<p>Simply limit the block target (i.e. 10 minutes) to the speed of light (300,000 km/s).</p>

<p>Unless there is some wormhole or other unknown to reduce this limit, Bitcoin is safe from honest mining from any planet outside of 180,000,000 kilometers away (300000km/s * 60 sec * 10 minutes) since any miner can't keep up with the blockchain and would get any block at about the time the next block comes out. </p>

<p>Someone with more advanced technology / more hashing power could create a 51% attack and submit it to disrupt the Bitcoin economy even outside of this radius.</p>

CC BY-SA 3.0
2014-08-13T18:34:25.503Z
That's probably the only way. Quite an ingenuous idea. But I don't know if a block time can be that short. Surly there's a reason all of them (afaik) being a few minutes or so.
Tyler
CC BY-SA 3.0
2014-08-13T19:57:34.387Z
The first attempt at drastically lowering the block time was GiestGeld, which had a block time of one or several seconds. This caused huge issues with massive blockchain forks and concurrency issues due to latency between nodes. It has been generally agreed that 60 seconds is the lowest reasonable time to target for generating a block, propagating to the network, and expecting most miners not to have found a block by then (if it takes 6 seconds to transmit a block to the whole network with a 60 sec target, 10% of blocks will be stale/rejected because they would be found during this time).
Mark
CC BY-SA 3.0
2015-01-06T14:15:25.580Z
As you mentioned, a small block interval does not protect from another planets mining efforts if it has more mining power. I've given an explanatory example in another [answer](http://bitcoin.stackexchange.com/a/34310/5406).
Murch
CC BY-SA 3.0
2014-08-13T01:46:31.260Z
2014-08-13T01:46:31.260Z
Mark
<p>If hashes are cheap on Earth, you want them exported to
Mars to improve the security of the Martian block chain.  Exporting
hashes is easy to do in the stateless manner we expect from
decentralized Bitcoin technology.</p>

<p>Each block on a block chain is protected against modification by the
proof of work it shows and by the proof of work shown by the blocks
that follow it. This proof of work is communicable---the work done in
one chain can be applied to another chain.</p>

<p>So, for example, we have Marscoin and Bitcoin.  Bitcoin (still focused
on and near Earth) is by far the stronger chain.  Marscoin is great for
local transactions on Mars, but nobody trusts it for serious savings
because its network difficulty is much lower than Bitcoin.</p>

<p>However, some people on Earth start to include hashes of recent Marscoin
block headers in their transaction outputs (maybe using
<a href="https://github.com/Blockstream/contracthashtool" rel="nofollow">ContractHashTool</a> to
do this as a part of their regular transactions).</p>

<p>Following a soft fork of Marscoin, Marscoin full nodes download both the
Bitcoin block chain and the Marscoin block chain. Looking at the Bitcoin
block chain, they find the first Marscoin header hash encoded in an
output and look at how much proof of work is protecting that transaction
on the Bitcoin block chain. They add that proof of work to the
corresponding block on the Marscoin block chain---now that block is
protected by the combined hash rate of two planets! For example:</p>

<blockquote>
  <p><strong>Bitcoin Block 10,000,000</strong></p>
  
  <ul>
  <li>Confirmed 100 times</li>
  <li>Protected by proof of work equal to 10^72 hashes</li>
  <li>Has a transaction that references Marscoin block 50,000</li>
  </ul>
  
  <p><strong>Marscoin Block 50,000</strong></p>
  
  <ul>
  <li>Confirmed 200 times</li>
  <li>Protected by native proof of work equal to 10^48 hashes</li>
  <li>External proof of work adds an extra 10^72 hashes</li>
  </ul>
</blockquote>

<p>The Marscoin full nodes perform the same calculation for any other
Marscoin header hashes found in the Bitcoin block chain. Now attacking
all but the most recent Marscoin blocks requires attacking Bitcoin at
the same time.</p>

<p>To prevent denial-of-service attacks against recent blocks, methods like
<a href="http://gavintech.blogspot.com/2012/05/neutralizing-51-attack.html" rel="nofollow">Gavin Andresen's require-priority
idea</a>
can ensure high-priority Martian transactions get through even if
malcontents try using cheap Terran hash rate in short-chain 51% attacks.</p>

<p>Now Terrans and Martians can live in peace and harmony---all thanks to
Bitcoin. :-)</p>

CC BY-SA 3.0
2015-01-06T19:47:02.570Z
2015-01-06T19:47:02.570Z
David A. Harding
<p><strong>TL;DR</strong>: Whatever block interval one employs, another planet with greater mining power will eventually overtake the local blockchain. It would only be possible to lock other miners out by adding <strong>low-depth checkpoints</strong>.</p>

<h2><em>Gedankenexperiment:</em></h2>

<p>MarsCoin starts a new blockchain with a 2 minute block interval, they have <strong>one Unit of Mining Power [UMP]</strong> at their disposal with a new MarsCoin specific mining algorithm.</p>

<p>Some traders from Earth start using MarsCoin for their transactions with Mars colonists. They get blocks with a 20 minute delay (as proposed in the question). After a while they realize that the MarsCoin mining rewards are quite juicy and technology is much cheaper on Earth than in the underdeveloped colony: They create mining equipment with <strong>2 UMP</strong>, allowing them to find blocks twice as fast than on Mars.</p>

<ol>
<li><strong>-20 minutes</strong>: Mars is at <em>Block X</em> and broadcasts it to Earth.</li>
<li>…</li>
<li><strong>0 minutes</strong>: Earth receives <em>Block X</em> from Mars, and starts their own mining effort. The difficulty has not increased yet, so they can mine one block per minute for the time being. Mars is at <em>Block X+10</em>.</li>
<li><strong>2 minutes</strong>: Earth receives <em>Block X+1</em> from Mars, but Earth is already on <em>Block X+2</em> on their own chain. Earth subsequently outperforms Mars and continues mining on their own chain. Mars is at <em>Block X+11</em>.</li>
<li>…</li>
<li><strong>21 minutes</strong>: Mars receives the first block <em>Block X+1</em> from Earth and discards it, because Mars found <em>Block X+20</em> at 20 minutes. Earth is at <em>Block X+21</em>, and received <em>Block X+10</em> last from Mars.</li>
<li>…</li>
<li><strong>61 minutes</strong>: Mars receives <em>Block X+41</em> from Earth, Mars is at <em>Block X+40</em>. Earth's chain's total proof of work is greater than Mars' chain's. Mars' chain is discarded in favor for Earth's. Mars' miners lose the mining reward of the last 40 blocks. Earth is at <em>Block X+61</em>.</li>
</ol>

<h2><em>Conclusion:</em></h2>

<p>Neither distance, nor block interval are sufficiently prohibitive. As long as the other planet has more mining power it will eventually overtake the local blockchain.</p>

<hr>

<h2><em>Addendum: Short block interval with checkpoints at low depth</em></h2>

<p>When I just read this question again, I thought of another thing. If one was hellbent to keep non-martian mining efforts from bearing fruits, one could introduce a sliding checkpoint scheme. In my above example, where I was prescribing a block interval of two minutes and terrestrian mining was twice as fast as martian mining, it would take a reorganization of more than forty blocks for the terrestrian chain to replace the martian.</p>

<p>If the community of Mars agreed upon forbidding chain reorganizations of more than 15 blocks (which at two minute intervals is less time than that of a radio wave roundtrip) by adding every block with 15 confirmations to the checkpoint list, terrestrian miners would be able to fork themselves from the Marscoin chain, but would never be able to take over the martian mining.</p>

CC BY-SA 3.0
2015-01-06T14:14:25.360Z
2017-07-12T04:19:47.703Z
2017-07-12T04:19:47.703Z
Murch
<p>This came from bubfranks on reddit:</p>

<p>"Oh fun! Let me try.
Given a random stream of bits that was impossible to predict, but agreed upon by local miners, reject blocks that do not include a valid bitstream. The probability of getting scooped by a cooperation between external hashpower and an internal node would be a function of the speed of light, distance between the attackers, and length of a valid bitstream."</p>

CC BY-SA 3.0
2015-01-06T23:49:07.757Z
I think this could be a good answer, but as it is presented right now, it is hard to understand and should be explained further.
Murch
CC BY-SA 3.0
2014-08-16T22:33:51.007Z
2014-08-16T22:33:51.007Z
user18953
<p>why don't we just increase block confirmation time from 10 minutes to 1 day for specific interstellar fork. I think the whole day of unconfirmed transactions is okay for interstellar network &amp; will expand the radius draasticaly.</p>

CC BY-SA 3.0
2015-01-07T11:07:47.770Z
That wouldn't stop another planet from taking over the chain with more mining power, leaving us with both drawbacks, the latency for confirmations and huge block intervals. So, there would be no benefit over just having to wait for 50 minutes to get the first confirmation on the Bitcoin blockchain.
Murch
CC BY-SA 3.0
2015-01-06T18:07:09.783Z
2015-01-06T18:07:09.783Z
Earl Fox
<p>I'm sure that cryptocurrencies will play a huge part of the economy across the solar system, when we have colonized many different planets, moons, and asteroids. But there are problems with using a single coin for the entire solar system. Let's just start with the example of a Mars colony. Long distances mean long delays in communication so it would take a very long time to get confirmations of Bitcoin transactions that originate on Mars. That could be problematic. Additionally, people on Mars could never hope to mine bitcoins because the average communications delay between Earth and Mars is 20 minutes one way, which is twice as long as the ten-minute blocks of Bitcoin. So I believe that a different coin would be best for each planet. But here's the crazy part. Even if Mars had its own coin (let's call it Marscoin), Earth would most likely have more computer processing power, so miners on Earth could download Marscoin's block chain and extend it faster than the miners on Mars. Then the miners on Mars would no longer be able to mine Marscoin. Then we would be back to the same problem of using Bitcoin on Mars. So, I think someone would have to create a coin that can somehow be planet-specific, so that longer block chains from other planets will always be rejected.</p>

CC BY-SA 3.0
2014-08-12T19:13:13.267Z
People on Mars could use a different mining algorithm.
Luca Matteis
CC BY-SA 3.0
2014-08-12T20:30:52.823Z
I think that would lessen the risk of having another planet taking over, but I don't think that eliminates the risk.
user18953
CC BY-SA 3.0
2015-01-06T17:01:46.627Z
Will this even be necessary in the future? I believe by the time we have terraformed and colonized Mars, Quantum Entanglement will be the primary method to instantaneously transmit data...
thedmeyer
CC BY-SA 3.0
2015-01-06T17:04:18.433Z
http://physics.stackexchange.com/a/78130
Nick ODell
CC BY-SA 4.0
2018-05-10T10:16:25.240Z
There are a few technical issues with your statements, however, using theoretical quantum links for data transfer will be instantaneous.
Willtech
<blockchain><mining-theory><latency><interplanetary>
How do you create a planet-specific cryptocurrency that will ignore longer block chains from other planets?
user18953
# How do you create a planet-specific cryptocurrency that will ignore longer block chains from other planets?

(Asked by: username_0 on 2014-08-12)

I'm sure that cryptocurrencies will play a huge part of the economy across the solar system, when we have colonized many different planets, moons, and asteroids. But there are problems with using a single coin for the entire solar system. Let's just start with the example of a Mars colony. Long distances mean long delays in communication so it would take a very long time to get confirmations of Bitcoin transactions that originate on Mars. That could be problematic. Additionally, people on Mars could never hope to mine bitcoins because the average communications delay between Earth and Mars is 20 minutes one way, which is twice as long as the ten-minute blocks of Bitcoin. So I believe that a different coin would be best for each planet. But here's the crazy part. Even if Mars had its own coin (let's call it Marscoin), Earth would most likely have more computer processing power, so miners on Earth could download Marscoin's block chain and extend it faster than the miners on Mars. Then the miners on Mars would no longer be able to mine Marscoin. Then we would be back to the same problem of using Bitcoin on Mars. So, I think someone would have to create a coin that can somehow be planet-specific, so that longer block chains from other planets will always be rejected.

**Tags:** blockchain, mining-theory, latency, interplanetary

> **Comments:**
> **username_1**: People on Mars could use a different mining algorithm.  
> **username_0**: I think that would lessen the risk of having another planet taking over, but I don't think that eliminates the risk.  
> **username_2**: Will this even be necessary in the future? I believe by the time we have terraformed and colonized Mars, Quantum Entanglement will be the primary method to instantaneously transmit data...  
> **username_3**: http://physics.stackexchange.com/a/78130  
> **username_4**: There are a few technical issues with your statements, however, using theoretical quantum links for data transfer will be instantaneous.  

---

## 5 answers

### Answer by username_5 on 2014-08-13. Score: 4

Simply limit the block target (i.e. 10 minutes) to the speed of light (300,000 km/s).

Unless there is some wormhole or other unknown to reduce this limit, Bitcoin is safe from honest mining from any planet outside of 180,000,000 kilometers away (300000km/s \* 60 sec \* 10 minutes) since any miner can't keep up with the blockchain and would get any block at about the time the next block comes out.

Someone with more advanced technology / more hashing power could create a 51% attack and submit it to disrupt the Bitcoin economy even outside of this radius.

> **Comments:**
> **username_6**: That's probably the only way. Quite an ingenuous idea. But I don't know if a block time can be that short. Surly there's a reason all of them (afaik) being a few minutes or so.  
> **username_5**: The first attempt at drastically lowering the block time was GiestGeld, which had a block time of one or several seconds. This caused huge issues with massive blockchain forks and concurrency issues due to latency between nodes. It has been generally agreed that 60 seconds is the lowest reasonable time to target for generating a block, propagating to the network, and expecting most miners not to have found a block by then (if it takes 6 seconds to transmit a block to the whole network with a 60 sec target, 10% of blocks will be stale/rejected because they would be found during this time).  
> **username_7**: As you mentioned, a small block interval does not protect from another planets mining efforts if it has more mining power. I've given an explanatory example in another [answer](http://bitcoin.stackexchange.com/a/34310/5406).  

---

### Answer by username_8 on 2015-01-06. Score: 2

If hashes are cheap on Earth, you want them exported to
Mars to improve the security of the Martian block chain. Exporting
hashes is easy to do in the stateless manner we expect from
decentralized Bitcoin technology.

Each block on a block chain is protected against modification by the
proof of work it shows and by the proof of work shown by the blocks
that follow it. This proof of work is communicable---the work done in
one chain can be applied to another chain.

So, for example, we have Marscoin and Bitcoin. Bitcoin (still focused
on and near Earth) is by far the stronger chain. Marscoin is great for
local transactions on Mars, but nobody trusts it for serious savings
because its network difficulty is much lower than Bitcoin.

However, some people on Earth start to include hashes of recent Marscoin
block headers in their transaction outputs (maybe using
[ContractHashTool](https://github.com/Blockstream/contracthashtool) to
do this as a part of their regular transactions).

Following a soft fork of Marscoin, Marscoin full nodes download both the
Bitcoin block chain and the Marscoin block chain. Looking at the Bitcoin
block chain, they find the first Marscoin header hash encoded in an
output and look at how much proof of work is protecting that transaction
on the Bitcoin block chain. They add that proof of work to the
corresponding block on the Marscoin block chain---now that block is
protected by the combined hash rate of two planets! For example:

> **Bitcoin Block 10,000,000**
>
> * Confirmed 100 times
> * Protected by proof of work equal to 10^72 hashes
> * Has a transaction that references Marscoin block 50,000
>
> **Marscoin Block 50,000**
>
> * Confirmed 200 times
> * Protected by native proof of work equal to 10^48 hashes
> * External proof of work adds an extra 10^72 hashes

The Marscoin full nodes perform the same calculation for any other
Marscoin header hashes found in the Bitcoin block chain. Now attacking
all but the most recent Marscoin blocks requires attacking Bitcoin at
the same time.

To prevent denial-of-service attacks against recent blocks, methods like
[Gavin Andresen's require-priority
idea](http://gavintech.blogspot.com/2012/05/neutralizing-51-attack.html)
can ensure high-priority Martian transactions get through even if
malcontents try using cheap Terran hash rate in short-chain 51% attacks.

Now Terrans and Martians can live in peace and harmony---all thanks to
Bitcoin. :-)


---

### Answer by username_7 on 2015-01-06. Score: 2

**TL;DR**: Whatever block interval one employs, another planet with greater mining power will eventually overtake the local blockchain. It would only be possible to lock other miners out by adding **low-depth checkpoints**.

## *Gedankenexperiment:*

MarsCoin starts a new blockchain with a 2 minute block interval, they have **one Unit of Mining Power [UMP]** at their disposal with a new MarsCoin specific mining algorithm.

Some traders from Earth start using MarsCoin for their transactions with Mars colonists. They get blocks with a 20 minute delay (as proposed in the question). After a while they realize that the MarsCoin mining rewards are quite juicy and technology is much cheaper on Earth than in the underdeveloped colony: They create mining equipment with **2 UMP**, allowing them to find blocks twice as fast than on Mars.

1. **-20 minutes**: Mars is at *Block X* and broadcasts it to Earth.
2. …
3. **0 minutes**: Earth receives *Block X* from Mars, and starts their own mining effort. The difficulty has not increased yet, so they can mine one block per minute for the time being. Mars is at *Block X+10*.
4. **2 minutes**: Earth receives *Block X+1* from Mars, but Earth is already on *Block X+2* on their own chain. Earth subsequently outperforms Mars and continues mining on their own chain. Mars is at *Block X+11*.
5. …
6. **21 minutes**: Mars receives the first block *Block X+1* from Earth and discards it, because Mars found *Block X+20* at 20 minutes. Earth is at *Block X+21*, and received *Block X+10* last from Mars.
7. …
8. **61 minutes**: Mars receives *Block X+41* from Earth, Mars is at *Block X+40*. Earth's chain's total proof of work is greater than Mars' chain's. Mars' chain is discarded in favor for Earth's. Mars' miners lose the mining reward of the last 40 blocks. Earth is at *Block X+61*.

## *Conclusion:*

Neither distance, nor block interval are sufficiently prohibitive. As long as the other planet has more mining power it will eventually overtake the local blockchain.

---

## *Addendum: Short block interval with checkpoints at low depth*

When I just read this question again, I thought of another thing. If one was hellbent to keep non-martian mining efforts from bearing fruits, one could introduce a sliding checkpoint scheme. In my above example, where I was prescribing a block interval of two minutes and terrestrian mining was twice as fast as martian mining, it would take a reorganization of more than forty blocks for the terrestrian chain to replace the martian.

If the community of Mars agreed upon forbidding chain reorganizations of more than 15 blocks (which at two minute intervals is less time than that of a radio wave roundtrip) by adding every block with 15 confirmations to the checkpoint list, terrestrian miners would be able to fork themselves from the Marscoin chain, but would never be able to take over the martian mining.


---

### Answer by username_0 on 2014-08-16. Score: -2

This came from bubfranks on reddit:

"Oh fun! Let me try.
Given a random stream of bits that was impossible to predict, but agreed upon by local miners, reject blocks that do not include a valid bitstream. The probability of getting scooped by a cooperation between external hashpower and an internal node would be a function of the speed of light, distance between the attackers, and length of a valid bitstream."

> **Comments:**
> **username_7**: I think this could be a good answer, but as it is presented right now, it is hard to understand and should be explained further.  

---

### Answer by username_9 on 2015-01-06. Score: -2

why don't we just increase block confirmation time from 10 minutes to 1 day for specific interstellar fork. I think the whole day of unconfirmed transactions is okay for interstellar network & will expand the radius draasticaly.

> **Comments:**
> **username_7**: That wouldn't stop another planet from taking over the chain with more mining power, leaving us with both drawbacks, the latency for confirmations and huge block intervals. So, there would be no benefit over just having to wait for 50 minutes to get the first confirmation on the Bitcoin blockchain.  

---

