity of the spacecraft using doppler techniques to some phenomenal accuracy: 10^-12 for measurement times of 10^3 seconds (see the paper I mention in one of the top level posts).

The range is measured by sending a specific signal, having the spacecraft process it and then send it back and measuring the time difference. This is not as accurate as the velocity measurement, but it doesn't need to be either. A few million kilometers either way is an error of a few hundredths of a percent in distance, and really isn't important. They can also cross check their distance results with their doppler results (integrate!), and these agree very well.

[ Parent ]

How they find the distance? (3.00 / 1) (#11)
by John Milton on Wed May 16, 2001 at 09:13:38 PM EST

To find the distance of the craft, they could measure the time it takes to bounce a signal back and forth between the probe and the earth. Since the probe is low on batteries by now, I don't think that would be effective. Another way is to measure the parallax shift. You look at the object from two widely seperated locations and measure the jump in its apparent position. Sort of like when you alternately close one eye and then the other. Since there are radio-telescopes on opposite sides of the world, the accuracy would be pretty good. Especially since, in the cosmic perspective, the probe is still so close to home.

"When we consider that woman are treated as property, it is degrading to women that we should Treat our children as property to be disposed of as we see fit." -Elizabeth Cady Stanton

[ Parent ]
Parallax is pretty inaccurate isn't it? (3.50 / 2) (#14)
by SIGFPE on Wed May 16, 2001 at 09:27:07 PM EST

Consider how the angle radiotelescope1-pioneer10-radiotelescope2 varies as a function of the distance of pioneer10. It has a pretty tiny derivative so I'd be amazed if much accurate could be obtained. (I've been doing photogrammetry at work so the problems are fresh in my mind!)
[ Parent ]
parallax measurements (3.00 / 1) (#21)
by jkominek on Wed May 16, 2001 at 11:34:50 PM EST

Actually, having radio telescopes on either side of the planet wouldn't be even close to sufficent. You'd wait sixth months and look at it again, so that you'd be looking from two positions 2AU apart, not just a few thousand km.

However, a radio telescope has very low resolution (disagree? consult an astrophysics textbook, I can suggest a title, email me). With hubble unable to spot the lunar lander on the moon, I don't see how a radio telescope could spot Pioneer unless it was a very active source, pointed straight at Earth.
- jay kominek unix is all about covering up the fact that you can't type.
[ Parent ]

That happens to me, too (3.85 / 7) (#33)
by DesiredUsername on Thu May 17, 2001 at 08:04:12 AM EST

Whenever *I* go walking on the outer fringes of the Solar System, I decelerate by 0.0000000098 Newtons to smell the roses.

Play 囲碁
The paper (5.00 / 2) (#37)
by krlynch on Thu May 17, 2001 at 12:31:40 PM EST

The actual paper is at the LANL archive:

Study of the anomalous acceleration of Pioneer 10 and 11
Authors: John D. Anderson, Philip A. Laing, Eunice L. Lau, Anthony S. Liu, Michael Martin Nieto, Slava G. Turyshev
Subj-class: General Relativity and Quantum Cosmology; Space Physics

Our previous analyses of radio Doppler and ranging data from distant spacecraft in the solar system indicated that an apparent anomalous acceleration is acting on Pioneer 10 and 11, with a magnitude $a_P \sim 8\times 10^{-8}$ cm/s$^2$, directed towards the Sun \cite{anderson,moriond}. Much effort has been expended looking for possible systematic origins of the residuals. We find that it is difficult to understand how any of these mechanisms can explain the magnitude of the observed behavior of the Pioneer anomaly. A detailed investigation of effects both external to and internal to thespacecraft, as well as those due to modeling and computational techniques, is provided. We also discuss the methods, theoretical models, and experimental techniques used to detect and study small forces acting on interplanetary spacecraft. These include the methods of radio Doppler data collection, data editing, and data reduction.

There are now further data for the Pioneer 10 orbit determination. The extended Pioneer 10 data set spans 3 January 1987 to 22 July 1998. [For Pioneer 11 the shorter span goes from 5 January 1987 to the time of loss of coherent data on 1 October 1990.] With these data sets and more detailed studies of all the systematics, we now give a result, of $ a_P = (8.74 \pm 1.25) \times 10^{-8} {cm/s^2}$. (Annual/diurnal variations on top of $a_P$, that leave $a_P$ unchanged, are also reported and discussed.)

The paper is about a month old, and is about 100 pages long, so maybe I'll read it and make a few comments of summary here later.

Update (5.00 / 4) (#41)
by krlynch on Thu May 17, 2001 at 01:51:34 PM EST

As promised, I've looked through the paper, and here is an update. First, I haven't read it in detail, but overall, it seems to be an extremely well done piece of work, and is extremely well presented (clearly and precisely). Their modelling and error analysis is very clean and believable, and they clearly point out every place that they might have gone wrong, and why they don't think that they did. Here's what they've considered as explanations:

  • Direct solar radiation pressure and mass
  • Solar Wind
  • Solar coronal effects and models
  • Electromagnetic Lorentz forces
  • Kuiper belt gravity
  • Planetary gravity
  • Phase and Frequency stability of the Deep Space Network (DSN)
  • DSN antenna complex changes
  • Radio beam reaction forces
  • RTG (the little "nuclear reactors" that provide power) heat reflecting off the spacecraft
  • Differential emissivity of the RTGs
  • non-isotropic radiative cooling of the spacecraft
  • Helium expelled from the RTGs
  • Gas leakage
  • Numerical stability of computer models
  • Accuracy and consistency of computer model test
  • New manifestations of old physics
  • New manifestations of NEW physics

The paper concludes that this anomalous acceleration is most likely something mundane that they've missed! Most likely some "gas leakage" or "heat leakage" that they don't understand, and NOT some new physics, since they have't been able to explain how the differences observed in all the deep space craft could arise from any new physics scenario.

Just an example of how detailed their work has been, they've considered all of the following possibilities, just for systematic effects impacting the DSN antenna systems: aging of the structures, long term antenna deformations due to gravity, ocean loading, wind loading, thermal expansion and contraction, long time drifts of equipment parameters, imperfect knowledge of antenna positions, errors from tropospheric and ionospheric models, errors modelling the Faraday rotation effects of the atmosphere. This is truly a phenomenal piece of work.

[ Parent ]

How about charge? (none / 0) (#46)
by leonbrooks on Tue May 22, 2001 at 04:47:25 AM EST

The probes will have built up a considerable whack of charge from the solar wind, maybe the ``background'' charge of the galaxy acting on this charge is enough to repel the vehicles gently sunwards?

Or perhaps they've picked up an opposite charge from a field not yet studied (solar equivalent of can Allen belts?) and are finding the sun more attractive than ever.

Just a thought.
-- If at first you don't succeed, try a shorter bungee
[ Parent ]

Mysterious force acting on deep space probes | 46 comments (21 topical, 25 editorial, 0 hidden)
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