Following the Falcon Heavy launch, the second stage + Roadster payload spent a number of hours 'drifting' through the Van Allen belt before performing the burn to put it into its current orbit.

Elon Musk via Twitter:

Upper stage restart nominal, apogee raised to 7000 km. Will spend 5 hours getting zapped in Van Allen belts & then attempt final burn for Mars.

Given that spending time there exposed the spacecraft to more radiation than it would receive had it performed its escape burn prior to passing through the belt, and this radiation was a source of potential risk that could have rendered the craft inoperable, what was the reason for this slow traversal?

It has been mentioned elsewhere that this was to show that it could be done to satisfy the DoD (e.g. this comment) but why would they want to put a spacecraft through extended exposure to the additional radiation encountered in the belts?

  • $\begingroup$ Had to meet seat belt requirements. $\endgroup$
    – Dan
    Commented Feb 20, 2018 at 21:38

1 Answer 1


Up until this flight of Falcon Heavy, officially, SpaceX could not fully deliver a satellite to [nearly circular] GEO (Geosynchronous Earth Orbit), but only to a [highly elliptical] GTO (Geosync Transfer Orbit) that expects the payload to circularize its own orbit once at the appropriate altitude.

This consumes fuel, and fuel for station keeping is one of the major determinants of satellite lifespan. Time is money, therefore the fuel required to get from GTO to GEO itself costs money.

Now many GEO operators are ok with this added cost, when they get a reduced launch cost overall, but it is a real factor.

The Department of Defense (DoD) has 8 reference orbits, of which direct insertion to GEO is one of them, and in order to compete with the Delta 4 Heavy for such launches, SpaceX needs to demonstrate that they can boost a payload to that orbit.

Getting to GEO direct, requires a coast phase after the initial orbital insertion, (as I've discussed in more detail in this answer). That is, first and second stage deliver the payload to orbit, and then a few hours later a second burn is initiated that finalizes the GEO orbit. One of the problems with the coast phase is radiation, another is the cold and lines freezing (which happened before to a Falcon 9 upper stage test).

The first Falcon Heavy flight was a demonstration flight testing many aspects. This can be seen in the loss of the center core due to lack of TEA-TEB ignitor fluid. They knew how much it takes to reignite the needed engines, and were clearly pushing the lower end of required ignitor and estimated low.

Thus this was just one more way to get some value out of a test flight. The payload was a media extraveganza and hilarious. The mission was a success. The second stage was able to test GEO orbit insertion. The center core test did not succeed, but it sets a lower level and now they have more information, that they did not have before.

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    $\begingroup$ So tl;dr: It's to show that the central core can go to geostationary itself without problems, rather than anything being placed in the belts for an extended period/orbit within the belts. Thanks. $\endgroup$
    – Baldrickk
    Commented Feb 19, 2018 at 17:01
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    $\begingroup$ @Baldrickk Upper stage, not center core. Center core tries to land. Side boosters are first stage. Center core is stage 1.5? Normal second stage is still second stage, third stage, or just simply upper stage? Whatever. PS: Baldrick from Black Adder? $\endgroup$
    – geoffc
    Commented Feb 19, 2018 at 17:07
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    $\begingroup$ Ah, yes, that would be a typo. I knew what I meant, but typed the wrong thing (doh). Baldrick is actually the origin of my real name, traced back to 1066 - Though I asked and found out from Tony Robinson himself the origin of Baldrick. The repetition of the 'k' is purely because the name is popular enough online that the original is usually owned by someone else. $\endgroup$
    – Baldrickk
    Commented Feb 19, 2018 at 17:18
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    $\begingroup$ Excellent answer. It might be an improvement to refer to orbital mechanics, because they are not like everyday mechanics. To get a circular orbit the way to do it is with 2 burns. First to get the apogee (the farthest point of the ellipsis) at the right orbit, then to get perigee (the closes point of the ellipsis) at the right orbit. A circular orbit is when periapsis ~= apoapsis The most efficient is a burn at apogee (raises periapsis) then wait until perigee to burn and raise apoapsis. There is usually a coasting period between the two burns. youtu.be/jCPHAkg4EWw?t=1097 $\endgroup$
    – Stian
    Commented Feb 19, 2018 at 22:55
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    $\begingroup$ @Baldr ... wait a second... I do know that. It's a constraint of other services however, and this way I keep my username consistent between sites. $\endgroup$
    – Baldrickk
    Commented Feb 21, 2018 at 9:26

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