This page describes every check the Kessler@home simulation has been put through: what was run, with which inputs and settings, what came out, and what the result does and does not show. Each setup is written out as a recipe, so that someone with the same reference models can repeat it. Unless a section says otherwise, the results are from the Kessler@home simulation as of 3 November 2026, application version 1.00. Versions 1.01 and 1.02 change only what a task reports and how the application is built; they compute the same thing. For what the Kessler@home simulation itself does, see The science.

Summary of the checks

ESA MASTER-8: objects in orbit today, by altitude

What it checks. Whether the population the Kessler@home simulation starts from holds as many objects at each altitude as ESA's reference model does. That population is the public catalog plus an estimate of the objects 10 cm and larger that the catalog misses at high altitude. The 1 to 10 cm debris, which is counted per altitude shell rather than tracked object by object, is estimated separately. Both estimates are made by re-creating 349 known breakups since 1961 with the Kessler@home simulation's own breakup model and following the fragments to the present.

Setup.

• Reference model: ESA's MASTER-8 (Meteoroid and Space Debris Terrestrial Environment Reference), command-line tool version 8.2.3, with the reference population files of February 2026, which are split by debris source.
• MASTER runs: spatial density against altitude, 200 to 2,000 km in 36 classes of 50 km, at 1 February 2026. Two runs were made, one for objects of 1 cm and larger and one for 10 cm and larger, each with every human-made source switched on and meteoroids off. A shell's count is its spatial density times the volume of the spherical shell (Earth radius 6,378.137 km).
• Sources: explosion fragments, collision fragments, and launch- and mission-related objects are compared directly. MASTER's sources that the Kessler@home simulation does not carry (sodium-potassium coolant droplets, solid rocket motor slag and dust, paint flakes, impact ejecta and multilayer insulation) are counted as "other" and included in MASTER's totals.
• Kessler@home side: the starting population of the sweep (the catalog as of October 2026). Each object is spread over the same 36 shells in proportion to the time it spends in each, from Kepler's equation; a near-circular orbit sits in one shell. The 1 to 10 cm debris is already a count per 50 km shell and is used as it is.
• Catalog completeness (chosen with this comparison): radar catalogs smaller objects nearer the ground, so above about 1,000 km the catalog misses some objects of 10 cm and larger, and the Kessler@home simulation adds an estimate of them. The estimate depends on the smallest size the catalog sees at each altitude, which isn't known exactly. Three assumptions were tried and the one closest to MASTER kept: 10 cm up to 1,000 km, rising to 15 cm at 1,500 km and 20 cm at 2,000 km. A larger assumption (10 cm to 600 km, 20 cm at 1,000 km, 30 cm from 1,500 km) gave 1.22 times MASTER's objects of 10 cm and larger and 1.87 times its 1 to 10 cm debris. Assuming the catalog misses nothing gave 0.94 and 1.12. Because of this choice, the total of objects 10 cm and larger agrees with MASTER partly by construction. The independent parts of the comparison are the intact objects and the cataloged fragments below 1,000 km.

Result. Objects 10 cm and larger resident in each band:

Objects of 1 to 10 cm:

What it shows.

• The intact objects (satellites, rocket bodies and mission-related objects) agree to 3%: 19,276 against MASTER's 18,706. They are the same objects, from the same catalog.
• The 200 to 400 km band holds three times MASTER's count. In the catalog of October 2026, 1,232 intact objects sit at 300 to 400 km, against MASTER's 290 in February. 923 of them are Starlink satellites, among them 637 of one type, launched in 2024 and 2025, that now operate at 350 to 375 km. Whether they were already there in February and are missing from MASTER's snapshot, or were moved down since, isn't known yet. Below 600 km as a whole the two counts agree within 2%, so the same satellites are in both, at different altitudes. SpaceX also lowered its 550 km shell to about 480 km during 2026 (about 4,400 satellites, announced in January 2026; MacDonald, 2026), which is why this population is short of MASTER's at 500 to 600 km.
• The cataloged fragments match MASTER at 600 to 1,000 km (6,861 against 7,038), so the catalog is complete to 10 cm there. Above 1,200 km MASTER carries more fragments, by a factor rising from 1.5 to 3 near 2,000 km. The estimated uncataloged objects recover about half of that gap, and the ratio stays at 0.64 to 0.84.
• The 1 to 10 cm debris is 1.27 times MASTER's overall: 1.1 to 1.25 times at 600 to 1,000 km and 1.4 to 1.55 times at 1,000 to 1,600 km, where the old Delta and Kosmos-3M clouds are. Below 600 km it is lower. Part of that is a source the Kessler@home simulation does not model: solid rocket motor slag, which MASTER estimates from a model of past solid motor firings. Slag is 4% of MASTER's 1 to 10 cm objects over 200 to 2,000 km, but 18% below 600 km, where small fragments don't last long. Comparing fragments with fragments, the Kessler@home simulation has 0.32 of MASTER's at 300 to 400 km, 0.54 at 400 to 500 km and 0.82 at 500 to 600 km; the reason for that deficit has not been found yet. Because this is the least certain input, the amount of 1 to 10 cm debris today is one of the swept settings (from 0.25 to 1.4 times this estimate).
• The size conventions differ. MASTER bins objects by diameter, while the breakup model counts fragments by its characteristic length, whose area-equivalent diameter is about 0.84 of it. A 15% difference in size moves a count by about 25%, so the comparison should be read at about that level.
• Not compared: the impact flux on a spacecraft, MASTER's main output. The Kessler@home simulation does not compute a flux on a test object; the spatial density is the quantity its collision method uses. The flux comparison is done against ORDEM, below.

NASA ORDEM 3.2: impact rates on spacecraft orbits

What it checks. The rate at which a spacecraft on a given orbit would be hit, computed from the Kessler@home simulation's population, against NASA's engineering model. This tests the same population as the MASTER comparison, through a different quantity: the flux combines how many objects there are, where, and how fast they cross the target's orbit.

Setup.

• Reference model: NASA's ORDEM 3.2 (Orbital Debris Engineering Model, version 3.2.0 of January 2022), obtained from NASA under its Software Usage Agreement. ORDEM's population files run from 2016 to 2050. The 2026 file used here is a projection, not an observation: ORDEM 3.2 keeps ORDEM 3.1's populations, adding only the fragments of the 2021 Cosmos 1408 test, and those hold real launches up to 2015 and then assume the 2008 to 2015 launch traffic repeats, with 90% of satellites and rocket bodies disposed of after their missions (Matney et al., 2019; Manis and Matney, 2022).
• ORDEM runs: spacecraft assessment for the year 2026, on circular orbits at the centers of the Kessler@home simulation's 50 km shells, 325 to 1,975 km (34 altitudes), at three inclinations, 51.6°, 74° and 98°, with random node and argument of perigee: 102 runs, each of 20 seconds to a few minutes. The direction-and-speed grid is IGLOO_30x30x2.SIG.
• Output compared: impacts per year on a sphere of 1 m² cross-section, for debris of 1 cm, 10 cm and 1 m and larger, split into intact objects and fragments. ORDEM's sizes are characteristic lengths, so the population's fragment sizes (area-equivalent diameters) are converted back to characteristic lengths first. ORDEM's sodium-potassium droplets, which the Kessler@home simulation does not carry, are left out. ORDEM's flux-weighted mean impact speed is compared as well.
• Kessler@home side: the flux on the same orbit from the starting population. Each object is spread over the shells by the time it spends in each, and over latitude by its inclination, with random node and perigee (the method of Kessler, 1981). Its speed relative to the target is averaged over the points where the two orbits cross, and the target is averaged over its shell and the latitudes it covers. The 1 to 10 cm flux is the one the Kessler@home simulation itself uses: each shell's count, split over inclination like the shell's tracked fragments, times the crossing-geometry factor for the target's inclination, over the shell's volume.
• Check of the method: a brute-force count, placing objects and targets at random positions on four test orbits (one of eccentricity 0.05), reproduces the computed flux to within 0.93 to 1.08, each within 1.2 standard errors of the sampling.

Result. Flux on the target, Kessler@home divided by ORDEM, summed over the shells of each band (population as rebuilt in November 2026):

What it shows.

• Where the intact population has not changed since ORDEM was built, the two agree: the intact flux is 0.90 to 0.98 of ORDEM's at 1,600 to 2,000 km, and within 1 to 22% in single shells at 825, 975, 1,425 and 1,475 km at all three inclinations. This checks the flux computation and its units end to end.
• Below 600 km the intact flux is 27 to 30 times ORDEM's, because nothing launched after 2015 is in ORDEM's population. Most of the difference is the working Starlink and OneWeb satellites: without them the ratio below 600 km is 7.4 to 8.8, and at 900 to 1,600 km, where the OneWeb shell is, 0.8 to 1.0. The other satellites launched since 2015 make up most of the rest; the dead satellites and rocket bodies alone still give 2.8 to 3.0 times ORDEM's intact flux. Below 600 km this comparison therefore tests ORDEM's projection rather than the Kessler@home simulation, and the catalog is the better source there.
• Fragments 10 cm and larger give 1.5 to 3.5 times ORDEM's fragment flux, most at 600 to 900 km. Two breakups since ORDEM's data, of Long March 6A upper stages in 2022 and 2024, account for 1,024 of the 5,721 cataloged debris objects there (18%), which explains part of it. The rest is a difference between ORDEM and the 2026 catalog, which MASTER-8 matches within 3% at 600 to 1,000 km. The catalog stays the source.
• The 1 to 10 cm debris is 2.2 to 6 times ORDEM's above 600 km, more so with altitude, and 0.9 to 1.1 times below 600 km. Against MASTER the same debris is 1.1 to 1.55 times, so the two reference models themselves differ by a factor of about 2 to 4 in this size range above 1,000 km, MASTER being the higher. This is the largest disagreement in the comparison. The swept setting for the amount of 1 to 10 cm debris was widened because of it: its options of 0.7 to 1.4 times bracketed MASTER only, and the added 0.25 and 0.5 reach ORDEM's level.
• The impact speeds of 1 to 10 cm debris agree within 0.7 km/s in every band and inclination. An earlier version of this comparison found that the Kessler@home simulation's fixed 10 km/s for small debris was up to a quarter too low for polar and sun-synchronous targets, which is why the Kessler@home simulation now takes the hit rate and speed from the crossing geometry.

The IADC comparison study: 200 years from 2009

What it checks. The Kessler@home simulation's whole long-term machinery (collisions, breakups, drag and the bookkeeping of the population) against the one published exercise in which several agency models projected the same scenario.

The study. In the Inter-Agency Space Debris Coordination Committee study "Stability of the Future LEO Environment" (Liou et al., 2013), six models (ASI SDM, ESA DELTA, ISRO KSCPROP, JAXA LEODEEM, NASA LEGEND and UKSA DAMAGE) projected the same scenario over 200 years. The scenario: the MASTER-2009 population of objects 10 cm and larger crossing LEO on 1 May 2009 (17,074 objects); launches repeating the 2001 to 2009 cycle; 8-year missions; 90% of payloads and rocket bodies disposed of within 25 years; no explosions; no collision avoidance. The models made 40 to 275 runs each, 725 in all. Results: the population grows by 30% on average by 2209 (model means +19% to +36%), 87% of runs end above the starting count, and a catastrophic collision happens every 5 to 9 years, mostly at 700 to 800 and 900 to 1,000 km. Lidtke, Lewis and Armellin (2017) later ran the DAMAGE version 25,000 times from 2013 and found a standard deviation of 3,377 objects in the final count.

Setup.

• Time: start 1 May 2009, 200 years, 5-day step, collision probabilities in 10 km cubes. The analytic rate used for dense constellation layers is switched off, since the study's models used the cube method throughout. Catastrophic breakup above 40 J/g.
• Starting population: the study's input is not public, so a synthetic population of 17,074 objects 10 cm and larger was built in the shape of the MASTER-2009 spatial density: a peak at 700 to 1,000 km holding the Fengyun-1C and Iridium–Cosmos clouds and a second peak at 1,400 to 1,500 km. Its makeup follows the published totals for the period (ESA's 2013 ranking study counts about 12,000 cataloged LEO objects, almost 9,000 of them fragments, and about 2,500 t in LEO; Liou, 2011, about 2,700 t): 600 working payloads, 1,300 dead payloads, 400 mission-related objects, 900 rocket bodies and 13,874 fragments, 2,700 t in all (rocket bodies 1,550 t, payloads 1,110 t). The fragments are drawn so that they decay as the real cataloged fragments of January 2009 did over the 17 years that followed: each takes the orbit shape (the eccentricity) of a real 2009 fragment at its altitude, and its area-to-mass comes from the breakup model with the lifetime weight fitted in the population hindcast below. The two collision clouds of February 2009 (Cosmos 2251 and Iridium 33), three months old at the start, are drawn without that weight. Followed under the measured solar activity with no collisions, the fragments that predate 2009 keep 87.1% after ten years, where the real ones kept 87.2%. This population replaced an earlier one in November 2026; what that changed is under "What it shows".
• Launches: an approximation of the 2001 to 2009 traffic, repeated throughout: 60 payloads a year (700 kg, 2.3 m) and 30 rocket bodies (1,500 kg, 3.8 m). Launch altitudes are 450 km (12%), 550 (15%), 650 (20%), 800 (25%), 900 (12%), 1,000 (8%) and 1,400 km (8%), each ±50 km. Inclinations are 98° (40%), 82.5° (12%), 74° (10%), 65° (8%), 51.6° (12%), 97° (8%), 86.4° (5%) and 48° (5%).
• Behavior: 8-year missions with station keeping. 90% of payloads at the end of their mission, and 90% of new rocket bodies at launch, are lowered to an orbit with 25 years of life left. No explosions, no collision avoidance, no debris removal.
• Atmosphere: the NRLMSISE-00 density table, with a cosine solar cycle: F10.7 mean 123, amplitude 60, period 11 years, maximum in 2014. Drag coefficient 2.2.
• Small debris: the 1 to 10 cm debris is off, since the study's models tracked objects 10 cm and larger only.
• Runs: 725, seeds 1 to 725, the same number as the study's six models made together (November 2026). Seeds 1 to 96 were run with the released application, version 1.03, and seeds 97 to 725 with the beta version 1.06 on graphics cards, which gives the same results bit for bit (checked on all of seeds 1 to 96 here).

Result.

The mean growth is known to about ±0.7 percentage points from 725 runs. Taken 24 at a time, the runs give between −3.0% and +11.8%, with 27.5 to 31.6 collisions: that is the spread to expect from 24 runs. The first 24 are the benchmark that is rerun after every change.

What it shows. The collision rate agrees. The run-to-run spread agrees too, including the long upper tail (final counts from 11,110 to 29,927). The trend agrees in direction but not in size: the population falls by 16% in the first decade (17,074 to 14,309), bottoms out near 13,200 around 2050 and recovers to 17,781, a gain of 4%, where the study's models grow by 30% without such a dip, and 56% of the runs end above the start against the study's 87%. The gap is about 4,400 objects over 200 years out of about 41,600 fragments created per run. It is the balance between fragments made and fragments decayed.

What is known about the gap. One possible cause has been tested, two have not, and one cannot be ruled out:

• The starting fragments (tested and corrected, November 2026). The synthetic population first used here gave −5.3%. The population hindcast below showed that its fragments decayed faster than the real 2009 fragments did: under the measured solar activity and with no collisions they kept 77.5% after ten years, against 87.2%. The cause was their orbit shape. They had been drawn with eccentricities spread evenly up to 0.08 (median 0.030), where the real fragments' median is about 0.010, and a more elongated orbit dips into denser air at its low point. The population was rebuilt with real orbit shapes and the refitted area-to-mass weight, and the runs repeated:

  Starting population	Runs	Its older fragments after ten years, drag only (real: 87.2%)	Final count (change)	Runs ending above the start	Catastrophic collisions
  First synthetic population (used until November 2026)	96	77.5%	16,165 (−5.3%)	34 of 96	27.2
  Real orbit shapes and the refitted area-to-mass weight	96	87.1%	17,614 (+3.2%)	47 of 96	29.9
  The same, with the two February 2009 clouds drawn without the weight (the population used now)	725	87.1%	17,781 (+4.1%)	403 of 725	29.5
  IADC study	725	—	about 22,200 (+30%)	87%	22 to 40

  On the same 96 runs as the first two rows, the population used now gives +5.3%. The correction was set by the real fragments' decay, not by the study's answer. It moved the result by about 10 of the 35 percentage points between the first result and the study (each figure is known to about ±0.7 to ±2 percentage points), and the collision rate stayed inside the study's range. About 26 percentage points remain between +4.1% and the study's +30%.
• The launch altitudes (not tested). The approximate traffic puts 27% of launches at 450 to 550 km, and 41% of the catastrophic collisions here happen at 400 to 700 km, while the study's are mostly at 700 to 800 and 900 to 1,000 km. Fragments made there decay within years instead of lasting at 700 to 1,000 km.
• The solar cycle (not tested). The cosine cycle, averaging 123, is stronger than the real cycle 24 that followed 2009, which averaged 98. Under it the Kessler@home simulation's Fengyun-1C cloud keeps 77% after ten years, against the catalog's 83%. The study's own solar assumptions are not known in this detail.
• The Kessler@home simulation itself (not ruled out). This comparison cannot exclude that the Kessler@home simulation grows debris more slowly than the study's six models. What speaks against a large error is that the collision rate matches the study's and that the hindcasts below reproduce the decay of real clouds and of a real population to within a few percentage points. But the remaining 26 percentage points have not been traced to any cause.

Only the study's own inputs, which are not public, would settle it. So this comparison validates the Kessler@home simulation's collision machinery but not the growth figure, and nothing from the synthetic population is used as a result about the real environment. The sweep itself starts from the real 2026 catalog, whose orbits are the cataloged ones.

Six real debris clouds, followed through the catalog

What it checks. The breakup model's fragment sizes and area-to-mass ratios, the drag model and the atmosphere together, against how real fragment clouds actually decayed. No synthetic population is involved.

Setup.

• Kessler@home side: the parent objects are placed at the breakup's date, altitude and inclination and broken up once with the NASA Standard Breakup Model: as a collision, from both masses and the impact speed, or as an explosion. The fragments 10 cm and larger are propagated with the same drag, atmosphere and Sun and Moon gravity as the full runs, with a 5-day step and seed 1. The observed monthly solar flux (F10.7) is used instead of the cosine cycle. Nothing else is in orbit: no launches, and no further collisions or explosions. The Kessler@home simulation's survival is the fraction of the fragments in orbit just after the breakup that are still in orbit at each anniversary.
• Catalog side: from CelesTrak's satellite catalog (SATCAT), every object of type debris carrying the parent's international designator, and the fraction of them with no decay date at each anniversary of the breakup.
• Parents: public approximations of mass and size (the sizes are the breakup model's characteristic lengths). The survival fraction is insensitive to them; the fragment count is not.

Both satellites of the 2009 collision are placed on Cosmos 2251's orbit. Iridium 33 was at 86.4°, but the atmosphere depends only on altitude and solar activity, so the inclination does not change how fast the fragments decay. For the two Briz-M explosions the fragment count is multiplied by 10 for a smoother fraction, and survival is counted from the fragments still in orbit 30 days after the breakup (with a 1-day step for those 30 days). A cloud with a 265 km perigee loses most of its fragments within weeks, before they can be cataloged, and the catalog's fraction is measured over the cataloged set.

Result. Percentage of the cloud still in orbit, Kessler@home simulation / catalog:

What it shows.

• Fengyun-1C is within about 2 percentage points of the catalog from year 8 to year 15, and the Cosmos 1408 decay has the right shape and timescale (30.3% against 25.2% after one year).
• In the first years the Kessler@home simulation loses more fragments than the catalog does (Fengyun-1C 6% against 0.6% in year 1). Fragments that reenter within weeks are never cataloged, so the catalog's first-year loss is biased low.
• The clouds of the 2009 collision decay faster than the Kessler@home simulation gives them: 65% of Cosmos 2251's fragments remain after ten years against its 77%, and 54% of Iridium 33's against 77%. The Kessler@home simulation gives every satellite the same area-to-mass distribution for its fragments, so it cannot tell a dense Soviet communications satellite from a light satellite with large panels. In the 2024 to 2026 solar maximum the cataloged clouds also decayed faster than in the Kessler@home simulation.
• For the Briz-M stage of 2012, the stage's own orbit is the cleaner test, since it does not depend on the breakup model. With a tumbling cross-section of 6.5 m², the Kessler@home simulation carries it from 265 × 5,008 km to 260 × 3,785 km in 14 years, against the catalog's 304 × 3,796 km today: a 1% error on the apogee lost through a solar maximum and a minimum. Its fragments, however, survive much better in the catalog than in the Kessler@home simulation; they behave like objects of area-to-mass below about 0.05 m²/kg, where the breakup model puts about 7% of its rocket-body fragments. The 2007 stage's fragments, on a higher orbit, stay within 9 percentage points of the catalog throughout, the Kessler@home simulation always the higher.
• Overall, the drag and propagation reproduce the observed decay to within a few percentage points for Fengyun-1C over a decade, 12 percentage points for Cosmos 2251 and 23 percentage points for Iridium 33, and within a factor of two at one to two years for Cosmos 1408. The remaining error is in the fragments' area-to-mass, one distribution per kind of parent, not in the propagation. It errs in both directions: satellite fragments live somewhat too long, and the fragments of the 2012 Briz-M stage too short.

A whole cataloged population, followed from 2009 and from 2017 to 2026

What it checks. How fast a real population decays in the Kessler@home simulation, against what the catalog shows actually happened. The debris clouds above follow fragments from their breakup. This check starts instead from every cataloged object in orbit on a past date, gives each one a mass and an area by the same rules that build the sweep's starting population, and follows them to the present. It tests two things: the drag on objects of known mass and area, and the rule that assigns an area-to-mass ratio to a cataloged fragment, which no public catalog records. It was run in November 2026 with the released application, version 1.03.

Setup.

• The 2017 cohort: every object that CelesTrak's satellite catalog (SATCAT) of 29 April 2017 lists in orbit with a perigee of 100 to 2,000 km. The file is the Internet Archive's copy of that date, the oldest one it holds. 14,310 objects are followed: 1,907 dead payloads and mission-related objects, 1,216 rocket bodies and 11,187 fragments.
• The 2009 cohort: the objects with an orbit dated 1 or 2 January 2009 in the file that MIT's public MOCAT-MC repository ships as its 2009 initial condition (the result of a Space-Track query, published there). It is used for this check only, and only counts are quoted here. 8,304 objects are followed: 1,560 dead payloads and mission-related objects, 873 rocket bodies and 5,871 fragments. It is not the complete catalog of that date, since it holds only the objects with an orbit determined in those two days.
• Set aside: the payloads operating on the start date (953 in 2017, 353 in 2009), because they hold or change their orbits, and objects without an orbit in the file.
• Masses and areas: by the rules of the sweep's starting population, from the object's orbit on the start date. An intact object has its mass from the GCAT catalog and its average cross-section from ESA's DISCOS database. A fragment has its size from its radar cross-section class, and its area-to-mass is drawn from the NASA Standard Breakup Model's distribution for that size. The draw is weighted toward values that decay slowly, with weight equal to the fragment's lifetime over 100 years (at most 1), on the reasoning that the fragments still in orbit are the ones that decay slowly. A second set is drawn without the weight, for comparison. Each draw was repeated ten times; the totals of the ten differ by 25 objects.
• Run: from the start date to 28 October 2026 with a 5-day step, the measured monthly solar flux (F10.7), drag coefficient 2.2 and the Sun's and Moon's gravity. Nothing else happens: no launches, no collisions, no explosions.
• Catalog side: CelesTrak's SATCAT of 28 October 2026, which gives each object's decay date or shows it still in orbit.

Result. Objects still in orbit on 28 October 2026, from 29 April 2017 (9.5 years):

From 2 January 2009 (17.8 years):

Fragments still in orbit by their altitude at the start, catalog against the Kessler@home simulation:

Year by year, the simulated count of the 2017 cohort stays within 30 objects of the real one until the end of 2021 (13,902 real, 13,915 simulated) and parts from it through the solar maximum that followed: 12,418 against 12,908 at the end of 2024, and 11,757 against 12,475 in October 2026.

The IADC comparison's synthetic fragments on the same run. The IADC comparison starts from a synthetic 2009 population. Its 11,924 fragments that predate 2009 were put through the 2009 run and compared with the real 2009 fragments, altitude band by altitude band. (Its other 1,950 fragments stand for the two collision clouds of February 2009, which the real set of January 2009 cannot contain; they are given separately below.) The last row is weighted to the real fragments' altitudes:

The first synthetic population, used for the IADC comparison until November 2026, lost its fragments too fast, most of all at 800 to 1,000 km. The cause was the orbit shape: its fragments were drawn with eccentricities spread evenly up to 0.08 (median 0.030), where the real fragments' median is about 0.010, and a more elongated orbit dips into denser air at its low point. At 800 to 900 km one in ten of its fragments came below 451 km; one in ten of the real ones comes below 702 km. Their area-to-mass differed much less. The corrected population gives each fragment the eccentricity of a real 2009 fragment at its altitude and uses the refitted area-to-mass weight described next; it follows the real decay to within 2 percentage points in every band from 700 km up. Its two February 2009 clouds, drawn without the weight because they were three months old, keep 71% after ten years, where the catalog shows 65% for Cosmos 2251 and 54% for Iridium 33; in the first population they kept 93%.

Does the fragment rule change the sweep's results? The rule was refitted to both cohorts: with weight equal to the square root of the lifetime over 1,000 years (at most 1), the Kessler@home simulation keeps 80.1% of the 2017 fragments against the catalog's 79.5%, and 74.8% of the 2009 fragments against 75.7%. A test copy of the sweep's starting population was built with that rule. It holds the same 35,865 objects; only the fragments' area-to-mass is redrawn, which moves their median at 600 to 700 km from 0.019 to 0.056 m²/kg and at 1,000 to 1,100 km from 0.123 to 0.072 m²/kg. It was run against the population as shipped on three scenarios:

A and B are two scenarios of the sweep in which about half of the runs end in runaway, so that a shift either way would show. Both use the nominal launch traffic without the two Guowang shells near 1,150 km, the 25-year disposal rule and a quarter of the estimated 1 to 10 cm debris. A has 90% disposal reliability, 20 debris removals a year, a strong solar cycle, no rocket-body explosions, and hits by 1 to 10 cm debris never end a satellite's mission; its runaway shells are all at 1,300 km and above, where there is almost no drag. B has 95% disposal reliability, 50 removals a year, a weak solar cycle, rocket-body explosions at 0.001 per rocket body per year, and such hits always end the mission; its runaway shells are mostly at 750 to 900 km, where a fragment's area-to-mass decides how long it stays, so B is the sharper test. For A and B the runs of the population as shipped are the sweep's own results, returned by volunteers' computers; 26 of them were repeated locally and came out identical. The runs with the refitted rule were made with the released application, most of them with its graphics-card version, which gives the same results bit for bit.

None of the differences is larger than the scatter to expect between two sets of runs. The standard error of the difference in runaway share is 3.6 percentage points for A and 3.5 for B, and the two differences have opposite signs. The collisions and final counts differ by less than two standard errors in every case.

What it shows.

• Intact objects, whose mass and area are known, come out within 3 percentage points of the catalog over 9.5 and 17.8 years, the Kessler@home simulation always losing slightly more. A run of the 2017 cohort with the drag coefficient lowered from 2.2 to 1.5 or 1.8, as a diagnostic, brings the rocket bodies to 90.0% and 89.4% against the catalog's 90.7%. The drag on these objects is therefore about 20 to 30% too strong in the product of drag coefficient, area and air density; which of the three is not known.
• For fragments the error is in the starting rule and not in the drag. For the fragments as a whole, the catalog's record lies between the weighted and the unweighted draw in both cohorts, and so do the Fengyun-1C and Cosmos 2251 clouds; Iridium 33's fragments decayed faster than either draw. With the weight, the Kessler@home simulation keeps too many fragments below 900 km and too few at 900 to 1,000 km. A weight proportional to the lifetime is the right correction for a population fed at a steady rate. A cloud of known age has lost only the fragments whose lifetime is shorter than its age, and a large share of the cataloged fragments belongs to a few such clouds (38% of the 2017 cohort's to the three named above).
• For the sweep: the cataloged fragments of its 2026 starting population are assigned by the same rule, so below 900 km they outlast real ones. Refitting the rule did not measurably change the result of the nominal scenario or of two scenarios at the boundary between stable and runaway. The sweep's starting population is therefore left as it is.
• For the IADC comparison: the fragments of the synthetic 2009 population first used there decayed faster than the real 2009 fragments did, because of their orbit shapes. With that corrected, the IADC result moved from −5.3% to +4.1%, against the study's +30%. So the starting fragments explain about 10 of the 35 percentage points of that comparison's missing growth, and not the rest.
• The limits of this check. Both cohorts end in October 2026, so both include the same 2024 to 2026 solar maximum. The 2009 cohort is incomplete. The refitted rule was fitted to these two cohorts and has not been tested on a third. And with about 400 runs on each side, a shift in a runaway share smaller than about 7 percentage points would not be detected.

MIT MOCAT-MC on the IADC scenario

What it checks. How an independent, open-source model behaves on the same scenario, and so how far the IADC growth figure constrains a model at all. MOCAT-MC was run in October 2026. The Kessler@home figures it is compared with are those of the IADC comparison above (725 runs, November 2026).

Setup.

• Code: MOCAT-MC from MIT ARCLab's public repository (MATLAB, MIT license), as of October 2026, run under GNU Octave 8.4. MOCAT-MC's own files are unchanged. MATLAB-only functions are replaced by 18 small stand-ins: dates as Julian day numbers in place of MATLAB's datetime type, discretize, histcounts, splitapply, isoutlier, a one-component Gaussian fit, and a Kepler solver in place of the compiled one.
• Driver edits: the two example driver files (initSim.m and main_mc.m of the Scenario_No_Launch example) were copied and changed in four places. Two changes affect results, because the files as published did not do what was intended:
  • The launch-repetition wrap-around compared the position in the launch window with one year instead of the window's length. From the second year on, it launched the window's first objects again at every step: 3,112 launches in two years against about 190 intended. It was changed to use the window's length.
  • The mission life of repeated launches was overwritten with the semi-major axis in Earth radii (about 1.1), which ended missions after about a year instead of eight. It was changed to write the intended column, the desired semi-major axis.
  The JB2008 density table shipped with MOCAT-MC runs from 2020 to 2224, so the density lookup of a 2009 start is shifted by eleven years (both years are solar minima). The save format is -v7 instead of -v7.3, and a script with a local function is split in two; Octave needs both, and neither changes a result.
• Scenario: MOCAT's own initial condition for 2009 (its two-line-element catalog of 1 January 2009: 8,632 objects with perigee between 200 and 2,000 km, of which 1,757 payloads, 876 rocket bodies, 5,755 fragments and 244 mission-related objects). Launches repeat 2001 to 2009 (727 objects per nine years, about 80 a year). Missions last 8 years, 90% of satellites are disposed of (MOCAT removes a disposed object at once), there is no collision avoidance and there are no explosions. 5-day step, MOCAT's default 50 km cubes, 200 years, seeds 1 to 4. One run takes about an hour on one core.
• Masses and sizes: known for only about a third of the objects. MOCAT's shipped method (fillMassRadius = 2) fills in the rest by sampling a Gaussian fitted to the known objects of the same class. For fragments this gives a median of 45 kg and 0.51 m, since the known ones are the large cataloged pieces.
• Variant B: every radius divided by 1.5, as Jang et al. did for their own reconstruction (here applied to the repeated launches too).

Result (objects 10 cm and larger, 2009 to 2208):

For MOCAT-MC a large collision is one that makes 100 fragments or more; for the IADC study and Kessler@home it is a catastrophic collision.

What it shows. With its shipped data MOCAT-MC runs away: a few thousand "fragments" of one meter and 45 kg behave like a population of small satellites. Dividing the radii by 1.5 brings its collisions to 28 per run, inside the study's 22 to 40 and close to Kessler@home's 30, while the growth stays at +163%. That is partly because the starting count is half the study's, so the same fragments made are twice the relative growth, and partly because 45 kg fragments keep collisions with intact objects catastrophic. Jang et al. reached +35% only by also adding 7,200 debris objects. Two independent codes given populations of the same mass and intact count put their large collisions in the same broad band and agree on the fragments per collision, and each can be moved across the study's growth range by changing the fragment population's sizes and masses, which neither code knows from public data. The growth figure therefore depends heavily on the input population: in MOCAT-MC the radii alone move it by 260 percentage points, and in Kessler@home correcting the synthetic fragments' orbits moved it by about 10 percentage points. That correction did not bring Kessler@home to the study's figure, though, so the input population has not been shown to be the whole explanation there. The collision rate for a given population is the quantity that tests a model's code.

Time step, cube size and number of runs

What it checks. Whether the numerical settings bias the result, and how many runs a scenario needs. The IADC scenario above, on its current starting population, was rerun with the step halved and with the cubes halved, 192 runs each (November 2026; seeds 1 to 192, the same applications as the IADC runs above). The reference row is the first 192 of the IADC comparison's 725 runs, so that the three cases use the same seeds.

The CPU times are from the released application, version 1.03, with the three cases running side by side on one computer.

• Step: no dependence can be detected. With the step halved a run has 30.4 catastrophic collisions against 29.4 (standard error of the difference 0.7) and ends with 18,185 objects against 17,824 (standard error 346), at twice the cost. The 5-day step stays.
• Cube size: no dependence can be detected. With 5 km cubes a run has 30.0 catastrophic collisions against 29.4 (standard error of the difference 0.65), so the two rates agree to 1.02 ± 0.02; the final count is 18,105 against 17,824 (standard error 330), and the collisions happen at the same altitudes. The computing time is 5% higher. One limit of the test: a cube eight times smaller makes a pair's collision probability in one step eight times larger, and one roll of the dice can represent a probability of at most 1. Pairs that could exceed 1 in a 5 km cube, which are pairs of large intact objects, are therefore tested in a coarser cube of about 11 km (32% of the pair tests here), and no pair exceeded 1 in any run. So the 5 km cube itself is tested by the collisions that involve a fragment, 23.4 per run against 22.0 (1.06 ± 0.03). At the default 10 km and 5 days no pair comes close to 1 in this scenario (the largest probability is 0.88), and every run reports whether any pair did.
• How small a dependence could hide: every difference in the table points the same way. The two finer settings give 2% to 3% more catastrophic collisions, 5% to 6% more collisions with a fragment and about 2% more objects at the end. None of these is beyond about two standard errors. The largest, the non-catastrophic collisions with 5 km cubes (14.0 against 13.0 per run), is 2.3 standard errors, and it comes from the first 96 runs alone: in the second 96 the three cases have 13.6, 13.8 and 13.7. So a dependence is not established, and one of a few percent in the collision rate cannot be excluded. That is small beside the uncertainty of the inputs the sweep varies.
• Number of runs: taken 24 at a time, the IADC comparison's 725 runs give mean growth between −3.0% and +11.8% and 9 to 19 runs ending above the start; taken 96 at a time, between −0.4% and +5.3%. The spread from run to run is 19 percentage points of growth and 6.0 collisions, so a scenario's mean growth is known to about ±4 percentage points from 24 runs, ±2 from 96 and ±1 from 400. A runaway probability, which behaves like a share of runs, is known to about ±4 percentage points from 150 runs and ±2.5 percentage points from 400. That is why every scenario gets 400 runs in the first pass and 2,000 or more where the outcome is close or rare.
• Earlier tests: the same tests on the first synthetic starting population gave the same answers: the step in October 2026 (29.2 against 29.4 collisions, 24 runs each) and the cubes in November 2026 (27.3 against 27.2, 96 runs each). A first cube test in October 2026, made before the coarser cube existed, gave 29% fewer collisions at 5 km, because pairs of large objects exceeded a probability of 1 there.

Benchmarks rerun after every change

What it checks. That a change to the Kessler@home simulation does not quietly change its answer. Two ensembles of 24 runs each are rerun after every change and compared with stored references:

• The IADC scenario above, which covers the collisions, breakups, drag, the 25-year disposal rule and the bookkeeping, but runs with several parts of the Kessler@home simulation switched off, as the study's models did;
• the nominal scenario on the real 2026 population with every part of the Kessler@home simulation switched on: the analytic rate for dense layers, the 1 to 10 cm debris, explosions, collision avoidance and the launch traffic model. Every one of its runs grows to the 2-million-object limit, near 2049, so the year it gets there is the outcome compared, with the collisions, the avoided collisions, and the breakups and mission losses caused by small debris.

Each run ends with a checksum of the complete final state of every object. If all 24 checksums are unchanged, the change did not touch that scenario. If they differ, the means must be within two standard errors of the reference, or the difference must be explained and written up before the new run becomes the reference. For example, a correction to one of the breakup model's area-to-mass parameters in November 2026 lowered the IADC collisions by 2.2 standard errors. Undoing each change of that revision in turn traced it to that correction, and the new rate was still inside the study's range. When the IADC comparison's starting population was replaced in November 2026, its reference was replaced with it.

The same result on every computer

What it checks. BOINC runs each task on two volunteers' computers and compares the results, so the Kessler@home simulation must give the same answer, bit for bit, on every computer. Each platform's release build of the Kessler@home application, in standalone mode, ran the same set of cases, and the final-state checksums were compared:

• Platforms: Linux x86-64; Linux ARM64 (an NVIDIA Jetson Orin Nano); Windows 11 (AMD Ryzen 7 7800X3D); macOS on Intel and on Apple Silicon (GitHub's hosted Mac machines).
• Cases: the real 2026 population for 3 years, and for 20 years through a developing cascade to 201,979 objects; a synthetic 2026 population for 5 years; the IADC population for 5 years and for the full 200 years; and a 2-year run stopped and resumed from its checkpoint, with and without the 1 to 10 cm debris. The full test suite also ran on Linux, Windows and the Jetson.
• Result: identical checksums on every platform. Two of the cases rerun with the combined multiply-and-add step computed in software instead of by the processor give the same checksums too. The graphics-card versions (NVIDIA CUDA and AMD HIP) are checked against the processor version by a built-in self-test, and two 200-year sweep cases, one ending with 1.4 million objects, came out identical on the processor and on three graphics cards.

Before the Kessler@home simulation used correctly rounded math functions (the CORE-MATH library) and its own number parsing, the three environments then available gave three different results, and in the cascading cases different object and collision counts within three years.

Component tests

Each build of the Kessler@home simulation runs an automated suite of 57,260 checks of the individual parts against known answers. Some of them:

References

• Liou, J.-C. et al. (2013). Stability of the future LEO environment. IADC-12-08, Inter-Agency Space Debris Coordination Committee. Summary (UNOOSA, 2013).
• Lidtke, A., Lewis, H. and Armellin, R. (2017). Statistical analysis of the inherent variability in the results of evolutionary debris models. Advances in Space Research.
• Jang, D. et al. (2025). A new Monte-Carlo model for the space environment. Journal of Spacecraft and Rockets. Code: MOCAT-MC, MIT ARCLab.
• ESA MASTER-8, Meteoroid and Space Debris Terrestrial Environment Reference, ESA Space Debris User Portal.
• NASA ORDEM 3.2, Orbital Debris Engineering Model, NASA Orbital Debris Program Office.
• Matney, M., Manis, A., Anz-Meador, P., Gates, D., Seago, J., Vavrin, A. and Xu, Y.-L. (2019). The NASA Orbital Debris Engineering Model 3.1: development, verification, and validation. 1st International Orbital Debris Conference.
• Manis, A. and Matney, M. (2022). ORDEM 3.2 flux assessment. Orbital Debris Quarterly News 26(2).
• Johnson, N. L., Krisko, P. H., Liou, J.-C. and Anz-Meador, P. D. (2001). NASA's new breakup model of EVOLVE 4.0. Advances in Space Research 28(9).
• Krisko, P. H. (2011). Proper implementation of the 1998 NASA breakup model. Orbital Debris Quarterly News 15(4).
• MacDonald, C. (2026). Starlink is lowering thousands of satellites' orbits to reduce risk of collisions. Engadget, 1 January 2026 (SpaceX's announcement by Michael Nicolls, its vice president of Starlink engineering).
• Kessler, D. J. (1981). Derivation of the collision probability between orbiting objects: the lifetimes of Jupiter's outer moons. Icarus 48.
• Matney, M. et al. (2013), on the Briz-M 2012-044C breakup. 6th European Conference on Space Debris.
• Bastida Virgili, B. and Krag, H. (2013), ESA's ranking study of objects in LEO. 6th European Conference on Space Debris.
• CelesTrak SATCAT, T. S. Kelso (the decay record of the fragment clouds).