Geometry (Node-first): ray/surface intercepts (raw.sincpt)

This recipe is Node.js-first and uses the native addon backend (backend: "node").

We’ll compute a surface intercept point on the Moon by tracing a ray from Earth toward the Moon.

Kernels required (and why)

• LSK (*.tls): required for kit.utcToEt().
• SPK (*.bsp): required to get Earth→Moon state/position.
• PCK (*.tpc): required for IAU_MOON + Moon radii (ellipsoid shape).

Load kernels from filesystem paths

In Node, kernel sources are just filesystem paths (strings):

import path from 'node:path'
import { spiceClients } from '@rybosome/tspice'

const kernelsDir = path.resolve('kernels')

const { spice, dispose } = await spiceClients.toAsync({ backend: 'node' })

try {
  await spice.kit.loadKernel(path.join(kernelsDir, 'naif0012.tls'))
  await spice.kit.loadKernel(path.join(kernelsDir, 'pck00011.tpc'))
  await spice.kit.loadKernel(path.join(kernelsDir, 'de432s.bsp'))

  // …geometry query…
} finally {
  await dispose()
}

Example: intercept the Moon along an Earth→Moon ray

const toDegrees = (rad: number): number => (rad * 180) / Math.PI

const et = await spice.kit.utcToEt('2024-01-01T00:00:00Z')

const method = 'ELLIPSOID'
const target = 'MOON'
const fixref = 'IAU_MOON'
const abcorr = 'NONE'
const observer = 'EARTH'

// Direction reference frame for the ray.
const dref = 'J2000'

// Aim the ray at the Moon by using the Earth→Moon position vector.
const { pos: earthToMoon } = await spice.raw.spkpos(target, et, dref, abcorr, observer)

const out = await spice.raw.sincpt(
  method,
  target,
  et,
  fixref,
  abcorr,
  observer,
  dref,
  earthToMoon,
)

if (!out.found) {
  throw new Error('Ray missed the target (no intercept found)')
}

// `spoint` is the intercept point in the target body-fixed frame (`fixref`).
const { spoint, trgepc, srfvec } = out

// Convert rectangular coordinates -> planetocentric lon/lat (radians).
const { radius, lon, lat } = await spice.raw.reclat(spoint)

console.log({
  trgepc,
  spointKm: spoint,
  observerToSurfaceKm: srfvec,
  lonDeg: toDegrees(lon),
  latDeg: toDegrees(lat),
  radiusKm: radius,
})

Interpreting the result

• found: false means the ray does not hit the target body.
• spoint is the intercept point in the target body-fixed frame you passed as fixref. In this example that’s IAU_MOON.
• srfvec is the vector from the observer to the surface point (in the fixref frame).
• trgepc is the “target epoch” (it can differ from your input et when you use aberration corrections other than "NONE").