*4.3. An Incomplete List of Pathfinder/Precursor Missions*

In the past, many projects in the broad area of nano-satellites for high-energy astrophysics were proposed and several were funded, in order to build demonstrators or pathfinders. Among these, we mention the following, without pretending to be complete.

#### 4.3.1. CAMELOT and GRBAlpha

One of the concepts developed to perform all-sky monitoring and timing-based localization of gamma-ray transients is called CAMELOT: Cubesats Applied for MEasuring and LOcalising Transients [24]. It is a constellation of 3U CubeSats equipped with large and thin (150 × 75 × 5 mm) CsI(Tl) scintillators read out by SiPM detectors, called multi-pixel photon counters (MPPCs), by Hamamatsu. The detectors are placed on two perpendicular walls of the satellites to maximize the effective photon collecting area on a CubeSat of this size.

The detector concept developed for the CAMELOT mission was first demonstrated on a 1U CubeSat, named GRBAlpha [25]. It carries a smaller, 75 × 75 × 5 mm CsI (Tl)

scintillator, which provides one-eighth of the expected effective area of the 3U CubeSats envisioned for the CAMELOT mission. GRBAlpha was successfully launched on 22 March 2021 to a sun-synchronous polar orbit with a Soyuz 2.1 rocket from Bajkonur. Following a short commissioning phase, the detector was switched on and regular measurements began. The on-board data acquisition software stack is periodically updated in orbit, continuously improving the capabilities of the science payload. The degradation of the SiPM detectors, which are protected by a 2 mm thick lead shield, is being monitored. About half of the polar orbit is plagued by a high particle background, and thus the duty cycle of the detector cannot be better than 50%, even if it is operated continuously. The satellite communicates on amateur radio frequencies and its ground segment is supported by the radio amateur community. The mission also takes advantage of the SatNOGS network for increased data downlink volume. Figure 1 shows GRBAlpha before the launch. One of the concepts developed to perform all-sky monitoring and timing-based localization of gamma-ray transients is called CAMELOT: Cubesats Applied for MEasuring and LOcalising Transients [24]. It is a constellation of 3U CubeSats equipped with large and thin (150 × 75 × 5 mm) CsI(Tl) scintillators read out by SiPM detectors, called multipixel photon counters (MPPCs), by Hamamatsu. The detectors are placed on two perpendicular walls of the satellites to maximize the effective photon collecting area on a CubeSat of this size. The detector concept developed for the CAMELOT mission was first demonstrated on a 1U CubeSat, named GRBAlpha [25]. It carries a smaller, 75 × 75 × 5 mm CsI (Tl) scintillator, which provides one-eighth of the expected effective area of the 3U CubeSats envisioned for the CAMELOT mission. GRBAlpha was successfully launched on 22 March 2021 to a sun-synchronous polar orbit with a Soyuz 2.1 rocket from Bajkonur. Following a short commissioning phase, the detector was switched on and regular measurements began. The on-board data acquisition software stack is periodically updated in orbit, continuously improving the capabilities of the science payload. The degradation of the SiPM detectors, which are protected by a 2 mm thick lead shield, is being monitored. About half of the polar orbit is plagued by a high particle background, and thus the duty cycle of the detector cannot be better than 50%, even if it is operated continuously. The satellite communicates on amateur radio frequencies and its ground segment is supported by the radio amateur community. The mission also takes advantage of the SatNOGS network for in-

time of writing, the mission detected five GRBs (four long and one short GRBs [26–30]),

creased data downlink volume. Figure 1 shows GRBAlpha before the launch.

*Galaxies* **2021**, *9*, x FOR PEER REVIEW 8 of 13

4.3.1. CAMELOT and GRBAlpha

In the past, many projects in the broad area of nano-satellites for high-energy astrophysics were proposed and several were funded, in order to build demonstrators or pathfinders. Among these, we mention the following, without pretending to be complete.

**Figure 1.** GRBAlpha, a 1U CubeSAT equipped with a gamma-ray detector (visible on the top, see [25] for details). GRBAlpha became the first nano-satellite to detect multiple confirmed GRBs. At the **Figure 1.** GRBAlpha, a 1U CubeSAT equipped with a gamma-ray detector (visible on the top, see [25] for details).

two of them over the course of a single night from 18 to 19 October. Figure 2 shows the light curves of GRB211018A observed by GRBAlpha in different energy bands [30]. GRB detectors developed for the CAMELOT mission are also going to be launched on VZLUSAT-2, which is a technological 3U CubeSat built by the Czech Aerospace Research Centre. The satellite will carry two perpendicular detectors the same size as GRBAlpha. VZLUSAT-2 is expected to be launched in January 2022 into a sun-synchronous polar orbit on a Falcon 9 rideshare mission. GRBAlpha became the first nano-satellite to detect multiple confirmed GRBs. At the time of writing, the mission detected five GRBs (four long and one short GRBs [26–30]), two of them over the course of a single night from 18 to 19 October. Figure 2 shows the light curves of GRB211018A observed by GRBAlpha in different energy bands [30]. GRB detectors developed for the CAMELOT mission are also going to be launched on VZLUSAT-2, which is a technological 3U CubeSat built by the Czech Aerospace Research Centre. The satellite will carry two perpendicular detectors the same size as GRBAlpha. VZLUSAT-2 is expected to be launched in January 2022 into a sun-synchronous polar orbit on a Falcon 9 rideshare mission. *Galaxies* **2021**, *9*, x FOR PEER REVIEW 9 of 13

4.3.2. HERMES Pathfinder

as a few years.

tions.

**Figure 2.** Light curves of GRB 211018A observed by GRBAlpha in different energy bands [30]. **Figure 2.** Light curves of GRB 211018A observed by GRBAlpha in different energy bands [30].

HERMES-Technologic and Scientific pathfinder (HERMES pathfinder) is an in-orbit demonstration consisting of a constellation of six 3U nano-satellites hosting simple but

The transient position is obtained by studying the delay time of arrival of the signal to different detectors hosted by nano-satellites on low Earth orbits [32]. To this purpose, particular attention is placed on reaching the best time resolution and time accuracy, with the goal of reaching an overall accuracy of a fraction of a micro-second [33]. The main goals of the project are: (1) join the multi-messenger revolution by providing the first miniconstellation for GRB localization with a total of six units (the first experiment of GRB triangulation with miniaturized instrumentation); (2) develop miniaturized payload technology for breakthrough science; (3) demonstrate COTS applicability to challenging missions, contribute to Space 4.0 goals, push and prepare for high-reliability large constella-

Figure 3 shows the HERMES pathfinder detector system during integration. The 60 GAGG scintillator crystals can be seen to the right and the 12 10 × 10 silicon drift detector mosaics used to read out the crystals to the left (see [33] and references therein for a

TP/SP is to prove that an accurate position of high-energy cosmic transients can be obtained using miniaturized hardware, with a cost at least one order of magnitude smaller than that of conventional scientific space observatories and a development time as short

detailed description of the HERMES pathfinder payload).
