Chapter 33 — Key Takeaways (Small Satellites, CubeSats, and Constellations)
A one-page reference. Reread this before an exam, or before you decide whether your mission should be a
small satellite, a constellation, or a traditional big bus.
The one idea
Two costs collapsed at once — capability (cheap, powerful electronics) and access (cheap launch) —
and together they turned the satellite from a monument (one exquisite, gold-plated spacecraft that
must work) into a product (cheap, mass-produced, disposable, flown by the thousand). This is theme 5
(reusability and cheap access change everything) in its purest form. Everything else in the chapter follows.
The vocabulary (terms this chapter owns)
Term
Definition (short)
Small satellite
A satellite below ~$500\ \text{kg}$; sub-classed mini / micro / nano / pico / femto by mass (conventions, not physics).
COTS
Commercial off-the-shelf parts — cheap, available, high-performance, not space-qualified; reliability bought back by redundancy, screening, short life, numbers.
CubeSat
A satellite built to the CubeSat Design Specification in stacked $10\ \text{cm}$ units, so it fits a standard deployer.
Unit (U)
The $10\ \text{cm}$ cube (1 litre), $\lesssim 1.3$–$2\ \text{kg}$; the building block. Sizes: 1U, 3U, 6U, 12U.
Rideshare
Many independent payloads share one rocket and split the cost (secondary payload, or dedicated rideshare like Transporter).
Deployer
The spring-loaded dispenser (e.g., P-POD) that houses a satellite during launch and ejects it at ~$1$–$2\ \text{m/s}$.
Constellation
Many coordinated satellites whose combined coverage exceeds any one's; arranged in shells (altitude, inclination, planes).
Mass classes (memorize the order of magnitude)
Class
Mass
Minisatellite
$100$–$500\ \text{kg}$
Microsatellite
$10$–$100\ \text{kg}$
Nanosatellite
$1$–$10\ \text{kg}$ (most CubeSats)
Picosatellite
$0.1$–$1\ \text{kg}$
Femtosatellite
$< 0.1\ \text{kg}$
CubeSat sizes
Size
Dimensions
Rough mass
Example
1U
$10\times10\times10\ \text{cm}$
$\lesssim 1.3\ \text{kg}$
student sats
3U
$10\times10\times30\ \text{cm}$
$\lesssim 4\ \text{kg}$
Planet Dove imagers
6U
$10\times20\times30\ \text{cm}$
$\lesssim 8$–$12\ \text{kg}$
MarCO (Mars!)
12U
$20\times20\times30\ \text{cm}$
$\lesssim 24\ \text{kg}$
larger tech demos
Coverage geometry — the chapter's calculation
Quantity
Relation
Meaning
Nadir angle
$\sin\eta = \dfrac{R_E}{R_E+h}\cos\varepsilon$
angle at the satellite between straight-down and the user
Earth-central angle
$\lambda = 90^\circ - \varepsilon - \eta$
footprint half-angle on the ground
Footprint fraction
$f = \dfrac{1-\cos\lambda}{2}$
share of Earth one satellite serves
Satellites (floor)
$N_{\min} \gtrsim 1/f$
instantaneous global coverage, no overlap
$\varepsilon$ = minimum elevation above the horizon; $h$ = altitude; $R_E = 6{,}371\ \text{km}$. Real
fleets need $2$–$3\times\,N_{\min}$ for overlap, and far more if sized for capacity (bandwidth), not
coverage. Worked anchor: $h = 550\ \text{km}$, $\varepsilon = 25^\circ \Rightarrow \lambda \approx 8.5^\circ$,
$f \approx 0.54\%$, $N_{\min} \approx 184$.
Decision aids
Which platform?
If the mission needs…
Lean toward…
one region covered from a fixed point, high per-satellite value
a single large satellite (GEO comsat)
continuous global coverage / low latency
a LEO constellation
a cheap, fast, single-purpose payload; short life OK
a CubeSat
a specific orbit/date
dedicated launch (pay for control)
lowest cost/kg, flexible orbit OK
rideshare (take the provider's orbit)
Higher vs lower constellation shell
Go higher (e.g., 1,200 km)
Go lower (e.g., 550 km)
fewer satellites (bigger footprints)
more satellites (smaller footprints)
higher latency
lower latency
dead sats persist centuries (debris) → need propulsion to deorbit
dead sats decay in years (self-cleaning)
Cost per kilogram to LEO (Tier 2, illustrative ratios)
Option
~Cost/kg
You control
Dedicated small launcher (Electron)
~$\$25{,}000$
orbit and schedule
Dedicated rideshare (Transporter)
~$\$5{,}000$
almost nothing
Whole Falcon 9 (if you can fill it)
~$\$2{,}900$
everything
Rideshare is ~$5\times$ cheaper per kg than a dedicated small launcher — you pay the premium only for
control over orbit and schedule.
Numbers worth remembering
$1\ \text{U} = 10\ \text{cm}$ cube, ~$1.3\ \text{kg}$; 3U CubeSat makes only a few watts orbit-average.
Deployer ejection ~$1$–$2\ \text{m/s}$; Transporter-1 deployed 143 satellites on one Falcon 9.
Latency: GEO ~0.24 s one-way vs LEO ~4 ms — the LEO broadband business case.
Sustainability: low shells self-clean (drag, Chapter 12); high shells persist for centuries → Chapter 35.
Common pitfalls
Pitfall
Reality
"$N_{\min} \approx 184$, so Starlink only needs 184."
$N_{\min}$ ignores overlap ($\times 2$–$3$) and capacity (the real driver of thousands).
"COTS is cheap because the parts are rad-hardened."
The opposite — COTS is not rad-hard; cheapness comes from commercial parts + accepting risk.
"A whole Falcon 9 is always cheapest (lowest $/kg)." | Only if you fill it; for one small sat the effective $/kg is enormous — rideshare wins.
"Raising elevation needs fewer satellites."
Higher $\varepsilon$ shrinks $\lambda$ and $f$ → more satellites.
"Small means LEO-only."
MarCO (6U) reached Mars; small ≠ local.
Threshold concept
Reliability moves from the unit to the system. A flagship achieves reliability by making every part
near-perfect (theme 2, everything must work). A constellation achieves it by flying enough cheap,
replaceable units that the system works even as individual satellites fail. Cheap-per-unit is what makes
redundancy-at-scale affordable — and it is what makes mega-constellations thinkable at all.
Mission / astrotools additions this chapter
MDR: a Platform Trade note — should this mission be a small satellite or a constellation? If a
constellation, a first $N_{\min}$ estimate and a disposal plan (feeds Chapter 35).