Astrobee

Christina Koch. Public domain (NASA / US government work).

Overview

Astrobee is a fleet of three free-flying robots operating inside the pressurized volume of the International Space Station: Honey, Bumble and Queen [12], [13]. They replaced SPHERES as the station’s intravehicular free-flyer facility. The design is modular and upgradeable, with propulsion by electric fan rather than stored propellant, built-in cameras, and a perching arm for grasping handrails.

The lineage runs through two earlier Ames programs. The Personal Satellite Assistant set out the mission role of a free-flying intravehicular monitor and an autonomy framework built on planners from Deep Space One’s Remote Agent, of which only the first two prototype models flew [12]. Smart SPHERES then flight-tested a SPHERES satellite carrying a smartphone as a ground-commanded camera payload; a December 2012 survey of the Kibo module showed every ground command executing cleanly across two loss-of-signal periods, but also two runs ending when the cold-gas propellant ran out and one ending when the satellite hit the ceiling hard enough to break its position estimate [9]. Astrobee’s charter absorbed those failure modes directly: propellant that recharges at a dock rather than a tank that runs out, and collision tolerance built into the mechanical design rather than into operational caution [11].

Specifications

Parameter	Value	Source
Envelope	12.5 in cube (about 30 cm)	[2], [1]
Mass	about 10 kg	[3]
Design life	5 years	[2]
Maximum velocity	0.5 m/s	[3]
Localization accuracy	1 cm	[3]
Payload bays	4, two on top and two on the bottom; up to 3 usable at once by guest science	[4]
Payload bay port	14.4 VDC nominal battery power and USB 2.0	[4]
Perching arm degrees of freedom	3, two joints plus gripper	[3], [4]

Mission profile

Parameter	Value	Source
Docking station launched	17 November 2018	[12]
Docking station installed in the JEM	15 February 2019	[12]
Bumble and Honey launched	17 April 2019	[12]
Queen and three perching arms launched	25 July 2019	[12]
Dock commissioned	11 February 2019	[4]
Bumble commissioned	30 April 2019, 135 sessions to 2025	[4]
Honey commissioned	30 October 2019, 59 sessions to 2025	[4]
Perching arms commissioned	4 February 2021, 23 sessions to 2025	[4]
Queen commissioned	20 September 2021, 21 sessions to 2025	[4]
On-orbit real-time operations through 2025	more than 165, of which more than 90 were guest science test sessions	[4]
Console support	more than 1200 hours, about 220 of them involving crew	[4]
Operating volume	Japanese Experiment Module, Node 2, US Laboratory	[3]

Queen and Bumble operated independently in separate modules on 7 April 2022 [12]. In September 2025 NASA awarded Arkisys, Inc. a reimbursable Space Act Agreement to sustain and maintain the platform.

Propulsion

Propulsion is electric and uses cabin air rather than stored propellant, which removes both resupply and any exhaust product that would contaminate the atmosphere. Two propulsion modules, right and left, each contain one centrifugal impeller feeding a single plenum that discharges through six variable-area nozzles, twelve in total, with the propulsion parameters below [2].

Parameter	Value
Impellers	2, one per module
Nozzles	12, 6 per module
Maximum thrust per nozzle	0.3 N
Maximum total thrust	3.6 N
Nozzle exit velocity	11 m/s
Nozzle discharge coefficient	0.9
Flapper travel	64 deg
Flapper backlash	1.8 deg
Nozzle servo	MKS DS95

Each nozzle exit area is set by a pair of gear-synchronized flappers driven by a hobby-grade radio-control servo [2]. Modulating twelve exit areas against two constant-speed impellers gives holonomic control: force in any direction plus torque about any axis, six degrees of freedom. Total thrust of 3.6 N against a 10 kg vehicle gives a maximum acceleration near 0.36 m/s2, which is four orders of magnitude above the milli-g disturbance levels the station structure sees from crew exercise and equipment [7].

Localization and navigation

Localization is six-degree-of-freedom and vision-based, matching wall texture features against a prior sparse feature map, with no beacons or added infrastructure in the module [1], [4]. Visual odometry allows continued navigation through regions where mapped features are absent [3]. Position accuracy is about 1 cm, which is what visual servoing tasks require.

Six cameras are carried [4]: NavCam for mapping and localization, SciCam, a 13 MP forward-facing RGB imager used to stream video of crew activities [1], and HazCam, SpeedCam, PerchCam and DockCam for obstacle detection, velocity estimation, arm operations and docking respectively [1], [3].

Astrobee’s on-orbit localizer has been replaced once. The original filter-based estimator was superseded by AstroLoc, a factor-graph localizer that folds visual odometry in as combined factors and marginalizes over a sliding window to hold runtime down, cutting average localization time from 1.13 s to 0.21 s against a baseline factor-graph implementation while eliminating the lost-localization events the earlier filter suffered on a public ISS activity dataset [8]. Evaluated against mapping-derived ground truth over twelve real ISS activities, AstroLoc gives the corpus’s best-supported on-orbit localization accuracy figures [4].

Compute

Flight computing is split across three processor boards by real-time requirement [1]:

Board	Role
Low-Level Processor	hard real-time motion control
Mid-Level Processor	core flight software, navigation, obstacle detection
High-Level Processor	payload interface and guest science code

Source: [1].

Guest science code runs on the high-level processor, isolated from the control loop, so a visiting researcher cannot destabilize the vehicle.

Perching arm

A stowable arm mounted in a payload bay grips a handrail so the robot can shut down propulsion and hold station at low power [3]. While perched the arm joints double as a pan and tilt unit for the forward-facing cameras, the SciCam among them [3], [4]. The flight arm has three actuated degrees of freedom, two joints plus the gripper, and stows completely inside the top aft payload bay. The gripper is under-actuated and compliant, closed passively by torsional springs in the finger joints and opened by a single motor reeling two tendons, so it holds a handrail through a long perch without drawing power [4]. The gripper is one degree of freedom, two fingers of two revolute joints each driven by a single tendon, and the end-effector module is swappable on orbit so that a guest scientist can fit a different gripper or add arm degrees of freedom [6].

Docking

The dock is 85 by 38 by 28 cm and provides two berths [3]. Retention is magnetic and alignment is visual, against fiducial markers on the dock. Self-docking to recharge is what permits multi-day operation with no crew involvement.

Communications

While flying, Astrobees link to the station local area network over the ISS Wi-Fi network, and when docked they communicate over wired Ethernet through the dock, which raises bandwidth and serves as a backup link [4]. Payloads have used both the 2.4 GHz and 5 GHz bands [5]. A free-flyer is itself a usable instrument for characterizing that network: an Astrobee Wi-Fi survey of the US Laboratory found received signal strength between -65 and -45 dBm over the optimal region, a 20 dB dynamic range within it, 35 dB of attenuation through a module hatch, and more than 20 dB of loss when the payload antenna sat orthogonal to the access point antenna.

SPHERES, the predecessor

Parameter	SPHERES	Astrobee	Source
Propulsion	cold gas thrusters at about 25 psi, exhaust velocity about 250 m/s	electric impeller at about 0.1 psi, exhaust velocity about 11 m/s	[1]
Localization	ultrasonic beacons installed in the module	onboard vision against a prior feature map, no added infrastructure	[4]

SPHERES cold gas operation consumed reaction mass that had to be resupplied, while an Astrobee draws on cabin air and recharges itself at the dock [1]. Astrobee’s requirement to dock and recharge without crew involvement, and to localize without beacons, follows directly from that difference.

Other ISS free-flyers

Astrobee is one of three intravehicular free-flyer programs to have flown crew-cooperative robots on the station, alongside DLR/Airbus’s CIMON and JAXA’s Int-Ball and Int-Ball2. A joint review written by members of all three teams finds that what separates the platforms that fly often from those needing crew setup at every session is operational autonomy through a docking station: Astrobee and Int-Ball2 dock and recharge unattended, while SPHERES, the first Int-Ball and CIMON do not. Communication redundancy also mattered in practice, including two Astrobee dock Ethernet failures recovered over Wi-Fi, and localization in a changing station interior is a shared failure mode across all three programs [10].

Operations

Astrobee runs experiment sessions, environmental surveys and guest payloads on the standardized bays [4]. Nearly 80 percent of console time across the first six years needed no crew at all. Each payload bay carries a blind-mate connector supplying 14.4 V nominal battery power and USB 2.0 data, engaged by two tool-free quarter-turn fasteners, and up to three payloads can be hosted at once [1]. Investigators develop against a simulator and then run on flight hardware.

What is not established

Pre-flight design numbers do not match as-flown hardware and should not be read as performance data: an early platform description gives mass as about 6 kg and a hardware thrust limit of 0.72 N with a 0.5 m/s overspeed cutoff, against the roughly 10 kg, 0.6 N and 50 cm/s cutoff the bumper testing later justified [1]. The corpus’s most complete operations record has three open gaps: localization error is measured against the authors’ own offline mapping pipeline rather than an independent metrology system, so the absolute error floor is not established; on-orbit station-keeping and pointing accuracy are described only qualitatively; and flight duration is stated inconsistently, 2.5 to 3 hours on one page against roughly 2 hours on four batteries elsewhere. Bumper impact loads and the worst-case window force are analysis and ground test, not flight experience [4].