Aerostat-Based Observation of Space Objects in the Stratosphere

Abstract

For the requirements of the multi-means observation and emergency monitoring of space objects, including space debris and near-earth asteroids, this paper analyzes the astronomical observation conditions in the stratosphere, which is the region of the earth’s atmosphere between 18 km and 55 km of altitude. The results reveal that near space has a significantly superior sky background and observation environment than ground-based observation, with the values of transmittance in the visible band and near-infrared bands more than 0.91 and 0.988, respectively. The sky background radiance at 20 km is 2.5% of the ground in the visible band and near-infrared band, which is practical for daytime observation, and there is an advantage in the availability of observable hours without the influence of aerosols and turbulence, etc. Based on near-space aerostats, such as a high-altitude balloon, a new method of space object floating observation has been proposed, including the observation facilities and scheme. The simulation shows that it has an all-weather/all-day ability while adopting multi-band observation. Applying a telescope with 9.5 mag detective ability located on the aerostat, debris with the size of about 0.36 m can be observed at a 1000 km distance and phase angle of 100°, while the near-earth asteroid with the size of about 980 km can be observed at a 5 million km distance and phase angle of 40° during the daytime. With these advantages, the aerostat-based observation would be a beneficial supplement to the ground-based observation network.

1. Introduction

Space debris has become a real threat to spacecraft in orbit, while near-earth asteroids (NEAs) pose a huge potential threat to the Earth and human society. Monitoring space objects, such as space debris and NEA, are related to the safety of human space assets and human beings themselves. At present, the main optical observation means to monitor space debris, and NEA refers to the ground observation networks and space-based observation platforms that are in development. Because of the weather phenomena, such as clouds, fog, rain, and snow, the available observation time of the ground-based optical observation station is restricted. Some threatening events of space debris or NEA need a long observational arc. In these circumstances, more ground-based telescopes located around the world are needed to track and monitor events all day. A multi-means collaborative observation can meet the monitoring requirements better. Aerostat-based monitoring with mobile deployment and an all-weather observation capability can increase the density of the observed geographic grid and the rapid response ability of space debris monitoring/early warning/re-entry with NEA emergency monitoring and other special events.

Near-space is generally considered to be an airspace term between the activity space of traditional airplanes and spacecraft, which is used by researchers and management organizations. Its spatial range refers to the airspace from a 20 km to 100 km altitude, including the atmospheric stratosphere from 18 km to 55 km in altitude, the mesosphere from 55 km to 85 km, and the partial thermosphere from 85 km to 100 km [1,2,3]. It is difficult for either airplanes or satellites to stay in the near space for a long time. With current technology means, aerostats (stratosphere airships, high-altitude balloons) and solar-powered drones can fly or float at low speeds in the stratosphere. These air vehicles can be used to carry out astronomical observations in near space. This paper discusses the space observation application using the aerostat platform with an optical telescope.

A high-altitude balloon is a near-space float platform [4] based on the principle of buoyancy and pressure control. The working height is the stratosphere above the dense atmosphere, where the electromagnetic radiation environment is close to space [5]; there are no weather phenomena such as clouds, fog, rain, and snow, which leads to better visibility and a darker sky background than ground-based astronomical observations. This platform has the characteristics of a long-time stay in near space, large load capacity, high flight altitude, and field deployment. Therefore, it can support the observation of space debris and NEA and form the ground–air–space-integrated monitoring network.

To research the feasibility of space debris observations based on the stratosphere aerostat, a method similar to the site selection of ground-based astronomical observation was adopted, including the analysis of the observation of environmental conditions in near-space, aerostat-based observing methods, the equipment needed for space debris observation, and the simulation of observation capability.

2. Astronomical Observation Conditions in Stratosphere

2.1. Atmospheric Transmittance and Sky Back-Ground

The aerostat for performing high-altitude detection tasks generally works in the altitude range of 18 km to 35 km in the stratosphere. The work height selection is directly related to the weight of the pod composed of the detection payload and flight control equipment [6]. The air is very thin in this altitude range. For example, at a 20 km altitude, the air density is 0.0888 kg/m³ and the pressure is 55.18 hpa, which are 7.24% and 5.44% [7] of the values at sea level, respectively. A rare air environment leads to good atmospheric transmittance for astronomical observation. According to the data published on the official website [8] of the European Southern Observatory (La Paranal) at 2635 m, the atmospheric transmittance at 0°, 30° and 60° zenith distances are drawn in the wavebands of 500 nm to 1700 nm. As shown in Figure 1, good atmospheric transmittance is available at this observation altitude, but several absorption bands exist in the near-infrared wave range that causes the atmosphere to be completely opaque. The LOWTRAN software (LOWTRAN7) was used to calculate the atmospheric transmittance in the visible band (500 nm~800 nm) and near-infrared band (900 nm~1700 nm) at a 20 km altitude in the stratosphere. LOWTRAN is a low-resolution propagation model and computer code for predicting atmospheric transmittance and background radiance by inputting atmospheric profiles and other parameters [9]. It can also be used to calculate solar/lunar scattered radiance. As shown in Figure 2, the results are much better than the station La Paranal, where the values of transmittance in the visible band and near-infrared band are more than 0.91 and 0.988, respectively, even though they are at different zenith distances. Therefore, the atmosphere in the stratosphere is very close to transparency for visible and near-infrared bands.

On the other hand, the sky background for stratosphere-based observations is also better than the ground-based observation because the atmosphere scattering in the stratosphere is much weaker than in the troposphere. This advantage is significant for astronomical observation during the daytime, especially in the morning and evening. Taking 02:00:00 (UTC) in a day around the summer solstice in Beijing as an example, the sky background magnitudes in the visible band (500 nm–800 nm) and near-infrared bands (900 nm–1700 nm) were calculated using LOWTRAN. As shown in Figure 3, the sky background in both the visible band and the near-infrared bands is lighter than 4 mag/arcsec² (hereafter, we use mag for short) on the ground, which greatly limits the observation ability during the daytime. As for the stratosphere observation, the sky background is darker than 6.5 mag for the visible band and 7.5 mag for the near-infrared band, which are 2.0~2.5 times darker than ground-based observations and practical for daytime observation.

The magnitude can be calculated by a simple relation from flux as follows:

$$E_2/E_1={10}^{-0.4(m_2-m_1)}$$

(1)

Here, $E$ is the flux of the object, $m$ is the magnitude, while the subscripts stand for different objects. In the visible and infrared bands, the sky background at a 20 km altitude is about 4 mag darker than the ground, which means that the sky background radiance at 20 km is 2.5% of the ground.

Figure 3. The magnitude of the sky background in visible and near-infrared bands on the ground and in the stratosphere (20 km) at 02:00:00 (UTC) around the summer solstice with LOWTRAN parameters: the visibility is set to 23 km; the sun is located at the azimuth of 109°; and the attitude is 56°.

Figure 3. The magnitude of the sky background in visible and near-infrared bands on the ground and in the stratosphere (20 km) at 02:00:00 (UTC) around the summer solstice with LOWTRAN parameters: the visibility is set to 23 km; the sun is located at the azimuth of 109°; and the attitude is 56°.

2.2. Aerosols

Aerosols are the dominant factor of sunlight/moonlight scattering, whose influence on the sky background also needs to be considered. The distribution of aerosols in the atmosphere with height is different from the trend of atmospheric density, which decreases whilst height increases. Figure 4 gives the vertical distribution [10] of the measured aerosol quantity density and mass concentration. These data were obtained from the in situ vertical distribution measurement experiment by the Institute of Atmospheric Sciences, Chinese Academy of Sciences (CAS) in 2018 in Golmud, Qinghai province, by carrying a portable optical particle spectrometer (Portable Optical Particle Spectrometer, POPS) on high-altitude balloons. This figure shows that there is a clear peak volume of the aerosol number density from 14 km to 18 km. At 18 km and 20 km, the aerosol concentration was about 0.05 μg m⁻³ and 0.02 μg m⁻³, respectively, which are very low levels and no longer change significantly when height increases. Therefore, the influence of aerosol on astronomical observations in the stratosphere can be neglected.

2.3. Temperature and Humidity

The environmental temperature in the stratosphere airspace is in the range of −70 °C to −50 °C [11]. Above 20 km, the environmental temperature shows an upward trend, which is contrary to the troposphere. In the scientific experiment of a high-altitude balloon flight conducted by the Institute of Space Information Innovation, CAS, on 16 August 2019, in Dachaidan district, Qinghai province, the temperature sensor was placed inside and outside of the pod and worked during the whole flight. This experiment lasted for 29 h in the stratosphere. The temperature inside the pod during the daytime was in the range of 10 °C to 30 °C, and the environmental temperature outside the pod was −50 °C to −40 °C as measured. The lowest temperature inside the pod at night was −30 °C, and the lowest environmental temperature outside the pod was −70 °C. The temperature inside and outside the pod differed greatly because the application load and supporting equipment in the pod were in the working or standby state, so the heat radiation made the temperature inside the pod significantly higher than the environmental temperature of the pod surface [12].

Water vapor in the atmosphere does not affect the infrared transmittance of optical astronomical observation and also sets additional requirements for the maintenance of equipment. The site selection of ground astronomical observation stations considers the humidity of the environment. For example, the atmospheric water vapor (Precipitable Water Vapor, PWV) of the Lenghu observation station in Qinghai Province, China, is lower than 2 mm [13] for 55% of the night. A humidity profile of the atmosphere is shown in Figure 5, which was obtained by the Anhui Institute of Optics and Precision Mechanics, CAS, with a meteorological balloon on 5 August 2020 in the Dachaidan district of Qinghai Province. As can be seen from the figure, the water vapor at about 10 km showed a decreasing trend and could not be detected at about 18 km (also reaching the limit of the sensitivity of the humidity detector). It can be seen that the stratosphere atmosphere is very dry, as analyzed in Section 2.1, which brings excellent atmospheric infrared transmittance for astronomical observation in this airspace, which can carry out infrared detection of space debris and NEA.

2.4. Seeing

Seeing is the sum of three effects of the centroid motion, blur, and flicker of star images, which indicates the wavefront tilt, phase distortion, and amplitude fluctuation of light transmission, respectively, because the light refraction is affected by turbulence in the atmosphere [14]. This is an important parameter that must be measured by the astronomical observation station represents the atmospheric optical quality and directly determines the effect of astronomical observations. The atmospheric coherence diameter r₀ (Fried parameter) or the refractive index structure constant $C_N^2$ (where N is the refractive index) are commonly used for characterization [15]. By means of these instruments, such as the single star scidar (SSS), the differential Image Motion Monitor (DIMM), or the analysis of weather research and forecast (WRF) [16], the seeing of ground observation stations can be measured and calculated. Because the influence on light refraction in the stratosphere is much smaller than that in the troposphere, the measurement sensitivity of the existing instruments makes it difficult to meet the requirements. Meteorological research methods, such as meteor-logical balloons, can obtain the height profile of atmospheric pressure, temperature, wind speed, and other parameters, reverse and calculate the $C_N^2$, and then analyze the changing trend of atmospheric turbulence [14]. The basic rule is that above the altitude of 11 km, the $C_N^2$ value always decreases, which means that the intensity of turbulence decreases correspondingly, and the $C_N^2$ value above 20 km is very small [13]. This means that the effect of atmospheric turbulence on stratospheric astronomical optical observation can be ignored, and seeing does not need to be a measurement indicator.

2.5. Available Observing Time

The aerostat-based detection platform working in the stratosphere without the limit of clouds, snow, rain, and other weather can detect space debris in all weather compared with the ground-based observation station, which means it can carry out observation tasks throughout the year. On the other hand, daytime observation is available because the stratospheric airspace has a good continuous sky background in the day and night and excellent transmittance in visible and infrared bands without the effect of atmospheric turbulence. In summary, this method has the advantage of the availability of observable hours and can carry out the observation of space objects all day and in all weather.

3. Aerostat-Based Observation Installation for Space Objects

The observation installation based on the aerostat for space objects in near space can be represented by the space objects floating observation system (SOFOS) shown in Figure 6. It includes the high-altitude balloon detection system and the ground support system. The former consists of a high-altitude balloon (including a parachute for pod recovery) and a pod that integrates with a power unit, control unit, application loads, and servicing devices. The latter consists of an observation planning and data processing subsystem, a release and recovery subsystem, and a control and data transmission subsystem.

3.1. Observation Platform

The high-altitude balloon has two kinds of zero-pressure balloons and over-pressure balloons, which can be selected and customized as an observation platform according to the application requirement. It can provide a load capacity of dozens of kilograms to more than one ton for observation payloads and can carry out a long-time flight of dozens of days [17]. The test high-altitude balloon can reach a long-time flight record of more than 100 days [18]. The aerostat-based observation installation can be moved onboard and released and recovered in the field, so it has the ability of mobile deployment and rapid response.

According to the requirements of floating observation tasks, a balloon pod (also known as a payload service module) should be developed. It serves as an integrated installation platform for application payloads and support equipment, which supply the energy source and control, data storage and transmission, and measurement and flight control. The pod is connected to the high-altitude balloon through a special cable and separated from the balloon by a remote-control command after the observation task and parachuted for recovery. The pod can be customized according to the application task and has a small limit on the weight and size of the observation payloads. It has significant advantages over both satellite and aircraft platforms. In the Strategic Priority Research Program of the Chinese Academy of Sciences, “Scientific Experiment System in Near Space”, special task pods for scientific experiments were designed and formed [19,20], including the pod for optical detection. From 2018 to 2022, this program carried out more than twenty scientific experiments in Inner Mongolia, Qinghai, Gansu, Xinjiang, and other places in China, in which all pods completed the tasks and were recovered successfully. These experiments fully verify the environmental adaptability of the pod in the stratospheric airspace. Figure 7 is an example of an atmospheric detection pod.

Compared with the ground observation telescope, the space object floating observation system is a mobile observation platform. The attitude control of the telescope fixed in the pod finds it difficult to detect dark space objects with a long integration time. Generally, at least two levels of control are required to achieve the tracking accuracy required for detection. The primary orientation control is provided through the pod, and the turntable provides the secondary orientation and pitch control, enabling the telescope to point to the object sky area and achieve stable object’s observation.

3.2. Observation Telescope

For aerostat-based observation and optical telescope, the observation payload in the SOFOS needs to adapt to the conditions of the floating platform. According to the stratospheric environmental conditions described in the previous section and the process of aerostat distribution and recovery, the telescope must meet the basic environmental conditions as follows:

(1)

Low pressure: 55 hpa @ 20,000 m.

(2)

Temperature range: −70 °C to +40 °C.

(3)

Impact overload: 5 g.

These conditions form special technical requirements for the design of the observation telescope and require environmental simulation tests to be carried out on the ground before the flight experiment.

According to the advantage of the astronomical observation conditions in the stratosphere, the observation telescope can select a multi-band detection mode with visible and infrared wavelengths to gain richer object information than ground observation and has a stronger detection capability than the ground telescope with the same aperture. As for space debris and oncoming NEA, small or medium-sized aperture telescopes, which are fixed with a stabilized turntable, can be used to observe such objects so that the weight and volume of the payloads can be reduced. It is convenient to design and install a low-cost SOFOS; therefore, a rapid response ability of emergency observation for space debris and oncoming NEA can be supplied.

If a space-based telescope has a sufficiently large field of view, it can observe the debris objects in the geosynchronous orbit and archive independent orbital determination as a single platform [21]. The same analysis can be used for the SOFOS. Utilizing its long flight ability in the stratosphere, SOFOS can be equipped with a large field optical telescope to provide long arc and high revisit observation for debris objects. It can improve the observation efficiency and accuracy for the important tracking and monitoring tasks of debris threat alarms, such as dangerous orbital rendezvous, disintegration, and big space debris reentry.

4. Observation Ability Analysis

Referring to the analysis for space-based optical observations [21], the observation ability of the telescope with the same aperture located in the stratosphere is 2~3 mag higher than that of the ground-based telescope. Taking a telescope with a field of view (FOV) of 20° × 20° and an aperture of 100 mm as an example, four telescopes located in the aerostat pod can form an optical array with a huge observation field of about 1500 square degrees. In this way, rapid wide-area surveys of space object observations can be carried out in near space. If equipped with a stabilized steerable mount, this optical array can also track and monitor threatening events, such as dangerous close approaches and large space debris reentry.

4.1. Observation Ability in the Visible Band

To evaluate the observation ability of the space debris of the SOFOS, it can be judged according to the signal-noise ratio (SNR). It is generally believed that the object can be detected when the SNR of the object is greater than three. The SNR of five is, therefore, set as the object-detectable condition for observation ability analysis. Using the telescope with a 20° × 20° large FOV as an example, whose focal length is 178 mm, optical efficiency is 70%, the CMOS detector (Gpixel Changchun Optotech Inc., Changchun, Jilin, China) has a 10 μm pixel size with a resolution of 6144 × 6144, peak quantum efficiency of 75% and readout noise of 4.2 e; meanwhile, considering that 80% energy of the starlight is concentrated in four pixels [22], assuming the sky background is 21.6 mag, it results in the decent value of a clean observation night without any light pollution. The calculation results for the observation ability of this telescope are shown in Figure 8. When the exposure time of the telescope is set to 1, 5, and 30 s, the observation ability reaches 14.5 mag, 15.6 mag, and 16.6 mag, respectively.

For the NEA observation, a telescope with a 500 mm aperture is used to analyze the observation ability of the SOFOS. Its focal length is 800 mm, FOV is 1° × 1°, and optical efficiency is 70%, while the CMOS detector has a pixel size of 11 μm with a resolution of 4096 × 4096, whose peak quantum efficiency is 70% with a readout noise of 1.6 e. Considering 80% energy of the starlight is concentrated on nine pixels, the results (see Figure 8) show that the observation ability of this telescope reaches 17.0 mag, 18.0 mag, and 18.9 mag when the exposure time is 1 s, 5 s, and 30 s, respectively.

Figure 8. The observation ability in the visible band of the telescopes with different apertures.

Figure 8. The observation ability in the visible band of the telescopes with different apertures.

4.2. Observation Ability in Infrared Band

4.2.1. Observation Ability Analysis

As described above, the intensity of the daytime sky background of the infrared band in the stratosphere is one mag darker than that of the visible band. It is favorable for the daytime observation of space debris and NEAs using SOFOS. When space debris is around the sun or NEA, coming from the orientation of the sun, the phase angle of the object is too large to detect in the visible band, but there is a chance to detect it in the infrared band: the NEA could be regarded as a stable thermal object and could be detected in the infrared band by means of the SOFOS in a long observation period during the daytime. However, the infrared characteristics of space debris change with the phase angle, so the detection opportunity during the daytime should be selected carefully.

Taking the same telescope with the 500 mm aperture mentioned above, a near-infrared detector of 900 nm~1700 nm bands is used to observe space objects in the stratosphere. The detector has a pixel size of 20 μm with a resolution of 640 × 512 and an average quantum efficiency of 80%. A short exposure time should be set to avoid saturation in the daytime due to the sky background and the well capacity. Setting the daytime sky background of 7.5 mag and the SNR of five as calculating conditions, the infrared detection ability is about 7.0 mag at the exposure time of 20 ms, as shown in Figure 9. If the optical design of the lens is changed to increase the focal length and reduce the FOV from 1° to 0.5°, then the detection ability can reach 7.7 mag at the same exposure time.

It should be pointed out that this is the detection ability of the single frame with a short exposure time. In order to increase the observation ability, multiple frames can be obtained in a short time continuously to obtain a higher SNR as follows:

$$SNR=\frac{M{S}_{\mathrm{obj}}}{\sqrt{M{S}_{\mathrm{obj}}+M{S}_{\mathrm{sky}}}}=\sqrt{M}\frac{S_{\mathrm{obj}}}{\sqrt{S_{\mathrm{obj}}+S_{\mathrm{sky}}}}$$

(2)

where M is the number of frames obtained by the telescope, $S_{obj}$ is the object flux, and $S_{sky}$ is the sky background flux.

This means that, with M frames co-added, the SNR is $\sqrt{M}$ times than that of the single frame, which corresponds to an enhancement of 2.5 lgM mag for the observation ability. Generally, the image is within a second, and no further shifting/rotating procedure is needed. As an example, when 10 frames are taken within a second, the ultimate observation ability can be enhanced by 2.5 mag to about 10.2 mag in the infrared band after the superposition of these images.

4.2.2. Observable Objects Analysis

Based on the detection ability in the infrared band analyzed above, we discuss what kind of space object can be detected during the daytime, thus demonstrating the application of the telescope located in the SOFOS. Under the diffuse sphere assumption, setting the object’s albedo to 0.1, the size of the observable object is calculated with a detection ability of 7.5 mag and 9.5 mag, respectively. Note that the detectable size is related to the distance and phase angle of the object.

As shown in Figure 10, the debris with a size of 0.2 m can be observed at a 1000 km distance and a phase angle of 40°. For the debris in geosynchronous orbit, an object of about 7.0 m can be observed at the phase angle of 40°.

For daytime observations for NEA, Figure 11 shows the observable object sizes in km from about one earth–moon distance to a 0.1 Astronomy Unit (AU). When the distance is 380,000 km/1.5 million km/5 million km/0.1 AU (about 15 million km) at a phase angle of 40°, the 9.5 mag ability telescope can observe the NEA object of about 80 m, 290 m, 980 km, and 2.9 km, respectively.

5. Conclusions

This paper analyzes the astronomical observation conditions in the stratosphere of near space, such as the sky background and atmospheric optical characteristics. The results show that all of these conditions are superior to ground-based observations. The superiority could supply the insufficiency of the ground stations by implementing daytime observations with infrared observation. The near-space floating platform, such as the high-altitude balloon, can work for a long time in the stratosphere airspace. Carrying an aerostat-based pod, it is able to install an aerostat-based observation system and further compose an SOFOS with a ground-supporting system. The analysis of the observation ability of the SOFOS shows that this method has all-weather and multi-band observation abilities for space debris and NEA. It also has the potential of daytime observation, which is a beneficial supplement to the ground observation network.

As for the requirement of the emergency monitoring of space debris and NEA, the collaboration observation mode of aerostat-based and ground-based networks could be studied in the future. Through the development of reasonable monitoring plans and equipment deployment of the SOFOS, the reasonable geographical location of the mobile monitoring station could be realized, and the relay observation or synchronous observation could be coordinated with ground stations to improve the arc length and revisit the rate of the observation. Then, all-time monitoring can be realized.