Gravity problem solver in Factor
I'm not quite skilled enough at Factor yet to review your code in full, but I would just like to note as a response to 


  I don't know the math well enough to test the edge cases, but I did check with this code


Factor has really slick builtin unit-testing. Here are the docs; I'll pull an example from my factor/work directory:

USING: tools.test mk-initialism ;
IN: mk-initialism.tests

{ "LASER" } [ "Light Ablah by S of Ed Radiof" &gt;initialism ] unit-test
{ "USA"   } [ "United S OF Americaof" &gt;initialism ]         unit-test
{ "USA"   } [ "united states and america" &gt;initialism ]     unit-test
{ "JTW"   } [ "Jordan Of the World" &gt;initialism ]           unit-test 


To create the test file, open the listener and do:

USE: tools.scaffold
"q.physics.gravitational-attraction" scaffold-tests


To run the tests, just run "q.physics.gravitational-attraction" test.