Astronomy & Celestial Mechanics
the sky, computed
Concept

Hypervelocity impact — where it appears

A collision fast enough, typically above a few kilometres a second, that the material behaves like a fluid and the projectile and target are shattered or vaporised. It is how satellites break up in collisions and how craters are excavated.

Named by 2 essays across 2 fields — each of them below, with the objects they name alongside it.

1,236 fragments anybody can see, and 63,397 that can kill. The cumulative fragment size distribution from the standard breakup model, for a catastrophic collision involving 1500 kilograms and for an explosion of the same object, both axes logarithmic. The exponents are −1.71 and −1.60, measured off the drawn curves; they are empirical, fitted to ground tests and to the observed clouds of real events, and they are steep. What follows is the reason a catalogue of tracked objects is not a catalogue of the hazard. Above ten centimetres — the size a ground radar can follow in low orbit — the collision makes about 1,236 pieces. Above one centimetre, which is the size that goes through a spacecraft at ten kilometres a second and cannot be shielded against, it makes 63,397. Above a millimetre, which shielding does stop but which erodes a surface, 3,251,385. The tracked population is under two per cent of the lethal one, and the difference is not a gap in the catalogue that better radars will close — objects of a centimetre at a thousand kilometres are beyond any sensor that has been proposed. An explosion makes fewer large pieces than a collision and a comparable number of small ones, because an explosion divides one object and a collision destroys two.

The fragments nobody can see and cannot shield against

A catastrophic collision in low orbit makes about a thousand pieces big enough to track and sixty thousand big enough to destroy a spacecraft. The catalogue is under two per cent of the hazard, the gap is not one better radars will close, and the fragments' lifetimes span a factor of twenty-five within a single event.

spaceflight · Orbital debris Two clouds with the same kicks: the explosion's centre at −2.3 m/s, the collision's at 10.9. The along-track kicks read from the periods of 1500 fragments, for an explosion (blue) and for a collision (orange) whose fragments received the same random kicks plus a shared velocity change of 12 m/s along the track — the centre-of-mass motion a projectile's momentum leaves behind. The two histograms have the same width and shape; the collision's is displaced. The number-weighted mean of the explosion's kicks is −2.3 ± 2.6 m/s, consistent with zero, and the collision's 10.9 ± 2.5 m/s. The width of a cloud says how violent the event was; its centre says whether anything arrived from outside.

A collision moves the cloud's centre, and an explosion cannot

However violently a satellite blows itself apart, its fragments' momenta add up to the parent's: the centre of the cloud keeps moving exactly as the satellite did. A collision is different, because the projectile brings momentum in, and a collision energetic enough to shatter its target needs a projectile heavy enough to move the centre by at least 80,000 joules per kilogram divided by the impact speed — eight metres a second in low orbit. The width of a debris cloud measures how violent the event was. Its centre measures whether anything arrived from outside.

orbits · Vis-viva

Named alongside it

The objects these essays reach for when they reach for this one.

Orbital debrisArea-to-mass ratioBallistic coefficientBreakup modelCentre of massConjunction assessmentConservation of momentumGabbard diagramKessler syndromeOrbital lifetimeOrbital periodSpace surveillance

All concepts