Section 3.3 - Resources: Exploration Methods
The energy required to move materials in space depends in large degree on the gravitational potentials that must be traversed. The deep gravity well of the Earth requires expending a large amount of energy to climb, as evidenced by conventional rockets. Therefore a primary way to reduce the difficulty and cost of space projects is to use local resources instead of bringing everything from Earth. To intelligently plan to use local resources, you first need to know what they are. The task of finding out what's there is called Resource Exploration both on Earth and in space. This section will discuss the methods of exploration, and a current inventory of known resources. The following sections will discuss uses for the resources and methods to implement them.
History and Process
All methods of finding resources involve first sensing the characteristics of a location, recording the measurements, and then interpreting the data to determine what it can be used for. Historically the sensing devices were the human organs of perception (eyes, ears, skin, taste, and smell). Paper and pen, and sometimes collected samples were used to record information, and then maps and written accounts made by humans interpreted the data. More recently, instruments with better accuracy and sensitivity than the human senses have been developed. Some instruments can detect properties that humans cannot sense at all. Integrating instruments with computers has automated the recording process, and is heading towards automation of the interpretation step. Humans are more in a supervisory role over the instruments and computers, though some amount of direct local sensing still goes on.
Scientific Exploration - Often measurements are made, such as wind speeds, for many years before a use, such as maps for wind turbine generators, were available. This goes to the distinction between scientific and commercial data. Scientific knowledge is sometimes described as good in and of itself. A less philosophical standpoint is that we do not know in advance what knowledge will prove useful. Therefore we accumulate all sorts of knowledge in the reasonable hope that some of it will. Space affects the Earth in many ways, and understanding the history of other planets helps us understand the particular one we live on better. Therefore astronomy and planetary science are expected to turn out useful in general.
Gathering data for "science" is also the first step towards more detailed exploration and local use, even if we don't yet know exactly what we will use it for. Without a definite use in mind, there is no benefit to keeping the data hidden or in duplicating effort, so science operates as a public and shared enterprise. That also provides better error detection and faster progress when more people can review and build upon past work of others.
Commercial Exploration - As uses become more local, private, and commercial, there is more of a tendency to keep data private rather than public. Individual gain and advantage now comes into play, and the relative effort of detailed exploration of one specific location is greater than general mapping or detection. So while similar equipment may be used for both public and private exploration, what gets done with the data is different. This is most evident in mining and other resource extraction industries. Some of the information gathered for commercial use will end up being released publicly and adding to the general store of knowledge. To date, almost all of the exploration of space has been done as science.
Progression by Distance
Exploration methods can be divided by the distance at which they are done, and the detail which they generate. These generally are inversely related - less distance generates more detail - because the instruments have a fixed sensor resolution. Greater distance from the interpreters of the data, as happens with unmanned planetary exploration, also has an inverse relation with transmission speed for the same reason. Devices like radio dishes to direct data back to Earth also have a fixed resolution, so greater distance makes for lower signal strength and bandwidth. So the advantage of smaller optics due to being close to the target has to be weighed against the need for more powerful transmitters and larger antennas due to being far from Earth.
The following sections discuss the various types of instruments classed into Long, Medium, and Short Range. Long range means the instrument is much closer to the storage and interpretation location than the target. Medium range means the instrument is much closer to the target than the storage/interpretation site, but not in immediate contact. Finally, short range means in immediate contact with the target location and able to interact with it.
Long Range Exploration (Astronomy)
Long Range Instruments
Prior to 1610, long range exploration was limited to the human senses, and thus little was known besides the brightness and position of objects in the sky. The development of the telescope for astronomy by Galileo changed that situation, a change which continues to this day with larger telescopes and a broadening of the wavelength bands in which they operate. A telescope generally consists of optics which gather and focus low intensity photons (light, or electromagnetic waves), and a sensor which then detects the concentrated photons. This allows detection of what would otherwise be too dim to see. Originally the detector was the human eye, followed by photography starting in 1840, and by electronic sensors starting in 1979. Electronic sensors are up to 50 times more sensitive than film in terms of photon efficiency. A photographic plate might represent 400 megapixels of image resolution, so it is only recently that arrays of sensors also matched plate resolution as well as sensitivity.
Recording the data has progressed from hand written logs and charts of what the astronomer saw, to photographic plates which served as both the sensor and recording medium, to automated measurement of the plates to convert to digital form, to transfer of the electronic sensor data to computer storage. Interpretation of the data is now semi-automated with computer software. For example, detection of Near Earth Objects is made by comparing electronic images from two times. Anything that changes between the images is a candidate NEO, and the software filters known objects and variable stars, leaving a list of detections for a human to examine. Final interpretation of astronomical data is by humans, in the form of maps, catalogs, technical papers, and books.
Optical telescopes have grown in size, and thus sensitivity, migrated to better observing locations (high altitude and dry, or even in space themselves), and developed adaptive optics to get around the blurring of the Earth's atmosphere. For other wavelengths absorbed by the atmosphere, high altitude or space is a requirement. For long wavelengths, such as radio bands, single instruments do not provide much resolution. Aperture Synthesis, the mathematical combination of data from widely separated telescopes, is used to provide higher resolution in that case.
Wikipedia has several articles listing large instruments. Optical Reflectors are used for all the largest instruments because of the difficulties with large lenses. The active detector is usually an electronic Charge Coupled Device, or CCD, which converts light to digital signals, and often a spectrometer is used to sort the incoming light by wavelength. Radio Telescopes use a number of designs, with steerable metal dishes of various sizes being used often, and aperture synthesis by combining signals from different instruments up to the diameter of the Earth to get higher resolution. The active detectors are typically cryogenically cooled solid state amplifiers to reduce noise. Space Telescopes cover a number of wavelength ranges with a variety of detector types. The ones that operate near visible light wavelengths are designed similar to ground optical telescopes.
Gravity bends electromagnetic waves, and natural lenses formed by massive objects such as stars and galaxies have been used both to observe distant objects behind the lens, and detect and measure the lensing object. A conventional telescope is still needed near the Earth to use this effect. Such natural gravity lenses are not steerable. In the future the Sun may be used as a steerable gravity lens by placing the observing telescope at the right location opposite what you want to look at. The angle the Sun bends light requires a distance of greater than 550 AU to reach a focus. The attraction of using the Sun is the enormous diameter, which leads to high resolution. In the interim, other large telescopes of increasing power will continue to be built, and existing telescopes are upgraded with better instruments and continue to be used.
First some nomenclature, since it can be confusing: an asteroid or meteoroid are objects while still in space. A meteor is the bright trail in the atmosphere as it heats up and melts, and a meteorite is the object after it has hit the ground. Meteorites are direct long range samples of the space environment, where nature has brought them to us, instead of us having to go there and acquire samples. They provide useful comparison spectra in the laboratory, to compare to telescopic spectra from objects still in space. We can also analyze them directly to determine their composition. Meteorites are more useful for this purpose if we can record the arrival trajectory, which lets us guess where it started from. A drawback to meteorites is we have no control over where or when they fall. Sample return missions function in a similar way to meteorites as far as bringing the material to Earth for study at long range from the origin. They provide much better information on the source location and better preservation by returning the samples in a container rather than meteorite's open passage through the atmosphere followed by possibly long periods of exposure on the ground before being picked up.
- Radar and Optical Ranging
Some objects get close enough to use radar or laser pulses to determine distance, and in some cases shape. The distance measurements are very accurate because of the timing accuracy of the detectors. Shape is obtained from time distribution of the return pulse, the parts of the object further away taking slightly longer. Several pulses at different times as the object rotates can be used to determine a three dimensional shape. Reflected pulse intensity varies as the inverse 4th power of distance, due to inverse square law both ways. So this method is strongly limited by distance, but the use of very large radio dishes and powerful transmitters has overcome that limit somewhat.
Long Range Data
The following types of information can be obtained by interpreting the long range instrument data:
Position/Orbit - Generally this is the first information found for a newly discovered object. The sky is mostly dark with stars and other objects showing up as bright spots to the sensors. Stars are slow moving relative to each other on human time scales, and the slower moving ones are referred to as the Fixed Stars, even though they are not if you wait long enough. For these types of stars their position in the sky is recorded in terms of latitude and right ascension projected onto a reference sphere assumed to be infinitely far away. Overlaid on the fixed stars are objects that move in short time scales. These are stars that exhibit parallax or orbital motion, and objects within our Solar System that show orbital motion. Parallax is a small shift in apparent position of relatively close stars vs farther stars caused by the motion of the Earth around the Sun. The width of the Earth's orbit, which is 2 Astronomical Units by definition, is small relative to the distance of even the nearest stars (260,000 AU). Nonetheless this allows direct determination of distance by simple trigonometry. All farther objects require estimating distance by indirect methods. Stars in binary or higher order systems (two or more stars), or ones with relatively heavy planets, can show a small motion caused by the collection of objects orbiting their common center of mass, rather than their own centers.
For objects within the Solar System, their orbital velocity is sufficient to show movement against the fixed stars in days or hours rather than months or years for stellar motions. Significant movement in a short time is the key feature to discover a Solar System object. Otherwise there is no way to distinguish it from the myriad stars which can be seen with a large telescope. By taking at least three measurements of position at known times, it's possible to determine the general parameters of an orbit. For discovery these are often at short intervals, such as successive nights. With additional measurements, and ones spaced out in time, which gives a longer baseline on the orbital path, the orbit parameters can be determined quite accurately.
Size - The fraction of sunlight reflected from an object is called Albedo. Albedo times the area of the object gives the total amount of light reflected from it. From the orbit parameters you can calculate the physical distance. The observed brightness and an assumed albedo then allows an estimate of the size. On initial discovery the object may occupy only a single pixel on the sensor, so measuring size by the image on the detector can't be done, and estimate by brightness has to suffice.
Medium Range Exploration
Medium Range Instruments
These operate the same way as long range telescopes on Earth, but due to being much closer to the target they get higher detail (measured as pixels/km or meter). Except for a few early film recorders, spacecraft have all used electronic sensors, whose data is usually recorded in a storage device, and then transmitted back to Earth. The storage is needed because the image recording time and when and how fast the transmission can be made are often different, especially when the telescope and transmitter are both fixed to the spacecraft body. Sensors designed for different wavelength bands can be used in the same instrument, and filters can be placed in front of the sensors to select specific wavelengths of interest. Farther infrared wavelengths measure thermal emission in addition to reflection of sunlight and the rate of change of a thermal map can indicate properties. Absorption of sunlight through an atmosphere can give it's composition.
- Non-Imaging Optical Instruments
Besides direct 2D imaging via telescope optics, several other types of data can be collected:
- Radiometer - This measures infrared brightness to determine surface temperature.
- Polarimeter - Measures the polarization of incoming light by means of polarizing plates or films
- Photometer - Measures the total brightness of an object to high accuracy, without necessarily making a 2D image
- Spectrometer - Separates incoming light by wavelength, and records the brightness in each range. A large amount of information about an object can be determined from its spectrum.
- Radar Instruments
Distance or altitude can be determined by the time a radar or laser pulse takes to bounce off the target. Synthetic Aperture Radar can determine altitude from the doppler frequency change vs return time.
Measures the magnetic fields around a target.
- High Energy Detectors
These instruments detect neutrons, alpha particles, ions, and gamma rays, which can provide composition information about a target. In some cases high energy natural radiation impacts the target, and secondary particles are emitted which are characteristic to the materials. In other cases particles are directly emitted by the target and detected.
- Gravity Mapping
This is not an instrument in itself, but uses doppler and timing information from radio signals to infer motion of the spacecraft caused by the gravity field of the target. Two sensors in orbit can determine their separation very accurately by interferometry. Finer details of the gravity field can be inferred from changes in this distance.
Medium Range Data
Short Range Exploration
Short Range Instruments
Cameras and telescopes are fundamentally similar devices, consisting of optics and an electronic sensor. The primary differences are at short range, large optics are not needed as much to get sufficient resolution or brightness, and targets are close enough to sometimes need focus closer than effective infinity. Two cameras separated by distance, or a single camera used from different positions can generate stereo data, from which three dimensional shapes can be inferred. When the optics are very short range and magnifying, the device is called a Microscope. Microscopic examination can determine mineral types.
- Mössbauer spectrometer
This uses a gamma ray source, such as a radioactive isotope, and measures the recoil of atoms in the target to determine their composition.
- Alpha Particle Spectrometer
This uses a combined alpha particle, proton, and X-ray source, such as a set of radioactive isotopes. The wavelengths and energies of the returned X-rays and particles are characteristic of the composition, which allows you to analyze a sample.
- Laser Spectometer
A medium power laser is used to vaporize a target, and the resulting emission spectrum can be used to determine composition.
- Chemical/Biological Sample Chambers
Samples of the target are collected and deposited in chambers, which are then subjected to various fluids and conditions such as heating. Various ways to analyze the result include: transmission spectra made by using a light on one side of the chamber and a spectrograph on the other, or gas chromatograph or mass spectrometer to determine composition of volatile gases that are released. Testing for biological activity from the target, or compatibility with Earth biology uses similar methods.
- Kinetic Prospecting
Traversing the rough terrain of a body such as the Moon is difficult and slow. An alternative is send a lander/rover to a high point such as a mountain or crater rim. It picks up a rock of suitable mass and uses a centrifuge arm to throw it very fast at a selected target. Then it observes the impact with a telescope and spectrometer to determine the composition of the target, and possibly other instruments for additional data. Repeat as many times as needed for other targets. This allows prospecting a large area without having to drive over all of it. Kinetic methods have been used in the Deep Impact and LCROSS missions.
- Ground Sensing Instruments
A Seismometer is very sensitive to motions of the ground. Natural or artificial impacts or movements generate seismic waves within the body. The timing of the waves as they arrive at the seismometer allows determining the internal structure and properties of the body. A Heat Probe inserted into the ground can measure total heat flow to and from the interior, and the thermal conductivity. A Gravimeter accurately measures the local acceleration of gravity, from which density of the surrounding ground can be calculated.
- Subsurface Examination
In Core Sampling a hollow drill is driven into the ground to separate a cylinder of nearly undisturbed material. The core sample is removed and examined above ground. Drilling uses larger machines to reach depths of up to several kilometers. The drill debris can be flushed to the surface and examined there, or instruments can ride behind the drill head or be lowered afterwards.