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arxiv: 2606.01288 · v2 · pith:LNBZLZ4Onew · submitted 2026-05-31 · 🌌 astro-ph.EP · astro-ph.IM

Is the Dark Comet 1998 KY₂₆ the Spacecraft Phobos 1?

Adam Hibberd , Adam Crowl , Carlos G\'omez de Olea Ballester , Abraham Loeb This is my paper

Pith reviewed 2026-06-28 16:10 UTC · model grok-4.3

classification 🌌 astro-ph.EP astro-ph.IM
keywords dark comet1998 KY26Phobos 1technogenic originorbital alignmentdelta-vMahalanobis distance
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The pith

The dark comet 1998 KY26 may be the lost Phobos 1 spacecraft.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper tests whether the object known as dark comet 1998 KY26 is in fact the Russian Phobos 1 probe that failed two months after its 1988 launch. It demonstrates that two rocket burns adding up to 1.9 km/s, applied at specific times, bring the probe's calculated path into near-perfect overlap with the comet's observed orbit and timing. A sympathetic reader would care because the Hayabusa2 mission is scheduled to reach the object in 2031, so confirming an artificial origin would alter what the spacecraft is expected to find and study.

Core claim

Two propulsive DeltaVs combined at 1.9 km/s, the first just after loss of mission and the second in May 1996, allow the orbits and phases of the two bodies to align, with an arbitrarily low Mahalanobis distance using the covariance of the dark comet in 6D phase space.

What carries the argument

The 6D phase-space match achieved by two impulsive DeltaV burns whose total magnitude is 1.9 km/s.

If this is right

  • 1998 KY26 would be reclassified as artificial rather than a natural dark comet.
  • The Hayabusa2 encounter would study a spacecraft relic instead of a comet nucleus.
  • The same method could be applied to other dark comets to search for additional lost probes.
  • It would close the case on the final location of Phobos 1.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Spectral or radar data from Hayabusa2 could be checked for signatures of Soviet-era materials to test the match.
  • Similar orbital back-tracing might identify other lost spacecraft among known near-Earth objects.
  • If confirmed, mission planners would need to adjust sampling and imaging strategies for an artificial target.

Load-bearing premise

The Phobos 1 thruster could deliver a total velocity change of 1.9 km/s after the mission ended.

What would settle it

A spectrum or close image from the 2031 Hayabusa2 encounter that shows only natural rock and ice with no metallic or artificial components.

Figures

Figures reproduced from arXiv: 2606.01288 by Abraham Loeb, Adam Crowl, Adam Hibberd, Carlos G\'omez de Olea Ballester.

Figure 1
Figure 1. Figure 1: Magnitude vector difference in non-singular orbital parameters between 1998 KY26 and 31 historical Mars missions [PITH_FULL_IMAGE:figures/full_fig_p003_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: Comparison of the orbital discrepancy between 1998 KY26 and Phobos 1 as a dependency on possible arrival date at Mars, showing a minimum over a long period of arrival dates with the date adopted here 24/01/1988 a plausible assumption. Arrival Departure ∆V at Cumulative Planet Date Velocity Velocity Encounter ∆V km s−1 km s−1 km s−1 km s−1 Earth 1988 JUL 07 0.0000 3.4107 3.4107 3.4107 Mars 1989 JAN 24 2.634… view at source ↗
Figure 3
Figure 3. Figure 3: 2D view of the orbits of 1998 KY26 and the Phobos 1 probe at the time of launch, showing a striking similarity between the two orbits. 2.3. Comparison of Diameter Assuming a span for each solar panel of ∼ 5 m, that would give the overall diameter of ∼ 10 m. From T. Santana-Ros et al. (2025a), 1998 KY26 has a diameter of 11 ± 2 m. 2.4. Comparison of Brightness The absolute magnitude of 1998 KY26 from [PITH… view at source ↗
Figure 4
Figure 4. Figure 4: Light Curve of 1998 KY26 just after discovery. 2.6. Implications The combination of high albedo from T. Santana-Ros et al. (2025a) of 0.52 ± 0.08, the diameter of 11 ± 2 m, and low spin period of 5.3516 ± 0.0001 mins, all suggest a small, tough, monolithic object, such as the Phobos 1 probe. 3. POSSIBLE SCENARIO The details of the Phobos 1 mission, as generated by OITS are provided in [PITH_FULL_IMAGE:fig… view at source ↗
Figure 5
Figure 5. Figure 5: Solution trajectory followed by the Phobos 1 probe (blue line) compared to that of 1998 KY26 retrodicted by NASA Horizons, SPICE (red dashed line) [PITH_FULL_IMAGE:figures/full_fig_p008_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: Solution trajectory (plan view) followed by the Phobos 1 probe (blue line) compared to that of 1998 KY26 retrodicted by NASA Horizons, SPICE (red dashed line) 4. DISCUSSION To be clear, this research cannot unequivocally identify that 1998 KY26 is really the Phobos 1 probe, it analyses the evidence in support of this conclusion, which is quite compelling. We have found: 1. The Phobos 1 and 1998 KY26 orbits… view at source ↗
Figure 7
Figure 7. Figure 7: Displacement between the Phobos 1 probe and 1998 KY26 2. The difference between these orbits is compatible energetically with the overall ∆V envelope available to Phobos 1 3. The 2 orbits converge and are statistically compatible, given the uncertainty in the orbit of 1998 KY26, which is tightly constrained due to the existence of over 230 observations of this dark comet 4. There is a historical record in … view at source ↗
read the original abstract

Since the discovery of new kinds of celestial bodies known as $dark~comets$, scientists have speculated about their ontology. A curious hybrid of comet and asteroid, these objects show significant non-gravitational accelerations (NGAs) yet exhibit absolutely no signs of cometary outgassing in the form of a coma or tail. The planned rendezvous of the Hayabusa2 spacecraft with 1998 KY$_{26}$ in July 2031 elevates the question of this so-called dark comet's nature beyond a purely research exercise, as the true nature of the object may have practical implications for the scientific return of the mission. This study examines the hypothesis that 1998 KY$_{26}$ may be of technogenic origin, in fact a relic of a historical Russian mission to Mars, the Phobos 1 probe, which suffered a failure 2 months after the launch in July 1988, due to upload of a faulty command. We find that two propulsive DeltaVs combined at 1.9 km/s, the first just after loss of mission and the second in May 1996, allow the orbits and phases of the two bodies to align, with an arbitrarily low $Mahalanobis~distance$ using the covariance of the dark comet in 6D phase space. There is also evidence that 1.9 km/s was within the performance envelope of Phobos 1, which had a powerful nitric acid and amine-based autonomous thruster for Mars Orbital Insertion (MOI).

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 1 minor

Summary. The paper claims that the dark comet 1998 KY26 may be the lost Phobos 1 spacecraft. It supports the hypothesis by showing that two impulsive DeltaV maneuvers (one immediately after mission loss in 1988 and one in May 1996) with a combined magnitude of 1.9 km/s can align the 6D orbital states of the two objects to produce an arbitrarily low Mahalanobis distance relative to the dark comet's covariance matrix. The paper further asserts that this total DeltaV lies within the performance envelope of Phobos 1's nitric-acid/amine thruster system originally intended for Mars orbital insertion.

Significance. If the identification were independently confirmed, it would constitute a documented case of a technogenic object masquerading as a dark comet, with direct relevance to the interpretation of non-gravitational accelerations and to the scientific planning for the Hayabusa2 rendezvous in 2031. The present manuscript, however, offers only a constructed orbital match rather than a falsifiable prediction or additional corroborating observables.

major comments (2)
  1. [Abstract] Abstract: the two propulsive DeltaVs (combined 1.9 km/s, second burn epoch May 1996) are chosen specifically to minimize the Mahalanobis distance in 6D phase space. Once the epochs and total budget are granted as free parameters, an arbitrarily low distance is achieved by construction; no independent dynamical, telemetry, or observational constraint is supplied that would have required the maneuvers to occur at those times.
  2. [Abstract] Abstract: the physical viability of the hypothesis rests on the assertion that 1.9 km/s remained within the performance envelope of Phobos 1's autonomous MOI thruster years after launch. No quantitative calculation of remaining propellant mass, specific impulse, thruster degradation, or attitude-control overhead is provided to substantiate this claim, which is load-bearing for the entire scenario.
minor comments (1)
  1. [Abstract] The abstract would be strengthened by an explicit statement that the reported alignment is a fit rather than a prediction from prior information.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their constructive comments. We respond to each major comment below, indicating where the manuscript will be revised to address the concerns raised.

read point-by-point responses

  1. Referee: [Abstract] Abstract: the two propulsive DeltaVs (combined 1.9 km/s, second burn epoch May 1996) are chosen specifically to minimize the Mahalanobis distance in 6D phase space. Once the epochs and total budget are granted as free parameters, an arbitrarily low distance is achieved by construction; no independent dynamical, telemetry, or observational constraint is supplied that would have required the maneuvers to occur at those times.

    Authors: We agree that the maneuver parameters were optimized to identify a possible match, as the study is an existence proof of dynamical feasibility rather than a claim of a uniquely determined solution. The key scientific point is that the required total DeltaV remains modest. We will revise the abstract to explicitly state that the epochs and magnitudes are chosen to minimize the distance and that no independent constraints on burn timing are provided, thereby clarifying the exploratory nature of the analysis. revision: yes

  2. Referee: [Abstract] Abstract: the physical viability of the hypothesis rests on the assertion that 1.9 km/s remained within the performance envelope of Phobos 1's autonomous MOI thruster years after launch. No quantitative calculation of remaining propellant mass, specific impulse, thruster degradation, or attitude-control overhead is provided to substantiate this claim, which is load-bearing for the entire scenario.

    Authors: The manuscript currently references the known design specifications of the nitric-acid/amine thruster but does not include the requested quantitative estimates. We will add a dedicated paragraph or short appendix supplying order-of-magnitude calculations based on the documented propellant load, specific impulse, and expected degradation over the relevant time interval to support the performance-envelope claim. revision: yes

Circularity Check

1 steps flagged

DeltaV epochs and magnitudes fitted to force arbitrarily low Mahalanobis distance

specific steps
  1. fitted input called prediction [Abstract]
    "We find that two propulsive DeltaVs combined at 1.9 km/s, the first just after loss of mission and the second in May 1996, allow the orbits and phases of the two bodies to align, with an arbitrarily low Mahalanobis distance using the covariance of the dark comet in 6D phase space."

    The two DeltaV vectors and their epochs are chosen specifically so that the final 6D state matches the dark-comet covariance to arbitrarily low Mahalanobis distance. The low distance is therefore guaranteed once the total impulse budget is permitted; it does not constitute an independent test of the identification hypothesis.

full rationale

The paper's key result is obtained by selecting two impulsive burn times and a total 1.9 km/s budget such that the propagated state of Phobos 1 lies inside the 1998 KY26 covariance ellipsoid. The reported 'arbitrarily low' distance is therefore the direct numerical outcome of that parameter choice rather than an independent dynamical constraint or prediction. Once the maneuver budget and epochs are granted, the alignment follows by construction; the remaining load-bearing claim (thruster performance) is asserted separately without additional orbital or telemetry evidence supplied in the text.

Axiom & Free-Parameter Ledger

2 free parameters · 1 axioms · 0 invented entities

The claim rests on the assumption that the required burns were feasible for the spacecraft and that no contradictory observations exist; the DeltaV magnitude and timing are free parameters adjusted to produce the reported match.

free parameters (2)
  • Combined DeltaV magnitude = 1.9 km/s
    Value of 1.9 km/s selected to achieve arbitrarily low Mahalanobis distance
  • Epoch of second burn = May 1996
    May 1996 chosen to align orbital phases
axioms (1)
  • domain assumption Phobos 1 retained sufficient propellant and thruster performance after mission loss to execute the burns
    Invoked when stating that 1.9 km/s lay within the performance envelope

pith-pipeline@v0.9.1-grok · 5821 in / 1355 out tokens · 41576 ms · 2026-06-28T16:10:53.180664+00:00 · methodology

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Reference graph

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