A New Perspective on Orbital Debris   

Planewave

The US DoD’s new S-band radar fence at Kwajalein may find 2-5cm objects in low Earth orbit (LEO), but its key feature is finding and then tracking most new objects during their first pass, rather than needing days to months to find objects using hit-or-miss ‘pencil-beam’ radars.

This fence may catalog ~20X more debris in low Earth orbit than now. But it raises a more basic question: What is ‘good enough’ debris detection? NASA provides a useful new perspective: ‘The major mission-ending risks for most operational spacecraft, however, come from impacts with debris just above the threshold of the protection shields (~5mm to 1cm)1 .

planewave telescopeIf true, then most direct LEO satellite losses from debris impact will be from ‘cm-class shrapnel’ that is below the 2-5cm LEO threshold of the new $1bn fence. This in turn raises a new question: Can we affordably find, track, and avoid most lethal debris in low Earth orbit? LEO includes average orbit altitudes of 200-2,000km. But there are few maneuverable satellites below ISS at ~400km, and few above Globalstar at 1,414km.

Radar sensitivity scales with the -4th power of range, and -6th power of small-target diameter, while telescopes seeing sunlit targets have a -2nd power sensitivity for both range and diameter. The S-band fence may be useful for ISS, but telescopes may beat radar at higher altitudes.

A cm-class LEO catalog may include ~1 million items, mostly shrapnel. Each item may require >100 updates/year, including both routine and ‘priority’ updates before worrisome conjunctions. A suitable network may need ~2 telescopes at each of 50 good sites, with each telescope slewing to and tracking new objects every few seconds after dusk and before dawn.

This would quickly wear out geared mounts, so agile direct-drive mounts seem necessary. Table 1 lists parameters for a ‘threshold brightness’ object at Iridium altitude, using a PlaneWave CDK500 telescope:

 

Shrapnel data

Line-of-sight sky condition

Telescope2

CMOS Camera3

778km alt. (Iridium)

45° elevation, 1,046km range

508mm aperture, f/6.8

1 e- per 4.6eV sun

5mm dia sphere

Atm. transparency = 0.735

17% area obstruction

1.6 e- eff. read noise

0.10 diffuse reflect.

Sky brightness = 500 S10

82% transmittance

0.4 e-/sec at -20C

45° illum.phase angle

FWHM seeing = 1.5 arc-sec

224eV/sec target spot

Noise Nb=4.6 e-/pix

Vis. magnitude = 19.6

<12th mag ref stars/frame: 6

RMS blur dia: 26 m

Sec(S/N=6) = 2 * 1.2

 

How about affordability?

Conjunction warnings are actionable only if they are accurate enough that acting on them saves satellite operators money on average. We don’t yet know either the total cost of such a network, or the likely cost of losing LEO (and GEO) satellites by not using such a network. To learn more about this concept, please download our 11-page white paper, ‘A Telescope Network for Avoidance of Most Lethal LEO Debris.’ 


PlaneWave Instruments will be exhibiting at Space Tech Expo USA 2019 on Booth 7001