William Harvey

From Wikipedia, the free encyclopedia

William Harvey
William Harvey
Born	1 April 1578 Folkestone, Kent, England
Died	3 June 1657 (aged 79) Roehampton, London, England
Nationality	English
Alma mater	Gonville and Caius College, Cambridge University of Padua
Known for	De Motu Cordis (on Circulation of the blood)
Scientific career
Fields	Medicine Anatomy
Doctoral advisor	Hieronymus Fabricius
Influenced	René Descartes[1]
Signature

William Harvey (1 April 1578 – 3 June 1657)^[2] was an English physician who made seminal contributions in anatomy and physiology. He was the first known physician to describe completely and in detail the systemic circulation and properties of blood being pumped to the brain and body by the heart, though earlier writers, such as Realdo Colombo, Michael Servetus, and Jacques Dubois, had provided precursors of the theory.^[3][4] In 1973 the William Harvey Hospital was constructed in the town of Ashford, a few miles from his birthplace of Folkestone.

Family

William's father, Thomas Harvey, was a jurat of Folkestone where he served the office of mayor in 1600. Records and personal descriptions delineate him as an overall calm, diligent, and intelligent man whose "sons... revered, consulted and implicitly trusted in him... (they) made their father the treasurer of their wealth when they acquired great estates...(He) kept, employed, and improved their gainings to their great advantage."^[5] Thomas Harvey's portrait can still be seen in the central panel of a wall of the dining-room at Rolls Park, Chigwell, in Essex. William was the eldest of nine children, seven sons and two daughters, of Thomas and his wife Joan Halke. Notable family connections include Heneage Finch, 1st Earl of Nottingham, who married William's niece Elizabeth Harvey, and the diplomat Sir Daniel Harvey.^{[citation needed]}

Biography

Early life and the University of Padua

Harvey's initial education was carried out in Folkestone, where he learned Latin. He then entered the King's School (Canterbury). Harvey stayed at the King's School for five years, after which he matriculated at Gonville and Caius College in Cambridge in 1593.

Harvey graduated as a Bachelor of Arts from Caius in 1597.^[6] He then travelled through France and Germany to Italy, where he entered the University of Padua, in 1599.

During Harvey's years of study there, he developed a relationship with Fabricius and read Fabricius's De Venarum Ostiolis.

Harvey graduated as a Doctor of Medicine at the age of 24 from the University of Padua on 25 April 1602. It reports that Harvey had

"conducted himself so wonderfully well in the examination and had shown such skill, memory and learning that he had far surpassed even the great hopes which his examiners had formed of him."^[7]

The College of Physicians, marriage and Saint Bartholomew's Hospital

After graduating from Padua, Harvey immediately returned to England where he obtained the degree of Doctor of Medicine from the University of Cambridge that same year, and became a fellow of Gonville and Caius College. Following this, Harvey established himself in London, joining the Royal College of Physicians on 5 October 1604.

A few weeks after his admission, Harvey married Elizabeth Browne, "daughter of Lancelot Browne Dr. Physic".^[8] They had no children.

Harvey was elected a Fellow of the Royal College of Physicians on 5 June 1607, which earned him the Post-nominal letters FRCP, and he then accepted a position at St Bartholomew's Hospital that he was to occupy for almost all the rest of his life. Succeeding a Dr Wilkinson on 14 October 1609, he became the Physician in charge at St Bartholomew's Hospital, which enjoined him, "in God's most holy name" to "endeavor yourself to do the best of your knowledge in the profession of physic to the poor then present, or any other of the poor at any time of the week which shall be sent home unto you by the Hospitaller... You shall not, for favor, lucre or gain, appoint or write anything for the poor but such good and wholesome things as you shall think with your best advice will do the poor good, without any affection or respect to be had to the apothecary. And you shall take no gift or reward... for your counsel... This you will promise to do as you shall answer before God... "^[9]

Harvey earned around thirty-three pounds a year and lived in a small house in Ludgate, although two houses in West Smithfield were attached as fringe benefits to the post of Physician. At this point, the physician's function consisted of a simple but thorough analysis of patients who were brought to the hospital once a week and the consequent writing of prescriptions.

Lumleian Lecturer

The next important phase of Harvey's life began with his appointment to the office of Lumleian lecturer on 4 August 1615. The Lumleian lectureship, founded by Lord Lumley and a Dr. Richard Caldwell in 1582, consisted in pronouncing lectures for a period of seven years, with the purpose of "spreading light" and increasing the general knowledge of anatomy throughout England.

Harvey began his lectures in April 1616. At this time, at the age of thirty-seven, he was described as "a man of lowest stature, round faced; his eyes small, round, very black and full of spirit; his hair as black as a raven and curling".^[10] The notes which he used at the time are preserved in the British Museum.

At the beginning of his lectures, Harvey laid down the canons for his guidance:

1. "To show as much as may be at a glance, the whole belly for instance, and afterwards to subdivide the parts according to their positions and relations.
2. To point out what is peculiar to the actual body which is being dissected.
3. To supply only by speech what cannot be shown on your own credit and by authority.
4. To cut up as much as may be in the sight of the audience.
5. To enforce the right opinion by remarks drawn far and near, and to illustrate man by the structure of animals.
6. Not to praise or dispraise other anatomists, for all did well, and there was some excuse even for those who are in error.
7. Not to dispute with others, or attempt to confute them, except by the most obvious retort.
8. To state things briefly and plainly, yet not letting anything pass unmentioned which can be seen.
9. Not to speak of anything which can be as well explained without the body or can be read at home.
10. Not to enter into too much detail, or in too minute dissection, for the time does not permit.
11. To serve three courses according to the glass [i.e. allot a definite time to each part of the body]. In the first day's lectures the abdomen, nasty yet recompensed by its infinite variety. In the second the parlour, [i.e. the thorax?]. In the third day's lecture the divine banquet of the brain."^[11]

Physician to James I

Harvey continued to participate in the Lumleian lectures while also taking care of his patients at St Bartholomew's Hospital; he thus soon attained an important and fairly lucrative practice, which climaxed with his appointment as 'Physician Extraordinary' to King James I on 3 February 1618. He seems to have similarly served various aristocrats, including Lord Chancellor Bacon.^[12][13] Bacon entirely failed to impress the more practical minded Harvey, who refused to regard him as a great philosopher. He said of him "He writes philosophy like a Lord Chancellor."^[14]

In 1628 he published in Frankfurt his completed treatise on the circulation of the blood, the De Motu Cordis. As a result of negative comments by other physicians Harvey "fell mightily in his practice",^[15] but continued advancing his career. He was re-elected 'Censor' of the College of Physicians in 1629, having been elected for the first time in 1613 and the second time in 1625. Eventually, Harvey was also elected Treasurer of the College.

Witchcraft trials

Harvey was a prominent sceptic regarding allegations of witchcraft. He was one of the examiners of four women from Lancashire accused of witchcraft in 1634, and as a consequence of his report, all of them were acquitted.^[16][17] Earlier, in 1632, while travelling with the King to Newmarket, he had been sent to investigate a woman accused of being a witch. Initially he told her that he was a wizard and had come to discuss the Craft with her, and asked whether she had a familiar. She put down a saucer of milk and called to a toad which came out and drank the milk. He then sent her out to fetch some ale, and killed the toad and dissected it, concluding that it was a perfectly ordinary animal and not supernatural in any way. When the woman returned she was naturally very angry and upset, but Harvey eventually silenced her by stating that he was the King's Physician, sent to discover whether she were a witch, and if she were, to have her apprehended.^[18]

Excursions abroad, election as physician to Charles I and the English Civil War

At the age of fifty-two, Harvey received commands by the king to accompany the Duke of Lennox during his trip abroad. This voyage – the first after his return from Padua – lasted three years, taking Harvey through the countries of France and Spain during the Mantuan War and Plague. During this journey he wrote to Viscount Dorchester:

"I can complain that by the way we could scarce see a dog, crow, kite, raven or any other bird, or anything to anatomize, only some few miserable people, the relics of the war and the plague where famine had made anatomies before I came. It is scarce credible in so rich, populous, and plentiful countries as these were that so much misery and desolation, poverty and famine should in so short a time be, as we have seen. I interpret it well that it will be a great motive for all here to have and procure assurance of settled peace. It is time to leave fighting when there is nothing to eat, nothing to be kept, and nothing to be gotten".^[19]

Having returned to England in 1632, Harvey accompanied King Charles I wherever he went as 'Physician in Ordinary'. In particular, Charles's hunting expeditions gave Harvey access to many deer carcasses; it was upon them that Harvey made many observations and consequent theories. Harvey returned to Italy in 1636, dining at the English College, Rome, as a guest of the Jesuits there, in October 1636. It is possible he met Galileo in Florence en route.^[20]

During the English Civil War a mob of citizen-soldiers against the King entered Harvey's lodgings, stole his goods, and scattered his papers. The papers consisted of "the records of a large number of dissections ... of diseased bodies, with this observations on the development on insects, and a series of notes on comparative anatomy."^[21] During this period, Harvey maintained his position, helped the wounded on several occasions and protected the King's children during the Battle of Edgehill.

The conflicts of the Civil War soon led King Charles to Oxford, with Harvey attending, where the physician was made "Doctor of Physic" in 1642 and later Warden of Merton College in 1645. "In Oxford he (Harvey) very soon settled down to his accustomed pursuits, unmindful of the clatter of arms and of the constant marching and countermarching around him, for the city remained the base of operations until its surrender... "^[22]

Harvey's later years, death and burial

The surrender of Oxford in 1645 marks the beginning of Harvey's gradual retirement from public life and duties. Now sixty-eight years old and childless, Harvey had lost three brothers and his wife by this time. He thus decided to return to London, and lived with his brothers Eliab and Daniel at different periods. Having retired from St Bartholomew's Hospital and his various other aforementioned positions, he passed most of this time reading general literature. Several attempts to bring Harvey back into the 'working world' were made, however; here is an excerpt of one of Harvey's answers:

"Would you be the man who should recommend me to quit the peaceful haven where I now pass my life and launch again upon the faithless sea? You know full well what a storm my former lucubrations raised. Much better is it oftentimes to grow wise at home and in private, than by publishing what you have amassed with infinite labour, to stir up tempests that may rob you of peace and quiet for the rest of your days."^[23]

Harvey died at Roehampton in the house of his brother Eliab on 3 June 1657. Descriptions of the event seem to show that he died of a cerebral hemorrhage from vessels long injured by gout: it is highly probable that the left middle cerebral artery malfunctioned, leading to a gradual accumulation of blood to the brain which eventually overwhelmed it. There exists a fairly detailed account of what happened on that day; according to the information at hand, Harvey:

"went to speak and found that he had the dead palsy in his tongue; then he saw what was to become of him. He knew there were then no hopes of his recovery, so presently he sends for his young nephews to come up to him. He then made signs (for seized with the dead palsy in his tongue he could not speak) to let him blood his tongue, which did him little or no good, and so ended his days, dying in the evening of the day on which he was stricken, the palsy giving him an easy passport."^[24]

His will distributed his material goods and wealth throughout his extended family and also left a substantial amount of money to the Royal College of Physicians.

Harvey was buried in Hempstead, Essex. The funeral procession started on 26 June 1657, leading Harvey to be placed in the 'Harvey Chapel' built by Eliab. The conditions of Harvey's burial are also known: "Harvey was laid in the chapel between the bodies of his two nieces, and like them he was lapt in lead, coffin less".^[25] On St. Luke's Day, 18 October 1883, Harvey's remains were reinterred, the leaden case carried from the vault by eight Fellows of the College of Physicians, and deposited in a sarcophagus containing his works and an inscription:

"The body of William Harvey lapt in lead, simply soldered, was laid without shell or enclosure of any kind in the Harvey vault of this Church of Hempstead, Essex, in June 1657. In the course of time the lead enclosing the remains was, from expose and natural decay, so seriously damaged as to endanger its preservation, rendering some repair of it the duty of those interested in the memory of the illustrious discoverer of the circulation of the Blood. The Royal College of Physicians, of which corporate body Harvey was a munificent Benefactor did in the years 1882–1883, by permission of the Representatives of the Harvey family, undertake this duty. In accordance with this determination the leaden mortuary chest containing the remains of Harvey was repaired, and was, as far as possible, restored to its original state... "^[26]

De Motu Cordis (On the Motion of the Heart and Blood)

An experiment from Harvey's de Motu Cordis

Published in 1628 in the city of Frankfurt (host to an annual book fair that Harvey knew would allow immediate dispersion of his work), the 72 page Exercitatio Anatomica de Motu Cordis et Sanguinis in Animalibus contains the matured account of the circulation of the blood. Opening with a dedication to King Charles I, the quarto has 17 chapters which give a clear and connected account of the action of the heart and the consequent movement of the blood around the body in a circuit. Having only a tiny lens at his disposal, Harvey was not able to reach the adequate pictures that were attained through such microscopes used by Antonie van Leeuwenhoek; thus he had to resort to theory – and not practical evidence – in certain parts of his book. After the first chapter, which simply outlines past ideas and accepted rules regarding the heart and lungs, Harvey moves on to a fundamental premise to his treatise, stating that it was important to study the heart when it was active in order to truly comprehend its true movement; a task which even he found of great difficulty, as he says:

"...I found the task so truly arduous... that I was almost tempted to think... that the movement of the heart was only to be comprehended by God. For I could neither rightly perceive at first when the systole and when the diastole took place by reason of the rapidity of the movement..."^[27]

This initial thought led Harvey's ambition and assiduousness to a detailed analysis of the overall structure of the heart (studied with less hindrances in cold-blooded animals). After this, Harvey goes on to an analysis of the arteries, showing how their pulsation depends upon the contraction of the left ventricle, while the contraction of the right ventricle propels its charge of blood into the pulmonary artery. Whilst doing this, the physician reiterates the fact that these two ventricles move together almost simultaneously and not independently as had been thought previously by his predecessors. This discovery was made while observing the heart of such animals as the eel and several other types of fish; indeed, the general study of countless animals was of utmost importance to the physician: among the ones already cited, one can add the study of the snail, the invisible shrimp, the chick before its hatching and even the pigeon. A digression to an experiment can be made to this note: using the inactive heart of a dead pigeon and placing upon it a finger wet with saliva, Harvey was able to witness a transitory and yet incontrovertible pulsation. He had just witnessed the heart's ability to recover from fatigue.

As early as the 17th century, William Harvey had already discerned the existence of the Ductus Arteriosus and explained its relative function. Here he says, "...in embryos, whilst the lungs are in a state of inaction, performing no function, subject to no movement any more than if they had not been present, Nature uses the two ventricles of the heart as if they formed but one for the transmission of the blood."^[28] However, the apex of Harvey's work is probably the eighth chapter, in which he deals with the actual quantity of blood passing through the heart from the veins to the arteries. Coming into conflict with Galen's accepted view of the liver as the origin of venous blood, Harvey estimated the capacity of the heart, how much blood is expelled through each pump of the heart, and the number of times the heart beats in a half an hour. All of these estimates were purposefully low, so that people could see the vast amount of blood Galen's theory required the liver to produce. He estimated that the capacity of the heart was 1.5 imperial fluid ounces (43 ml), and that every time the heart pumps, ​¹⁄₈ of that blood is expelled. This led to Harvey's estimate that about ¹⁄₆ imperial fluid ounce (4.7 ml) of blood went through the heart every time it pumped. The next estimate he used was that the heart beats 1,000 times every half an hour, which gave 10 pounds 6 ounces of blood in a half an hour, and when this number was multiplied by 48 half hours in a day he realized that the liver would have to produce 498 pounds of blood in a day, more than the weight of the whole body.

Having this simple but essential mathematical proportion at hand – which proved the overall impossible aforementioned role of the liver – Harvey went on to prove how the blood circulated in a circle by means of countless experiments initially done on serpents and fish: tying their veins and arteries in separate periods of time, Harvey noticed the modifications which occurred; indeed, as he tied the veins, the heart would become empty, while as he did the same to the arteries, the organ would swell up.

This process was later performed on the human body (in the image on the right): the physician tied a tight ligature onto the upper arm of a person. This would cut off blood flow from the arteries and the veins. When this was done, the arm below the ligature was cool and pale, while above the ligature it was warm and swollen. The ligature was loosened slightly, which allowed blood from the arteries to come into the arm, since arteries are deeper in the flesh than the veins. When this was done, the opposite effect was seen in the lower arm. It was now warm and swollen. The veins were also more visible, since now they were full of blood. Harvey then noticed little bumps in the veins, which he realized were the valves of the veins discovered by his teacher, Hieronymus Fabricius. Harvey tried to push blood in the vein down the arm, but to no avail. When he tried to push it up the arm, it moved quite easily. The same effect was seen in other veins of the body, except the veins in the neck. Those veins were different from the others – they did not allow blood to flow up, but only down. This led Harvey to believe that the veins allowed blood to flow to the heart, and the valves maintained the one way flow.

Contrary to a popular misconception, Harvey did not predict the existence of capillaries. His observations convinced him that direct connection between veins and arteries are unnecessary; he wrote "blood permeates the pores" in the flesh and it is "absorbed and imbibed from every part" by the veins.^[29]

Views of the circulation of blood before Harvey

At the time of Harvey's publication, Galen had been an influential medical authority for several centuries. Galen believed that blood passed between the ventricles by means of invisible pores. According to Galen's views, the venous system was quite separate from the arterial system, except when they came in contact through the unseen pores. Arabic scholar Ibn al-Nafis had disputed aspects of Galen's views, providing a model that seems to imply a form of pulmonary circulation in his Commentary on Anatomy in Avicenna's Canon (1242). Al-Nafis stated that blood moved from the heart to the lungs, where it mixed with air, and then back to the heart, from which it spread to the rest of the body.^[30] Harvey's discoveries inevitably and historically came into conflict with Galen's teachings and the publication of his treatise De Motu Cordis incited considerable controversy within the medical community. Some doctors affirmed they would "rather err with Galen than proclaim the truth with Harvey."^[31][32] Galen incompletely perceived the function of the heart, believing it a "productor of heat", while the function of its affluents, the arteries, was that of cooling the blood as the lungs "...fanned and cooled the heart itself".^[33] Galen thought that during dilation the arteries sucked in air, while during their contraction they discharged vapours through pores in the flesh and skin.

Until the 17th century, two separate systems were thought to be involved in blood circulation: the natural system, containing venous blood which had its origin in the liver, and the vital system, containing arterial blood and the 'spirits' which flowed from the heart, distributing heat and life to all parts. Like bellows, the lungs fanned and cooled this vital blood.

Independently of Ibn Al-Nafis, Michael Servetus identified pulmonary circulation, but this discovery did not reach the public because it was written down for the first time in the Manuscript of Paris in 1546.^[34] It was later published in the theological work which caused his execution in 1553, almost all copies of which were destroyed. In: Christianismi Restitutio, Book V, the Aragonese Miguel Servet (Michel de Villeneuve, 1509?–1553) wrote: 'The blood is passed through the pulmonary artery to the pulmonary vein for a lengthy pass through the lungs, during which it becomes red, and gets rid of the sooty fumes by the act of exhalation'.

Pulmonary circulation was described by Renaldus Columbus, Andrea Cesalpino and Vesalius, before Harvey would provide a refined and complete description of the circulatory system.

On Animal Generation

Harvey's other major work was Exercitationes de generatione animalium (On Animal Generation_, published in 1651. He had been working on it for many years but might never have finished it without the encouragement of his friend George Ent.^[2]

The book starts with a description of development of the hen's egg. The major part is theoretical, dealing with Aristotle's theories and the work of the physicians following Galen and up to Fabricius. Finally he deals with embryogenesis in viviparous animals especially hinds and does. The treatment is generally Aristotelian and limited by use of a simple magnifying lens.

Needham claims the following achievements for this work.^[35]

• His doctrine of omne vivum ex ovo (all life comes from the egg) was the first definite statement against the idea of spontaneous generation. He denied the possibility of generation from excrement and from mud, and pointed out that even worms have eggs.
• He identified the citricula as the point in the yolk from which the embryo develops and the blastoderm surrounding the embryo.
• He destroyed once and for all the Aristotelian (semen-blood) and Epicurean (semen-semen) theories of early embryogeny.
• He settled the long controversy about which parts of the egg were nutritive and which was formative, by demonstrating the unreality of the distinction.

Legacy

William Harvey on a 1957 Soviet postage stamp

A final allusion to the rules established and followed by the physician throughout his life can be made:

1. "That none be taken into the Hospital but such as be curable, or but a certain number of such as are curable.
2. That none lurk here for relief only or for slight causes.
3. That the Chirurgions, in all difficult cases or where inward physic may be necessary, shall consult with the Doctor, at the times he sitteth once in the week and then the Surgeon himself relate to the Doctor what he conceiveth of the cure and what he hath done therein.
4. That no Chirurgion or his man do trepan the head, pierce the body, dismember, or do any great operation on the body of any but with the approbation and the direction of the Doctor..."^[36]

Arthur Schlesinger Jr. included William Harvey in a list of "The Ten Most Influential People of the Second Millennium" in the World Almanac & Book of Facts.

The main lecture theatre of the School of Clinical Medicine, University of Cambridge is named after William Harvey, who was an alumnus of the institute.

William Harvey Research Institute at Barts and The London School of Medicine and Dentistry is a research facility focussing on biochemical pharmacology, orthopaedic diseases, endocrinology, genomics, clinical pharmacology and translational medicine and therapeutics.

Harvey's whalebone demonstration rod, tipped with silver, resides in the silver room of the museum of the Royal College of Physicians. He used it to point to objects during his lectures.^[37]

William Harvey Hospital in Ashford, Kent is named after him. Harvey's hometown of Folkestone, Kent also has a statue of him.^{[citation needed]}

Personality

In terms of his personality, information shows that William Harvey was seen as a "...humorous but extremely precise man...",^[38] how he was often so immersed in his own thoughts that he would often suffer from insomnia (cured with a simple walk through the house), and how he was always ready for an open and direct conversation. He also loved the darkness, for it is said that it was there where "...he could best contemplate", thus sometimes hiding out in caves. A heavy drinker of coffee, Harvey would walk out combing his hair every morning full of energy and enthusiastic spirit through the fields. We have also come to understand Harvey's somewhat unorthodox method of dealing with his gout, here cited completely: "...his [Harvey's] cure was thus: he would sit with his legs bare...put them into a pail of water till he was almost dead with cold, then betake himself to his stove, and so 'twas gone".^[39] Apart from the already mentioned love of literature, Harvey was also an intense and dedicated observer of birds during his free time: several long and detailed passages of citations could be written delineating his observations in such places as the "Pile of Boulders" (a small island in Lancashire) and 'Bass Rock' (island off the East Coast of Scotland).

Gallery

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  William Harvey
  
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  Color portrait
  
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  William Harvey, after a painting by Cornelius Jansen
  
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  William Harvey
  

Works

• Harvey, William (1889). On the Motion of the Heart and Blood in Animals. London: George Bell and Sons.
• Harvey, William; Franklin, Kenneth J. (translator); Wear, Andrew (introduction) (1993). The Circulation of the Blood and Other Writings. London: Everyman: Orion Publishing Group. ISBN 0-460-87362-8.
• The Works of William Harvey. Robert Willis (translator). London: Sydenham Society. 1847. Includes:
  • An anatomical disquisition on the motion of the heart and blood in animals
  • 2 disquisitions addressed to John Riolan, including refutations to objections to the circulation of the blood
  • Anatomical exercises on the generation of animals. To which are added:
    • On Parturition
    • On the uterine membranes and humours
    • On conception
  • Anatomical examination of the body of Thomas Parr
  • Letters

Gravity assist

From Wikipedia, the free encyclopedia

The trajectories that enabled NASA's twin Voyager spacecraft to tour the four giant planets and achieve velocity to escape the Solar System

In orbital mechanics and aerospace engineering, a gravitational slingshot, gravity assist maneuver, or swing-by is the use of the relative movement (e.g. orbit around the Sun) and gravity of a planet or other astronomical object to alter the path and speed of a spacecraft, typically to save propellant and reduce expense. Gravity assistance can be used to accelerate a spacecraft, that is, to increase or decrease its speed or redirect its path. The "assist" is provided by the motion of the gravitating body as it pulls on the spacecraft.^[1] The gravity assist maneuver was first used in 1959 when the Soviet probe Luna 3 photographed the far side of Earth's Moon and it was used by interplanetary probes from Mariner 10 onwards, including the two Voyager probes' notable flybys of Jupiter and Saturn.

Explanation

Example encounter^[2]

A gravity assist around a planet changes a spacecraft's velocity (relative to the Sun) by entering and leaving the gravitational sphere of influence of a planet. The spacecraft's speed increases as it approaches the planet and decreases while escaping its gravitational pull (which is approximately the same), but because the planet orbits the Sun the spacecraft is affected by this motion during the maneuver. To increase speed, the spacecraft flies with the movement of the planet (taking a small amount of the planet's orbital energy); to decrease speed, the spacecraft flies against the movement of the planet. The sum of the kinetic energies of both bodies remains constant (see elastic collision). A slingshot maneuver can therefore be used to change the spaceship's trajectory and speed relative to the Sun.

A close terrestrial analogy is provided by a tennis ball bouncing off the front of a moving train. Imagine standing on a train platform, and throwing a ball at 30 km/h toward a train approaching at 50 km/h. The driver of the train sees the ball approaching at 80 km/h and then departing at 80 km/h after the ball bounces elastically off the front of the train. Because of the train's motion, however, that departure is at 130 km/h relative to the train platform; the ball has added twice the train's velocity to its own.

Translating this analogy into space: in the planet reference frame, the spaceship has a vertical velocity of v, while the planet is at rest. After the slingshot occurs and the spaceship leaves the planet, it will still have a velocity of v, but in the horizontal direction, as the effects of gravity cancel out.^[2] In the Sun reference frame, the planet has a horizontal velocity of v, and by using the Pythagorean Theorem, the spaceship initially has a total velocity of √2v. After the spaceship leaves the planet, it will have a velocity of v + v = 2v, gaining around 0.6v.^[2]

Possible outcomes of a gravity assist maneuver depending on the frame of reference

This oversimplified example is impossible to refine without additional details regarding the orbit, but if the spaceship travels in a path which forms a hyperbola, it can leave the planet in the opposite direction without firing its engine. This example is also one of many trajectories and gained speeds the spaceship can have.

This explanation might seem to violate the conservation of energy and momentum, apparently adding velocity to the spacecraft out of nothing, but the spacecraft's effects on the planet must also be taken into consideration to provide a complete picture of the mechanics involved. The linear momentum gained by the spaceship is equal in magnitude to that lost by the planet, so the spacecraft gains velocity and the planet loses velocity. However, the planet's enormous mass compared to the spacecraft makes the resulting change in its speed negligibly small. These effects on the planet are so slight (because planets are so much more massive than spacecraft) that they can be ignored in the calculation.^[3]

Realistic portrayals of encounters in space require the consideration of three dimensions. The same principles apply, only adding the planet's velocity to that of the spacecraft requires vector addition, as shown below.

Two-dimensional schematic of gravitational slingshot. The arrows show the direction in which the spacecraft is traveling before and after the encounter. The length of the arrows shows the spacecraft's speed.

 

Play media

A view from MESSENGER as it uses Earth as a gravitational slingshot to decelerate to allow insertion into an orbit around Mercury.

Due to the reversibility of orbits, gravitational slingshots can also be used to reduce the speed of a spacecraft. Both Mariner 10 and MESSENGER performed this maneuver to reach Mercury.

If even more speed is needed than available from gravity assist alone, the most economical way to utilize a rocket burn is to do it near the periapsis (closest approach). A given rocket burn always provides the same change in velocity (Δv), but the change in kinetic energy is proportional to the vehicle's velocity at the time of the burn. So to get the most kinetic energy from the burn, the burn must occur at the vehicle's maximum velocity, at periapsis. Oberth effect describes this technique in more detail.

Historical origins

In his paper “Тем кто будет читать, чтобы строить” (To those who will be reading [this paper] in order to build [an interplanetary rocket]),^[4] published in 1938 but dated 1918–1919,^[5] Yuri Kondratyuk suggested that a spacecraft traveling between two planets could be accelerated at the beginning and end of its trajectory by using the gravity of the two planets' moons. In his 1925 paper "Проблема полета при помощи реактивных аппаратов: межпланетные полеты" [Problems of flight by jet propulsion: interplanetary flights],^[6] Friedrich Zander made a similar argument.

The gravity assist maneuver was first used in 1959 when the Soviet probe Luna 3 photographed the far side of Earth's Moon. The maneuver relied on research performed under the direction of Mstislav Keldysh at the Steklov Institute of Mathematics.^[7][8]

Egorov’s work is mentioned in: Boris V. Rauschenbakh, Michael Yu. Ovchinnikov, and Susan M. P. McKenna-Lawlor, Essential Spaceflight Dynamics and Magnetospherics (Dordrecht, Netherlands: Kluwer Academic Publishers, 2002), pages 146–147.^[9]

Purpose

Plot of Voyager 2's heliocentric velocity against its distance from the Sun, illustrating the use of gravity assist to accelerate the spacecraft by Jupiter, Saturn and Uranus. To observe Triton, Voyager 2 passed over Neptune's north pole resulting in an acceleration out of the plane of the ecliptic and reduced velocity away from the Sun.^[10]

A spacecraft traveling from Earth to an inner planet will increase its relative speed because it is falling toward the Sun, and a spacecraft traveling from Earth to an outer planet will decrease its speed because it is leaving the vicinity of the Sun.

Although the orbital speed of an inner planet is greater than that of the Earth, a spacecraft traveling to an inner planet, even at the minimum speed needed to reach it, is still accelerated by the Sun's gravity to a speed notably greater than the orbital speed of that destination planet. If the spacecraft's purpose is only to fly by the inner planet, then there is typically no need to slow the spacecraft. However, if the spacecraft is to be inserted into orbit about that inner planet, then there must be some way to slow it down.

Similarly, while the orbital speed of an outer planet is less than that of the Earth, a spacecraft leaving the Earth at the minimum speed needed to travel to some outer planet is slowed by the Sun's gravity to a speed far less than the orbital speed of that outer planet. Thus, there must be some way to accelerate the spacecraft when it reaches that outer planet if it is to enter orbit about it. However, if the spacecraft is accelerated to more than the minimum required, less total propellant will be needed to enter orbit about the target planet.^{[clarification needed][dubious – discuss]} In addition, accelerating the spacecraft early in the flight reduces the travel time.

Rocket engines can certainly be used to increase and decrease the speed of the spacecraft. However, rocket thrust takes propellant, propellant has mass, and even a small change in velocity (known as Δv, or "delta-v", the delta symbol being used to represent a change and "v" signifying velocity) translates to a far larger requirement for propellant needed to escape Earth's gravity well. This is because not only must the primary-stage engines lift the extra propellant, they must also lift the extra propellant beyond that, which is needed to lift that additional propellant. Thus the liftoff mass requirement increases exponentially with an increase in the required delta-v of the spacecraft.

Because additional fuel is needed to lift fuel into space, space missions are designed with a tight propellant "budget", known as the "delta-v budget". The delta-v budget is in effect the total propellant that will be available after leaving the earth, for speeding up, slowing down, stabilization against external buffeting (by particles or other external effects), or direction changes, if it cannot acquire more propellant. The entire mission must be planned within that capability. Therefore, methods of speed and direction change that do not require fuel to be burned are advantageous, because they allow extra maneuvering capability and course enhancement, without spending fuel from the limited amount which has been carried into space. Gravity assist manoeuvers can greatly change the speed of a spacecraft without expending propellant, and can save significant amounts of propellant, so they are a very common technique to save fuel.

Examples:

• The Messenger mission used gravity assist maneuvering to slow the spacecraft on its way to Mercury; however, since Mercury has almost no atmosphere, aerobraking could not be used for insertion into orbit around it.
• Journeys to the nearest planets, Mars and Venus, often use a Hohmann transfer orbit, an elliptical path which starts as a tangent to one planet's orbit round the Sun and finishes as a tangent to the other. When other bodies are unavailable for gravity assists, this often takes the minimum amount of propellant.
• Even using a Hohmann transfer orbit, travel to the outer planets (Jupiter, Saturn, Uranus, and Neptune) would require an extremely large delta-v budget and powerful (or very long-burning) rockets to escape the Sun's gravity, and a very high speed to complete the journey in years rather than decades. Gravitational assist maneuvers offer a way to gain a very high speed without using propellant, therefore as of 2017, all missions to the outer planets have used them.^{[citation needed]}
  • The 1997 Cassini–Huygens mission to Saturn is an example of a mission to the outer Solar System. It used repeated gravity assist manouvres - Venus twice, and Earth and Jupiter once each - to travel 2.1 billion miles in a little over 6 years, arriving in 2004, which was far faster and more fuel-economical than attempting to travel the "straight line" 0.89 billion miles to Saturn directly without gravitational assistance.

Limits

The Planetary Grand Tour trajectory of Voyager 2

The main practical limit to the use of a gravity assist maneuver is that planets and other large masses are seldom in the right places to enable a voyage to a particular destination. For example, the Voyager missions which started in the late 1970s were made possible by the "Grand Tour" alignment of Jupiter, Saturn, Uranus and Neptune. A similar alignment will not occur again until the middle of the 22nd century. That is an extreme case, but even for less ambitious missions there are years when the planets are scattered in unsuitable parts of their orbits.

Another limitation is the atmosphere, if any, of the available planet. The closer the spacecraft can approach, the faster its periapsis speed as gravity accelerates the spacecraft, allowing for more kinetic energy to be gained from a rocket burn. However, if a spacecraft gets too deep into the atmosphere, the energy lost to drag can exceed that gained from the planet's gravity. On the other hand, the atmosphere can be used to accomplish aerobraking. There have also been theoretical proposals to use aerodynamic lift as the spacecraft flies through the atmosphere. This maneuver, called an aerogravity assist, could bend the trajectory through a larger angle than gravity alone, and hence increase the gain in energy.

Interplanetary slingshots using the Sun itself are not possible because the Sun is at rest relative to the Solar System as a whole. However, thrusting when near the Sun has the same effect as the powered slingshot described as the Oberth effect. This has the potential to magnify a spacecraft's thrusting power enormously, but is limited by the spacecraft's ability to resist the heat.

An interstellar slingshot using the Sun is conceivable, involving for example an object coming from elsewhere in our galaxy and swinging past the Sun to boost its galactic travel. The energy and angular momentum would then come from the Sun's orbit around the Milky Way. This concept features prominently in Arthur C. Clarke's 1972 award-winning novel Rendezvous With Rama; his story concerns an interstellar spacecraft that uses the Sun to perform this sort of maneuver, and in the process alarms many nervous humans.

A rotating black hole might provide additional assistance, if its spin axis is aligned the right way. General relativity predicts that a large spinning mass produces frame-dragging—close to the object, space itself is dragged around in the direction of the spin. Any ordinary rotating object produces this effect. Although attempts to measure frame dragging about the Sun have produced no clear evidence, experiments performed by Gravity Probe B have detected frame-dragging effects caused by Earth.^[11] General relativity predicts that a spinning black hole is surrounded by a region of space, called the ergosphere, within which standing still (with respect to the black hole's spin) is impossible, because space itself is dragged at the speed of light in the same direction as the black hole's spin. The Penrose process may offer a way to gain energy from the ergosphere, although it would require the spaceship to dump some "ballast" into the black hole, and the spaceship would have had to expend energy to carry the "ballast" to the black hole.

Timeline of notable examples

Mariner 10 – first use in an interplanetary trajectory

The Mariner 10 probe was the first spacecraft to use the gravitational slingshot effect to reach another planet, passing by Venus on February 5, 1974, on its way to becoming the first spacecraft to explore Mercury.

Voyager 1 – farthest human-made object

As of September 18, 2017, Voyager 1 is over 139.9 AU (20.9 billion km) from the Sun,^[12] and is in interstellar space.^[13] It gained the energy to escape the Sun's gravity completely by performing slingshot maneuvers around Jupiter and Saturn.^[14][15]

Galileo – a change of plan

The Galileo spacecraft was launched by NASA in 1989 aboard Space Shuttle Atlantis. Its original mission was designed to use a direct Hohmann transfer. However, Galileo's intended booster, the cryogenically fueled Centaur booster rocket was prohibited as a Shuttle "cargo" for safety considerations following the loss of Space Shuttle Challenger. With its substituted solid rocket upper stage, the IUS, which could not provide as much delta-v, Galileo did not ascend directly to Jupiter, but flew by Venus once and Earth twice in order to reach Jupiter in December 1995.

The Galileo engineering review speculated (but was never able to prove conclusively) that this longer flight time coupled with the stronger sunlight near Venus caused lubricant in Galileo's main antenna to fail, forcing the use of a much smaller backup antenna with a consequent lowering of data rate from the spacecraft.

Its subsequent tour of the Jovian moons also used numerous slingshot maneuvers with those moons to conserve fuel and maximize the number of encounters.

The Ulysses probe changed the plane of its trajectory

In 1990, NASA launched the ESA spacecraft Ulysses to study the polar regions of the Sun. All the planets orbit approximately in a plane aligned with the equator of the Sun. Thus, to enter an orbit passing over the poles of the Sun, the spacecraft would have to eliminate the 30 km/s speed it inherited from the Earth's orbit around the Sun and gain the speed needed to orbit the Sun in the pole-to-pole plane — tasks that are impossible with current spacecraft propulsion systems alone, making gravity assist maneuvers essential.

Accordingly, Ulysses was first sent toward Jupiter, aimed to arrive at a point in space just ahead and south of the planet. As it passed Jupiter, the probe fell through the planet's gravity field, exchanging momentum with the planet. This gravity assist maneuver bent the probe's trajectory northward relative to the Ecliptic Plane onto an orbit which passes over the poles of the Sun. By using this maneuver, Ulysses needed only enough propellant to send it to a point near Jupiter, which is well within current capability.

MESSENGER

The MESSENGER mission (launched in August 2004) made extensive use of gravity assists to slow its speed before orbiting Mercury. The MESSENGER mission included one flyby of Earth, two flybys of Venus, and three flybys of Mercury before finally arriving at Mercury in March 2011 with a velocity low enough to permit orbit insertion with available fuel. Although the flybys were primarily orbital maneuvers, each provided an opportunity for significant scientific observations.

The Cassini probe – multiple gravity assists

The Cassini probe passed by Venus twice, then Earth, and finally Jupiter on the way to Saturn. The 6.7-year transit was slightly longer than the six years needed for a Hohmann transfer, but cut the extra velocity (delta-v) needed to about 2 km/s, so that the large and heavy Cassini probe was able to reach Saturn, which would not have been possible in a direct transfer even with the Titan IV, the largest launch vehicle available at the time. A Hohmann transfer to Saturn would require a total of 15.7 km/s delta-v (disregarding Earth's and Saturn's own gravity wells, and disregarding aerobraking), which is not within the capabilities of current launch vehicles and spacecraft propulsion systems.

Cassini's speed related to Sun. The various gravity assists form visible peaks on the left, while the periodic variation on the right is caused by the spacecraft's orbit around Saturn. The data was from JPL Horizons Ephemeris System. The speed above is in kilometers per second. Note also that the minimum speed achieved during Saturnian orbit is more or less equal to Saturn's own orbital velocity, which is the ~5 km/s velocity which Cassini matched to enter orbit.

Parker Solar Probe

NASA's Parker Solar Probe mission, scheduled for launch in 2018, will use multiple gravity assists at Venus to remove the Earth's angular momentum from the orbit, in order to drop down to a distance of 8.5 solar radii (5.9 Gm) from the Sun. Parker Solar Probe's mission will be the closest approach to the Sun by any space mission.

Rosetta – first spacecraft to match orbit with a comet

The Rosetta probe, launched in March 2004, used four gravity assist maneuvers (including one just 250 km from the surface of Mars) to accelerate throughout the inner Solar System - enabling it to match the velocity of the 67P/Churyumov–Gerasimenko comet at their rendezvous point in August 2014.

Interplanetary Transport Network

From Wikipedia, the free encyclopedia

This stylized depiction of the ITN is designed to show its (often convoluted) path through the Solar System. The green ribbon represents one path from among the many that are mathematically possible along the surface of the darker green bounding tube. Locations where the ribbon changes direction abruptly represent trajectory changes at Lagrange points, while constricted areas represent locations where objects linger in temporary orbit around a point before continuing on.

The Interplanetary Transport Network (ITN)^[1] is a collection of gravitationally determined pathways through the Solar System that require very little energy for an object to follow. The ITN makes particular use of Lagrange points as locations where trajectories through space are redirected using little or no energy. These points have the peculiar property of allowing objects to orbit around them, despite lacking an object to orbit. While they use little energy, the transport can take a very long time. Shane Ross has said "Due to the long time needed to achieve the low energy transfers between planets, the Interplanetary Superhighway is impractical for transfers such as from Earth to Mars at present."^[2]

History

Interplanetary transfer orbits are solutions to the gravitational "restricted three-body problem", which, for the general case, does not have exact solutions, and is addressed by numerical analysis approximations. However, a small number of exact solutions exist, most notably the five orbits referred to as "Lagrange points", which are orbital solutions for circular orbits in the case when one body is significantly more massive.

The key to discovering the Interplanetary Transport Network was the investigation of the nature of the winding paths near the Earth-Sun and Earth-Moon Lagrange points. They were first investigated by Jules-Henri Poincaré in the 1890s. He noticed that the paths leading to and from any of those points would almost always settle, for a time, on an orbit about that point.^[3] There are in fact an infinite number of paths taking one to the point and away from it, and all of which require nearly zero change in energy to reach. When plotted, they form a tube with the orbit about the Lagrange point at one end.

The derivation of these paths traces back to mathematicians Charles C. Conley and Richard P. McGehee in 1968.^[4] Hiten, Japan's first lunar probe, was moved into lunar orbit using similar insight into the nature of paths between the Earth and the Moon. Beginning in 1997, Martin Lo, Shane D. Ross, and others wrote a series of papers identifying the mathematical basis that applied the technique to the Genesis solar wind sample return, and to Lunar and Jovian missions. They referred to it as an Interplanetary Superhighway (IPS)^[5]

Paths

As it turns out, it is very easy to transit from a path leading to the point to one leading back out. This makes sense, since the orbit is unstable, which implies one will eventually end up on one of the outbound paths after spending no energy at all. Edward Belbruno coined the term "weak stability boundary"^[6] or "fuzzy boundary"^[7] for this effect.

With careful calculation, one can pick which outbound path one wants. This turned out to be useful, as many of these paths lead to some interesting points in space, such as the Earth's Moon or between the Galilean moons of Jupiter.^[8] As a result, for the cost of reaching the Earth–Sun L₂ point, which is rather low energy value, one can travel to a number of very interesting points for a little or no additional fuel cost. But the trip from Earth to Mars or other distant location would likely take thousands of years.

The transfers are so low-energy that they make travel to almost any point in the Solar System possible.^{[citation needed]} On the downside, these transfers are very slow. For trips from Earth to other planets, they are not useful for manned or unmanned probes, as the trip would take many generations. Nevertheless, they have already been used to transfer spacecraft to the Earth–Sun L₁ point, a useful point for studying the Sun that was employed in a number of recent missions, including the Genesis mission, the first to return solar wind samples to Earth.^[9] The network is also relevant to understanding Solar System dynamics;^[10][11] Comet Shoemaker–Levy 9 followed such a trajectory on its collision path with Jupiter.^[12][13]

Further explanation

The ITN is based around a series of orbital paths predicted by chaos theory and the restricted three-body problem leading to and from the unstable orbits around the Lagrange points – points in space where the gravity between various bodies balances with the centrifugal force of an object there. For any two bodies in which one body orbits around the other, such as a star/planet or planet/moon system, there are three such points, denoted L₁ through L₃. For instance, the Earth–Moon L₁ point lies on a line between the two, where gravitational forces between them exactly balance with the centrifugal force of an object placed in orbit there. For two bodies whose ratio of masses exceeds 24.96,^[14] there are two additional stable points denoted as L₄ and L₅. These five points have particularly low delta-v requirements, and appear to be the lowest-energy transfers possible, even lower than the common Hohmann transfer orbit that has dominated orbital navigation in the past.

Although the forces balance at these points, the first three points (the ones on the line between a certain large mass, e.g. a star, and a smaller, orbiting mass, e.g. a planet) are not stable equilibrium points. If a spacecraft placed at the Earth–Moon L₁ point is given even a slight nudge towards the Moon, for instance, the Moon's gravity will now be greater and the spacecraft will be pulled away from the L₁ point. The entire system is in motion, so the spacecraft will not actually hit the Moon, but will travel in a winding path, off into space. There is, however, a semi-stable orbit around each of these points, called a halo orbit. The orbits for two of the points, L₄ and L₅, are stable, but the halo orbits for L₁ through L₃ are stable only on the order of months.

In addition to orbits around Lagrange points, the rich dynamics that arise from the gravitational pull of more than one mass yield interesting trajectories, also known as low energy transfers.^[4] For example, the gravity environment of the Sun–Earth–Moon system allows spacecraft to travel great distances on very little fuel^{[citation needed]}, albeit on an often circuitous route.

Missions

Launched in 1978, the ISEE-3 spacecraft was sent on a mission to orbit around one of the Lagrange points.^[15] The spacecraft was able to maneuver around the Earth's neighborhood using little fuel by taking advantage of the unique gravity environment. After the primary mission was completed, ISEE-3 went on to accomplish other goals, including a flight through the geomagnetic tail and a comet flyby. The mission was subsequently renamed the International Cometary Explorer (ICE).
The first low energy transfer using what would later be called the ITN was the rescue of Japan's Hiten lunar mission in 1991.^[16] Another example of the use of the ITN was NASA's 2001–2003 Genesis mission, which orbited the Sun–Earth L₁ point for over two years collecting material, before being redirected to the L₂ Lagrange point, and finally redirected from there back to Earth. The 2003–2006 SMART-1 of the European Space Agency used another low energy transfer from the ITN. In a more recent example, the Chinese spacecraft Chang'e 2 used the ITN to travel from lunar orbit to the Earth-Sun L₂ point, then on to fly by the asteroid 4179 Toutatis.