OrbitalHub

Two years after it left Cape Canaveral on a Falcon 9, the European Space Agency's Hera spacecraft turned its main science camera toward the place it has been chasing across the inner solar system and saw, for the first time, what it came to see. Didymos appeared as a single bright pixel in a 1020-by-1020 image, surrounded by background stars that the camera was tracking to subtract out. The asteroid was 236,500 kilometers away at the time, far enough that the image looked almost like a software test pattern. The date was 7 October 2026, exactly two years after launch. The first braking maneuver, BRM-1, is scheduled for 15 October 2026, the mission's 12 km/s relative velocity has been bleeding away in tiny burns, and full arrival at the binary system is planned for next month, almost a month earlier than the original timeline. The image, published by ESA in early October 2026, is the first resolved optical sighting of the asteroid whose small moonlet Dimorphos NASA hit with the DART impactor four years ago, and it is the start of the science campaign the whole mission was built around, covered in a fresh Universe Today feature by David Dickinson and in ESA's own early-arrival announcement.

The reason Hera is going to Didymos is the question DART left open. In September 2022, NASA crashed a refrigerator-sized kinetic impactor into Dimorphos at roughly 6.6 kilometers per second, in the first deliberate attempt by any space agency to change the trajectory of a celestial body. The impact shortened Dimorphos's 11 hour 55 minute orbit around its parent by about 32 minutes, a much larger change than the pre-impact models had predicted. The extras came from ejecta, the spray of rock and dust thrown off the surface at the moment of contact, which acted like an unpropelled rocket exhaust pushing the moonlet the other way. The pre-impact momentum budget was therefore incomplete, and the right number to plug into a future deflection plan depends on details that ground-based telescopes could not measure: the mass of Dimorphos before the hit, the size and depth of the crater DART carved, the porosity of the surface, the density of the interior. Hera's job is to measure all of these, with enough precision that the next time the world considers deflecting a real asteroid, the engineers will know what the impact will do.

Hera almost did not happen. The original plan, in the early 2010s, was a joint ESA-NASA mission called AIDA, in which NASA would supply DART and ESA would supply an orbiter called AIM that would arrive at Didymos first and watch the impact happen in real time. ESA cancelled AIM in 2016 because the agency's member states would not underwrite the cost, and for several years the only follow-up to DART was a network of ground-based observatories that the LICIACube flyby camera and Hubble time had to substitute for. ESA's member states eventually funded a stripped-down replacement called Hera in 2019, and the agency awarded the prime contract to OHB in Bremen, with a launch contract to SpaceX. The spacecraft left Earth on 7 October 2024 from Cape Canaveral's SLC-40, riding a Falcon 9 first stage, booster B1061-23, that had already flown 22 missions and was expended to give Hera enough energy for a direct transfer, as documented in the mission's Wikipedia timeline.

The cruise was longer than a Mars transfer and shorter than a trip to a comet. Hera flew by Mars on 12 March 2025, using the planet's gravity to bend its trajectory and align it for the Didymos encounter, and took the opportunity to image the smaller Martian moon Deimos from a few hundred to a thousand kilometers away, the closest observations of that body since Viking 2 in 1977. Those images later fed a Nature Astronomy study in 2026 arguing that Deimos is a rubble-pile body reshaped by an oblique impact of a 320-meter asteroid, useful as a sanity check on the radar instruments aboard the Juventas CubeSat. Through 2025 and the first half of 2026, the spacecraft's flight dynamics team used Hera's Asteroid Framing Camera, a pair of baffle-protected 1020-by-1020 monochrome sensors built by Jena-Optronik in Germany, to practice the eventual arrival on two main-belt asteroids, Otero and Kellyday, that happened to be near the spacecraft's line of travel. Tracking Otero for three hours and the much fainter Kellyday in July 2025 taught the team how to keep a tiny, slowly-moving point of light centered in a camera designed to image objects thousands of times closer, in the camera test ESA reported in August 2025. In October 2025, with the cruise going smoothly, ESA announced that the arrival had been pulled forward by a month. In March 2026, Hera performed its largest single deep-space maneuver, burning 123 kilograms of hydrazine for a 367 m/s change in velocity. In June and July 2026 the flight software was upgraded, the asteroid phase began.

The arrival sequence itself starts with the first pixel and ends with a network of small spacecraft hovering above a pair of dark, fast-spinning rocks. On 2 October 2026, controllers executed a short test burn to verify the braking configuration. BRM-1, the first real braking maneuver, is set for 15 October 2026, two days after the publication of this post, and a series of further burns will put Hera into orbit around Didymos. From orbit, the spacecraft will map both bodies in detail. The Asteroid Framing Camera, working alongside the Thermal Infra-Red Imager TIRI and the Planetary Altimeter PALT, will return global shape models of Didymos at a few-meter resolution and a 10-centimeter view of the DART impact site, the first close-up of the crater NASA made. The HyperScout-H visible and near-infrared mapper will look for compositional fingerprints across the surface. A Radio Science Experiment will track Hera's orbit closely enough to pin down the gravity field of the binary, which is the cleanest way to get the mass of Dimorphos and therefore the cleanest measurement of the momentum DART actually transferred.

The CubeSats will do the part of the science Hera cannot do from a safe orbit. Juventas, built by GomSpace in Luxembourg, carries a low-frequency radar called JuRa, a laser altimeter, and a gravimeter, the first radar sounder ever aimed at an asteroid's interior, and the data it returns will tell the team whether Dimorphos is a uniform rubble pile, a fractured monolith, or a coherent body with a deep layer of regolith over a denser core. If conditions allow, Juventas will also touch down on the surface and use its accelerometers to measure the mechanical response of the regolith, the first in-situ measurement of asteroid surface stiffness. Milani, built by Tyvak International in Italy, is a hyperspectral imager that will survey the surface mineralogy of both bodies and characterize the dust around the binary. Both CubeSats will be released after Hera is in a stable orbit and will operate on their own for a few months before their trajectories carry them away. Once the science is done, the plan is to repurpose the mission as a software laboratory for autonomous deep-space operations, running new code on the spare core of the spacecraft's LEON3 processor and using the binary system as a long-duration test bench.

The reason the planetary defense community is watching this closely is that the DART impact was a real-world test of a method that only existed in textbooks before 2022, and the textbooks are being rewritten. If Hera measures a momentum transfer close to the simple mass-times-velocity prediction, the kinetic-impactor method works about as well as the engineers hoped, and a future asteroid on an Earth-crossing orbit could be pushed off course with a single heavy spacecraft launched a few years in advance. If the momentum transfer is much larger, the ejecta boost is doing more of the work than the impactor itself, and the design of a real deflection mission should emphasize surface coupling rather than impactor mass. The point of Hera is to give the answer in numbers that can be written into a flight plan. Didymos, a 780-meter rock discovered by the Spacewatch survey in 1996 with a 151-meter moonlet found seven years later, is the laboratory. The first photograph of it from the spacecraft built to study it is, on its own, just a single bright pixel. The work begins now.

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;