The lecture discusses the consequences of decreasing atmospheric density trends in the thermosphere. It is explained that decreasing density leads to increased satellite orbital lifetimes as there is less air drag. Two approaches are presented: how density trends affect satellite orbits and lifetimes, and using changes in satellite orbits over time to estimate density trend rates. An activity uses an orbital lifetime calculator to predict satellite lifetimes with and without density trends, demonstrating their impact.

1. Space Environment
Lecture 17 – Geospace Climate
Consequences of density trends
Professor Hugh Lewis
SESA3038 Space Environment

2. Overview of lecture 17
• Having gained an understanding of the drivers of climate change in the
upper atmosphere, in this lecture we start to look at the consequences of a
density trend
• We take two different approaches
– We see how the density trend affects satellite orbits, in particular the
orbital lifetimes
– We see how we can measure the change in satellite orbits over time to
estimate the rate of change of density in the thermosphere
• We end with an activity that uses a simple atmospheric model to predict
the orbital lifetimes of a satellite with, and without, the density trend
Space Environment – Geospace Climate

3. Predictions of density changes
(reminder)
WACCM-X model predictions
for the years 2000 and 2095
based on RCP8.5
By the year 2095 atmospheric
density at 400 km altitude could
be at only 20% of the year 2000
level
Space Environment – Geospace Climate

4. Decrease in orbit height due to air drag
Reminder:
The change in orbital altitude/size due to air drag is
proportional to the atmospheric density:
The cooling trend in the thermosphere is leading to a
decreasing trend in atmospheric density and, hence,
smaller changes in orbital altitude
Satellite orbital lifetimes will increase
𝛿𝛿𝛿𝛿 = −2𝜋𝜋𝜋𝜋
𝑆𝑆𝐶𝐶𝐷𝐷
𝑀𝑀
𝑎𝑎2
Space Environment – Geospace Climate

5. Satellite decay Space Environment – Geospace Climate
0
100
200
300
400
500
600
1973 1978 1983 1988 1993 1998 2003
Year
Perigee
Altitude
(km)
Unmodified CIRA-72
Modified CIRA-72
δa increases at solar max due to atmospheric heating by EUV irradiance but
decreases due to atmospheric cooling by CO2 (Lewis et al., 2005)
Expected lifetime: 25 years
Actual lifetime: 27 years

6. Satellite decay Space Environment – Geospace Climate
Results from Brown et al., 2019 for RemoveDebris:

7. Satellite decay Space Environment – Geospace Climate
Results from Brown et al., 2019 for other satellites:

8. Satellites as sensors
Space Environment – Geospace Climate
Free International Network
of Density Sensors (FINDS)

9. Satellites as sensors Space Environment – Geospace Climate
𝛿𝛿𝛿𝛿
Ideal (unperturbed) orbit
Actual orbit (perturbed by air drag)
Predicted orbit*
* Perturbed orbit predicted using atmospheric model (e.g. NRLMSIS-00)
𝛿𝛿𝛿𝛿∗
𝑡𝑡 + 𝛿𝛿𝛿𝛿 𝑡𝑡 E.g. see Doornbos et al.
2009, Emmert 2009,
Saunders et al. 2011
𝛿𝛿𝛿𝛿 = −2𝜋𝜋𝜋𝜋
𝑆𝑆𝐶𝐶𝐷𝐷
𝑀𝑀
𝑎𝑎2

10. Satellites as sensors Space Environment – Geospace Climate

11. Activity
• A orbital lifetime calculator in Microsoft
Excel is available in the “Course
Contents” folder. It computes the orbital
lifetime of a satellite in a circular orbit
with and without the density trend
– Based on a simple exponential
atmospheric model, with a
sinusoidal approximation for the
solar activity, and the formula
found on slide 4 of this
presentation
• Use this to explore the effect of the
density trend (by varying the
parameters in the tables at the top of the
worksheet)
Space Environment – Space Weather