Dec 11, 2017

Last week, OSIRIS-REx continued Outbound Cruise operations. The spacecraft is currently 47.6 million km from Earth and is executing a program designed to study and reduce the presence of water on the spacecraft.

During routine in-flight testing of the spacecraft’s thermal properties earlier this year, the mission’s navigation team noticed an unexpected minor acceleration of the spacecraft when the Sample Return Capsule (SRC) was exposed to sunlight. The mission team determined that this small thrust was caused by the outgassing of water that had been adsorbed by the SRC’s heat shield and backshell before launch. Retention of water in blanketing and other materials – and the subsequent outgassing of this water – occurs with all spacecraft. For OSIRIS-REx, it was determined that when the SRC is exposed to the Sun at a distance of <1.0 AU, this trapped water escapes and imparts a small thrust. While this small thrust would not be a problem for other missions, the gravity at the target asteroid Bennu is low enough that even this small amount of thrust could make orbital operations more difficult for OSIRIS-REx.

To better understand the outgassing effects on the spacecraft’s trajectory – and to bake out much of the remaining water before the spacecraft arrives at Bennu – the OSIRIS-REx mission team designed an outgassing program for execution starting earlier this fall. The choice of timing took into account both the spacecraft’s proximity to the Sun (<1.0 AU) and the fact that there were no science activities planned during this period. The outgassing program is being run concurrently with Outbound Cruise operations and does not affect the timing of the spacecraft’s arrival at Bennu.

Starting in mid-October, the spacecraft has been placed into various attitudes to expose different parts of the SRC to direct sunlight and initiate outgassing. Priority is given to the portions of the SRC that will face the Sun during asteroid proximity operations. The mission team has been able to detect and measure the rate of outgassing at each attitude and has determined that water is being removed as expected. The goal is to reduce the outgassing to the point where the spacecraft can fly the planned baseline trajectories around Bennu without modifications, and preliminary indications show that the program is progressing toward this goal. The program is scheduled to run through early January 2018.