To reinforce kotlin patterns, instead of

 ​val​ astronomyService = Retrofit.Builder()​   
      .client(createHttpClient().build())
   ​   .baseUrl(​"https://api.nasa.gov/planetary/"​)​   
      .addConverterFactory(MoshiConverterFactory.create())
   ​   .build().create<AstronomyService>()

why not the kotlin builder pattern?

 ​val​ astronomyService = Retrofit.Builder()​.apply {   
         client = createHttpClient().build()
   ​      baseUrl = ​"https://api.nasa.gov/planetary/"​)
         converterFactory += MoshiConverterFactory.create()
   ​   }
     .build().create<AstronomyService>()
// Or, if those don't have setters
 ​val​ astronomyService = Retrofit.Builder()​.apply {   
         client(createHttpClient().build())
   ​      baseUrl(​"https://api.nasa.gov/planetary/"​)​   
         addConverterFactory(MoshiConverterFactory.create())
   ​   }
     .build().create<AstronomyService>()