
In my previous Space Sunday update, I covered the (then) upcoming attempt by NASA’s Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer (OSIRIS-REx) to snag samples of material from the surface of asteroid 101955 Bennu, a carbonaceous near-Earth asteroid.
The attempt was successfully made on Tuesday, October 20th – although just how successful it was did not become apparent until a few days later, when mission engineers realised they now had a slight problem.
The mission required OSIRIS-REx slowly descended from its close-in altitude of 770m, a sample gathering called TAGSAM (Touch-And-Go Sample Acquisition Mechanism) extended beneath it. This was intended to make very brief contact with the surface of Bennu, absorbing the spacecraft’s momentum in springs, and allowing it to fire a nitrogen jet to blast material up from the asteroid some of which would hopefully be caught in the arm’s sampler head, prior to the arm “pushing off” from Bennu once more, allowing OSIRIS-REx to gently back away to a point where it could examine what it has gathered.

The entire operation was scheduled to take some 4.5 hours from start to back-away and parking. The event was live-streamed, but due to the current distance between Earth and Bennu, those on Earth were witnessing events 18.5 minutes after they had actually occurred. This also meant the entire operation was carried out autonomously, the software controlling it having been previously uploaded to the satellite.
OSIRIS-REx, following Bennu’s rotation about its axis, struck the asteroid a metre one metre away from its intended contact point, which lay within a shallow crater on Bennu that has been christened “Nightingale”. It remained in contact with the surface for 6 seconds – very slightly longer than had been anticipated.

Whilst there was a camera on the robot arm recording the operation, the footage could not immediately be sent back to Earth. Instead, mission controllers relied on the telemetry OSIRIS-REx did immediately transmit back to Earth. This revealed that everything had apparently gone as planned: TAGSAM made contact, the gas was fired and regolith (surface material blasted upwards. The telemetry then confirmed OSIRSIS-REx was backing away from the asteroid towards the point where analysis of the amount of captured material could be carried out.
This was transcendental. I can’t believe we actually pulled this off. The spacecraft did everything it was supposed to do. Even though we have some work ahead of us to determine the outcome of the event, this was a major accomplishment for the team. I look forward to analysing the data to determine the mass of sample collected.
– OSIRIS-REx Principal Investigator Dante Lauretta
Then came the first of the surprises. When the video footage captured by the TAGSAM arm camera was received and processed (above right) on October 21st, it revealed that the sample head hadn’t so much touched the surface of Bennu as smashed straight through it to an estimated depth of almost 50 centimetres – and in doing so, had pulverized a rock roughly 20 cm across which, when first viewed in the footage, caused the mission team to worry it might prevent sample gathering and damage the sample head.
The next step in the operation was to analyse the state of the sample head once TAGSAM had been returned to its stowed position against the spacecraft. To do this, one of the star tracker cameras used for navigation was tasked to capture an image of the sample head. When this was returned to Earth, mission staff had a second surprise: the sample head was “leaking” material.
Following the sample gathering operation, a Mylar diaphragm should have rotated over the opening of the sample head to seal any material gathered inside it – but the star tracker camera revealed this had failed to sit correctly, and a small cloud of material was forming around the sample head as it persistently “leaked” out. Given the force of the contact with Bennu, the mission team realised that, rather than just collecting 60 grams of material, the sample head had likely been filled to capacity, preventing the Mylar cover from correctly sealing it.

With material slowly but steadily escaping, the decision was been taken to cancel the attempt to estimate the amount of material gathered, and instead move to transferring the sample head to the Sample-Return Capsule (SAC). This is the unit that will return the sample to Earth when OSIRIS-REx return here in 2023. As the SAC is sealable, moving the sample head there as soon as possible – in this case, October 27th – will ensure the remaining material from Bennu is preserved.
In the meantime, and while OSIRIS-REx cannot start on its return to Earth until March 2021, the decision has been made not to return the vehicle to a low-level “hover” orbiting Bennu, but to instead allowing it to continue away from the asteroid at around 44 metres per hour until it reaches a more extended orbital position.
Continue reading “Space Sunday: asteroid sampling & starship building”





















