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HaloSat (Soft X-ray Surveyor)

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HaloSat is a 6U CubeSat astronomical science mission of NASA that will measure soft X-ray emissions from the halo of the Milky Way galaxy. The sum of baryons observed in the local universe falls short of the number measured at the time of the cosmic microwave background—the "missing baryon" problem. HaloSat should help determine if the missing baryons reside in the hot halos surrounding galaxies. 1) 2)

The HaloSat mission is led by the University of Iowa with Philip Kaaret as PI (Principal Investigator). The team from the University of Iowa in collaboration with NASA/GSFC ( Goddard Space Flight Center), and JHU/APL (Johns Hopkins University/Applied Physics Laboratory) will be developing the science instrument for the HaloSat mission. BCT (Blue Canyon Technologies) of Boulder, CO, will build the 6U CubeSat. Nagoya University of Japan is responsible for the Scattering measurement. 3)

Mission goal: Measuring the mass of the X-ray halo in our Galaxy.

• 6U CubeSat with a size of 10 x 20 x 34 cm3

• Observation:~75 % of the sky in 6 months

• FOV (Field of View): ~10º

• Three SDDs (Silicon Drift Detectors): ~80 eV @ 0.45 keV

• Iowa University (PI: Kaaret Philip), NASA/GSFC, Nagoya University.

Scientific Motivation: Observation of the Soft X-ray Sky.

- SXDB (Soft X-ray Diffuse Background) in the range 0.1 - 2 keV

- Soft X-ray sky is still full of mysteries

Scientific Goal (1) : Missing baryon problem

- Only ~5 % of the energy density in the Universe is in a form of baryons

- Census of baryons in the Universe today finds ~2/3 of that number

- To measure the mass of the X-ray-emitting hot gas halo in our Galaxy is important in cosmology.

Scientific Goal (2) : M band problem

- SXDB should reduce in the Galactic disk due to an absorption. However, no reduction is observed (M band problem).

- Existence of an unresolved SXDB was suggested in the Galactic disk.

Halosat allows us to study a spatial distribution of "each" SXDB component.

• Goal: measure the mass of the Milky Way's halo

- Determine the geometry of the halo -is it extended or disk-like?

- Measure how much radiation is made by the halo - set by the gas mass

• Requirement: measure hot gas at ~106 K

- Detect X-rays from oxygen atoms

- O VII at 561 eV, O VIII at 653 eV

- Sensitive near 600 eV with 100 eV energy resolution

• Requirement: determine the geometry of the halo (observe whole sky)

• Requirement: obtain sufficient X-ray counts

- Long duration mission

- View large part of the sky (10º x 10º fields)

- Allows use of small detectors (25 mm2)

Table 1: Mission goal and science requirements


Figure 1: Scientific motivation: where are the missing Baryons? (image credit: University of Iowa, Ref. 2)

Operate the 6U CubeSat for 213 days (required) to 365 days (goal).

Some background: Baryonic matter makes up almost 5% of the total massenergy of the Universe today. However, observations of luminous matter fail to locate a substantial fraction of the predicted baryons. One of the possible reservoirs of the missing baryons may be halos of hot gas surrounding galaxies. The closest such hot halo is the one extending around our Milky Way galaxy. The gas within this halo is at a temperature of ~106 K. 4) Thus, it should readily emit in the X-ray band. 5)

Line emission and absorption from highly ionized species present in the gaseous halo is the primary diagnostics to study such hot gas. Using absorption lines as the diagnostic tool can only probe the properties of the halo along a limited number of lines of sight simply because the number of X-ray bright extragalactic sources is limited. In contrast, emission lines can be measured in any direction and thus provide a means to study the full geometry of the halo. In particular, emission from highly ionized oxygen, O+6 (OVII) and O+7 (OVIII), can be used as a diagnostic tool for studying the properties of the hot galactic halo. The scientific goal of the HaloSat mission is to constrain the mass and spatial distribution of the hot gas that surrounds our Milky Way by mapping the emission from OVII (561 eV) and OVIII (653 eV).




In 2016, BCT (Blue Canyon Technologies) has been awarded a contract to build and test a new 6U-class satellite. Funded by NASA's Science Mission Directorate, BCT will deliver the spacecraft bus, ready for instrumentation, to the HaloSat project, which is funded by NASA/GSFC (Goddard Space Flight Center) Wallops Flight Facility. 6)

BCT's XB spacecraft is a high-performance small satellite that includes an ultra-precise attitude control system that allows for accurate knowledge and fine-pointing of the satellite payload. This mission builds upon the success of BCT's recent performance on the MinXSS spacecraft, which is completing its 6th month on-orbit. BCT is currently working on over fifteen different spacecraft missions that use its high-performance XB spacecraft bus.

HaloSat is a CubeSat that will measure soft X-ray emissions from the halo of our Milky Way galaxy. The sum of baryons observed in the local universe falls short of the number measured at the time of the cosmic microwave background—the "missing baryon" problem. HaloSat should help determine if the missing baryons reside in the hot halos surrounding galaxies. The HaloSat mission is led by the University of Iowa Principal Investigator (PI), Philip Kaaret. The team from the University of Iowa in collaboration with NASA/GSFC, and Johns Hopkins University will be developing the science instrument for the HaloSat mission.

Located in Boulder, Colorado, the company is developing a new environmental test facility, increasing manufacturing output, and creating a satellite ground-station for single to constellation-sized missions. Working with a grant from the state of Colorado, BCT has been increasing its spacecraft production and test capabilities. BCT is now providing end-to-end mission solutions for their customers.


• Avionics system is XACT (fleXible ADCS Cubesat Technology) from Blue Canyon Technologies.

• Provides high performance pointing system in a 0.5 U package including star trackers, reaction wheels, inertial measurement unit, magnetometers, torque rods.


Figure 2: The XACT unit uses high performance components that can be used for a wide range of missions (image credit: BCT)

Spacecraft pointing accuracy

±0.003º (1σ) for 2 axes; ±0.007º (1σ) for 3rd axis


0.91 kg


10 x 10 x 5 cm (0.5U)

Reaction wheel voltage

12 V

Data interface

RS-.422, RS-485 &SPI

Slew rate

≥10º/s (4 kg, 3U CubeSat)

Table 2: Parameters of XACT


Figure 3: Illustration of the 6U CubeSat (image credit: University of Iowa)


Figure 4: Illustrationof the 6U CubeSat configuration (image credit: BCT)


Figure 5: Artist's rendition of the deployed nanosatellite (image credit: BCT)


Launch: The HaloSat CubeSat was launched on 21 May 2018 (08:44 UTC) on the Cygnus CRS-9E flight of Orbital ATK (OA-9E), ELaNa-23 flight of NASA to the ISS. The launch vehicle was Antares 230 and the launch site was MARS (Mid-Atlantic Regional Spaceport) LP-0A, Wallops Island, VA, USA. 7)

Orbit: Near circular orbit, altitude of ~400 km, inclination = 51.6º.


Figure 6: The Orbital ATK Antares rocket, with the Cygnus spacecraft onboard, launched from Pad-0A, Monday, May 21, 2018 at NASA's Wallops Flight Facility in Virginia. Orbital ATK's ninth contracted cargo resupply mission with NASA to the International Space Station will deliver approximately 3352 kg of science and research, crew supplies and vehicle hardware to the orbital laboratory and its crew (image credit:NASA/Aubrey Gemignani)

The ELaNa 23 (Education Launch of Nanosatellites 23) initiative payloads of NASA on OA-9E are: 8)

• HaloSat (Soft X-ray Surveyor), a 6U CubeSat of the University of Iowa (12 kg), Iowa City, Iowa.

• TEMPEST-D1 (Temporal Experiment for Storms and Tropical Systems Technology - Demonstration 1) , a 6U CubeSat of CSU (Colorado State University), Fort Collins, CO.

• EQUISat, a 1U CubeSat of Brown University, Providence, R.I.

• MemSat, a 1U CubeSat of Rowan University, Glassboro, N.J.

• CaNOP (Canopy Near-IR Observing Project), a 3U CubeSat of Carthage College, Kenosha, WIS, USA.

• RadSat, (Radiation-tolerant SmallSat Computer System), a 3U CubeSat of MSU (Montana State University), Bozeman, Montana.

• RaInCube (Radar In a CubeSat), a 6U CubeSat of NASA/JPL (Jet Propulsion Laboratory), Pasadena, CA.

• SORTIE (Scintillation Observations and Response of the Ionosphere to Electrodynamics), a 6U CubeSat of ASTRA (Atmospheric & Space Technology Research Associates), Boulder, CO.

• CubeRTT (CubeSat Radiometer Radio Frequency Interference Technology) Validation Mission , a 6U CubeSat of OSU (Ohio State University), Columbus, Ohio.

• AeroCube-12A and -12B, a pair of 3U CubeSats of the Aerospace Corporation, El Segundo , CA, to demonstrate a the technological capability of new star-tracker imaging, a variety of nanotechnology payloads, advanced solar cells, and an electric propulsion system on on one of the two satellites (AC12-B).

• EnduroSat One, a 1U CubeSat of Bulgaria, developed by Space Challenges program and EnduroSat collaborating with the Bulgarian Federation of Radio Amateurs (BFRA) for the first Bulgarian Amateur Radio CubeSat mission.

• Lemur-2, four 3U CubeSats (4.6 kg each) of Spire Global Inc., San Francisco,CA.

On 16 July 2018, the Cygnus spacecraft of Orbital ATK (OA-9E) flight was raised to over 480 km after departing the International Space Station before further CubeSats were released. NanoRacks deployed the following satellites: 9)

- Lemur-2 (Four 3U CubeSats) of Spire Global, Inc.

- AeroCube 12A and AeroCube-12B of The Aerospace Corporation.


Figure 7: The University of Iowa HaloSat team attended the satellite's launch at NASA's Wallops Flight Facility. From left to right: Daniel LaRocca, Anna Zajczjk, Philip Kaaret, William Fuelberth, Hannah Gulick and Emily Silich. Kay Hire (center) holds the University of Iowa's tiki totem statue (image credit: Alexis Durow, Ref. 11)



Mission status

• August 18, 2018: The HaloSat science instrument was turned on for the first time on-orbit on August 18. All three detectors are working great! We turned on during a pass while the instrument was pointed at the Earth's atmosphere and the spectrum recorded by HaloSat (Figure 8) shows X-rays from Nitrogen and Oxygen in the atmosphere. 10)


Figure 8: First light image of the X-ray surveyor of Earth's atmosphere from HaloSat (image credit: University of Iowa)

• July 18,2018: HaloSat will help scientists search for the universe's missing matter by studying X-rays from hot gas surrounding our Milky Way galaxy. 11)

- CMB (Cosmic Microwave Background) is the oldest light in the universe, radiation from when it was 400,000 years old. Calculations based on CMB observations indicate the universe contains: 5 percent normal matter protons, neutrons and other subatomic particles; 25 percent dark matter, a substance that remains unknown; and 70 percent dark energy, a negative pressure accelerating the expansion of the universe.

- As the universe expanded and cooled, normal matter coalesced into gas, dust, planets, stars and galaxies. But when astronomers tally the estimated masses of these objects, they account for only about half of what cosmologists say should be present.

- "We should have all the matter today that we had back when the universe was 400,000 years old," said Philip Kaaret, HaloSat's principal investigator at the University of Iowa (UI), which leads the mission. "Where did it go? The answer to that question can help us learn how we got from the CMB's uniform state to the large-scale structures we see today."

- Researchers think the missing matter may be in hot gas located either in the space between galaxies or in galactic halos, extended components surrounding individual galaxies.

- HaloSat will study gas in the Milky Way's halo that runs about 2 million degrees Celsius. At such high temperatures, oxygen sheds most of its eight electrons and produces the X-rays HaloSat will measure.

- Other X-ray telescopes, like NASA's NICER (Neutron star Interior Composition ExploreR) and the Chandra X-ray Observatory, study individual sources by looking at small patches of the sky. HaloSat will look at the whole sky, 100 square degrees at a time, which will help determine if the diffuse galactic halo is shaped more like a fried egg or a sphere.

- "If you think of the galactic halo in the fried egg model, it will have a different distribution of brightness when you look straight up out of it from Earth than when you look at wider angles," said Keith Jahoda, a HaloSat co-investigator and astrophysicist at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "If it's in some quasi-spherical shape, compared to the dimensions of the galaxy, then you expect it to be more nearly the same brightness in all directions."

- The halo's shape will determine its mass, which will help scientists understand if the universe's missing matter is in galactic halos or elsewhere.

- HaloSat will be the first astrophysics mission that minimizes the effects of X-rays produced by solar wind charge exchange. This emission occurs when the solar wind, an outflow of highly charged particles from the Sun, interacts with uncharged atoms like those in Earth's atmosphere. The solar wind particles grab electrons from the uncharged atoms and emit X-rays. These emissions exhibit a spectrum similar to what scientists expect to see from the galactic halo.

- "Every observation we make has this solar wind emission in it to some degree, but it varies with time and solar wind conditions," said Kip Kuntz, a HaloSat co-investigator at Johns Hopkins University in Baltimore. "The variations are so hard to calculate that many people just mention it and then ignore it in their observations."

- In order to minimize these solar wind X-rays, HaloSat will collect most of its data over 45 minutes on the nighttime half of its 90-minute orbit around Earth. On the daytime side, the satellite will recharge using its solar panels and transmit data to NASA's Wallops Flight Facility in Virginia, which relays the data to the mission's operations control center at Blue Canyon Technologies in Boulder, Colorado.

• July 13, 2018: NanoRacks successfully completed the 14th CubeSat Deployment mission from the Company's commercially developed platform on the International Space Station. Having released nine CubeSats into low-Earth orbit, this mission marks NanoRacks' 185th CubeSat released from the Space Station, and 217th small satellite deployed by NanoRacks overall. 12)

- The CubeSats deployed were launched to the Space Station on the ninth contracted resupply mission for Orbital ATK (now Northrop Grumman Innovation Systems) from Wallops Island, Virginia in May 2018.

- NanoRacks offered an affordable launch opportunity, payload manifesting, full safety reviews with NASA, and managed on-orbit operations in order to provide an end-to-end solution that met all customer needs.

- The satellites deployed were: CubeRRT, EQUiSat, HaloSat, MemSat, RadSat-g, RainCube, TEMPEST-D, EnduroSat One, Radix (the last two entries are commercial CubeSats).

- The CubeSats mounted externally to the Cygnus spacecraft from the May 2018 launch are scheduled to be deployed on Sunday, July 15th, pending nominal operations.

Figure 9: HaloSat, a new CubeSat mission to study the halo of hot gas surrounding the Milky Way, was released from the International Space Station over Australia on July 13, 2018 (image credit: NanoRacks/NASA)

• On May 24, 2018, the Cygnus spacecraft successfully docked to the ISS. HaloSat is scheduled to be deployed from the ISS in late July to mid-August 2018 and start collecting science data one month later.



Sensor complement: X-ray Detector

In order to achieve the scientific goals of the mission, a science instrument that meets the following requirements had to be built (Ref. 5):

• be equipped with X-ray detector(s) sensitive in an energy band of 0.3 – 2 keV with an energy resolution (defined as full width at half maximum) of ≤100 eV near 600 eV;

• achieve a statistical accuracy of ±0.5 LU (LU = photons/cm2/s/ster) for a field with an oxygen line strength of 5 LU by:

- using detectors with sufficient effective area;

- operating for 6-12 months;

- viewing large part of the sky at a time (have field of view with a diameter of 10°);

• be able to observe the whole sky.

X-ray Detector: The FAST SDDs (Silicon Drift Detectors) from Amptek Inc. are used by the HaloSat instrument to detect X-rays. The radiation sensing element with its multilayer collimator and 2-stage thermoelectric cooler is encapsulated in a TO-8 package. The detector has an active area of 17 mm2. The entrance window is a C-Series C1 window made of Si3N4 covered with a thin layer of aluminum. The C1 window has good transmission properties around 600 eV (transmission is around 40% for 600 eV X-rays) and provides sensitivity down to 0.3 keV.


Figure 10: X-ray detector assembly. SDD detector, passive shield and baseplate are indicated with yellow, semi-transparent gray and blue color, respectively (image credit: HaloSat Team)

Passive Shielding: In order to minimize the background from events resulting from cosmic ray interactions and the diffuse X-ray background, the SDD detector is surrounded on 5 sides by a passive shield made of copper-tungsten alloy electroplated with a thin layer of gold (Figure 10). The sixth side of the shield has a circular cutout to provide an unobstructed path for the X-rays from the source to reach the detector (green element in Figure 10).


Figure 11: Sensor assembly (image credit: University of Iowa)

Signal Processing Electronics: An X-ray impinging on the detector chip is converted into an electron cloud with a charge that is proportional to the energy of that X-ray. The liberated charge is then drifted down a field gradient applied between the drift rings towards centrally located anode. The charge that accumulates at the anode is converted to a voltage signal by the FET preamplifier. The signal then goes through a preamplifier and a shaping amplifier circuit, followed by lower and upper level discriminators and a resettable peak hold circuit. In the next step, the signal from the detector is digitized and sent to the spacecraft bus. The silicon drift detector, due to its method of operation, has no imaging capabilities, however, its main advantage is low noise and thus good energy resolution.

Use of X-ray detectors from Amptek Inc. with an active area of 25 mm2 behind a Si3N4 window.

• The SDDs (Silicon Drift Detectors) are inside a sealed can and cooled by a TEC (Thermoelectric Cooler)

• The power for cooling is a large fraction of the power budget

• Same detectors as used for NICER

• Lab testing with Ti-L shows ΔE ~80 eV FWHM at 451 eV.


Figure 12: Illustration of the Amptek SDD with TEC in TO-8 can (0.55" diameter), image credit: University of Iowa

• The SDDs view the sky through a 13.3 mm diameter hole that is 135 mm away (9.2º - 13.4º)

• Aspect control ±1.0º << FOV

• To veto charged particle background, SDD is enclosed in a scintillator readout with APDs (Avalanche Photo Diodes)

• Three identical detectors.


Figure 13: Functional diagram: independent electronics for each detector assembly (image credit: University of Iowa)


Figure 14: Mechanical design of the X-ray detector (image credit: University of Iowa)


Observing strategy: Once the spacecraft crosses the dusk terminator, the science payload will be turned on and pointed towards selected target. The science payload will be switched off right before the spacecraft crosses the dawn terminator. Per each orbit, during its night-side, two science targets will be observed with approximately 1300 seconds of exposure time devoted to each target. The selected pair of the targets will be observed for ten consecutive orbits after which a new pair of targets will be selected. There are 330 HaloSat targets that are evenly spread across the sky. The targets cover 98.5% of the sky. A minimum of 8000 detector-seconds will be accumulated for each target.


Figure 15: Operations of the X-ray detector (image credit: University of Iowa)

Legend to Figure 15:

• Observations on the night side, two ~1000 s exposures per orbit

• Accumulate 10,000 detector s for each of ~ 4000 targets

• Scheduled to minimize helio/magnetospheric background.

Mission operations: The CADET radio onboard the HaloSat spacecraft will be used to downlink telemetry and receive commands. The NASA Wallops Flight Facility ground station will be used to communicate with the spacecraft. Blue Canyon Technologies will run the Mission Operation Center, while the Science Operation Center will be run at the University of Iowa.

Archiving and distribution of data: All the telemetry (including X-ray event data, housekeeping, spacecraft pointing and attitude) will be captured and converted to FITS (Flexible Image Transport System) format. The data will then be archived at the HEASARC (High Energy Astrophysics Science Archive Research Center) and made publicly available within 5 months from mission completion. In addition to the telemetry data, calibration files and software required for analysis of the instrument science data will also be archived at the HEASARC.

1) P. Kaaret, K. Jahoda, B. Dingwall, "HaloSat – A CubeSat to Study the Hot Galactic Halo," URL:

2) Philip Kaaret, "HaloSat Overview," The University of Iowa, August 17, 2016, URL:

3) Ikuyuki Mitsuishi, Masashi Ishihara, Kazuki Sugimoto, Shinya Nakano, Keisuke Tamura, Kikuko Miyata, Yuzuru Tawara, Koji Matsushita, Kazushi Tachibana, Philip Kaaret, Donald Kirchner, William Robison, Anna Zajczyk, Daniel LaRocca, William Fuelberth, Ross McCurdy, Keith White, Keith Jahoda, Thomas Johnson, Luis Santos, Michael Matthews, K. D. Kuntz, "HaloSat - Soft X-ray Surveyor," Proceedings of the 68th IAC (International Astronautical Congress), Adelaide, Australia, 25-29 Sept. 2017, paper: IAC-17-B4.2.1

4) David B. Henley, Robin L. Shelton, "An XMM-Newton survey of the soft-x-ray background. III The galactic halo-X-ray emission," The Astrophysical Journal, Volume 773, Number 2, 20 August 2013,, URL:

5) A. Zajczyk, P. Kaaret, D. L. Kirchner, D. LaRocca, W. T. Robison, W. Fuelberth, H. C. Gulick, J. Haworth, R. McCurdy, D. Miles, R. Wearmouth, K. White, K. Jahoda, T. E. Johnson, M. Matthews, L. H. Santos, S. L. Snowden, K. D. Kuntz, S. Schneider, C. Esser, T. Golden, K. Hansen, K. Hanslik, D. Koutroumpa, "HaloSat: a search for missing baryons with a CubeSat," Proceedings of the 32nd Annual AIAA/USU Conference on Small Satellites, Logan UT, USA, Aug. 4-9, 2018, paper: SSC18-WKIX-01, URL:

6) "Blue Canyon Technologies Building New HaloSat XB6 Spacecraft," BCT, 7 November 2016, URL:

7) "NASA Sends New Research on Orbital ATK Mission to Space Station," NASA/JPLRelease 18-037, 21 May 2018, URL:

8) "Upcoming ELaNa CubeSat Launches," NASA CubeSat Launch Initiative, URL:

9) "NanoRacks Completes Fifth External Cygnus Deployment, Six More CubeSats in Orbit," NanoRachs, 16, july 2018, URL:

10) "HaloSat first light," University of Iowa, 18 August 2018, URL:

11) Jeanette Kazmierczak, Rob Garner, "NASA's New Mini Satellite Will Study Milky Way's Halo," NASA, 18 July 2018, URL:

12) "NanoRacks Completes 14th CubeSat Deployment Mission from International Space Station," NanoRacks, 13 July 2018, URL:

The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (

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