Error detection and correction

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Error_detection_and_correction

To clean up transmission errors introduced by Earth's atmosphere (left), Goddard scientists applied Reed–Solomon error correction (right), which is commonly used in CDs and DVDs. Typical errors include missing pixels (white) and false signals (black). The white stripe indicates a brief period when transmission was interrupted.

In information theory and coding theory with applications in computer science and telecommunication, error detection and correction (EDAC) or error control are techniques that enable reliable delivery of digital data over unreliable communication channels. Many communication channels are subject to channel noise, and thus errors may be introduced during transmission from the source to a receiver. Error detection techniques allow detecting such errors, while error correction enables reconstruction of the original data in many cases.

Definitions

Error detection is the detection of errors caused by noise or other impairments during transmission from the transmitter to the receiver.

Error correction is the detection of errors and reconstruction of the original, error-free data.

History

In classical antiquity, copyists of the Hebrew Bible were paid for their work according to the number of stichs (lines of verse). As the prose books of the Bible were hardly ever written in stichs, the copyists, in order to estimate the amount of work, had to count the letters. This also helped ensure accuracy in the transmission of the text with the production of subsequent copies. Between the 7th and 10th centuries CE a group of Jewish scribes formalized and expanded this to create the Numerical Masorah to ensure accurate reproduction of the sacred text. It included counts of the number of words in a line, section, book and groups of books, noting the middle stich of a book, word use statistics, and commentary. Standards became such that a deviation in even a single letter in a Torah scroll was considered unacceptable. The effectiveness of their error correction method was verified by the accuracy of copying through the centuries demonstrated by discovery of the Dead Sea Scrolls in 1947–1956, dating from c. 150 BCE-75 CE.

The modern development of error correction codes is credited to Richard Hamming in 1947. A description of Hamming's code appeared in Claude Shannon's A Mathematical Theory of Communication and was quickly generalized by Marcel J. E. Golay.

Principles

All error-detection and correction schemes add some redundancy (i.e., some extra data) to a message, which receivers can use to check consistency of the delivered message and to recover data that has been determined to be corrupted. Error detection and correction schemes can be either systematic or non-systematic. In a systematic scheme, the transmitter sends the original (error-free) data and attaches a fixed number of check bits (or parity data), which are derived from the data bits by some encoding algorithm. If error detection is required, a receiver can simply apply the same algorithm to the received data bits and compare its output with the received check bits; if the values do not match, an error has occurred at some point during the transmission. If error correction is required, a receiver can apply the decoding algorithm to the received data bits and the received check bits to recover the original error-free data. In a system that uses a non-systematic code, the original message is transformed into an encoded message carrying the same information and that has at least as many bits as the original message.

Good error control performance requires the scheme to be selected based on the characteristics of the communication channel. Common channel models include memoryless models where errors occur randomly and with a certain probability, and dynamic models where errors occur primarily in bursts. Consequently, error-detecting and correcting codes can be generally distinguished between random-error-detecting/correcting and burst-error-detecting/correcting. Some codes can also be suitable for a mixture of random errors and burst errors.

If the channel characteristics cannot be determined, or are highly variable, an error-detection scheme may be combined with a system for retransmissions of erroneous data. This is known as automatic repeat request (ARQ), and is most notably used in the Internet. An alternate approach for error control is hybrid automatic repeat request (HARQ), which is a combination of ARQ and error-correction coding.

Types of error correction

There are three major types of error correction:

Automatic repeat request

Automatic repeat request (ARQ) is an error control method for data transmission that makes use of error-detection codes, acknowledgment and/or negative acknowledgment messages, and timeouts to achieve reliable data transmission. An acknowledgment is a message sent by the receiver to indicate that it has correctly received a data frame.

Usually, when the transmitter does not receive the acknowledgment before the timeout occurs (i.e., within a reasonable amount of time after sending the data frame), it retransmits the frame until it is either correctly received or the error persists beyond a predetermined number of retransmissions.

Three types of ARQ protocols are Stop-and-wait ARQ, Go-Back-N ARQ, and Selective Repeat ARQ.

ARQ is appropriate if the communication channel has varying or unknown capacity, such as is the case on the Internet. However, ARQ requires the availability of a back channel, results in possibly increased latency due to retransmissions, and requires the maintenance of buffers and timers for retransmissions, which in the case of network congestion can put a strain on the server and overall network capacity.

For example, ARQ is used on shortwave radio data links in the form of ARQ-E, or combined with multiplexing as ARQ-M.

Forward error correction

Forward error correction (FEC) is a process of adding redundant data such as an error-correcting code (ECC) to a message so that it can be recovered by a receiver even when a number of errors (up to the capability of the code being used) are introduced, either during the process of transmission or on storage. Since the receiver does not have to ask the sender for retransmission of the data, a backchannel is not required in forward error correction. Error-correcting codes are used in lower-layer communication such as cellular network, high-speed fiber-optic communication and Wi-Fi, as well as for reliable storage in media such as flash memory, hard disk and RAM.

Error-correcting codes are usually distinguished between convolutional codes and block codes:

• Convolutional codes are processed on a bit-by-bit basis. They are particularly suitable for implementation in hardware, and the Viterbi decoder allows optimal decoding.
• Block codes are processed on a block-by-block basis. Early examples of block codes are repetition codes, Hamming codes and multidimensional parity-check codes. They were followed by a number of efficient codes, Reed–Solomon codes being the most notable due to their current widespread use. Turbo codes and low-density parity-check codes (LDPC) are relatively new constructions that can provide almost optimal efficiency.

Shannon's theorem is an important theorem in forward error correction, and describes the maximum information rate at which reliable communication is possible over a channel that has a certain error probability or signal-to-noise ratio (SNR). This strict upper limit is expressed in terms of the channel capacity. More specifically, the theorem says that there exist codes such that with increasing encoding length the probability of error on a discrete memoryless channel can be made arbitrarily small, provided that the code rate is smaller than the channel capacity. The code rate is defined as the fraction k/n of k source symbols and n encoded symbols.

The actual maximum code rate allowed depends on the error-correcting code used, and may be lower. This is because Shannon's proof was only of existential nature, and did not show how to construct codes that are both optimal and have efficient encoding and decoding algorithms.

Hybrid schemes

Main article: Hybrid ARQ

Hybrid ARQ is a combination of ARQ and forward error correction. There are two basic approaches:

• Messages are always transmitted with FEC parity data (and error-detection redundancy). A receiver decodes a message using the parity information and requests retransmission using ARQ only if the parity data was not sufficient for successful decoding (identified through a failed integrity check).
• Messages are transmitted without parity data (only with error-detection information). If a receiver detects an error, it requests FEC information from the transmitter using ARQ and uses it to reconstruct the original message.

The latter approach is particularly attractive on an erasure channel when using a rateless erasure code.

Types of error detection

Error detection is most commonly realized using a suitable hash function (or specifically, a checksum, cyclic redundancy check or other algorithm). A hash function adds a fixed-length tag to a message, which enables receivers to verify the delivered message by recomputing the tag and comparing it with the one provided.

There exists a vast variety of different hash function designs. However, some are of particularly widespread use because of either their simplicity or their suitability for detecting certain kinds of errors (e.g., the cyclic redundancy check's performance in detecting burst errors).

Minimum distance coding

A random-error-correcting code based on minimum distance coding can provide a strict guarantee on the number of detectable errors, but it may not protect against a preimage attack.

Repetition codes

A repetition code is a coding scheme that repeats the bits across a channel to achieve error-free communication. Given a stream of data to be transmitted, the data are divided into blocks of bits. Each block is transmitted some predetermined number of times. For example, to send the bit pattern 1011, the four-bit block can be repeated three times, thus producing 1011 1011 1011. If this twelve-bit pattern was received as 1010 1011 1011 – where the first block is unlike the other two – an error has occurred.

A repetition code is very inefficient and can be susceptible to problems if the error occurs in exactly the same place for each group (e.g., 1010 1010 1010 in the previous example would be detected as correct). The advantage of repetition codes is that they are extremely simple, and are in fact used in some transmissions of numbers stations.

Parity bit

Main article: Parity bit

A parity bit is a bit that is added to a group of source bits to ensure that the number of set bits (i.e., bits with value 1) in the outcome is even or odd. It is a very simple scheme that can be used to detect single or any other odd number (i.e., three, five, etc.) of errors in the output. An even number of flipped bits will make the parity bit appear correct even though the data is erroneous.

Parity bits added to each word sent are called transverse redundancy checks, while those added at the end of a stream of words are called longitudinal redundancy checks. For example, if each of a series of m-bit words has a parity bit added, showing whether there were an odd or even number of ones in that word, any word with a single error in it will be detected. It will not be known where in the word the error is, however. If, in addition, after each stream of n words a parity sum is sent, each bit of which shows whether there were an odd or even number of ones at that bit-position sent in the most recent group, the exact position of the error can be determined and the error corrected. This method is only guaranteed to be effective, however, if there are no more than 1 error in every group of n words. With more error correction bits, more errors can be detected and in some cases corrected.

There are also other bit-grouping techniques.

Checksum

Main article: Checksum

A checksum of a message is a modular arithmetic sum of message code words of a fixed word length (e.g., byte values). The sum may be negated by means of a ones'-complement operation prior to transmission to detect unintentional all-zero messages.

Checksum schemes include parity bits, check digits, and longitudinal redundancy checks. Some checksum schemes, such as the Damm algorithm, the Luhn algorithm, and the Verhoeff algorithm, are specifically designed to detect errors commonly introduced by humans in writing down or remembering identification numbers.

Cyclic redundancy check

Main article: Cyclic redundancy check

A cyclic redundancy check (CRC) is a non-secure hash function designed to detect accidental changes to digital data in computer networks. It is not suitable for detecting maliciously introduced errors. It is characterized by specification of a generator polynomial, which is used as the divisor in a polynomial long division over a finite field, taking the input data as the dividend. The remainder becomes the result.

A CRC has properties that make it well suited for detecting burst errors. CRCs are particularly easy to implement in hardware and are therefore commonly used in computer networks and storage devices such as hard disk drives.

The parity bit can be seen as a special-case 1-bit CRC.

Cryptographic hash function

Main article: Cryptographic hash function

The output of a cryptographic hash function, also known as a message digest, can provide strong assurances about data integrity, whether changes of the data are accidental (e.g., due to transmission errors) or maliciously introduced. Any modification to the data will likely be detected through a mismatching hash value. Furthermore, given some hash value, it is typically infeasible to find some input data (other than the one given) that will yield the same hash value. If an attacker can change not only the message but also the hash value, then a keyed hash or message authentication code (MAC) can be used for additional security. Without knowing the key, it is not possible for the attacker to easily or conveniently calculate the correct keyed hash value for a modified message.

Error correction code

Main article: Error correction code

Any error-correcting code can be used for error detection. A code with minimum Hamming distance, d, can detect up to d − 1 errors in a code word. Using minimum-distance-based error-correcting codes for error detection can be suitable if a strict limit on the minimum number of errors to be detected is desired.

Codes with minimum Hamming distance d = 2 are degenerate cases of error-correcting codes and can be used to detect single errors. The parity bit is an example of a single-error-detecting code.

Applications

Applications that require low latency (such as telephone conversations) cannot use automatic repeat request (ARQ); they must use forward error correction (FEC). By the time an ARQ system discovers an error and re-transmits it, the re-sent data will arrive too late to be usable.

Applications where the transmitter immediately forgets the information as soon as it is sent (such as most television cameras) cannot use ARQ; they must use FEC because when an error occurs, the original data is no longer available.

Applications that use ARQ must have a return channel; applications having no return channel cannot use ARQ.

Applications that require extremely low error rates (such as digital money transfers) must use ARQ due to the possibility of uncorrectable errors with FEC.

Reliability and inspection engineering also make use of the theory of error-correcting codes.

Internet

In a typical TCP/IP stack, error control is performed at multiple levels:

• Each Ethernet frame uses CRC-32 error detection. Frames with detected errors are discarded by the receiver hardware.
• The IPv4 header contains a checksum protecting the contents of the header. Packets with incorrect checksums are dropped within the network or at the receiver.
• The checksum was omitted from the IPv6 header in order to minimize processing costs in network routing and because current link layer technology is assumed to provide sufficient error detection (see also RFC 3819).
• UDP has an optional checksum covering the payload and addressing information in the UDP and IP headers. Packets with incorrect checksums are discarded by the network stack. The checksum is optional under IPv4, and required under IPv6. When omitted, it is assumed the data-link layer provides the desired level of error protection.
• TCP provides a checksum for protecting the payload and addressing information in the TCP and IP headers. Packets with incorrect checksums are discarded by the network stack and eventually get retransmitted using ARQ, either explicitly (such as through three-way handshake) or implicitly due to a timeout.

Deep-space telecommunications

The development of error-correction codes was tightly coupled with the history of deep-space missions due to the extreme dilution of signal power over interplanetary distances, and the limited power availability aboard space probes. Whereas early missions sent their data uncoded, starting in 1968, digital error correction was implemented in the form of (sub-optimally decoded) convolutional codes and Reed–Muller codes. The Reed–Muller code was well suited to the noise the spacecraft was subject to (approximately matching a bell curve), and was implemented for the Mariner spacecraft and used on missions between 1969 and 1977.

The Voyager 1 and Voyager 2 missions, which started in 1977, were designed to deliver color imaging and scientific information from Jupiter and Saturn. This resulted in increased coding requirements, and thus, the spacecraft were supported by (optimally Viterbi-decoded) convolutional codes that could be concatenated with an outer Golay (24,12,8) code. The Voyager 2 craft additionally supported an implementation of a Reed–Solomon code. The concatenated Reed–Solomon–Viterbi (RSV) code allowed for very powerful error correction, and enabled the spacecraft's extended journey to Uranus and Neptune. After ECC system upgrades in 1989, both crafts used V2 RSV coding.

The Consultative Committee for Space Data Systems currently recommends usage of error correction codes with performance similar to the Voyager 2 RSV code as a minimum. Concatenated codes are increasingly falling out of favor with space missions, and are replaced by more powerful codes such as Turbo codes or LDPC codes.

The different kinds of deep space and orbital missions that are conducted suggest that trying to find a one-size-fits-all error correction system will be an ongoing problem. For missions close to Earth, the nature of the noise in the communication channel is different from that which a spacecraft on an interplanetary mission experiences. Additionally, as a spacecraft increases its distance from Earth, the problem of correcting for noise becomes more difficult.

Satellite broadcasting

The demand for satellite transponder bandwidth continues to grow, fueled by the desire to deliver television (including new channels and high-definition television) and IP data. Transponder availability and bandwidth constraints have limited this growth. Transponder capacity is determined by the selected modulation scheme and the proportion of capacity consumed by FEC.

Data storage

Error detection and correction codes are often used to improve the reliability of data storage media. A parity track capable of detecting single-bit errors was present on the first magnetic tape data storage in 1951. The optimal rectangular code used in group coded recording tapes not only detects but also corrects single-bit errors. Some file formats, particularly archive formats, include a checksum (most often CRC32) to detect corruption and truncation and can employ redundancy or parity files to recover portions of corrupted data. Reed-Solomon codes are used in compact discs to correct errors caused by scratches.

Modern hard drives use Reed–Solomon codes to detect and correct minor errors in sector reads, and to recover corrupted data from failing sectors and store that data in the spare sectors. RAID systems use a variety of error correction techniques to recover data when a hard drive completely fails. Filesystems such as ZFS or Btrfs, as well as some RAID implementations, support data scrubbing and resilvering, which allows bad blocks to be detected and (hopefully) recovered before they are used. The recovered data may be re-written to exactly the same physical location, to spare blocks elsewhere on the same piece of hardware, or the data may be rewritten onto replacement hardware.

Error-correcting memory

Main article: ECC memory

Dynamic random-access memory (DRAM) may provide stronger protection against soft errors by relying on error-correcting codes. Such error-correcting memory, known as ECC or EDAC-protected memory, is particularly desirable for mission-critical applications, such as scientific computing, financial, medical, etc. as well as extraterrestrial applications due to the increased radiation in space.

Error-correcting memory controllers traditionally use Hamming codes, although some use triple modular redundancy. Interleaving allows distributing the effect of a single cosmic ray potentially upsetting multiple physically neighboring bits across multiple words by associating neighboring bits to different words. As long as a single-event upset (SEU) does not exceed the error threshold (e.g., a single error) in any particular word between accesses, it can be corrected (e.g., by a single-bit error-correcting code), and the illusion of an error-free memory system may be maintained.

In addition to hardware providing features required for ECC memory to operate, operating systems usually contain related reporting facilities that are used to provide notifications when soft errors are transparently recovered. One example is the Linux kernel's EDAC subsystem (previously known as Bluesmoke), which collects the data from error-checking-enabled components inside a computer system; besides collecting and reporting back the events related to ECC memory, it also supports other checksumming errors, including those detected on the PCI bus. A few systems also support memory scrubbing to catch and correct errors early before they become unrecoverable.

Conversion to Christianity

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Conversion_to_Christianity 

Conversion on the Way to Damascus (1601) by Caravaggio (Santa Maria del Popolo, Rome) depicts the conversion of Paul the Apostle to Christianity according to the events narrated in the ninth chapter of the Book of Acts (Acts 9:1-22)

Conversion to Christianity is the religious conversion of a previously non-Christian person that brings about changes in what sociologists refer to as the convert's "root reality" including their social behaviors, thinking and ethics. The sociology of religion indicates religious conversion was an important factor in the emergence of civilization and the making of the modern world. Conversion is the most studied aspect of religion by psychologists of religion, but there is still very little actual data available. Neurological studies have determined that conversion is not the result of pathology.

Christianity is growing rapidly in the global South and East, primarily through conversion. Different methods of conversion have been practiced historically. There is evidence of coercion by secular leaders in the Early and Late Middle Ages, though coercion as a method has never been approved or even supported by any majority of Christian theologians.

Different Christian denominations may perform various different kinds of rituals or ceremonies of initiation into their community of believers. The primary ritual of conversion is baptism, but this is not universally seen as necessary. For some it is an aspect of conversion, and for others it is merely a testimony to conversion that has already occurred.

Denominational switching, or "religious mobility", is not generally considered as a "true" religious conversion, though it can be seen as an aspect (a subset) of it. Confirmation is a controversial practice with many different approaches to whether or not it is necessary for conversion.

Individual conversion

James P. Hanigan writes that individual conversion is the foundational experience and the central message of Christianity, adding that Christian conversion begins with an experience of being "thrown off balance" through cognitive and psychological "disequilibrium", followed by an "awakening" of consciousness and a new awareness of God. Hanigan compares it to "death and rebirth, a turning away..., a putting off of the old..., a change of mind and heart". The person responds by acknowledging and confessing personal lostness and sinfulness, and then accepting a call to holiness thus restoring balance. This initial internal conversion is followed by practices that further the process of conversion, which according to Hanigan, will include ethical changes.

In examples of conversion from the New Testament, such as Peter's conversion and Paul's, Hanigan perceives this same common "death and rebirth" experience. He says these individuals did not respond out of a sense of guilt, but from awe, reverence, and holy fear of what they perceived as God's presence.

Jan Frans van Geel and Jan Baptist van Hool - The calling of St. Peter and St. Andrew, detail of the pulpit in the St. Andrew's Church, Antwerp

Comparative studies of the early twenty-first century offer the insight that religious conversion provides a new locus of self-definition, moral authority and social identity through the acceptance of religious actions that seem more fitting and true to the recipient.

Anthropologist Robert Hefner adds that "Conversion assumes a variety of forms... because it is influenced by a larger interplay of identity, politics and morality". The message of Truth, a redemptive identity, and acceptance into a social organization whose purpose is the propagation of that message has proven to be a revolutionary force in its own right.

Theology

According to sociologist Ines W. Jindra, there is a “theological dimension” to conversion. Karl Rahner makes a distinction between fundamental theology, which Avery Dulles defines as "a reflection on the structures of ... conversion to Christianity," and formal theology, which shifts away from the personal into more abstract concepts.

Dulles defines conversion by quoting Bernard Lonergan who wrote that conversion is "a radical shift in a person's apprehensions and values, accompanied by a similar radical change in oneself, in one's relations with other persons, and in one's relations to God. The subject of theology, then, is the person undergoing conversion to God".

Lonergan has written that, "As conversion is basic to Christian living, so an objectification of conversion provides theology with its foundations... Reflection on the person who converts supplies theology with a foundation which is "concrete, dynamic, personal, communal, and historical. Religious conversion manifestly possesses each of these five properties."

Religious historian David W. Kling's History of Christian Conversion lists nine broad themes common to conversion narratives. Jindra describes the first theme as “human cognizance of divine presence,” while Kling says, "God becomes real to people" through conversion. Conversion always has "context": humans are "socially constituted" beings and religious conversion always occurs in a social context. Jindra writes that, while all conversion accounts vary, they all show evidence of being based upon personal internal experiences of crisis expressed through the specific historical context in which the converts lived.

There are aspects of both "movement and resistance" in conversion. Christianity has, from its beginnings, been an evangelical mission oriented religion which has spread through conversion. However, people naturally tend toward inertia, toward the familiar, unless otherwise motivated toward change, making conversion the exception not the rule in history.

There is both "continuity and discontinuity" in the conversion process. Conversion can be disruptive and cause a rupture with the past, but rupture is rarely complete. Aspects of the past are frequently kept, resulting in a kind of "hybrid" faith. Gender also plays a direct role in how people do or do not convert.

In the more famous conversion stories, such as Augustine's and Martin Luther's, it is apparent the conversion story was later used, not only for personal insight and transformation, but also for drawing in potential converts. Kling writes that "the influence of [such] personal testimonies on the history of conversion cannot be over-estimated." Testimonies and narratives provide the vocabulary of conversion. Indications from Jandra's twenty-first century research indicates this is also true for more ordinary, less famous, conversions. Conversion produced change in the lives of most converts in important and positive ways: Jindra says "they became more stable, found meaning in life, tackled their former problematic biographical trajectories, and improved their relationships (Jindra, 2014)".

Conversion has historically been impacted by how personal "identity" and sense of self is defined. This can determine how much intentional action on the part of the individual convert has directed outcome, and how much outside forces may have impinged upon personal agency instead. In Christian conversion, there is nearly always a network of others who influenced the convert prior to conversion. Jindra writes that the specific context, which includes the ideology of the group being joined, the individual convert's particular crisis, "and the degree of agency vs. the influence of others" are important aspects influencing whether converts change or do not change after a conversion.

These factors overlap with research psychologist Lewis Rambo's stages of conversion. Rambo's model of conversion includes context, crisis (involving some form of searching by the prospective convert), encounter, and interaction, (with someone who believes in the new religious belief system). This is followed by commitment and its results.

Social science

In his book Sociology of Religion, German sociologist Max Weber writes that religious conversion begins with the prophet, as the voice of revelation and vision, calling others to break with tradition and bring their lives into conformity with his "world-building truth." Weber believed that prophetic ideals can become, through the conversion of a community of followers, "a force for world transformation as powerful as anything in human history.

Calling conversion and Christianization "twin phenomena", Hefner has written that religious conversion was an important factor in the emergence of civilization and the making of the modern world. According to Hefner, the "reformulation of social relations, cultural meanings and personal experience" involved in conversion carries with it an inherent "world building aspect".

In the late nineteenth century, the development of world religions (Judaism, Christianity, Islam and Buddhism) was seen as part of the inevitable march toward human enlightenment in a linear upward evolution. Anthropology effectively demonstrated the failure of this model to provide explanation for religious variations.

The world religions developed institutions capable of standardizing knowledge and some have argued that this helped them survive while "empires and economic orders have come and gone". But in fact, only a few religions have been successful in propagating themselves over the long term, and standardized doctrine does not necessarily impact individual conversion and belief.

One of the most influential works in sociology of religion from the 1960s is Robert Bella's (1964) Religious Evolution, which argued that world religions all proclaim the existence of a transcendental realm that is superior to everyday reality, thereby legitimizing salvation/conversion experiences designed to link humans with that world. Bella describes the possibility of redemption/conversion under these terms as "world-shaking in its consequences". The tension between ordinary reality and the transcendent creates recognition of a need for social reform, driven by a redemptive vision, that remakes the world rather than passively accepting it. In this way, Hefner says, world religions loosened the grip of tradition and laid the foundation for human freedom.

Psychology

While conversion is the most studied aspect of religion by psychologists of religion, there is little empirical data on the topic, and little change in method since William James' classic Varieties of Religious Experience in 1902.  James Scroggs and William Douglas have written on seven current concerns in the psychology of conversion.

1) Definition. Calling this the "oldest issue in the field", Scroggs and Douglas indicate psychologists ask whether conversion requires a sudden about-face or gradual change. There is no consensus. The word connotes a sudden about-face, but psychologists are unwilling to let go of the possibility of gradual conversion.

2) Pathology. Freud saw religion as a pathology, and those who follow his school of thought have continued to do so. Empirical studies indicate religion is associated with good mental health among women, that it aids with depression and overcoming serious problems like heroin addiction, and that generally, there are significant links between religion and spirituality and good physical and mental health. In Scroggs and Douglas's view, which view a psychologist takes depends on their training and personal commitment to faith or non-faith.

3) Type of person. Many wonder if there is one kind of person that is more likely to be converted than others. Sociologists stress the importance of such variables as social class, group expectations, and social change (as in American frontier society or contemporary China). According to Scroggs and Douglas, William "James regarded the sick soul as the most likely candidate for conversion. The sick soul lives 'close to the pain threshold.' He is generally introverted and pessimistic in outlook, taking the evil of the world profoundly to heart. The sick soul is brooding, steeped in existential angst. He is Kierkegaard's man who is in despair and knows he is in despair".

Trauma and existential crisis can lead to conversion. For the already converted, trauma is also often associated with "beneficial changes in self-perception, relationships, and philosophy of life, and positive changes in the realm of existential, spiritual, or religious matters" according to a study by psychologists Rosemary de Castella and Janette Simmonds.

A 2011 study indicates conversion can take either an inward form, wherein religion becomes the primary guiding principle and goal of the convert's life, or it can take an outward form where religion mostly serves other purposes, such as political or economic goals, which are more important to that individual than religion. For those who experience inward conversion, lower levels of depression, anxiety and stress are associated, while higher levels are associated with those who practice outward conversion only. 

4) Age. Scroggs and Douglas say that early writers on the psychology of conversion were unanimous in regarding adolescence as the most probable age for conversion.

In surveys of three churches, psychologist Robert Ferm found the average age of conversion to be 43, 46, and 41 years respectively.(Ferm, Robert, The Psychology of Christian Conversion. Westwood, N. J., Fleming Revell, I959,p. 218.) Converts made by Graham's first British campaign averaged in their middle twenties. Jung emphasized mid to late thirties... Hiltner writes that conversion "is most important, most likely, and most cultivatable in the thirties, rather than being regarded primarily as an adolescent phenomenon".

Accordingly Ferm writes that, "It is probably fair to conclude from Erikson's theories that both the identity crisis in adolescence and the integrity crisis in the middle years constitute ripe moments for conversion".

5) Conscious or unconscious. How much of the conversion experience is brought on by conscious control, and how much by unconscious factors behind or even beyond an individual?  Forces beyond conscious control are cited by the majority of converts. "Most psychologists agree the role of unconscious factors is extensive and often decisive in conversion, and that a long period of subconscious incubation precedes sudden conversions" write Scroggs and Douglas. Allport, Maslow, Rogers, and others stress the role of conscious decision.

6) Science-versus-religion. Psychologists as social scientists tend to operate according to a nothing-but reductionism. Conversion must be described as a natural process. Theologians and others who accept the possibility of the supernatural, have tended to take a something-more, hands-off-the-sacred-preserve approach to studying conversion. Different worldviews can bias interpretations. Scroggs and Douglas write that "No solution to this very difficult problem appears in the immediate purview", but they do suggest that acknowledging bias and incorporating both views in "not only interdisciplinary but interbias research is necessary".

7) Which approach? Because there are different schools of psychology with conflicting theories, determining which is most appropriate to the study of conversion is one of the issues Scroggs and Douglas perceive. "Behaviorism, operationalism, and learning theory have rarely been applied to the study of religious conversion," and the overwhelming majority of works have been written from a single perspective: "functionalism" which defines what is true as what works.

Neurology

Kelly Bulkeley in The Oxford Handbook of Religion Conversion has written that, as of 2014, no neuro-scientific research focused specifically on religious conversion has been done. Nor is there a single consensus on how the brain/mind system works, and researchers take many different approaches.  There is controversy over the mind/body problem, as well as whether the brain is simply modular (composed of separate parts), or if that is too limited an explanation for what Bulkeley calls the complex, "global, synthetic, whole-is-greater-than-the-sum-of-its-parts aspects of brain function". There is disagreement over determinism vs. free will, the use of brain imaging, first-person reports of conversion, and the applications of quantum physics.

The phenomenon of conversion is based on the belief that humans have the ability to change the way they mentally perceive and experience the world. Research on the plasticity of the brain has shown that the brain's ability to create new neural pathways remains with us throughout our lives. Bulkeley writes that "Cognitive neuroscience in relation to religious conversions, where people undergo a basic reordering of the assumptions and expectations that frame their perceptions of the world, may lead to new evidence regarding the latent potential of brain/mind development".

Studies on prayer and meditation show they alter the brain's functioning in measurable, material, ways.

"Several implications flow from that basic finding. One is that at least some aspects of religion are not generated by pathological brain functioning. Current [cognitive neuroscience] research refutes the idea that religion ... stems from faulty brain/mind processes. The best available scientific evidence indicates that people who engage in religiously motivated contemplative practices have normal, healthy brains. Perhaps other forms of religion can be more directly tied to neuropathology, but in the case of meditation and prayer the CN literature supports a pragmatic appreciation of the effectiveness of religious practices in shaping the healthy interaction of brain and mind".

Statistics

According to a 2001 study by religion professor David B. Barrett of Columbia University and historian George Thomas Kurian, approximately 2.7 million people were converted to Christianity that year from another religion, while approximately 3.8 million people overall were converting annually. In the first decades of the twenty-first century, Pentecostalism is the largest and fastest growing form of Christianity. Professor of religion Dyron B. Daughrity quotes Paul Freston: "Within a couple of decades, half the world's Christians will be in Africa and Latin America. By 2050, on current trends, there will be as many Pentecostals in the world as there are Hindus, and twice as many Pentecostals as Buddhists". This growth is primarily due to religious conversion.

Historian Philip Jenkins observes that Christianity is also growing rapidly in China and some other Asian countries. Sociologist and specialist in Chinese religion Fenggang Yang from Purdue University writes that Christianity is "spreading among the Chinese of South-East Asia", and "Evangelical and Pentecostal Christianity is growing more quickly in China". More than half of these converts have university degrees.

Countries by percentage of Protestants 1938

Christian distribution globally

Social Anthropologist Juliette Koning and sociologist Heidi Dahles of Vrije Universiteit Amsterdam, agree there has been a "rapid expansion of charismatic Christianity from the 1980s onwards. Singapore, China, Hong Kong, Taiwan, Indonesia, and Malaysia are said to have the fastest-growing Christian communities and the majority of the new believers are "upwardly mobile, urban, middle-class Chinese". Allan Anderson and Edmond Tang have reported in their book Asian and Pentecostal: The Charismatic Face of Christianity in Asia that "Asia has the second largest number of Pentecostals/charismatics of any continent in the world, and seems to be fast catching up with the largest, Latin America." The World Christian Encyclopedia estimated 135 million in Asia compared to 80 million in North America.

It has been reported also that increasing numbers of young people are becoming Christians in several countries such as China, Hong Kong, Indonesia, Iran, Japan, Singapore, and South Korea.

The Council on Foreign Relations says the "number of Chinese Protestants has grown by an average of 10 percent annually since 1979". Award-winning historian of Christianity, Todd Hartch of Eastern Kentucky University, has written that by 2005, around 6 million Africans were converting annually to Christianity. According to Iranian historian Ladan Boroumand "Iran today is witnessing the highest rate of Christianization in the world".

The gospel in South India - or the religious life, experience, and character of the Hindu Christians (1880)

While the exact number of Dalit converts to Christianity in India is not available, religion scholar William R. Burrow of Colorado State University has estimated that about 8% of Dalits have converted to Christianity. According to a 2021 study by the Pew Research Center, Christianity has grow in India in recent years due to conversion. Most converts are former Hindus, though some are former Muslims.

Since the 1960s, there has been a substantial increase in the number of conversions from Islam to Christianity, mostly to the Evangelical and Pentecostal traditions of Christianity. The 2015 study Believers in Christ from a Muslim Background: A Global Census estimated that 10.2 million Muslims converted to Christianity. Countries with the largest numbers of Muslims converted to Christianity include Indonesia (6,500,000), Nigeria (600,000), Iran (500,000 versus only 500 in 1979), the United States (450,000), Ethiopia (400,000), and Algeria (380,000). Indonesia is home to the largest Christian community of converts from Islam. Since the mid and late 1960s, between 2 and 2.5 million Muslims converted to Christianity.

Methods of conversion

Coercion

While Christian theologians, such as the fourth century Augustine and the ninth century Alcuin, have long maintained that conversion must be voluntary, there are historical examples of coercion in conversion to Christianity. Constantine used both law and force to eradicate the practice of sacrifice and repress heresy though not specifically to promote conversion. Theodosius also wrote laws to eliminate heresies, but made no requirement for pagans or Jews to convert to Christianity. However, the sixth century Eastern Roman emperor Justinian I and the seventh century emperor Heraclius attempted to force cultural and religious uniformity by requiring baptism of the Jews. In 612, the Visigothic King Sisebut, prompted by Heraclius, declared the obligatory conversion of all Jews in Spain. In the many new nation-states being formed in Eastern Europe of the Late Middle Ages, some kings and princes pressured their people to adopt the new religion. And in the Northern crusades, the fighting princes obtained widespread conversion through political pressure or military coercion even though the theologians continued to maintain that conversion must be voluntary.

Baptism

In all varieties of Christianity, baptism is the initiation rite for entrance into the Christian community. Almost all baptisms share in common the use of the Trinitarian formula (in the name of the Father, Son, and Holy Spirit) by the minister while baptizing the convert. Two aspects of baptism are sources of disagreement: mode and meaning. In Understanding Four Views on Baptism editors have written that Christians disagree on the meaning of baptism and whether it is a necessary aspect of conversion or simply demonstration of a conversion that has already taken place.

Baptism of a child by affusion

There are also different modes of baptism in Christianity. These include immersion (dunking), affusion (pouring), and aspersion (sprinkling). The most common practice in the ancient church was baptism by immersion of the whole head and body of an adult. It remained common into the Middle Ages and is still found in the Eastern church, the Anglican and Roman Catholic Churches, and in most Protestant denominations.

Historian Philip Schaff has written that sprinkling, or pouring of water on the head of a sick or dying person, where immersion was impractical, was also practiced in ancient times and up through the twelfth century, and is currently practiced in most of the West. However, according to the Oxford Dictionary of the Christian Church affusion has become the most common practice of the Western churches.

Infant baptism was controversial for the Protestant Reformers, and remains so for some Protestants, but according to Schaff, it was practiced by the ancients and is neither required nor forbidden in the New Testament.

The mode of baptism often depends on the denomination one enters, and in some cases, personal choice. Many Anglicans and Lutherans baptize by affusion. Presbyterians and Congregationalists accept baptism by pouring or sprinkling. Steven W. Lemke writes that the Presbyterian Westminster Confession says, “Dipping of the person into the water is not necessary". Baptists disagree. Many Evangelical Protestants, such as Baptists, insist that only full immersion baptism is valid. The Second London and Philadelphia confessions of the Baptists affirm that “immersion, or dipping of the person in water, is necessary". Baptism by immersion is again affirmed in Article 7 of the BF&M [Baptist Faith and Message]". Others, like Methodists, may conduct all three forms of baptism.

Denominational switching

Switching from one Christian denomination, such as Presbyterianism, to another Christian denomination, such as Catholicism, has not generally been seen by researchers as conversion to Christianity. Mark C. Suchman says this is because most sociologists and other scientists have defined conversion as "radical personal change, particularly change involving a shift in one's sense of 'root reality'." However, in Suchman's view, this produces a form of 'selection bias' within the research. He writes that the study of "everyday" religious mobility is not a substitute for analyses of "true conversion," but the denominational switching that he refers to as "religious mobility" can be seen as an aspect of conversion.

Suchman describes six types, or causes, of "religious mobility" as a supplement and complement to the more traditionally limited concept of conversion. He draws on theories from the sociology of deviance where there is some recognition that "a change of religious affiliation generally represents a break with previous norms and a severing of social commitments - even when it does not involve a radical personality realignment".

Theories of deviance define what can be considered as the variables and determinants involved and what kind of mobility can be seen as random. "Strain theory" argues that those who are unhappy in their religious affiliation will generally "engage in deviance" from that group. Those who are not well integrated in their religious social group, those who become enmeshed in social relations outside the group with participants in deviant cultures, and those whose ethnicity and traditional background differs from their current affiliation are candidates for switching. Intermarriage, with partners of different religions and/or denominations, is also associated with religious switching.

Confirmation

Main article: Confirmation

Theologian Knut Alfsvåg writes that confirmation was first introduced by Pope Innocent I in the 5th century as part of the unified sacrament of baptism, chrismation (confirmation) and first communion that was commonly accepted by the 12th century. It was formally designated a sacrament in 1274 by the Council of Lyon. Baptism, along with the declaration and instruction involved in confirmation, and the Eucharist, have remained the essential elements of initiation in all Christian communities, however, Alfsvåg writes that confirmation has differing status in different denominations.

Confirmation class of 1918 at Cape Mount

Some see baptism, confirmation, and communion as elements of a unified sacrament through which one becomes a Christian and part of the church. Also known as Chrismation by eastern Christians, under some circumstances, confirmation may be administered immediately after baptism. When an adult decides to convert to the Catholic or Orthodox Church, they become a "catechumen" and attend classes to learn what conversion means and requires. Once classes are completed and the candidate is baptized, adults can then be confirmed immediately following baptism. A clergy member will anoint their forehead, (or in the case of Byzantine Christians, the forehead, eyes, nostrils, mouth, ears, breast, hands, and feet), with the chrisma (oil) calling upon the Holy Spirit to seal the convert with the gifts of the Spirit.

In Western churches that practice infant baptism, (Catholic Church, the Church of England, Anglicans, Lutherans, Presbyterians, Congregationalists, Methodists, Nazarenes, Moravians, and United Protestants), infants who are baptized are not generally confirmed immediately except in cases of emergency such as illness or impending death. Otherwise, child candidates must wait till they are old enough to make a decision for themselves. Confirmation cannot occur until the candidate has participated in confirmation classes, demonstrated an adequate understanding of what they are agreeing to, and are able to profess "with their own mouth" their desire to be confirmed in their faith. In the Eastern Churches (Eastern Orthodoxy, Oriental Orthodoxy, and the Church of the East), the rite is called chrismation, and is done immediately after baptism, regardless of age.

To be fully in communion with the Catholic Church, (a phrase used since c. 205), the Catholic Church requires a convert to have professed faith and practice the sacraments - baptism, confirmation and the Eucharist. The Orthodox Church also maintains the tradition of baptism, chrismation and first communion as a united rite till this day, referring to chrismation as "the Pentecost of the individual" (a reference to the Holy Spirit).

The practice of confirmation was criticized during the Reformation by those who do not consider confirmation a condition for conversion to Christianity or being a fully accepted member of the church. Luther saw confirmation as “a churchly rite or sacramental ceremony,” but for Luther, it was baptism that was necessary and not confirmation. John Wesley removed the rite altogether leaving Methodism with no rite of confirmation from 1785 to 1965. These see confirmation as a combination of intercessory prayer and as a graduation ceremony after the period of instruction.

Postpartum confinement

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Postpartum_confinement 

Isabella of Hainault rests after having given birth to the future Louis VIII of France.

Postpartum confinement is a traditional practice following childbirth. Those who follow these customs typically begin immediately after the birth, and the seclusion or special treatment lasts for a culturally variable length: typically for one month or 30 days, 26 days, up to 40 days, two months, or 100 days.  This postnatal recuperation can include care practices in regards of "traditional health beliefs, taboos, rituals, and proscriptions." The practice used to be known as "lying-in", which, as the term suggests, centres on bed rest. In some cultures, it may be connected to taboos concerning impurity after childbirth.

Overview

A mother and her newborn rest in bed, breastfeeding

Postpartum confinement refers both to the mother and the baby. Human newborns are so underdeveloped that pediatricians such as Harvey Karp refer to the first three months as the "fourth trimester". The weeks of rest while the mother heals also protect the infant as it adjusts to the world, and both learn the skills of breastfeeding.

Almost all countries have some form of maternity leave. Many countries encourage men to take some paternal leave, but even those that mandate that some of the shared parental leave must be used by the father ("father's quota") acknowledge that the mother needs time off work to recover from the childbirth and deal with the postpartum physiological changes.

A 2016 American book describes the difficulties of documenting those "global grandmotherly customs" but asserts that "like a golden rope connecting women from one generation to the next, the protocol of caring for the new mother by unburdening her of responsibilities and ensuring she rests and eats shows up in wildly diverse places". These customs have been documented in dozens of academic studies, and commonly include support for the new mother (including a release from household chores), rest, special foods to eat (and ones to avoid), specific hygiene practices, and ways of caring for the newborn.

Martha Wolfenstein [Wikidata] and Margaret Mead wrote in 1955 that the postpartum period meant a "woman can be cherished and pampered without feeling inadequate or shamed". The 2016 review that quoted them cites customs from around the world, from Biblical times to modern Greece:

From the data it seems that women were housebound for a number of days after the birth and the length of this period of seclusion varied by caste or ethnic group [in Nepal]. This is a phenomenon found across the globe, including in high-income countries in the recent past. The length of time a woman is secluded or rested varied across different countries and the principles underpinning this isolation (to heal vs. being unclean) also seem to differ greatly. After the period of seclusion there is often a ceremony to purify women to publicly accept them back into daily life. The literature supports the concept of a resting – a lengthy lie-in or lying-in period, a period of seclusion, as women need to rest in order to heal, yet it may mean that they are neglected.

Health effects

One meta-review of studies concluded, "There is little consistent evidence that confinement practices reduce postpartum depression."

By region

Asia

China

Chinese painting of a woman breastfeeding her baby, surrounded by supporters

Postpartum confinement is well-documented in China, where the custom is known as "Sitting the month": 坐月子 "Zuò yuè zi" in Mandarin or 坐月 "Co5 Jyut2" in Cantonese. The period of confinement ranges from 30 to 100 days. Confinement for one month (30 days) is considered "mini confinement" while confinement for three months (100 days) is considered "full confinement". After 100 days, the Hundred Days Banquet (百日宴) is held to celebrate the baby reaching 100 days old.

The Chinese custom of postpartum confinement is documented as far back as the year 960. Women are advised to stay indoors for recovery from the trauma of birth and for feeding the newborn baby. Aspects of traditional Chinese medicine are included, with a special focus on eating foods considered to be nourishing for the body and helping with the production of breastmilk.

In Guangdong and neighboring regions, new mothers are barred from visitors until the baby is 12 days old, marked by a celebration called 'Twelve mornings' (known as 十二朝). From this day onwards, Cantonese families with a new baby usually share their joy through giving away food gifts, while some families mark the occasion by paying tribute to their ancestors.

In ancient China, women of certain ethnic groups in the South would resume work right after birth, and allow the men to practice postpartum confinement instead. (See Couvade.)

Aspects of the practice of "sitting the month" continue today, especially in Hong Kong and Taiwan, and mother and baby sometimes spend the month in special clinics. At the end of this period, when the "month is fulfilled" (manyue), the mother receives relatives and friends who bring special foods such as red eggs.

Everyday habits and personal hygiene practices

Traditional in China, the mother and child were kept separate from the rest of the household. The mother was not permitted to bathe, wash her hair, or weep, because these activities were believed to affect the breast milk.

Nowadays, however, new mothers may wash their hair or take a bath or shower infrequently during the postpartum period, but it is claimed to be important to dry their body immediately afterwards with a clean towel and their hair properly using a hair dryer. It is also claimed to be important for women to wrap up warm and minimize the amount of skin exposed, as it was believed that they may catch a cold during this vulnerable time.

Special foods

Pork knuckle with ginger and black vinegar

The custom of confinement advises new mothers to choose energy and protein-rich foods to recover energy levels, help shrink the uterus, and for the perineum to heal. This is also important for the production of breastmilk. Among the traditionally recommended galactogogues were rich porridge, fish soup, and hard-boiled eggs. Sometimes, new mothers only begin to consume special herbal foods after all the lochia is discharged.

A common dish is pork knuckles with ginger and black vinegar as pork knuckles are believed to help replenish calcium levels in women. Ginger is featured in many dishes, as it is believed that it can remove the 'wind' accumulated in the body during pregnancy. Meat-based soup broths are also commonly consumed to provide hydration and added nutrients.

Indian subcontinent

In parts of India it is called jaappa (also transliterated japa); in North India and Pakistan, sawa mahina ("five weeks").

Most traditional Indians follow the 40-day confinement and recuperation period also known as the jaappa (in Hindi). A special diet to facilitate milk production and increase hemoglobin levels is followed. Sex is not allowed during this time. In Hindu culture, this time after childbirth was traditionally considered a period of relative impurity (asaucham), and a period of confinement of 10–40 days (known as purudu) was recommended for the mother and the baby. During this period, she was exempted from usual household chores and religious rites. The father was purified by a ritual bath before visiting the mother in confinement.

In the event of a stillbirth, the period of impurity for both parents was 24 hours.

Many Indian subcultures have their own traditions after birth. This birth period is called Virdi (Marathi), which lasts for 10 days after birth and includes complete abstinence from puja or temple visits.

In Pakistan, postpartum tradition is known as sawa mahina ("five weeks").

Iran

In Persian culture it is called chilla, i.e. "forty days".

Korea

Korean women spend samchil-il (three seven days, 21 days) in confinement, receiving sanhujori (postpartum care). In the past, during the samchil-il period, geumjul (taboo rope) made with saekki and various symbolic objects, such as chili peppers (for a boy) and coal (for a girl), was hung over the gate to denote the childbirth and restrict visitor access.

Thailand

Thailand has various customs. New mothers used to be encouraged to lie in a warm bed near the fire for 30 days, a practice known as yu fai. This has been adapted into a form of Thai massage. Kao krachome is a type of herbal medicine in which the steam from the boiled plants is inhaled. Ya dong involves herbal medicine taken internally. Thai immigrants to Sweden report using the steam bath to heal after childbirth, although the correct ingredients are not easy to find. Thai Australians who had had caesarian sections felt that they did not need to - in fact, ought not to - undergo these rituals.

Europe

Main article: Lying-in

A mother in Florence lying-in, from a painted desco da parto or birth tray of c. 1410. As women tend to the child, expensively dressed female guests are already arriving.

The term used in English, now old-fashioned or archaic, was once used to name maternity hospitals, for example the General Lying-In Hospital in London. A 1932 Canadian publication refers to lying-in as ranging from two weeks to two months. These weeks ended with the re-introduction of the mother to the community in the Christian ceremony of the churching of women.

Lying-in features in Christian art, notably Birth of Jesus paintings. One of the gifts presented to the new mother in Renaissance Florence was a desco da parto, a special form of painted tray. Equivalent presents in contemporary culture include baby showers and push presents.

Special foods included caudle, a restorative drink. "Taking caudle" was a metonym for postpartum social visits.

Americas

Latin America

In Latin American countries, it is called la cuarentena ("forty days," a cognate with the English word "quarantine". It is practised in parts of Latin America and amongst in communities in the United States. It is described as "intergenerational family ritual that facilitated adaptation to parenthood", including some paternal role reversal.

Aerocapture

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Aerocapture 

Schematic showing the various phases of the aerocapture maneuver. Atmospheric height is greatly exaggerated for clarity.

Aerocapture is an orbital transfer maneuver in which a spacecraft uses aerodynamic drag force from a single pass through a planetary atmosphere to decelerate and achieve orbit insertion.

Aerocapture uses a planet's or moon's atmosphere to accomplish a quick, near-propellantless orbit insertion maneuver to place a spacecraft in its science orbit. The aerocapture maneuver starts as the spacecraft enters the atmosphere of the target body from an interplanetary approach trajectory. The aerodynamic drag generated as the vehicle descends into the atmosphere slows the spacecraft. After the spacecraft slows enough to be captured by the planet, it exits the atmosphere and executes a small propulsive burn at the first apoapsis to raise the periapsis outside the atmosphere. Additional small burns may be required to correct apoapsis and inclination targeting errors before the initial science orbit is established.

Compared to conventional propulsive orbit insertion, this nearly fuel-free method of deceleration could significantly reduce the mass of an interplanetary spacecraft, as a substantial fraction of the spacecraft mass is often propellant used for the orbit insertion burn. The saving in propellant mass allows for more science instrumentation to be added to the mission, or allows for a smaller and less-expensive spacecraft, and, potentially, a smaller, less-expensive launch vehicle.

Because of the aerodynamic heating encountered during the atmospheric pass, the spacecraft must be packaged inside an aeroshell (or a deployable entry system) with a thermal protection system. The vehicle also requires autonomous closed-loop guidance during the maneuver to enable the vehicle to target the desired capture orbit and command the vehicle to exit the atmosphere when sufficient energy has been dissipated. Ensuring that the vehicle has enough control authority to prevent the spacecraft penetrating too deep into the atmosphere or exiting prematurely without dissipating enough energy requires either the use of a lifting aeroshell, or a drag-modulation system which can change the vehicle's drag-producing area during flight.

Aerocapture has been shown to be feasible at Venus, Earth, Mars, and Titan using existing entry vehicles and thermal protection system materials.^[4] Until recently, mid-L/D (lift-to-drag) vehicles were considered essential for aerocapture at Uranus and Neptune, due to the large uncertainties in entry state and atmospheric density profiles. However, advances in interplanetary navigation and atmospheric guidance techniques have shown that heritage low-L/D aeroshells such as Apollo offer sufficient control authority for aerocapture at Neptune. Aerocapture at Jupiter and Saturn is considered a long-term goal, as their huge gravity wells result in very high entry speeds and harsh aerothermal environments, making aerocapture a less attractive, and, perhaps, infeasible option at these destinations. However, it is possible to use an aerogravity assist at Titan to insert a spacecraft around Saturn.

Brief History of Aerocapture

Histogram showing the number of publications addressing aerocapture since the 1960s, classified by target planet.

Aerocapture has been studied for planetary missions since the early 1960s. London's pioneering article on using aerodynamic maneuvering to change the plane of a satellite in Earth orbit, instead of using a propulsive maneuver is considered a precursor for the concept of aerocapture. The aerocapture concept was then referred to as aerodynamic braking or "aerobraking", and was investigated as a potential orbit insertion method for Mars and Venus missions by Repic et al. In modern terminology, aerobraking refers to a different "aeroassist" maneuver and is not to be confused with aerocapture. Cruz's 1979 article was the first to use the word aerocapture, and was followed by a series of studies focusing on its applications to Mars Sample Return (SR).

In the late 1980s, the Aeroassist Flight Experiment (AFE) was conceived to use a Shuttle-launched payload to demonstrate aerocapture at Earth. The project resulted in a number of significant developments including guidance flight software, but was eventually cancelled due to cost overruns and was never flown. In the late 1990s, aerocapture was considered for the Mars Odyssey mission (then referred to as Mars 2001 Surveyor), but was later dropped in favor of aerobraking due to cost reasons and heritage with other Mars missions. In the early 2000s, aerocapture was identified as the focus area by the NASA In-Space Propulsion Technology (ISPT) program. A multi-center Aerocapture Systems Analysis Team (ASAT) was put together under this project to define reference aerocapture missions at various Solar System destinations and identify any technology gaps to be closed before implementation on a flight project. The ASAT team led by Mary Kae Lockwood at the NASA Langley Research Center studied in substantial detail aerocapture mission concepts to Venus, Mars, Titan, and Neptune. Since 2016, there is renewed interest in aerocapture particularly with respect to small satellite orbit insertion at Venus and Mars, and Flagship-class missions to Uranus and Neptune in the upcoming decade.

Benefits of aerocapture

NASA technologists are developing ways to place robotic space vehicles into long-duration scientific orbits around distant Solar System destinations without the need for the heavy fuel loads that have historically limited vehicle performance, mission duration, and mass available for science payloads.

A study showed that using aerocapture over the next best method (propellant burn and aerobraking) would allow for a significant increase in scientific payload for missions ranging from Venus (79% increase) to Titan (280% increase) and Neptune (832% increase). Additionally, the study showed that using aerocapture technology could enable scientifically useful missions to Jupiter and Saturn.

Aerocapture technology has also been evaluated for use in crewed Mars missions and found to offer significant mass benefits. For this application, however, the trajectory must be constrained to avoid excessive deceleration loads on the crew. Although there are similar constraints on trajectories for robotic missions, the human limits are typically more stringent, especially in light of the effects of prolonged microgravity on acceleration tolerances.

Aerocapture spacecraft designs

The aerocapture maneuver can be accomplished with three basic types of systems. The spacecraft can be enclosed by a structure covered with thermal protection material also known as the rigid aeroshell design. Similarly another option is for the vehicle to deploy an aerocapture device, such as an inflatable heat shield, known as the inflatable aeroshell design. The third major design option is of an inflatable, trailing ballute—a combination balloon and parachute made of thin, durable material towed behind the vehicle after deployment in the vacuum of space.

Blunt body, rigid aeroshell design

The blunt body, rigid aeroshell system encases a spacecraft in a protective shell. This shell acts as an aerodynamic surface, providing lift and drag, and provides protection from the intense heating experienced during high-speed atmospheric flight. Once the spacecraft is captured into orbit, the aeroshell is jettisoned.

NASA has used blunt aeroshell systems in the past for atmospheric entry missions. The most recent example is the Mars Exploration Rovers, Spirit and Opportunity, which launched in June and July 2003, and landed on the Martian surface in January 2004. Another example is the Apollo Command Module. The module was used for six uncrewed space flights from February 1966 to April 1968 and eleven crewed missions from Apollo 7 in October 1968 through the final crewed Apollo 17 lunar mission in December 1972. Because of its extensive heritage, the aeroshell system design is well understood. Adaptation of the aeroshell from atmospheric entry to aerocapture requires mission-specific customization of the thermal protection material to accommodate the different heating environments of aerocapture. Also, higher-temperature adhesives and lightweight, high temperature structures are desired to minimize the mass of the aerocapture system.

Inflatable aeroshell design

The inflatable aeroshell design looks much like the aeroshell or blunt body design. The inflatable aeroshell is often referred to as a hybrid system, with a rigid nosepiece and an inflated, attached decelerator to increase the drag area. Just prior to entering the atmosphere, the inflatable aeroshell extends from a rigid nose-cap and provides a larger surface area to slow the spacecraft down. Made of thin-film material and reinforced with a ceramic cloth, the inflatable aeroshell design could offer many of the same advantages and functionality as trailing ballute designs. While not as large as the trailing ballute, the inflatable aeroshell is roughly three times larger than the rigid aeroshell system and performs the aerocapture maneuver higher in the atmosphere, reducing heating loads. Because the system is inflatable, the spacecraft is not enclosed during launch and cruise, which allows more flexibility during spacecraft design and operations.

Trailing ballute design

One of the primary inflatable deceleration technologies is a trailing ballute configuration. The design features a toroidal, or donut-shaped, decelerator, made of a lightweight, thin-film material. The ballute is much larger than the spacecraft and is towed behind the craft, much like a parachute, to slow the vehicle down. The "trailing" design also allows for easy detachment after the aerocapture maneuver is complete. The trailing ballute design has performance advantages over the rigid aeroshell design, such as not constraining the spacecraft size and shape, and subjecting the vehicle to much lower aerodynamic and thermal loads. Because the trailing ballute is much larger than the spacecraft, aerocapture occurs high in the atmosphere where much less heat is generated. The ballute incurs most of the aerodynamic forces and heat, allowing the use of minimal thermal protection around the spacecraft. One of the primary advantages of the ballute configuration is mass. Where the rigid aeroshell may account for 30–40% of the mass of a spacecraft, the ballute mass fraction could be as little as 8–12%, saving mass for more science payload.

In practice

Aerocapture has not yet been tried on a planetary mission, but the re-entry skip by Zond 6 and Zond 7 upon lunar return were aerocapture maneuvers, since they turned a hyperbolic orbit into an elliptical orbit. On these missions, since there was no attempt to raise the perigee after the aerocapture, the resulting orbit still intersected the atmosphere, and re-entry occurred at the next perigee.

Aerocapture was originally planned for the Mars Odyssey orbiter, but later changed to aerobraking for reasons of cost and commonality with other missions.

Aerocapture has been proposed and analyzed for arrival at Saturn's moon Titan.

In fiction

Aerocapture within fiction can be read in Arthur C. Clarke's novel 2010: Odyssey Two, in which two spacecraft (one Russian, one Chinese) both use aerocapture in Jupiter's atmosphere to shed their excess velocity and position themselves for exploring Jupiter's satellites. This can be seen as a special effect in the movie version in which only a Russian spacecraft undergoes aerocapture (in the film incorrectly called aerobraking).

Players of the video game Kerbal Space Program often employ aerocapture, particularly when exploring the satellites of Jool (a gas giant that serves as the game's Jupiter analogue).

In the television serial Stargate Universe, the ship Destiny's autopilot employs aerocapture within the atmosphere of a gas giant at the edge of a star system. This puts the ship on a direct heading into the star at the center of the system.

In the sci-fi novel Delta-v, asteroid miners use a purpose-built aerocapture ship in a desperate attempt to return to Earth from the asteroid Ryugu.

Related methods

Aerocapture is part of a family of "aeroassist" technologies being developed by NASA for science missions to any planetary body with an appreciable atmosphere. These destinations could include Mars, Venus and Saturn's moon Titan, along with the outer planets.

Aerobraking is another aeroassist maneuver that shares some similarities but also some important differences with aerocapture. While aerocapture is used for inserting a spacecraft into orbit from a hyperbolic trajectory, aerobraking is used for reducing the apoapsis of a spacecraft that is already in orbit.

Comparison of aerocapture and aerobraking

	Aerocapture	Aerobraking
Starting trajectory	Interplanetary	High orbit
Atmospheric passes over duration	1 over hours to days	100–400 over weeks to months
Depth of atmospheric entry	Relatively dense mid-atmosphere	Sparse outer atmosphere
Hardware requirements	Heavy heat shield	No heat shield

One of the main advantages of using an aerocapture technique over that of an aerobraking technique is that it enables mission concepts for human spaceflight due to the rapid process of transitioning to the desired orbit, shortening the length of the mission by months.

Software

• Aerocapture Mission Analysis Tool (AMAT) provides rapid mission analysis capability for aerocapture and Entry, Descent, and Landing (EDL) mission concepts to atmosphere-bearing destinations in the Solar System.