space
A human-built robotic probe is launched on a credible interstellar mission by 2126.
Last revised Sep 11, 2026.
Current estimate
probability this has happened by each date
A probe meeting the credibility test below has been launched by 31 December 2035. No beam director, no funded programme and no metre-scale sail exists today, and none could be built and flown in nine years from a standing start.
A probe meeting the credibility test below has been launched by 31 December 2050. This requires a multi-gigawatt directed-energy facility to be built inside about twenty-four years, most plausibly as a by-product of an array funded for planetary defence, power beaming or a military purpose.
A probe meeting the credibility test below has been launched by 31 December 2100.
A robotic spacecraft is launched with (a) a named target stellar system, (b) a design cruise velocity sufficient to arrive within 200 years of launch, which for Proxima Centauri is about 1,343 AU per year or roughly 2 percent of the speed of light, and (c) a communications design intended to return data from that system. Launch is required; arrival and data return are not. A mission to the heliopause or the very local interstellar medium does not qualify, whatever it is named.
Resolution criteria
- 2035
- A probe meeting the credibility test below has been launched by 31 December 2035. No beam director, no funded programme and no metre-scale sail exists today, and none could be built and flown in nine years from a standing start.
- 2050
- A probe meeting the credibility test below has been launched by 31 December 2050. This requires a multi-gigawatt directed-energy facility to be built inside about twenty-four years, most plausibly as a by-product of an array funded for planetary defence, power beaming or a military purpose.
- 2100
- A probe meeting the credibility test below has been launched by 31 December 2100.
- 2126
- A robotic spacecraft is launched with (a) a named target stellar system, (b) a design cruise velocity sufficient to arrive within 200 years of launch, which for Proxima Centauri is about 1,343 AU per year or roughly 2 percent of the speed of light, and (c) a communications design intended to return data from that system. Launch is required; arrival and data return are not. A mission to the heliopause or the very local interstellar medium does not qualify, whatever it is named.
Probability history
| Revised | State | p(2035) | p(2050) | p(2100) |
|---|---|---|---|---|
hf-forecast-0027 | researched | 0.5% no prior revision | 3% no prior revision | 13% no prior revision |
hf-forecast-0014 | initial prior | — | — | — |
Strongest evidence for
- The first measurable radiation-pressure displacement in a subwavelength lightsail was reported in June 2026: millimetre-wide nanoscale-thickness silicon nitride membranes at 99 percent reflectivity, surviving intensities comparable to the Sun's surface, a 50,000-fold improvement in optomechanical response (hf-evidence-0055).
- The architecture's marginal cost per probe is small once the fixed asset exists: about $6M of energy per sail against an $8.0B beam director, so the first launch does not need a per-mission justification, only a facility (hf-evidence-0054).
- The directed-energy route's data return is about 25 years from launch, which is the only interstellar architecture whose payoff lands inside a working career (hf-evidence-0054).
- The beam director has non-interstellar justifications — planetary defence, power beaming, debris removal — so it could be built for a reason unrelated to this forecast and then repurposed.
- A hundred years permits more than one attempt, and abundant evidence that something has not happened yet is weak evidence that it will not happen within a century (hf-day-0004-takeaway-02).
Strongest evidence against
- The only programme ever organised around this goal is on hold, and the $100 million announced in 2016 was never delivered: roughly $4.5 million was actually distributed, about 4.5 percent, per its own executive director and lead investigator (hf-evidence-0053).
- The demonstrated sail is millimetre-wide and tethered against a 4.1 m free-flying requirement — roughly four thousand times the linear dimension (hf-evidence-0054, hf-evidence-0055).
- At 0.2c the interstellar medium erodes about 0.5 mm of surface en route to Alpha Centauri, roughly 500 times the thickness of a micrometre-class sail, so shielding and orientation are load-bearing rather than refinements (hf-evidence-0056).
- The fastest object ever launched would take about 77,000 years to reach Proxima, and the most ambitious conventional concept ever seriously designed about 37,300 years (hf-evidence-0057).
- The one funded advanced-propulsion flight demonstrator was cancelled, and DARPA's deputy director named the reason: falling launch costs made the R&D return negative (hf-evidence-0058).
- No space agency has ever funded a to-another-star mission past study level, and NASA's own language keeps such work explicitly outside its mission set (hf-evidence-0058).
- The single longest deep-space operating record is 48 years of managed decline at about 4 watts lost per year, down from ten instruments to three (hf-evidence-0060).
Present uncertainties
- Whether a multi-gigawatt directed-energy array is ever built for any reason. Almost all of the remaining probability mass sits on that single question, and it is a question about defence and energy policy rather than about spaceflight.
Next discriminating observation
A funded construction start — not a study — on a directed-energy array above roughly one gigawatt, for any stated purpose. Secondarily: a free-flying laser-driven sail of at least 0.1 m demonstrating sustained acceleration outside a tether, which would close the largest engineering gap in the chain.
Sources
- Breakthrough Starshot is on hold, and the $100 million was never delivered
Starshot's executive director Pete Worden is quoted saying 'We have put the program on hold and are working to transition portions to others.' Lead investigator Philip Lubin is quoted on the 2016 pledge: 'Unfortunately that, by the way, never came true. There was no $100 million,' and says his group 'got less than $200,000 spread out over eight years.' The article reports roughly $4.5 million actually distributed across about 30 contracts, against the $100 million announced on 12 April 2016 — approximately 4.5 percent.
Limit: This is journalism quoting named primary actors, not a programme document. No official cancellation exists. 'Paused' should not be upgraded to 'cancelled' without a new primary statement, and a pause is reversible.
Fetched directly 2026-09-11; publication date 16 September 2025, author Sarah Scoles. The 2016 pledge amount is confirmed in the original announcement release. Breakthrough Initiatives' own Starshot page was fetched and carries no dated status update in either direction, which is itself the corroborating observation: the programme has not announced a wind-down and has not announced activity.
- The Breakthrough Starshot system model: an $8.0B ground beam director and $6M of energy per probe
Parkin's cost-optimised 0.2c Alpha Centauri design is a 4.1 m diameter sail accelerated for 9 minutes, with '$8.0B capital cost for the ground-based beam director' and '$6M' of energy per sail launched, assuming $0.01/W lasers, $500/m2 optics and $50/kWh storage. A 0.01c solar-system precursor falls to a 19 cm sail, $517M capital and $8k per launch. Lubin's roadmap states the same architecture reaches 'gram level complete spacecraft on a wafer that reach more than 1/4c and reach the nearest star in 20 years'.
Limit: These are cost models resting on laser and optics unit costs far below today's, not quotations for a facility. The capital cost is the fixed asset; the marginal cost per probe is small, which is the structurally important part rather than the absolute number.
Both abstracts fetched directly 2026-09-11 from arXiv. A search-engine synthesis citing the ScienceDirect version returned $8.3B / 4.2 m / 10 min for the same design; ScienceDirect blocked direct access, so the discrepancy is unresolved and the arXiv figures are used as the higher-confidence reading. The difference does not change any conclusion drawn here.
- First measurable radiation-pressure displacement in a subwavelength lightsail, at millimetre scale
A June 2026 experiment reports 'the largest subwavelength tethered lightsails to date: nanoscale-thickness, millimeter-wide silicon nitride membranes patterned with billions of holes', achieving '99% reflection through resonant photonic modes' and 'radiation-pressure displacements of up to 1.75 micrometer, a 50,000-fold increase over previous lightsail optomechanical responses', while withstanding 'directed laser intensities comparable to optical intensities at the surface of the Sun'. Atwater et al. 2018 had framed the same problem as requirements only: 'We predict that major research advances in photonic design and materials science will enable us to define the pathways needed to realize laser-driven lightsails.' Separately, NASA's ACS3 deployed an 80 square metre solar sail on 7 metre composite booms after an April 2024 launch and remains active.
Limit: The demonstrated sail is millimetre-wide and tethered; Parkin's design needs 4.1 m and free flight, roughly four thousand times the linear dimension and seven orders of magnitude in area. Solar-sail deployment at 80 square metres is real engineering but solar photon pressure cannot approach relativistic velocity, so ACS3 evidences structures, not speed.
The arXiv abstract, the Atwater abstract mirror and the NASA ACS3 page were each fetched directly 2026-09-11. Nature's full text was behind an authentication wall; the abstract text is from an institutional repository mirror.
- At 0.2c the interstellar medium erodes roughly 0.5 mm of surface on the way to Alpha Centauri
'For a spacecraft speed v=0.2c, we find that dust bombardment can erode a surface layer of ~0.5 mm thickness after the spacecraft has swept a column density of N_H~3x10^17 cm^-2, assuming the standard gas-to-dust ratio of the interstellar medium.' Gas bombardment adds track damage 'to a depth of ~0.1 mm for quartz material after traversing a gas column of N_H~2x10^18 cm^-2 along the path to alpha Centauri, whereas the effect is much weaker for graphite material.'
Limit: A published comment exists (arXiv:1701.04319) disputing the treatment; it was located but not read, so the magnitude is quantified-but-contested rather than settled. The paper models a forward-facing surface, and mitigations — edge-on transit, a graphite shield, folding the sail — are proposed in the literature. The relevant comparison is that the eroded depth is roughly 500 times a one-micrometre sail thickness, which makes orientation and shielding load-bearing rather than optional.
The IOP abstract and the arXiv preprint were both fetched directly 2026-09-11 and agree.
- The fastest thing we have ever launched would take about 77,000 years; the fastest thing we have designed, about 37,000
NASA states Voyager 1 is 'moving at a velocity of 38,026.79 mph (17.0 km/second) relative to the Sun' as of 21 August 2024, at a 'speed of about 3.5 AU per year'. The most ambitious studied conventional-propulsion concept, the NASA-led Interstellar Probe, targets 'an escape speed twice that of the fastest solar-system escaping spacecraft, Voyager 1, of up to 7.2 au/year' on an SLS Block 2 baseline.
Limit: Neither speed is an upper bound on what a century of engineering could do, and a solar or thermal Oberth manoeuvre could exceed both. The figures bound what has been launched and what has been seriously designed, which is what a base rate is for.
Both pages fetched directly 2026-09-11.
- DRACO was cancelled because launch costs fell, and interstellar work stays explicitly non-mission
DARPA Deputy Director Rob McHenry, on cancelling the DRACO nuclear thermal propulsion flight demonstration: 'When DRACO was originally conceived of, that was pre- the precipitous decrease in launch costs that has been driven largely by SpaceX capabilities'; 'as the launch costs came down, the efficiency gain from nuclear thermal propulsion relative to the massive R&D costs necessary to achieve that technology started to look like less and less of a positive ROI'; 'if we can launch enough propellant cheaply enough, it's going to take a long time to earn back that efficiency.' The programme was zeroed in NASA's fiscal 2026 budget request. Separately, NASA's 2026 NIAC release states 'The NIAC projects are about early-stage concept development and are not considered official NASA missions', with 18 Phase I awards totalling $3.2 million at up to $175,000 each.
Limit: A cancelled nuclear thermal demonstrator is evidence about funding decisions, not about physics, and nuclear thermal propulsion was never a route to another star. Its value here is the stated reason, which names the mechanism rather than leaving it to be inferred.
Breaking Defense (27 June 2025, Theresa Hitchens) and the NASA NIAC release (29 July 2026) were fetched directly 2026-09-11. The SpaceNews report returned HTTP 429 on direct fetch and is cited as the independent second report of the same named quotation, located via search; the quotation itself rests on the fetched Breaking Defense text.
- In agency usage 'interstellar' means past the heliopause, about 122 AU, not en route to a star
NASA describes Voyager 1 as having 'entered interstellar space on Aug. 25, 2012, becoming the first human-made object to do so' at about 122 AU, having crossed the heliopause. The proposed Interstellar Probe inherits the same usage — it 'would reach 350 au after 50 years' — and its authors state plainly 'We do not expect an Interstellar Probe to reach the pristine VLISM in its lifetime.'
Limit: This is a vocabulary observation, not a claim about capability. It matters only because a reader tracking 'interstellar mission' headlines will count heliophysics milestones as progress toward a question they do not address.
The Frontiers article was fetched directly 2026-09-11; the NASA heliopause language was retrieved by a delegated worker from nasa.gov and matches NASA's standard published phrasing across its Voyager pages.
- The only 48-year deep-space data point is managed decline, not stable endurance
NASA states both Voyagers 'lose about 4 watts of power each year'; as of March 2025 'Three science instruments will continue to operate on each spacecraft' against ten at launch, and 'engineers believe the two probes could have enough electricity to continue operating with at least one science instrument into the 2030s.' Voyager 1 launched 5 September 1977, so the single longest-running deep-space record is roughly half a century of progressive instrument shutdown.
Limit: A 20-year relativistic cruise is a shorter endurance problem than Voyager's, so this constrains slow missions far more than fast ones. It also says nothing about what a purpose-built century-duration design could achieve, only that nothing has ever been asked to.
The NASA Science blog post (5 March 2025) was fetched directly 2026-09-11. The JPL release is the same announcement from the operating centre and is cited as the second publication of record.