X-ray observations in near-target missions in the past have been challenging. Some of the challenges originate from incomplete prelaunch ground calibration of the instruments and inadequate onboard calibration system (usually due to tight assembly schedule and budget constraints). As a result, the data analysis was complex and often led to somewhat ambiguous interpretation. On the other hand, a cause of many challenges in the past planetary X-ray observation lies in the very nature of non-focusing optics of collimator instruments as described below.
REgolith X-ray Imaging Spectrometer (REXIS) onboard OSIRIS-REx, which is scheduled to launch in 2016 to collect samples from Bennu, was designed to take into account lessons learned from the previous missions (Inamdar et al. 2014). For instance, REXIS will be equipped with one of the most comprehensive onboard calibration systems in space X-ray instruments: a set of \({}^{55}\)Fe radioactive sources are strategically mounted to monitor any potential drifts or changes of spectral gain and resolution of every node of four X-ray CCDs onboard. In addition, the team plans to conduct a series of extensive ground and flight calibrations to understand the detector response and to track any variation during the observation.
REXIS utilizes coded-aperture imaging to identify ∼50 m scale surface variation of the elemental composition of Bennu. The addition of imaging capability is the first of its kind (coded-aperture imaging) in planetary X-ray observations (see below about the Bepicolumbo mission). Coded-aperture imaging is basically shadowgram imaging using a mask coded with a pattern of open and opaque elements in front of the detector. It is a novel technique to enable imaging without focusing optics, and thus allows a relatively compact, simple telescope design (REXIS fits in an envelope of \({\sim}10\,\hbox {cm}\times 10\,\hbox {cm}\times 20\,\hbox {cm}\)). However, coded-aperture imaging still operates under the basic principle of collimator instruments, facing the same challenges. Thus, its imaging sensitivity is relatively low compared to focusing instruments and it has been mainly employed to identify point sources in astrophysics in the past.
When observing large planetary bodies such as the Moon, a simple collimator-based X-ray instrument can efficiently collect the X-rays from the surface and identify spatial variations as the instrument orbits around the target. The large gravity and the spherical geometry of the target with a small eccentricity allow stable circular orbits very close to the surface (Fig. 3a). In addition, the field of view (FoV) of the instrument is fully occupied by the target, so that no external background can contaminate the data.
In the case of relatively small planetary bodies such as asteroids and comet nuclei, the small gravity and the irregular shape of the target often limit stable orbital configurations near the target (Fig. 3b, c). To collect faint X-ray emission from the target, the FoV of the instrument is usually designed to fill the target, and thus the collimator-type instrument cannot efficiently identify the surface feature of small scales. At the same time, the exposure to the background sky in the FoV should be limited because the X-ray sky is not dark, but it shines with many diffuse and point X-ray sources, often brighter than the target itself. For instance, our Galactic plane glows brightly in X-ray, which is known as Galactic ridge X-ray emission from the early days of the X-ray astronomy. The sky at high Galactic latitude is also filled with X-ray emission from unresolved Active Galactic Nuclei (Fig. 4). These components contribute to bright Cosmic X-ray background (CXB), which can often dominate X-ray emission from the target in the collimator-type instruments when their FoV is exposed to the background sky. The continuum component of the CXB is about 10 ph cm\({}^{-2}\) s\({}^{-1}\) sr\({}^{-1}\) keV\({}^{-1}\) at 1 keV, which is about 100–1000 times brighter than the example in Fig. 1.
X-ray instruments in space also experience additional internal X-ray background which occurs due to interactions between cosmic-ray particles and the instruments. The internal background can contain XRF lines originated from the instrument, which can complicate the elemental identification of the target. The sensitivity of a typical collimator instrument is proportional to a square root of the detector area (instead of being linearly proportional to the detector area) because the internal background increases as the detector area increases. On the other hand, focusing telescopes allow a small focal plane with a large effective area, so that the internal background can be 10- to 100-fold smaller than the collimator instruments of similar collecting area.
Due to high CXB (and the large internal background), observations with collimator instruments are susceptible to changes in observing configurations (e.g. changes in pointing directions or observing distances), which can severely narrow the observing windows. Focusing telescopes, on the other hand, can start meaningful observations much farther out (e.g. during early phases of approach and debris scouting) by simply resolving and removing most of both external and internal backgrounds.
Figure 5 compares relative minimum exposures for XRF detection from an asteroid by well-calibrated XRS of NEAR-Shoemaker and Hayabusa with an example MiXO telescope (MiXO-70B, see “Example MiXO configurations and performance” section) as a function of standoff distance or orbit radius (R
\({}_{\rm O}\)) normalized to the target size (radius, R
\({}_{\rm T}\)). It also shows a scaled example orbital/approach profile from OSIRIS-REx (the orange histogram). In situ or near-target experiments studying small asteroids (R
\({}_{\rm O}\)
\(\lesssim 100\,\hbox {km}\)) often cannot stay in a stable orbit at close proximity (i.e. R
\({}_{\rm O}\)/R
\({}_{\rm T} >> 1\)) due to low gravity, debris, or an irregular asteroid shape. In addition, the multiple stages of approach to the target make the overall orbital profiles to spread over a wide range of distances to the target.
Imaging by focusing X-ray optics enables a more forgiving orbit configuration for observation, and focusing telescopes can continue to accumulate useful data throughout the mission (R
\({}_{\rm T}\)/R
\({}_{\rm O}\)
\(\lesssim\) 300 for global measurements, R
\({}_{\rm T}\)/R
\({}_{\rm O}\)
\(\lesssim\) 100 for mapping). As shown in Fig. 5, the MiXO telescope would have completed the same measurements as NEAR-Shoemaker or Hayabusa even before they reached their observing distance. A longer observing period enabled by MiXO also enhances a chance of success and new discoveries, since detection of hard X-rays from heavy elements often relies on relatively rare strong solar flares. In addition to the global measurement, with MiXO, the accumulated data over \({\sim}\)2 months (at 1 AU) during the quiet sun state alone will be sufficient for detection of \({\sim }\)1 % spatial variation for major elements (e.g. O-K, Fe-L) over the entire surface.
The advantages of focusing optics extend to observations of large planetary bodies as well. For instance, at Mercury, the high thermal loads force spacecraft to be at highly elliptical orbits, limiting the observing duty cycle of collimator instruments. With focusing optics less susceptible to changes in observing conditions, the observing duty cycle can be maintained high. In the case of the Jovian system, high radiation environment (1 krad per an orbit, depending on shielding, etc.) severely limits the detector and shield size, and thus, the focusing optics is required to achieve large light-collection power with a small detector.