  The stellar halo of the Milky Way is built up, at least in part, from debris
from past mergers. Stars from such merger events define substructures in
phase-space, for example in the form of streams, which are groups of stars
moving on similar trajectories. The nearby Helmi streams discovered more than
two decades ago are a well-known example. Using 6D phase-space information from
the Gaia space mission, Dodd et al. (2022) have recently reported that the
Helmi streams are split into two clumps in angular momentum space. Such
substructure can be explained and sustained in time if the dark matter halo of
the Milky Way takes a prolate shape in the region probed by the orbits of the
stars in the streams. Here, we explore the behaviour of the two clumps
identified in the Helmi streams in a Modified Newtonian Dynamics (MOND)
framework to test this alternative model of gravity. We perform orbit
integrations of Helmi streams member stars in a simplified MOND model of the
Milky Way and using the more sophisticated Phantom of RAMSES simulation
framework. We find with both approaches that the two Helmi streams clumps do
not retain their identity and dissolve after merely 100 Myr. This extremely
short timescale would render the detection of two separate clumps as very
unlikely in MONDian gravity. The observational constraints provided by the
streams, which MOND fails to reproduce in its current formulation, could
potentially also be used to test other alternative gravity models.
