Launch Watch · Analysis

Published 4 October 2026, 08:54 HST (UTC−10)

Why one asteroid has two speeds in JPL’s close-approach data

Check v_rel and v_inf across 482 JPL records, including 2024 XA’s 4.574 km/s gap, and see why the two speed fields cannot be swapped.

One JPL record gives asteroid 2024 XA a close-approach speed of 13.566 kilometers per second and a second speed of 8.992 kilometers per second. The 4.574-kilometer-per-second gap is not a disagreement between two telescopes. The fields describe different quantities: speed at closest approach and speed relative to a hypothetical massless Earth.

We checked those fields in 482 records returned by a saved JPL Close Approach Data API query. The query uses date limits of 1 January and 31 December 2024, selects Earth approaches within 0.01 astronomical unit, and sorts by nominal distance. It was retrieved on 4 October 2026. These are orbit-solution results available in that snapshot, not a reconstruction of what observers knew on each encounter date.

What the two speed fields mean

JPL’s API documentation defines v_rel as velocity relative to the approach body at close approach. It defines v_inf as velocity relative to a massless body. Both are in kilometers per second. With Earth as the approach body, the second field provides the comparison without Earth’s gravitational contribution to the encounter speed.

For 2024 XA, the nominal distance is 0.0000516452821681997 astronomical unit. Using JPL’s astronomical-unit conversion, 149,597,870.7 kilometers per astronomical unit, gives 7,726.024 kilometers. The CNEOS table’s column descriptions specify Earth-center to object-center distance and define the comparison speed relative to a massless Earth. This is not an altitude above the ground.

Method: test the relationship in every returned row

The two-body energy relation provides a direct consistency check: subtract v_inf² from v_rel², multiply by nominal center-to-center distance, and divide by two. With speed in kilometers per second and distance in kilometers, the result has the units of Earth’s gravitational parameter, cubic kilometers per second squared.

For 2024 XA, the calculation using the API’s unrounded values gives 398,600.435507 cubic kilometers per second squared. That matches the Earth GM value on JPL’s constants page at the precision shown. Repeating the same calculation for all 482 rows gives a largest fractional difference below 0.000000000006. The script retains the source precision; the shorter numbers in the article are rounded for readability.

This striking agreement tests the interpretation and arithmetic of the returned fields. It does not independently validate JPL’s orbit accuracy: the fields come from the same computed record and their relationship is built into the comparison. It would be a mistake to describe 482 matching calculations as 482 new physical measurements of Earth’s mass.

The close-up is not the whole sample

All 482 records have v_rel greater than v_inf. But the median gap across the full query is only 0.061 kilometers per second, compared with 4.574 for 2024 XA, the largest gap in this saved set. The ten nearest records make the difference easy to see; they should not be presented as typical of every returned encounter.

The relation also explains why distance alone does not order the gaps. At the same nominal distance, the change in speed depends on the starting comparison speed as well as Earth’s gravitational parameter. The chart shows this clearly: 2024 UZ6 has a larger speed gap than several closer-listed objects, even though its distance is greater.

Choose the field before making the headline

If a sentence is about speed at closest approach to Earth, v_rel is the relevant field in this API. Keep v_inf labeled when discussing the massless-body comparison. Switching between them without explanation can change the headline number substantially, as 2024 XA demonstrates.

The selected database rows are not a census of all 2024 flybys, and the number of records says nothing by itself about discovery completeness. Nominal distance and speed also do not establish an impact probability. For this audit, the defensible outcome is a checked interpretation of two fields, a reproducible relation across the saved query, and a clear warning against interchanging their meanings.