The rear hatch (or liftgate) on a 1998 Chevrolet Astro van stays up due to
gas-charged struts (also called gas springs, lift supports, or lift-gate shocks). These are hydraulic cylinders filled with pressurized nitrogen gas. When the hatch is opened, the struts extend, and the pressurized gas inside provides the force needed to hold the hatch in the open position. When you close the hatch, the struts compress.
If your Astro's hatch isn't staying up, one or both of these struts are likely worn out or broken. They're a common wear item and need replacing over time.
