Education
LessonUpper secondary and early undergraduate (ages 15+) · 50 min

Why Low Orbits Decay

Atmospheric drag never stops acting in low Earth orbit. Students read the drag term and perigee from real records, rank objects by how fast they will come down, and learn why the answer is a band rather than a date.

dragdecayreentrybstarspace-weatherperigee
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Overview

At 400 km the atmosphere has not ended, it has only thinned. There is still enough gas there to push back against a spacecraft moving through it at roughly 7.7 km/s. The force is tiny. It also never stops, and that is what matters.

ESA puts the consequence simply: depending on the altitude, after a few weeks, years or even centuries, this resistance decelerates the satellite sufficiently so that it reenters the atmosphere. Above about 800 km, drag becomes so weak that objects generally remain in orbit for many decades.

Three things set the rate. How low the object goes, because density climbs steeply as altitude falls. How much drag area it has for its mass, because a light object with a big cross-section is slowed much more than a dense compact one. And what the Sun is doing, because solar activity heats and inflates the upper atmosphere, raising the density at any given altitude.

That last one is why lifetimes are honest only as bands. NOAA notes that the largest uncertainty in determining orbits for satellites in low Earth orbit is atmospheric drag, and that drag is the most difficult force to model, mainly because of the complexity of neutral atmosphere variations driven by the Sun. Under calm conditions a satellite may need an orbital boost around four times a year; near solar maximum, it may need one every two to three weeks.

Catalog records carry a field that bundles the object-specific part of this together: B*, the drag term used by the SGP4 model. A larger B* means the model expects drag to bite harder on that particular object. It is a model coefficient fitted to tracking data, not a measured area, so treat it as a comparative indicator rather than a physical constant.

At a glance

Learning objectives

  • Explain orbital decay as the cumulative effect of a very small drag force acting continuously.
  • Locate the perigee altitude and the B* drag term on a catalog record and say what each one contributes.
  • Rank several real objects by expected orbital lifetime and justify the ranking from the record fields.
  • Explain why an orbital lifetime is quoted as an order of magnitude and not as a date.

Prerequisites

  • Comfort reading numbers in scientific notation.
  • No calculus required.

Required software

  • A web browser and a spreadsheet.

Dataset version

OrbitalWiki live catalog. Record the dataset-release label from /datasets when available, or the exact access date for a live lookup.

Student materials

Decay comparison worksheet (five objects, ranking and bands)CSV, opens in any spreadsheet app
Download

Student instructions

  1. 1Choose five objects from the catalog with clearly different perigee altitudes, ideally at least one below 300 km and one above 700 km. Record the NORAD ID, name, perigee, apogee, and the access date.
  2. 2For each object, record the B* drag term and the element epoch exactly as displayed. If a field is blank, write "not available" rather than zero.
  3. 3Rank your five objects from shortest to longest expected orbital lifetime. Write one sentence per object justifying its position from the fields you recorded, not from what the object is called.
  4. 4Using the figure, assign each object to a band: days, months, years, decades, or centuries. State the band, never a date.
  5. 5Find any object in your set whose ranking by perigee disagrees with its ranking by B*. Explain what that disagreement tells you about the object’s shape or mass. If there is no disagreement in your set, say so.
  6. 6Write a two-sentence limitation covering solar activity, using the NOAA source, and explaining why you gave bands instead of dates.

Expected output

  • A five-row table with NORAD ID, perigee, apogee, B*, element epoch, and access date.
  • A justified ranking by expected lifetime, with reasoning drawn from record fields.
  • A lifetime band for each object, and no predicted reentry date anywhere in the work.
  • A stated limitation naming solar activity as the dominant source of uncertainty.
[teacher]

Teacher materials, not student-facing

Teacher answer keyCSV, marking guidance including the "no dates" rule
Download

Teaching notes

  • The failure mode to watch for is a student writing "this will reenter in March". Nothing in this lesson supports a date. Reentry prediction is a specialist activity with large error bars even a day out; the honest output here is a band and a reason.
  • B* is a fitted SGP4 coefficient, not a physical drag coefficient or an area. It can even be negative in a fitted element set. If a student finds a negative or zero value, that is a genuine finding about how element sets are produced, worth discussing, not an error to correct.
  • Crewed stations and many operational satellites are actively boosted, so their observed altitude does not follow the natural decay curve at all. If a student picks one, the "why is this not decaying as predicted?" discussion is more valuable than a clean answer.
  • Perigee, not mean altitude, is the field that drives decay, because that is where the object meets the densest air on each revolution. Students often reach for apogee first.
  • If the class has covered the solar cycle, this is a good place to connect: the same satellite has a very different lifetime launched at solar minimum versus solar maximum.

Answer key

Which record field matters most for decay rate, perigee or apogee?
Perigee. The object passes through the densest air it will meet at its lowest point, and drag there dominates the energy loss.
What does a larger B* indicate?
That the SGP4 model expects drag to affect this object more strongly, which usually means more cross-sectional area for its mass. It is a fitted model coefficient, not a measured area.
Why is orbital lifetime given as a band?
Because upper-atmosphere density varies with solar activity by a large factor and is the hardest force to model, so the same starting altitude yields answers that differ by an order of magnitude or more.
Two objects share a perigee but one decays much faster. Why?
It has a larger drag area for its mass, so the same air density removes energy from it faster. A light object with a big cross-section decays sooner than a dense compact one.
An object at 800 km, roughly how long?
Decades to centuries. Drag is very weak at that altitude, which is exactly why debris there is a long-term problem.
Why does a station need boosting more often near solar maximum?
Solar activity heats and expands the upper atmosphere, so the density at station altitude rises and drag increases. NOAA describes boosts moving from roughly four times a year in calm conditions to every two to three weeks at the peak of the cycle.

How to cite

Cite each record by NORAD ID with the displayed element epoch and your access date, and cite the ESA and NOAA pages for the physical claims. Do not cite a lifetime band as if it came from the catalog; it is your own reasoning from the fields.

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