Author Topic: Deep Space Probes??  (Read 13125 times)

Offline DMeader

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Re: Deep Space Probes??
« Reply #20 on: 10/15/2008 06:55 pm »
I don't think Dawn and Cassini are what you were talking about, right?  While both are great missions, I wouldn't classify them as "deep space probes."

It's a little ambiguous. I consider anything outside of Earth orbit "deep space", that's why I answered the way I did. Others may have different criteria. I see now that the OP was speaking of spacecraft that would eventually leave the solar system.

Offline Namechange User

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Re: Deep Space Probes??
« Reply #21 on: 10/15/2008 07:09 pm »
Other than Voyagers, is there anything currently out there?? Any plans for such a mission?? (Not that I could see) Seeing as they would be relatively economical, I'm curious as to why we aren't sending them out. Perhaps one every 3 to 5 years (10 years?) so they could effectively form a "relay chain" for data transmission?? Be nice to be doing something while we're waiting for new launch vehicles......

If they all follow the same path, what would be the point in sending one after another? 

By the way, Buck Rodgers according to TV when I was a kid was launched in 1997 on "America's last deep space probe".  So he should be on ice out there somewhere. 
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Offline Analyst

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Re: Deep Space Probes??
« Reply #22 on: 10/15/2008 07:29 pm »

Does anyone know what sort of data transfer rates do we get from Voyagers?

The Voyagers are still amazing in terms of data rates. Using a 3.7 m HGA and a X-band TWTA with 18 W RF output they achieve(d) (max.):

- 115.2 kbps from Jupiter (5 AU),
- 44.8 kbps from Saturn (10 AU),
- 29.9 kbps from Uranus (19 AU), with DSN arraying,
- 21.6 kbps from Neptune (30 AU), with DSN arraying,
- 1.4 kbps now from 110 AU during DTR playback, with DSN arraying, this will work until about 2015 (~ 135 AU),
- 160 bps now regular, using a 34m HEF, will work as long as they have power to operate the transmitter (~2025 plus).

Compare this with New Horizons (2.1 m HGA) little more than 1 kbps from Pluto (32 AU).

We have to keep in mind Voyager has been (and still is) primarely a real time mission, it transmitted most data directly to earth, as they were taken. No SSR in the 1970ies, just one DTR (tape) with 536 Mbits (not bytes!) capacity (96 pictures maximum). For me still the most amazing mission ever.

Analyst
« Last Edit: 10/15/2008 07:38 pm by Analyst »

Offline kevin-rf

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Re: Deep Space Probes??
« Reply #23 on: 10/15/2008 07:37 pm »
By the way, Buck Rodgers according to TV when I was a kid was launched in 1997 on "America's last deep space probe".  So he should be on ice out there somewhere. 

Would that have been STS-86? and has everything been done in the old lunar lander sound stage since then ;) 

Or was it the last flight of a mythical area 51 bird that was then lost? Remember there was only one person on Buck's shuttle, and the smallest shuttle crew was two. That or the rest of the crew really did taste like chicken ;)
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Offline Cbased

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Re: Deep Space Probes??
« Reply #24 on: 10/16/2008 11:44 am »
The Voyagers are still amazing in terms of data rates.
Brilliant! Thank you very much Analyst!
What controls the data transfer rates on the Voyager side? Is it just programming from Earth? I mean how does it "know" to slow down? Is it as simple as a command from Earth "from now on your downlink speed it X kbps?

Also, I've heard when Earth uplinks the commands to Voyagers we send these packets multiple times each time slightly changing the frequency. Is it still the case? Do we have to cover broader frequency ranges since the spacecrafts are further away?

btw, announcement just in time for this conversation:

http://www.nasa.gov/mission_pages/ibex/index.html

IBEX mission. Not exactly a deep space mission (in terms of probe location), but certainly the one that will contribute to our knowledge of deep space environment.

Offline Analyst

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Re: Deep Space Probes??
« Reply #25 on: 10/16/2008 03:46 pm »

1) What controls the data transfer rates on the Voyager side? Is it just programming from Earth? I mean how does it "know" to slow down? Is it as simple as a command from Earth "from now on your downlink speed it X kbps?

2) Also, I've heard when Earth uplinks the commands to Voyagers we send these packets multiple times each time slightly changing the frequency. Is it still the case? Do we have to cover broader frequency ranges since the spacecrafts are further away?

1) Yes. They calculate their link margins on earth and tell the probe to select a certain data rate.

2) This is the case for Voyager 2 only because of its receiver problem. The receiver can only listen in a very narrow frequency band, and this band depends on the temperature of the receiver. So you have to account for this in advance, e.g. predict the temperature (and Doppler of course). They are sending the same command at 7 different frequencies to assure at least some get through. Usually 3 or 4 get through. This has nothing to do with with the distance from earth.

You may find this useful:
http://descanso.jpl.nasa.gov/DPSummary/Descanso4--Voyager_new.pdf

Analyst
« Last Edit: 10/16/2008 03:52 pm by Analyst »

Offline Cbased

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Re: Deep Space Probes??
« Reply #26 on: 10/18/2008 12:18 am »
Perfect. Wealth of information. Thank you Analyst

Offline HIPAR

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Re: Deep Space Probes??
« Reply #27 on: 10/18/2008 02:11 am »
NASA JPL has a nice Voyager page:

http://voyager.jpl.nasa.gov/

There are status reports current through about June 08 there. Basically, the spacecrafts are still in remarkably good health with good power reserves and adequate propellant to keep them pointing towards Earth.  Instruments are still returning good data.  The Deep Space Network communicates with them regularly.

Now we should know enough about what's happening out there to intelligently decide what to measure and how to measure it with new or improved types of instruments. 

I have to wonder how many people who worked on the spacecrafts are currently with us keeping them working and interpreting the data.  I would think maintaining the continuity for a mission that lasts a century will be the major challenge for operating any future interstellar probe.

---  CHAS   

Offline amaturespacecase

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Re: Deep Space Probes??
« Reply #28 on: 10/18/2008 05:44 am »
Speaking of deep space... the IBEX is launching on Sunday to study the heliosphere/termination shock/bow shock ect.... Very interesting stuff.

Would there be any value what so ever in launching a few simple probes, maybe 3 or 4, on different trajectories out of the solar system to study the heliosphere at different positions? 
NASA's FY 2009 budget request- Ok bend over!

Offline Blackstar

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Re: Deep Space Probes??
« Reply #29 on: 10/18/2008 09:31 pm »
Speaking of deep space... the IBEX is launching on Sunday to study the heliosphere/termination shock/bow shock ect.... Very interesting stuff.

Would there be any value what so ever in launching a few simple probes, maybe 3 or 4, on different trajectories out of the solar system to study the heliosphere at different positions? 

IBEX is basically remote sensing--looking at it from very far away.  The Interstellar Probe mission that I linked to above is a proposal to send a spacecraft there.

To answer your question, yes and no.  The heliophysics community ranks a mission to the heliosphere very highly.  So there is indeed scientific value.  But they would settle for one spacecraft and don't require 3-4.  More than one would be great, but they are already asking for a lot simply by asking for one.

That said, this is not an easy mission.  The flight times to the target distance (200 A.U.) are very long, ranging upwards of 25 years.  How do you flight qualify a spacecraft for 25+ years of operation?  And of course you have to power it with RTGs.  That requires a fair amount of power (because it needs to work at 25+ years).  So there is no such thing as a "simple" probe to such a distance.  The instruments are relatively simple, but getting out that distance and having everything still work is not.

Offline amaturespacecase

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Re: Deep Space Probes??
« Reply #30 on: 10/19/2008 06:37 am »
Speaking of deep space... the IBEX is launching on Sunday to study the heliosphere/termination shock/bow shock ect.... Very interesting stuff.

Would there be any value what so ever in launching a few simple probes, maybe 3 or 4, on different trajectories out of the solar system to study the heliosphere at different positions? 

IBEX is basically remote sensing--looking at it from very far away.  The Interstellar Probe mission that I linked to above is a proposal to send a spacecraft there.

To answer your question, yes and no.  The heliophysics community ranks a mission to the heliosphere very highly.  So there is indeed scientific value.  But they would settle for one spacecraft and don't require 3-4.  More than one would be great, but they are already asking for a lot simply by asking for one.

That said, this is not an easy mission.  The flight times to the target distance (200 A.U.) are very long, ranging upwards of 25 years.  How do you flight qualify a spacecraft for 25+ years of operation?  And of course you have to power it with RTGs.  That requires a fair amount of power (because it needs to work at 25+ years).  So there is no such thing as a "simple" probe to such a distance.  The instruments are relatively simple, but getting out that distance and having everything still work is not.

Thanks for the reply Blackstar... I have had an amateur interest in NASA and astronomy since the 6th grade when they took us out to the Cape on a field trip. When reports started coming in that Voyager 1 had reached the termination shock I was well, "shocked." Our cosmic neighborhood just keeps getting smaller and smaller. I mean, how long did it take New Horizons to fly past Jupiter, all of 13 months?

I hope the heliophysics community gets their probe. Since a mission like this is inherently a major operation, would a mission to the heliosphere reach the level of a Flagship mission?
NASA's FY 2009 budget request- Ok bend over!

Offline khallow

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Re: Deep Space Probes??
« Reply #31 on: 10/19/2008 09:34 am »

 How do you flight qualify a spacecraft for 25+ years of operation?

I guess some RTG's would be flight-qualified now (having served on Pioneer 10/11 or Voyager 1/2. But for most components, I doubt it makes sense to speak of flight-qualifying them. Technology is changing to fast for us to do that. Actually testing a system for a significant length of time means you'd be using systems much older than the current state of the art. It might take a extraordinary effort to keep in production a proven, old system.
Karl Hallowell

Offline Analyst

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Re: Deep Space Probes??
« Reply #32 on: 10/19/2008 04:26 pm »
I'm still not quite sure what kind of mission you are referring to, but you should take a look at this:

http://interstellar.jpl.nasa.gov/interstellar/probe/index.html

Thanks Blackstar. Amasing... 25bps (bit/sec) data transfer rate... not much at all. No video feeds for us then :-/
Does anyone know what sort of data transfer rates do we get from Voyagers?

This would be the data sampling rate of all instruments combined, not the downlink data rate to earth. The latter would be higher because you save the data on board and transmit them during a DSN pass. Maybe once per day or once per week. Considering a standard 8h pass, you need a download rate of 75 bps if you download once per day, 525 bps once per week. I assume somthing in between. A 34m HEF DSN station can capture 160 bps from more than 150 AU. This would mean a download every other day.

No chance for another real time mission like Voyager.

Analyst
« Last Edit: 10/19/2008 04:27 pm by Analyst »

Offline Stan Black

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Re: Deep Space Probes??
« Reply #33 on: 10/19/2008 05:43 pm »
Why not have a chain of probes? The outer does not communicate directly with Earth but with a chaser? It's mechanical things that fail. The redundancy comes from having multiple units. 

Offline Analyst

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Re: Deep Space Probes??
« Reply #34 on: 10/19/2008 07:11 pm »
1) Why not have a chain of probes? The outer does not communicate directly with Earth but with a chaser?
2) It's mechanical things that fail. The redundancy comes from having multiple units. 

1) The "recieving probe" would have to be the "recieving power" of a DSN station. Not workable and unneeded. Transmit directly to earth. But heh, you can roll a cable behind you. :)
2) Redundancy is nice. But no budget is the reality. Last time we had more than one probe: MER 2003. Last time before: Voyager 1977. You get the idea.

Analyst

Offline Blackstar

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Re: Deep Space Probes??
« Reply #35 on: 10/19/2008 11:00 pm »

 How do you flight qualify a spacecraft for 25+ years of operation?

I guess some RTG's would be flight-qualified now (having served on Pioneer 10/11 or Voyager 1/2. But for most components, I doubt it makes sense to speak of flight-qualifying them. Technology is changing to fast for us to do that. Actually testing a system for a significant length of time means you'd be using systems much older than the current state of the art. It might take a extraordinary effort to keep in production a proven, old system.


You're mixing together a bunch of issues.  The issue is not simply flight qualifying the RTG's (and we don't use the same RTGs as Pioneer or Voyager anymore), it is flight qualifying the spacecraft.  You have to figure out how to determine that the thing will still work reliably at 25+ years.  That's not easy. 

As for the RTGs, the first issue is determining how much power you will have available at 25 years.  Remember that they are like batteries--they run down.  I don't have the power curve data in front of me, but off the top of my head I think that Cassini's RTGs were about 886 watts at beginning of mission and something like 635 at end of mission.  Not quite sure what the duration is there, but you can get a rough idea--a 25 year mission means that you will probably have to build a spacecraft with twice as much initial power just so that you have enough when it gets out to its destination.  RTGs are not cheap, so you end up with a very expensive power source.

All this harks back to the earlier comment about building a bunch of "small, cheap" spacecraft.  Interstellar Probe is not suited for that, because it is a very challenging mission because of the distance.  Simply put, you cannot build a simple or cheap spacecraft for this mission (although the instruments are relatively simple).

As for your latter comment, there is an issue there.  NASA is about to transition from RTGs to a new technology known as the ASRG.  That technology has not been flight qualified yet.  We know that RTGs never break down, but we don't know that for ASRGs.  The value of the ASRG is that it uses less fuel, making it more attractive for an interstellar probe mission.  The downside is that it has moving parts and control electronics.

Incidentally, the concept of an interstellar probe to the edge of the solar system goes back to at least the 1960s.  There have been several recent studies of it.  I can point you to a recent book that contains some more information on it.  You can also find some information here:

http://www.nap.edu/catalog.php?record_id=12201

« Last Edit: 10/19/2008 11:01 pm by Blackstar »

Offline punkboi

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Re: Deep Space Probes??
« Reply #36 on: 10/20/2008 12:09 am »
2) Redundancy is nice. But no budget is the reality. Last time we had more than one probe: MER 2003. Last time before: Voyager 1977. You get the idea.

Analyst

True.  New Horizons would've been the latest with New Horizons 2 (which would've went to Uranus), but the lack of an available RTG in time for a 2008-09 launch--from what I read 3 years ago--was what prevented the NH2 mission from ever taking place.
« Last Edit: 10/20/2008 12:12 am by punkboi »

Offline Analyst

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Re: Deep Space Probes??
« Reply #37 on: 10/20/2008 06:44 am »
Somewhere I have a cost study about New Horizons 2, ca. 2004. According to this, it would have cost about the same than New Horizons. I have no idea if these numbers are true or not. But apparently it was not only the RTG.

Analyst

PS: Such cost studies are a funny thing. When NASA / Congress does not want a mission, they include everything as cost. Just look at the HST-SM4 study ca. 2005: They added ~$1 billion (fixed costs) for extending the Shuttle manifest an extra 3 months. When - on the other hand - they want a mission, there are studies showing it a bargin. STS-94 did cost less than $100 million. You can find similar studies for unmanned probes. Take them all with a grain of salt.

Offline Blackstar

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Re: Deep Space Probes??
« Reply #38 on: 10/20/2008 11:50 am »
True.  New Horizons would've been the latest with New Horizons 2 (which would've went to Uranus), but the lack of an available RTG in time for a 2008-09 launch--from what I read 3 years ago--was what prevented the NH2 mission from ever taking place.

No, it was more complicated than that.  NH was part of the New Frontiers program.  There was no Uranus mission listed as a New Frontiers priority.  Also, there was no additional money in the New Frontiers program.

Offline toddbronco2

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Re: Deep Space Probes??
« Reply #39 on: 10/20/2008 03:13 pm »
Also, New Horizons launched on the Atlas V 551 with a Star 48 booster.  This is one of the most powerful rockets in the U.S. inventory and is by no means "cheap."  If you wanted to send a clone of the New Horizons hardware on a second mission you'd have to purchase a second launch vehicle and then for the next two decades there will be yet another probe competing for time on the DSN. 

I was hoping somebody on the forum might have some more information about the supply of RTG fuel.  When I was at APL this summer, some of their engineers told stories about the Department of Energy scrounging around all of the national labs and universities trying to come up with enough Plutonium...should I call them "pellets" or "blocks" or whatever, to power New Horizons.  They told us that New Horizons ended up launching with less power from its RTG than the probe had originally be designed for.  The point of the story was that Plutonium for RTG's was in short supply nationally and that our country wasn't producing a lot.  Obviously there's enough for MSL, and I saw that in the NASA authorization bill that's been mentioned on NSF, there was specific reference to restarting production of RTG fuel.  Can anybody elaborate?  What is the current state of RTG fuel stockpiles and production?
« Last Edit: 10/20/2008 03:15 pm by toddbronco2 »

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