Virtual flying lessons for Hera asteroid mission

As ESA’s Hera spacecraft for planetary protection goes by way of pre-flight testing, the system that may steer it round its goal binary asteroid system can also be present process its closing checks for space.

Validation of the mission’s Steerage Navigation and Management system’s readiness for proximity operations inside this difficult, ultra-low gravity setting by way of an extended collection of digital maneuvers, carried out in parallel in Spain and Germany.

On the headquarters of Steerage Navigation and Management (GNC) system developer GMV in Madrid, a reproduction of Hera’s On-Board Pc is at present being put by way of proximity operations round a mannequin asteroid imaged with a digital camera, for max realism, with different sensors and actuators emulated utilizing personalized “check-out” tools.

In the meantime, on the premises of spacecraft producer OHB in Bremen, exams are happening utilizing a full-scale {hardware} duplicate of the spacecraft, known as the Hera Avionics Take a look at Bench.

“The system for Hera’s interplanetary cruise phase—which after all is probably the most essential to be prepared for launch—is now totally examined utilizing the precise spacecraft flight mannequin,” explains ESA GNC engineer Jesus Gil Fernandez.

“This phase will finish at asteroid arrival when digital camera photographs can be used to tell apart the asteroid from background stars by recognizing its gradual movement throughout successive photographs. GNC for the follow-on proximity operations phase is what we’re concentrating on now, involving the spacecraft initially coming as shut as 30 km from the asteroid pair, then a lot nearer in a while, right down to 1 km.”

Alien, ultra-low gravity setting

Following its lift-off this October, Hera is headed for a distinctively alien setting. After a two-year cruise by way of space, together with a Mars flyby that can be used to take science observations of Deimos, the spacecraft will rendezvous with the Didymos binary asteroid system: the Dimorphos moonlet, concerning the dimension of the Nice Pyramid of Giza, is in orbit about 1.2 km away from the mountain-sized Didymos principal physique.

The mixed gravity fields of those two asteroids are tens of 1000’s of instances weaker than Earth’s.

Including to the unique nature of this vacation spot, Dimorphos has already undergone a change of orbit round Didymos, after NASA’s DART spacecraft impacted with it in September 2022. And this impression is prone to have reshaped the asteroid in dramatic vogue.

Information fusion for environmental mapping

To function safely round Didymos, Hera has a excessive diploma of onboard autonomy. Its Steerage, Navigation and Management (GNC) system is designed to fuse knowledge from numerous sources to construct up a coherent image of its environment, in an analogous method to self-driving automobiles.

“Its principal knowledge supply can be its principal Asteroid Framing Digital camera, whose photographs are getting used each for science and navigation,” provides Jesus. “These photographs can be mixed with different inputs to make a sturdy estimate of its place, notably the mission’s PALT-H laser altimeter, which bounces down laser pulses to the asteroid’s floor, in addition to inertial sensors. This GNC system is designed to be operated manually from the bottom initially, however as soon as Hera’s CubeSats are deployed, autonomous navigation can be wanted to meet core mission goals.”

Throughout proximity operations, Hera will maintain Didymos framed in its digital camera as an total reference level, detecting the distinction between the asteroid’s edges and the deep space round it. The detected form can be in contrast with a predicted spherical mannequin. Afterward, when the spacecraft comes nearer than about 10 km from Didymos and greater than 2 km above Dimorphos, a picture processing method known as “heart of brightness” can be used, targeted on the typical place of sun-illuminated pixels, because of the smaller asteroid’s advanced and unsure form.

Hyperbolic arcs to keep up place

The gravity ranges of the 2 asteroids are too low for the spacecraft to enter orbit in any conventional sense. As a substitute (borrowing a method from ESA’s Rosetta comet-chaser) Hera will fly in “hyperbolic arcs”—resembling a collection of alternating flybys, reversed by common thruster firings each three to 4 days. Within the case of any regular mission, this quantity of repeat velocity modifications would quickly exhaust its propellant tanks, however the gravity degree round Didymos is so low that Hera will solely be flying at a typical relative velocity of round 12 cm per second.

“Hera’s hyperbolic arcs are designed in order that if a thruster firing has a small error then the spacecraft would maintain at a protected distance from the asteroids anyway,” provides Jesus. “Nevertheless, the low velocities concerned imply that the orbital maneuvers that convey Hera very near the asteroids should be executed very precisely, in any other case there may nonetheless be a collision danger. Thus, the GNC contains an autonomous trajectory correction system, plus an autonomous collision danger estimation system empowered to carry out collision avoidance maneuvers as wanted.”

Floor characteristic monitoring

Hera’s self-driving autonomy will actually come into its personal because the spacecraft nears the asteroids later in its mission, Jesus explains, “As soon as we come nearer than 2 km then Dimorphos will fill the digital camera’s subject of view. Then comes probably the most bold navigation mode of all, based mostly on autonomous floor characteristic monitoring with no absolute reference. This can be a matter of imaging the identical options—comparable to boulders and craters—in successive footage to achieve a way of Hera’s altitude and trajectory with respect to the floor.”

Characteristic identification and mapping can even be used to derive the mass of Dimorphos, though this system can be carried out from the bottom fairly than aboard the spacecraft.

Mission controllers will measure the “wobble” the moonlet causes to its guardian, relative to the widespread heart of gravity of the general Didymos system. This can be achieved by figuring out small meter-scale variations within the rotation of fastened landmarks round this heart of gravity over time.

GNC testing of a few of the modes on this closing experimental phase will proceed after launch, to arrange the spacecraft forward of its October 2026 arrival at Didymos.