test

1. Space Environment
Lecture 4 – Effects on spacecraft
Atomic Oxygen & atmospheric drag
Professor Hugh Lewis
SESA3038 Space Environment

2. Overview of lecture 4
• In this lecture we will complete our high-level look at the key
characteristics of the space environment and their effects on spacecraft,
with a focus here on:
– Atomic Oxygen and how it can affect the drag coefficient
– Thus playing a role in the drag force experienced by spacecraft (and
recalling SESA2024 Astronautics, the rate at which spacecraft orbits
will decay)
– The importance of Atomic Oxygen is such that we will see it again when
we look at the geospace climate, where it plays a crucial role in global
geospace climate change
Space Environment – Environmental effects

3. Environmental effects
• High vacuum
• Solar radiation
• Particle radiation
• Atomic Oxygen
Space Environment – Environmental effects

4. Atomic Oxygen reaction & erosion Space Environment – Environmental effects
• Can affect spacecraft drag indirectly by affecting material properties:
Reminder: principal effects of drag upon the orbit are to reduce its size and to
circularise it (both semi-major axis, a, and eccentricity, e, decrease)
Original orbit
Successive orbits
Earth’s
atmosphere

5. Drag force (an aside) Space Environment – Environmental effects
• Drag force acting is:
– Where: 𝜌𝜌 = atmospheric density
𝐕𝐕𝑎𝑎 = velocity relative to atmosphere
𝑆𝑆 = reference area (area perpendicular to 𝐕𝐕𝑎𝑎)
𝐶𝐶𝐷𝐷 = drag coefficient
– Value of 𝐶𝐶𝐷𝐷 ~ 2 to 4 depending on gas-surface interaction (this is what
Atomic Oxygen changes)
𝐃𝐃 =
1
2
𝜌𝜌𝑆𝑆𝐶𝐶𝐷𝐷𝑉𝑉
𝑎𝑎
2
−�
𝐕𝐕𝑎𝑎

6. Drag force (an aside) Space Environment – Environmental effects
• For LEO spacecraft, particles are:
• Incident at 𝐕𝐕𝑎𝑎 ≈ 7 km/s
• Accommodated for a short interval (this is where Atomic Oxygen plays a role)
• Re-emitted at thermal velocity 𝑉𝑉𝑡𝑡𝑡 ≈ 2 km/s
• Estimate of value of 𝐶𝐶𝐷𝐷:
• Assume 𝑉𝑉
𝑎𝑎 ≫ V𝑡𝑡𝑡~0
• Let 𝐕𝐕 be velocity of particles relative to spacecraft (𝐕𝐕 = −𝐕𝐕𝑎𝑎)

7. Drag force (an aside) Space Environment – Environmental effects
• Estimate of value of 𝐶𝐶𝐷𝐷:
• The incident momentum per unit of time on
surface S is:
• The reflected momentum is: 𝑚𝑚𝐕𝐕𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 = 0
• Therefore the force on surface S is the change
of momentum in unit time:
𝑚𝑚𝐕𝐕𝑖𝑖𝑖𝑖𝑖𝑖 = 𝜌𝜌 𝑉𝑉𝑉𝑉 𝐕𝐕 = 𝜌𝜌𝑉𝑉2
𝑆𝑆�
𝐕𝐕
𝐅𝐅 = ∆ 𝑚𝑚𝐕𝐕 = 𝑚𝑚𝐕𝐕𝑖𝑖𝑖𝑖𝑖𝑖 − 𝑚𝑚𝐕𝐕𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 = 𝑚𝑚𝐕𝐕𝑖𝑖𝑖𝑖𝑖𝑖
S

8. Drag force (an aside) Space Environment – Environmental effects
• Estimate of value of 𝐶𝐶𝐷𝐷:
• Therefore the force on surface S is the change
of momentum in unit time:
• And:
• Hence:
𝐅𝐅 = ∆ 𝑚𝑚𝐕𝐕 = 𝑚𝑚𝐕𝐕𝑖𝑖𝑖𝑖𝑖𝑖 − 𝑚𝑚𝐕𝐕𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 = 𝑚𝑚𝐕𝐕𝑖𝑖𝑖𝑖𝑖𝑖
= 𝜌𝜌𝑉𝑉2
𝑆𝑆�
𝐕𝐕
S
𝐅𝐅 ≡ 𝐃𝐃 =
1
2
𝜌𝜌𝑉𝑉
𝑎𝑎
2
𝑆𝑆𝐶𝐶𝐷𝐷 −�
𝐕𝐕𝑎𝑎
𝜌𝜌𝑉𝑉2
𝑆𝑆�
𝐕𝐕 = 𝐃𝐃 =
1
2
𝜌𝜌𝑉𝑉2
𝑆𝑆𝐶𝐶𝐷𝐷
�
𝐕𝐕

9. Drag force (an aside) Space Environment – Environmental effects
• Estimate of value of 𝐶𝐶𝐷𝐷:
• Hence:
• Recall that this is if particles “stick” to the
spacecraft surface (V𝑡𝑡𝑡~0)
S
𝜌𝜌𝑉𝑉2
𝑆𝑆�
𝐕𝐕 = 𝐃𝐃 =
1
2
𝜌𝜌𝑉𝑉2
𝑆𝑆𝐶𝐶𝐷𝐷
�
𝐕𝐕
𝐶𝐶𝐷𝐷 = 2

10. Drag force (an aside) Space Environment – Environmental effects
• Estimate of value of 𝐶𝐶𝐷𝐷:
• Assume particles are specularly reflected V𝑡𝑡𝑡 = −𝑉𝑉
𝑎𝑎
• Again, let 𝐕𝐕 be velocity of particles relative to spacecraft (𝐕𝐕 = −𝐕𝐕𝑎𝑎)
• The reflected momentum is now: 𝑚𝑚𝐕𝐕𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 = −𝑚𝑚𝐕𝐕 = −𝜌𝜌𝑉𝑉2
𝑆𝑆�
𝐕𝐕
• The force on surface S (the change of momentum in unit time) is now:
𝐅𝐅 = ∆ 𝑚𝑚𝐕𝐕 = 𝑚𝑚𝐕𝐕𝑖𝑖𝑖𝑖𝑖𝑖 − 𝑚𝑚𝐕𝐕𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟
= 𝜌𝜌𝑉𝑉2
𝑆𝑆�
𝐕𝐕 − −𝜌𝜌𝑉𝑉2
𝑆𝑆 �
𝐕𝐕
= 2𝜌𝜌𝑉𝑉2
𝑆𝑆�
𝐕𝐕
S

11. Drag force (an aside) Space Environment – Environmental effects
• Estimate of value of 𝐶𝐶𝐷𝐷:
• Hence:
• Recall that this is if particles do not “stick” to the spacecraft surface at all
(V𝑡𝑡𝑡 = −𝑉𝑉
𝑎𝑎)
• The “stickiness” of the spacecraft surface is characterised using an
accommodation coefficient that depends on the material properties
• Atomic Oxygen changes the material properties
S
2𝜌𝜌𝑉𝑉2
𝑆𝑆�
𝐕𝐕 = 𝐃𝐃 =
1
2
𝜌𝜌𝑉𝑉2
𝑆𝑆𝐶𝐶𝐷𝐷
�
𝐕𝐕
𝐶𝐶𝐷𝐷 = 4

12. Atomic Oxygen erosion Space Environment – Environmental effects
• Some materials are unacceptable for long-duration use in LEO
spacecraft. Selection of protective mechanism based on:
• Resistant to AO attack
• Thin, light weight and adhere strongly to base material
• Free from defects, pores and scratches
• Non-contaminating
• Should not alter basic properties of the base material
• Should withstand ground handling, manufacturing loads and orbit loads
• Stable in LEO space environment:
– Particulate radiation
– UV radiation
– Micrometeoroids and orbital debris
– Thermal cycling
– Charging

13. Recap of lectures 2 to 4
• In these lectures we took a high-level look at the key characteristics of the
space environment and their effects on spacecraft:
– High vacuum
– Solar radiation
– Particle radiation
– Atomic Oxygen, including how it affects the drag coefficient
– The drag coefficient is partly determined by the spacecraft materials,
Atomic Oxygen changes the properties of these materials, thereby
affecting, indirectly, the drag coefficient
– We will see Atomic Oxygen again when we discuss the geospace climate,
where it plays a crucial role in global geospace climate change
• In our next lectures we will look at the effects of the space environment on
humans
Space Environment – Environmental effects

14. Activity
• A reminder that you can read the “Spacecraft
Environment and its effect on design” chapter
in Fortescue, Stark & Swinerd:
• Read this chapter (pages 11-47) to support your
learning of this topic (and those to come)
• Access to the e-book is available via the Library
website:
https://onlinelibrary.wiley.com/doi/book/10.1002/9
781119971009
Space Environment – Environmental effects