OrbitalHub

On 21 November 2026, a spacecraft that has been decelerating since 2018 will finally stop. Partway through the day, more than 200 million kilometres from the people flying it, the stacked BepiColombo orbiters will turn their chemical thrusters against their own motion and fire. If the burn behaves, the joint ESA-JAXA mission becomes the first to hold two spacecraft in orbit around Mercury simultaneously. If it does not, eight years of trajectory design has to be rebuilt from scratch around a rock that most spacecraft are barely engineered to survive.

That frame is among the last things the mission photographed with the hardware it no longer needs. M-CAM 1 is bolted to the Mercury Transfer Module, the ESA-built propulsion stage that carried both science orbiters across the inner Solar System and was discarded on 3 September 2026 at a point 63 million kilometres from the Sun (ESA, September 2026). What remains is roughly 1,500 kg of spacecraft still moving fast enough that Mercury cannot simply grab it.

The module carried three of those 1,024 by 1,024 pixel black-and-white cameras, which exist less for science than for triage: they let engineers check hardware health from a distance when the main science instruments are pointed the wrong way or blocked. They also became the mission's accidental souvenir shop, recording Earth, both Venus encounters and all six Mercury passes. The final swing-by on 8 January 2025 came within about 295 km of the surface and was the last gravitational tug before the long approach.

Mercury's reputation as a hostile destination is mostly a matter of velocity. It orbits at 0.387 astronomical units, where the Sun's pull is roughly seven times stronger than at Earth, and a spacecraft launched from Earth arrives there carrying far too much speed. Run the numbers with the vis-viva equation, v^2 = GM(2/r - 1/a), and a direct transfer from Earth's orbit to Mercury's comes out near 57 km/s at the arrival point, against Mercury's own orbital speed of about 48 km/s. That surplus has to go somewhere. Entering a low orbit around a small planet means surrendering most of it to the planet's gravity, and the only cheap way to surrender velocity is to let a planet take it away for you.

That is what the nine flybys were for. BepiColombo launched from Kourou on an Ariane 5 on 20 October 2018 at 4,100 kg, comprising a 1,230 kg orbital module carrying 85 kg of instruments and a 255 kg Japanese magnetospheric orbiter carrying 45 kg (ESA fact sheet). It swung past Earth once, Venus twice and Mercury six times, shedding heliocentric energy in small instalments, and used solar-electric thrust between encounters to trim the orbit inward. It is named for the Italian engineer Giuseppe Colombo, whose 1970s work on gravity-assist trajectories let Mariner 10 visit Mercury three times. ESA notes that orbit insertion at Mercury demands more energy than sending a probe past Pluto, which is why a nearly $2 billion mission needed a nine-stop itinerary.

The itinerary almost did not survive 2024. On 26 April, while the module was mid-manoeuvre, its power system began drawing unexpected currents between the solar array and the unit that extracts and distributes power, starving the ion engines. Engineers at ESOC in Darmstadt restored thrust to about 90 percent within eleven days but could not recover full power, and the flight dynamics team concluded in September that the reduced thrust could no longer reach the December 2025 insertion date (ESA, September 2024). The hardware was unfixable, so the trajectory moved. The fourth Mercury flyby on 4 September 2024 was flown roughly 35 km lower than planned, a small shave of altitude that conserved propellant and let a redesigned path carry the spacecraft to Mercury a year late while keeping the science programme intact (ISAS).

September 2026 was the handover. When the transfer module let go, the remaining pair had to inherit power generation, propulsion, pointing and thermal control in one shot, at a distance where a radio round trip takes hours. Confirmation came through Doppler shift as the two halves changed velocity, read out from Cebreros in Spain and Malargüe in Argentina (ESA, 3 September 2026). By mid-September the instruments that had spent years staring at the back of a dead engine deck were working. The BepiColombo Laser Altimeter fired more than 700,000 pulses over 21.5 hours, MERTIS returned about a gigabit during a 22-hour look at empty space through its planet and calibration ports, and SIMBIO-SYS returned its first image in space (ESA, 6 October 2026). A thruster rehearsal on 24 September produced 11.4 m/s over 8 minutes 42 seconds, the first trajectory correction of the arrival sequence.

The propulsion that carried it here deserves the credit. ESA loaded 581 kg of xenon into the module's three tanks at Kourou in June 2018, and that fuel went through four QinetiQ T6 gridded ion thrusters, 22 cm across and rated around 4.5 kW each, producing 75 to 145 mN of thrust at a specific impulse up to 4,300 seconds (Hofer et al., AIAA 2010-7114). Do that through Tsiolkovsky's rocket equation, Δv = v_e × ln(m_0 / m_f), with an effective exhaust velocity near 39 km/s, and burning a bit over a fifth of the stack's mass buys on the order of 10 km/s of velocity change. The same job on a chemical engine with a 3.3 km/s exhaust would demand a mass ratio near 20 to one, meaning a spacecraft more propellant than spacecraft. That arithmetic is the whole reason solar-electric propulsion exists.

The other constraint was sunlight. At 63 million kilometres the solar flux is roughly 5.6 times what Earth receives, and heat limits how squarely the array can face the Sun, so ESA canted the wings and accepted a deployed span near 30 metres to collect the power four thrusters needed (ESA fact sheet). The September 2024 power fault sat uncomfortably close to that limit, which is why the recovery plan was flown conservatively rather than pushed.

What happens on 21 November is closer to a chemical burn, because capture cannot be done slowly. Arrival commands go up days ahead and execute autonomously, because at Mercury's distance there is no opportunity to watch a burn and correct it. After capture, the pair separates on 9-10 December, MPO settles into a 480 by 1,500 km polar orbit with a 2.3-hour period, and Mio stretches out to 590 by 11,640 km to study the magnetosphere from outside. Sixteen instruments begin formal operations in April 2027 (ESA fact sheet).

The paired-orbit arrangement matters more than it sounds. MPO works close in, repeatedly crossing the planet's surface to build topography, map composition and weigh the interior from gravity and radio tracking. Mio works far out, crossing the magnetopause and sampling the solar wind that Mercury sits inside rather than outside. No single spacecraft can do both jobs at once, and building two cost an extra decade and an extra module to carry them.

What MESSENGER left open in 2015 is what these two orbiters are built to close, and the arrival itself is an argument for slowness in a field that rewards speed. BepiColombo spent five years producing thrust no desk lamp could outrun, because that was the cheapest way to buy the velocity change it needed, and the bill for that patience came due in a single burn that nobody will be able to touch.

Abstract

The blogpost explores relativistic effects on proof-of-work based cryptocurrency protocols. Cryptocurrencies are here to stay and it is quite plausible that future human colonists spread across the solar system and beyond will use a decentralized cryptocurrency as opposed to a fiat currency issued by a central authority. The low transaction fees, the ubiquitous access, not being bound by exchange rates or interest rates, not being controlled by financial institutions who are serving foreign interests -- these are some of the advantages cryptocurrencies will enjoy in the thriving exo-economy.

Motivation

At present, on a cryptocurrency network, the information exchanged by the nodes in the network reaches each node almost instantaneously. The speed at which the TCP/IP packets travel on the network and the fact that the Internet spans only the Earth and the LEO makes this possible. However, once the region the network is spread across will reach certain boundaries, the network size would have negative impact on the network: increasing communication failures due to network delays, more frequent and longer blockchain forks as part of the proof-of-work protocol, network segregation into local sub-networks (let us call them topological forks), just to name a few.

Nullius in verba... as they say. Let's go deeper into details and figure out a feasible solution for a truly interplanetary cryptocurrency.

Cryptocurrency 101

You can think of a cryptocurrency as a digital money ecosystem. Plain and simple. A collection of technologies are part of this ecosystem, all of them the result of years of research in the cryptography and distributed systems fields: a decentralized network of computers, a public transaction ledger -- also known as a blockchain, a protocol that consists of a set of rules for transaction validation, a decentralized consensus mechanism. [ANTO]

A decentralized network of computers ensures the resilience of the network. We can think of both computing power and data storage capabilities. As the computing power is distributed across the network, any disruption can be successfully handled by the network. Transaction data resides on all nodes of the network. This implies that even physical damage done to network nodes will not take out the network.

The blockchain is a public distributed ledger which stores all transactions handled by the network. As the name suggests, the blockchain is a list of data blocks. Each of these blocks contains a set of transactions, as many as can fit in the block, given the maximum size of the block (a characteristic of the network). Transactions contain sender and receiver info, and the amount/asset that is changing ownership, and they are broadcasted and added to blocks by network nodes. Blocks are linked to the previous block in the chain by a cryptographic hash (the hash of the previous block becomes part of the current block). This backward link leads all the way to the first block in the chain, the Genesis block. Each cryptocurrency blockchain has one. The cryptographic hashes have an important role in protecting the blockchain from tampering attempts.

The transaction validation rules enforced by the nodes in the network ensure that the content of each block in the blockchain is valid. By far, the most frequent form of fraud is double spending. The validation process makes sure that the inputs on the transactions exist and that they have not been already spent. The transactions marked as invalid are rejected by the network and do not make it on the blockchain.

The consensus mechanism is designed so that all the nodes in the network can agree on the set of transactions to be included in the current block. It shifts the authority and the credibility required by a central clearing house to a network of nodes. Important to mention here that, inherently, the nodes do not trust each other, and they do not have to because the trust is enforced by the consensus mechanism itself.

The Proof-of-Work

We mentioned above that one of the key innovations that make cryptocurrencies possible is the decentralized consensus mechanism. Currently, the de facto consensus mechanism is the proof-of-work.

The proof-of-work consensus mechanism was proposed by Satoshi Nakamoto in 2008 [NAKA]. In general, a proof-of-work is a piece of data that requires costly and time consuming computational effort, but it is very easy to verify. A very good analogy is one of your semester projects in college... it takes you a whole semester to finalize it, but it takes very little time and effort for your supervisor to evaluate and grade it. Similarly, proof-of-work is used by cryptocurrencies for block generation. For the network to accept the block, nodes must complete a proof-of-work which also guarantees the integrity of the transactions included in the block. The difficulty of the proof-of-work is adjusted by the network so that a new block can be generated every fixed time interval. This fixed time interval is characteristic to every cryptocurrency. Most notably, for Bitcoin this time interval is set to 10 minutes.

Satoshi Nakamoto's solution, emobodied by the proof-of-work algorithm above, achieves consensus on the network without a central trusted authority (hence the name decentralized consensus mechanism).

Enter the Ansible

For the astute reader, it is quite obvious at this point that communication delays between the network nodes have a direct effect on the protocol described above. An important assumption made when the protocol was designed is that these delays are small, and this is why they can be handled by the network when the nodes choose to follow and validate the blocks on the longest blockchain fork. Also, this is the reason why all the nodes in the network can have a say in the network consensus. They all can find a solution for the current block, and they all can be rewarded when they find the solution. The communication delays witnessed by the peripheral nodes will cripple their ability to find solutions and these nodes will not be incentivized to remain in the network. Hence, the segregation of the network effect mentioned earlier.

Unfortunately, given the current technological level of our civilization, we do not have at our disposal a technology that allows us to communicate fast and reliable over the large expanse of space. It would take a hell of a wait time to proces a payment made by a mining corporation in the TRAPPIST-1 system, 12 parsecs within the constellation of Aquarius, to a planetary engineering corporation located in the Sol system... twice 39 years plus the block confirmation time.

Fortunately, the Sci-Fi literature already offers a solution for our problem. For those of you, Sci-Fi nerds like myself, that have already read Rocannon's World [LEGU] and Ender's Game [CARD], the Ansible device must sound very familiar. The ansible is a fictional device capable of faster-than-light communication. To word this differently, an operator of such device can send and receive messages to and from another device over any distance with no delay.

Hence, even if only in the realm of science fiction, we will be able to devise a solution for the problem that the future cryptocurrency enthusiasts, living in the outer space colonies, will eventually face.

The Special Relativity Theorem

Before elaborating more on a required upgrade of the network protocol, we have to discuss the special relativity theorem and its implications on how time and distance are perceived in reference systems.

Albert Einstein was awarded the Nobel Prize in Physics in 1921. He received it for his contributions to the understanding of the photoelectric effect, after publishing a paper on it in 1905. At that point his contributions to the understanding of gravity through his theory of relativity were well known, but the new perspective on gravity offered by Einstein's theory was so controversial that the Nobel Prize Committee members chose to protect their reputation. They decided that it was appropriate to award Einstein the Nobel Prize for "his services to theoretical Physics, and especially for his discovery of the law of the photoelectric effect."

During his research Einstein attempted to reconcile the principle of relativity with the principle of the constancy of the velocity of light. This attempt led Einstein to the discovery of the special relativity theorem. Einstein's Gedankenexperiment (thought experiment) with a test subject travelling by train is very well known in the scientific community. If our human subject is walking towards the front of the train with velocity w with respect to the train, and the train is moving with velocity v with respect to the embankment, then an observer on the embankment will measure as W = v + w the velocity of our subject on the train. If instead of our traveller, we consider a beam of light propagating with velocity c, the velocity measured by the observer on the embarkment would be v + c. However, this violates the principle that the velocity of light is constant in any inertial reference system and equal with c. Einstein found a solution for this problem and thus resolved the incompatibility.

One direct consequence of the special relativity theorem is the Lorentz transformation.

Before the relativity theorem, time in physics had an absolute significance, independent of the state of motion of the reference system. However, every inertial reference system has its own particular time. We always have to be told which inertial reference system the statement of time refers to. If you look at your watch, the time you read is the time as measured in the Earth inertial reference system. By the way, the Earth circles the Sun on an almost circular orbit, which means that any body maintaining a constant position in the Earth reference system should experience some centripetal force. However, this centripetal component is negligible compared to the gravitational pull of the Earth. Hence, engineers use this aproximation when calculating satellite orbits.

Back to our original trail of thoughts... the axiom that lays the foundation for the Lorentz transformation states that every ray of light possesses the velocity of transmission c relative to any inertial reference system. That is the velocity of transmission in vacuo (in a vacuum).

If following Einstein's thought experiments [EINS], the above-mentioned axiom leads to a set of four equations that explain the relation between dimensional and temporal coordinates in two inertial reference systems:

  x' = (x - vt)/sqr(1 - v²/c²)
  y' = y
  z' = z
  t' = (t - x*v/c²)/sqr(1 - v²/c²)

where:

 x, y, z, t coordinates in inertial reference system K,
 x', y', z' , t' coordinates in inertial reference system K',
 v the relative velocity between inertial reference systems;

also, the expression 1//sqr(1 - v²/c²) is known as the Lorentz factor.

The Lorentz transformation equations are a more general case of the Galilean transformation:

  x' = x - vt
  y' = y
  z' = z
  t' = t

which is the basis of the assumptions of the classical mechanics as to the absolute character of both dimensional and temporal coordinates. The above are the result of replacing velocity c with ∞ in the Lorentz transformation equations.

The Lorentz transformation has a few corollaries which have an impact on time dilation, length contraction, relativistic mass, relativistic momentum, and relativistic kinetic energy. The one that concerns us is the time dilation corollary:

 Δt = γΔt'
 
where γ is defined by

 α = 1/γ = sqr(1 - v²/c²)

Assuming a clock at rest in inertial reference system K, and moving with the velocity v in the inertial reference system K', the time Δt' between two ticks as measured in the frame K' is longer than the time Δt between the same ticks as measured in the rest frame of the clock, K.

Relativistic Effects on Proof-of-Work

To better understand the relativistic effects on how relative time is perceived in two inertial reference systems (or Galilean reference systems [EINS]) we have to mention the twin paradox. The twin paradox is a thought experiment that involves two identical twins. One of the twins makes a journey into space onboard a relativistic spaceship, and upon his return to Earth discovers that his twin has aged much more than he did. Depending on how fast the spaceship moves through space and/or how long the journey was, our traveller could return to Earth and realize that several generations have passed meanwhile.

As a direct application of the twin paradox, we have Mazer Rackham, International Fleet Admiral and Ender Wiggin's mentor, as portrayed by Orson Scott Card [CARD]. The fighter pilot who destroyed the Formic Fleet Flagship, killed the Hive Queen, and ended the Second Formic Invasion, Mazer Rackham is sent on a journey on a relativistic spaceship and returns to Earth 100 years later in order to assist the International Fleet. Upon his return he has barely aged a few years.

Similarly, if a subnetwork is moving relative to the rest of the network, the nodes in the subnetwork would experience time dilation as predicted by the special relativity theorem. Hence, they are at a disadvantage compared to the rest of the nodes because they have less time available to find a solution for the proof-of-work problem. In order to make thing square for all the nodes in the network, the difficulty of the problem should be adjusted in each subnetwork using the Lorentz factor: in order to compensate for time dilation, the relativistic nodes should have to solve a simpler problem. Also, their block confirmation time should decrease as well.

One element is missing still... how do ansibles synchronize? What value has the Lorentz factor for each one of them? One simple solution for this problem is having one of the ansibles (let us say the Earth-bound one) broadcasting a beacon every fixed number of seconds. The ansibles interfacing to subnetworks would pick up the broadcast, and by measuring the time interval between the beacons, infer the Lorentz factor. Once the Lorentz factor is determined, the nodes mining on the subnetworks will have their proof-of-work difficulty and the block confirmation time adjusted accordingly.

A more Down-to-Earth Solution for the Problem

We will fork the storyline (pun intended) at Enter the Ansible paragraph, and suggest a more down-to-earth solution for the problem. The peripheral nodes are not to participate in the consensus. Their function would be just to relay messages (a.k.a. payment information) across the network. In our multi-planetary scenario, the nodes working on the proof-of-work would reside on/around the Earth, and the network nodes on Mars or any other region in the Sol system would have to wait 2x (network packets travel time) + (block confirmation time) in order to confirm a payment. However, such a solution would have an Achilles heel very hard to defend... if starting its mining engine, any peripheral node could take over the local network and force a topological fork of the network. For a node, the incentive of staying honest on the current network must be stronger than any reward obtained by cheating.

Concluding remarks

While the blogpost contains some elements of fiction, it accurately describes the cryptocurrency ecosystem, the special relativity theorem, the Lorentz transformation and its corollaries. We would like to thank the readers that went through this exercise with us. Undeniably, cryptocurrencies are disrupting the global economy and they are here to stay. Undeniably, we -- as a species -- will explore and migrate farther than the LEO. History has taught us that decentralization is the key to survival and prosperity. Let us together make the future happen.

References

[ANTO] Antonopoulos, Andreas M., Mastering Bitcoin, Second Edition, O'Reilly Media, Inc., June 2017;
[CARD] Card, Orson Scott, Ender's Game, A Tor Teen Book, 2014;
[EINS] Einstein, Albert, Relativity, The Special and the General Theory, Princeton University Press, 2015;
[LEGU] Le Guin, Ursula K., Rocannon's World, Harper & Row, 1977;
[NAKA] Nakamoto, Satoshi, Bitcoin - A Peer-to-Peer Electronic Cash System, www.bitcoin.org, 2008;