Voyager 2 to Switch to Backup Thruster Set | Hacker News Reader

Voyager 2 to Switch to Backup Thruster Set

nasa.gov

96 pointsJ3L240456 comments

From my wikipedia dive: the Voyager spacecraft are powered by "radioisotope thermoelectric generators", each of which is essentially a barrel full of plutonium spheres. As the plutonium decays, it generates heat. The resulting heat differential (one area is now hot, some other part of the craft remains cold) is then used to generate electricity via the Seebeck effect.

http://en.wikipedia.org/wiki/Voyager_2#Power

http://en.wikipedia.org/wiki/Radioisotope_thermoelectric_gen...

Also known as "things we really hope the press never hears about".

"Collectively, the RTGs supply the spacecraft with 470 watts at launch and will allow operations to continue until at least 2020."

That's interesting. I always thought it was a small nuclear reactor onboard.

I don't see why the press/public would freak out to know that nuclear power is being used on a space probe 9 billion miles away.

Because it's launched from the earth, and rockets have an unfortunate tendency to blow up at times. That's why the RTG's are designed to re-enter safely even if there's an explosion.

Even after it leaves orbit, the probes loop back to use the Earth for gravitational assist and there's a minuscule chance that it will crash; with odds something like 1:1000000. In the absolute let's-imagine-the-impossible worst case scenario, an additional few thousand people are expected to get cancer.

Summarize that as "but if it crashes into the Earth, thousands of people will die!"

Even the hugely improbable worst case scenario of an RTG powered spacecraft impacting the Earth during a flyby is unlikely to lead to any increased deaths per se. In such a scenario (again, astronomically unlikely) the RTGs fuel would vaporize in the upper atmosphere and ultimately add a very tiny additional contribution to the global background radiation level.

This is a pretty thorough analysis of the risks/outcomes along those lines: http://www.dartmouth.edu/~chance/course/Syllabi/97Dartmouth/...

That document estimates 120 extra cancer deaths, should the nearly-impossible flyby reentry occur. However, a better text is the Cassini Final Environmental Impact Statement, chapter 2 (http://saturn.jpl.nasa.gov/spacecraft/safety/chap2.pdf). Table 2-8 estimates 4500 deaths, assuming there is no lower limit for radiation exposure, or 15 deaths if you exclude those with under 1/1000 rem exposure.

> In such a scenario (again, astronomically unlikely) the RTGs fuel would vaporize in the upper atmosphere and ultimately add a very tiny additional contribution to the global background radiation level.

In that case, the media would spend all their time talking about how many billions of becquerels of plutonium got released into the ocean (or atmosphere, or wherever), not having any directly-attributable deaths to point to or any other large measurements to use.

The media can't do math, they'd try to compare it to some other thing (like Three Mile Island, Chernobyl, or Hiroshima) and probably get it wrong by a factor of 1,000 one way or another.

What's funny is that they even get Three Mile Island wrong, everyone regularly confuses it with a movie The China Syndrome that was released around the same time.

How about if the rocket blows up on launch? Wouldn't the launch pad area be covered it aerosolized plutonium?

No. Most of it would be in ceramic chunks, which are much less dangerous.

I don't think anyone cares about nuclear power being used on a space probe 9 billion miles away. People do care about said space probe flying up and out of our atmosphere, just like they did with the Cassini probe[1].

[1] http://motherjones.com/politics/1997/09/cassini-controversy

The only production spacecraft with actual reactors were the Soviet RORSAT (US/A) radar surveillance satellites. The US also launched one test reactor, SNAP-10A.

The press has long known about this. There were and are a number of protests each time an RTG-based probe is launched.

There was a huge outcry for Cassini's launch.

Of course, I doubt very many of those folks would own up to having been against Cassini now with all of the fantastic results we've received.

The outcry for Cassini was because they were going to use Earth for a slingshot boost. People were worried they would miss and it would crash (despite a gravitational slingshot never having failed in that way in any other launch).

In response NASA ordered Cassini to navigate slightly away from Earth, and NASA would transmit orders for it to redirect to earth at the last minute.

It made no real sense, but I suppose this way just in case NASA vanished into a alien portal Cassini would not crash into the earth :)

If I recall correctly, the Galileo probe (around 1989) had the same problems from anti-nuke folks that feared a radiation fall-out if the shuttle exploded while bringing the satellite into space.

As did Cassini in 1997. The protesters have a point (although as ever overstate the problem) but there’s not much option for power that far out from the sun.

RTGs have been powering deep space probes since the beginning of our travels into space. It's really the only technology that will keep a craft powered for many years/decades. Without them you simply can't do these kinds of missions.

And they're not that scary. There's no fission (or fusion) happening here. It's just a radioactive sample that heats a surrounding material by it's natural radioactive decay.

When isotopes decay, the process is known as fission, so there is certainly fission going on.

He means there is no sustained chain reaction going on. It's neither critical nor supercritical. I have a chunk of uranium ore on my bookshelf doing it right now.

Where from, what cost, any requirements for potential owners?

Here you go http://unitednuclear.com/index.php?main_page=index&cPath... Shipped direct from Oak Ridge!

I got it from a geologist buddy of mine a few years ago, I think he said he found it lying around in Arizona. It's fairly harmless, just looks like a rock really (unless you have a gieger counter).

As far as I know, there are no restrictions on such things. You can pick up pieces of uranium glass and old fiestaware on ebay for a couple of bucks (both about as radioactive as my rock :)

Not all isotopic decay is fission. When an alpha particle is emitted it can be considered fission in the broadest sense (though that's somewhat unusual nomenclature). However, when a beta particle is emitted or absorbed it's not fission.

You're certainly correct - sadly, that never seems to matter much to the press.

It doesn't matter that it is not scary. Most people are unilaterally afraid of anything to do with radioactivity.

Which is, in my opinion, like being scared of a house cat because you've seen lions before.

Just wait until they hear about the RTG powered pacemakers, first implanted in 1970. ( http://home.comcast.net/~dprutchi/nuclear_pacemakers.pdf ) [PDF]

I don't know if it's so bad if the press hears about the nuclear batteries. It might distract them from the larger risks involved with launching a satellite. For example, the tank of hydrazine used for the propulsion system. If that springs a leak there is a highly unstable and toxic gas all over your launch site - even after the giant fireball it causes destroys your satellite. Next I would worry about the ready to explode rocket stack beneath the satellite. If that explodes on your town it will hurt many, many more people than the thermoelectric generator that was incinerated in the explosion.

> If that springs a leak there is a highly unstable and toxic gas all over your launch site - even after the giant fireball it causes destroys your satellite.

This actually happened at least once to a Soviet Proton (UR-500) rocket; it was a big factor in the cancellation of the much larger UR-700.

"The main element of these deep-space NASA missions that fuels the conspiracy is the RTGs, or radioisotope thermoelectric generators, that power space probes such as Cassini, Galileo, Voyager, and others. Past Mars there's not enough sunlight to provide the power a spacecraft needs, and so these RTGs are the only option we have. We've mentioned them before on Skeptoid: Russia has used similar generators to power about 150 lighthouses along its extremely remote northern coast. Heat from a radioactive element, usually plutonium-238, goes through a thermocouple, which is a material that produces a direct electrical current when heat is applied to it. RTGs have no moving parts and are extremely simple and reliable."

Brian Dunning writes this in 'The Lucifer Project', http://skeptoid.com/episodes/4143. Some people "conjecture that such a payload of radioactive material would act like an atomic bomb in the high-pressure depths of a gas giant, and they suppose that this would somehow ignite the entire planet, turning the whole thing into a small star."

The Lucifer Project is about turning Jupiter into a second Sun.

This is a good animation of them working (animated GIF): http://www.grc.nasa.gov/WWW/TECB/videos/animation_TDC_01_jef...

What's the information security posture on these things like?

What I mean is, what was considered "secure" in 1977 (DES based encryption was state of the art back then, right?) would be considered laughable today... I wonder what precautions are taken by NASA to prevent someone from intercepting and decoding NASA's commands to the probe and then beaming their own commands to Voyager 2.

From what I've read, the signals from these probes are so weak that you need large receivers coupled with extremely low temperature electronics to detect them. That puts it out of the reach of everyone except the extremely well funded. In the end, it would be a short list of suspects.

Yes, you quite literally need one of these to communicate with Voyager:

http://www2.jpl.nasa.gov/files/images/browse/jpl9020ac.gif

Does it blow anyone else's mind that we have a piece of technology 9 billion miles away and we are actually talking to it?!

Does anyone know any details on what kind of software runs on Voyager 2 and how they program it from Earth?

Is there just a list of predefined commands (activate this, deactivate that, send images) or do they overwrite the 'firmware' with each upload or ...

Firmware, see: http://www.space-travel.com/reports/NASA_Fixes_Bug_On_Voyage...

In this case they downloaded the firmware from Voyager 2 and found a flipped bit.

It surprises me how little information about the Voyagers is on the internet. I'd love to know everything about what makes them tick.

Try exploring http://www.nasa.gov/mission_pages/voyager/index.html and http://voyager.jpl.nasa.gov/

The later has weekly mission reports all the way back to 1995. I also just submitted information about its onboard computers:

http://news.ycombinator.com/item?id=3204224

I found something: http://voyager.jpl.nasa.gov/faq.html search for "What kind of computers are used on the Voyager spacecraft?"

Really cool to think we sent this guy out there which is now 9 billion miles away, and we are still in contact/command with it.

Yeah, and the fact that it takes DAYS for a command to reach it.

13.4 hours http://www.google.com/search?q=9+billion+miles+/+speed+of+li...

Consider that there are almost certainly several undiscovered worlds as large as or larger than Pluto closer to us than Voyager 2 is now.

That these things still work is an amazing feat of engineering.

The computer on that thing probably has the longest uptime in the solar system.

What are they using the thrusters for at this time? Just attitude adjustment to get the antenna pointed at the Earth?

First sentence of linked article: NASA's Deep Space Network personnel sent commands to the Voyager 2 spacecraft Nov. 4 to switch to the backup set of thrusters that controls the roll of the spacecraft.

The Voyagers are 3-axis stabilized (vs spin stabilized) and they use the thrusters for attitude control. Without the thrusters the antenna would drift away from being pointed at Earth and we'd lose contact.

More modern spacecraft use non-propulsive systems such as momentum wheels and gyrodines to maintain 3-axis attitude control and use thrusters to reset only when those systems become saturated.

Do the thrusters use the same power source as the instruments?

No. The thrusters are chemically powered. The instruments are powered by radio-thermal generators.

"Voyager 1 changed to the backup for this same component after 353,000 pulses in 2004."

Impressive.

I'm amazed they can keep it pointed for so many years without running out of fuel. They must put out about the same thrust as a burping flea.

"Slow to impulse, standard orbit, Mr. (Crusher|Sulu)"

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