TAGSAM

Erika Blumenfeld & Joseph Aebers. Public domain (NASA / US government work).

Overview

TAGSAM is the sampling mechanism carried by OSIRIS-REx. It is an articulated arm ending in an annular collection head that contacts the surface for a few seconds, releases nitrogen gas into the regolith beneath it, and captures the fluidized material in an internal reservoir [1].

Specifications

Parameter	Value	Source
Arm extension from the spacecraft	2.8 m	[11]
Wrist articulation	15 deg in any direction, compliant U-joint	[1], [2]
Head height	about 7 cm	[2]
Head base plate diameter	32 cm	[2]
Head opening diameter	21 cm	[2]
Contact pad samplers	24 pads of about 1.75 cm diameter, stainless steel Velcro loop, 57.42 cm2 total, encircling the base plate as a backup sampler and as a record of material exposed at the surface	[1], [2], [6]
Contact pad grain capture	up to 1 mm diameter [11]	[11]
Bulk capture particle limit	about 2 cm	[1], [2]
Nitrogen bottles	3, independently controlled, 3 possible attempts	[1], [2]
Gas release timing	1 s after contact	[2]
Contact duration	up to 8 s on a soft surface	[11]
Head collection chamber capacity	at least 150 g	[7]
Demonstrated ground and parabolic collection	over 600 g	[11]

Mission profile

Parameter	Value	Source
Host	OSIRIS-REx	[1]
Target	asteroid Bennu	[1]
Contact site	Nightingale, in the 20 m Hokioi crater	[2]
Contact date	20 October 2020	[2]
Sample return capsule landing	Utah Test and Training Range, 24 September 2023	[2], [7]
Sample requirement	60 g bulk plus 26 cm2 of fine-grained contact pad material	[1], [7]

Mechanism

The head is a simple annulus with a wire mesh filter screen around its outer circumference [1]. There is no digging tool and no closing jaw. Nitrogen injected into the interior of the annulus entrains fines and pebbles up to 2 cm [1] and, if the head is firmly seated, the only path for gas and entrained regolith is through the filter, where the material is retained. A compressible constant-force spring in the forearm buffers the contact dynamics [2].

Cross-section of the collection head with its dimensions and gas path. Nitrogen enters through the central manifold and exits at the outer rim, so once the head is seated on the surface the only route out for gas and entrained regolith is through the filter, where the material is retained. Source: [2]. CC BY 4.0.

The architecture follows from the force scaling at asteroid gravity. Cohesion between grains exceeds their weight over most of the relevant particle size range, and gravity provides no useful normal force for a tool to react against [5]. Gas transfers momentum to the regolith without the spacecraft having to react a digging load, which is what makes collection possible without landing.

The 15 deg wrist compliance was allocated as a hard constraint on site selection: the mission requirement capped facet tilt at 14 deg, with the remaining 1 deg reserved for other error sources [2].

Descent and contact sequence

Event	Parameter
Departure orbit	1 km
Checkpoint burn	Places the spacecraft on an intersecting trajectory at 16 cm/s approach
Checkpoint to Matchpoint	About 600 s
Matchpoint altitude	55 m
Contact velocity target	10 cm/s vertical, zero lateral to +/- 2 cm/s
Back-away burn	0.7 m/s
Delivery requirement	Head within 25 m of the aim point at over 98 percent probability

Source: [1].

Trajectory dispersion after the deorbit burn is too large for predefined Checkpoint and Matchpoint maneuvers, so the spacecraft state is updated onboard at Checkpoint and both burns are adjusted autonomously [1]. Two independent guidance methods were carried. LIDAR-guided TAG detects a range-threshold crossing and takes a range measurement at a fixed time to correct the maneuvers. Natural Feature Tracking renders features from an onboard shape model built in flight, using predicted camera pointing and Sun position, and correlates them against live imagery to produce a position and velocity update relative to the surface. During contact the reaction wheels and attitude control thrusters are enabled to damp imparted torques and keep any part of the spacecraft other than the head off the surface. The solar arrays are moved into a Y-wing configuration for sampling, to keep dust off the cells and increase ground clearance if the spacecraft rotates on contact.

A post-mission reassessment of the guidance chain against the flown Nightingale contact found the delivered accuracy consistent with preflight dispersion analysis, with the LIDAR range-trigger and Natural Feature Tracking updates each contributing separately identifiable corrections to the touchdown point [9]. Earlier operational experience with Natural Feature Tracking, gained during the approach and reconnaissance phases at Bennu, had already shown the technique tolerant of the shape model errors and lighting changes that a first flight of the method could not be fully bounded against on the ground [10].

What the contact measured

The touchdown is the only direct measurement of the geotechnical properties of a rubble pile near subsurface. The arm spring took no compression, so the load path from the head to the spacecraft inertial measurement unit was rigid and the 200 Hz accelerometer record is a force record, with the contact measurements below [3].

Quantity	Measured value
Contact velocity	10 cm/s, negligible lateral
Peak acceleration before gas release	0.014 m/s2
Peak contact force	10 to 15 N
Depth at 1.2 s	2 to 3 cm
Depth at end of the pre-gas interval	5.95 to 6.91 cm in 0.605 to 0.705 s
Head tilt taken up by a 5 cm rock	about 7 deg
Disturbed area	0.51 m2 against a 0.08 m2 footprint
Derived near-subsurface bulk density	440 to 600 kg/m3
Derived packing fraction	0.2 to 0.45
Derived compressive strength	2 to 200 Pa
Derived bulk cohesion	0.2 to 20 Pa

No initial acceleration spike appeared, which is the signature of granular impact in the laboratory; the rise was gradual because contact was off-center against a 5 cm rock that tilted the head [3]. A 40 cm rock was levered upward and millimeter debris was lofted from its surface under weak forces, which itself argues for minimal cohesive bonding.

Interpretation runs through a modified granular intrusion force law with a quasi-static term F_pressure = 80 mu rho a^2 g |z| and an inertial term F_drag = 3.2 rho a^2 U^2 [4]. For weakly cohesive regolith the response splits on packing fraction: at or below about 0.5 the force stays under about 60 N and is drag dominated, while at or above 0.6 the force exceeds 60 N for friction angle above 20 deg and penetration stops within a few centimeters above 30 deg [3], [4]. The measured 10 to 15 N places Bennu firmly in the loose regime.

Site selection

Bennu was found to have no ponded deposits of particles small enough to sample, which is the surface type the mechanism had been designed against [2]. The mission requirement was a site with at least 80 percent probability of acquiring at least 60 g per attempt. Meeting it after arrival required a sampleability algorithm fitted to ground test results relating collection mass to observable surface properties.

Three map products drove the decision: a deliverability map giving navigation precision to a target point, a safety map identifying boulders that could endanger the spacecraft, and a sampleability map [2]. Particle mapping had to resolve grains at or below the 21 cm head opening, against an imaging requirement to cover more than 80 percent of the surface at better than 21 cm resolution. Reconnaissance flybys at 250 to 350 m produced pixel scales below 2 cm, and digital terrain models were built at 2 cm facet spacing, comparable to the 32 cm head diameter [2]. A sequence of TAG rehearsals was flown after site selection, each demonstrating one step of the collection sequence before the flight system was committed to the surface [7].

Contact itself was triggered by a compression indicator on the arm, with the guidance LIDAR taking the spacecraft to about 5 m before the final approach [7].

At the actual contact point, latitude 55.9 deg and longitude 41.8 deg, particle mapping found a minimum particle size of 7.65 mm and a size-frequency distribution power law slope of -2.09 +/- 0.008 over 521 particles within a 1.5 m radius [2]. Rock tilt efficiency was 0.434 because a rock larger than the 21 cm opening overlapped the sampling location. Predicted collection ranged from 32.4 g using resolved particles only under low-mobility scoring to 249.9 g including unresolved material under high-mobility scoring [2]. Imagery during the event showed the head flush with the surface with no effective tilt, which raises the tilt efficiency to 1.0 and the predictions to 258 g and 575 g.

The lesson recorded by the team is that the site choice depended on data that could only be obtained after arrival: Nightingale and Osprey diverged in their fraction of unresolved facets only as progressively finer imagery was analyzed, and an earlier decision on lower-resolution imagery could have selected a different site [2].

Sample mass measurement and stowage

Collected mass was determined from the change in spacecraft moment of inertia, with the arm extended along the spacecraft X axis and the measurement compared against an identical one made before contact [1]. SamCam then refocused through a diopter lens in its filter wheel to image the head and contact pads at a range of incidence and emission angles for visual verification. In parallel [1], the contact force profile was reconstructed from OCAMS, TAGCAMS and guidance telemetry, incorporating head motion and forearm pogo spring compression, and matched against an atlas of discrete element simulation outcomes calibrated on 1 g and microgravity low-velocity impact experiments. OCAMS itself is a three-camera suite built for this purpose: a wide-angle survey camera for site imaging, a map camera for the digital terrain models used to build the sampleability map, and the narrow-angle SamCam pointed down the arm to record the contact and stowage sequence [8].

Enough material entered the reservoir that the retaining flap jammed open and particles escaped, and stowage was advanced ahead of schedule [11]. The bulk sample collected was estimated from imaging at 250 plus or minus 101 g, against a mission requirement of at least 60 g [1], [6].

Return

Event	Value	Source
Bennu departure	March 2021, 931 days after arrival	[7]
Capsule mass	46 kg	[1]
Capsule dimensions	81 cm diameter, 50 cm tall	[1]
Capsule release	4 h before entry interface	[7]
Entry velocity	12.2 km/s	[7]
Entry flight path angle	-8.2 deg	[7]
Kinetic energy removed by the aeroshell	over 99 percent	[7]
Sample temperature limit through entry	below 75 C	[7]
Drogue deployment	Mach 1.4 at about 30 to 33 km	[1], [7]
Main chute deployment	about 3 km	[1], [7]
Terminal descent speed	4.6 m/s	[1]
Riser cutter trigger	10 g switch at ground impact	[1]

The capsule is a Stardust-derived blunt-nosed cone with a PICA heat shield [1], [7]. The Stardust aerogel grid was replaced by a capture ring that locks the TAGSAM head during stowage [1]. Two backshell vents allow depressurization on ascent and repressurization on descent.

Contamination control

Sensitive surfaces were held to IEST-STD-CC1246D level 100A/2, and TAGSAM and capsule cleanliness was monitored so that no surface accumulated more than 180 ng/cm2 of total amino acids and hydrazine [1]. Over 300 material coupons and an extensive witness plate catalog were collected through assembly, test and launch operations and archived at NASA JSC, with further witness plates mounted on the mechanism and capsule to record in-flight contamination.

Curation and allocation

The sample canister was purged with high-purity gaseous nitrogen inside a temporary ISO 7 cleanroom at the Utah Test and Training Range, then flown to Ellington Field and trucked to NASA JSC under a continuous nitrogen purge, planned for the day after recovery [6]. Disassembly and curation use four custom nitrogen gloveboxes in an ISO 5 cleanroom, certified in November 2021 and held at 22 C plus or minus 1 C: one for the capsule canister, one for disassembly of the TAGSAM head, one for the contact pads and one for the witness plates. The catalog was to be released in late March 2024, about six months after return, with up to 25 percent by mass allocated to the science team for early analysis [6].

The 24 contact pads are a distinct sample stream from the bulk reservoir: they were designed both as a backup should regolith fail to mobilize into the chamber and to capture material exposed at the surface for direct comparison against the spacecraft’s remote sensing, and they are removed and processed in their own glovebox [6].

Radiation

No TAGSAM-specific radiation design data is published. The interplanetary environment the mechanism was exposed to over seven years is that of the Badhwar-O’Neill galactic cosmic ray model, with solar modulation derived from time-delayed sunspot number [12].

Technologies developed

Touch-and-go sampling with onboard hazard-relative guidance is now the reference approach for small-body sample return [1], [11]. The Natural Feature Tracking implementation, an onboard shape model correlated against live imagery to produce a surface-relative state update, was flown here for the first time on an asteroid approach and is the direct antecedent of terrain-relative navigation on later small-body missions [1]. The contact itself produced the granular force law calibration that any successor sampler will be sized against [3], [4].