Asteroids are minor planets of the inner Solar System. Larger asteroids have been called planetoids; these terms have been applied to any astronomical object orbiting the Sun that did not resolve into a disc in a telescope and was not observed to have characteristics of an active comet such as a tail. As minor planets in the outer Solar System were discovered that were found to have volatile-rich surfaces similar to comets, these came to be distinguished from the objects found in the main asteroid belt. In this article, the term "asteroid" refers to the minor planets of the inner Solar System, including those co-orbital with Jupiter. There exist millions of asteroids, many the shattered remnants of planetesimals, bodies within the young Sun's solar nebula that never grew large enough to become planets; the vast majority of known asteroids orbit within the main asteroid belt located between the orbits of Mars and Jupiter, or are co-orbital with Jupiter. However, other orbital families exist with significant populations, including the near-Earth objects.
Individual asteroids are classified by their characteristic spectra, with the majority falling into three main groups: C-type, M-type, S-type. These were named after and are identified with carbon-rich and silicate compositions, respectively; the sizes of asteroids varies greatly. Asteroids are somewhat arbitrarily differentiated from meteoroids. In the case of comets, the difference is one of composition: while asteroids are composed of mineral and rock, comets are composed of dust and ice. Furthermore, asteroids formed closer to the sun; the difference between asteroids and meteoroids is one of size: meteoroids have a diameter of one meter or less, whereas asteroids have a diameter of greater than one meter. Meteoroids can be composed of either cometary or asteroidal materials. Only one asteroid, 4 Vesta, which has a reflective surface, is visible to the naked eye, this only in dark skies when it is favorably positioned. Small asteroids passing close to Earth may be visible to the naked eye for a short time.
As of February 2020, the Minor Planet Center had data on 858,000 objects in the inner and outer Solar System, of which about 542,000 had enough information to be given numbered designations. The United Nations declared 30 June as International Asteroid Day to educate the public about asteroids; the date of International Asteroid Day commemorates the anniversary of the Tunguska asteroid impact over Siberia, Russian Federation, on 30 June 1908. In April 2018, the B612 Foundation reported "It's 100 percent certain we'll be hit, but we're not 100 percent sure when." In 2018, physicist Stephen Hawking, in his final book Brief Answers to the Big Questions, considered an asteroid collision to be the biggest threat to the planet. In June 2018, the US National Science and Technology Council warned that America is unprepared for an asteroid impact event, has developed and released the "National Near-Earth Object Preparedness Strategy Action Plan" to better prepare. According to expert testimony in the United States Congress in 2013, NASA would require at least five years of preparation before a mission to intercept an asteroid could be launched.
The first asteroid to be discovered, was considered to be a new planet. This was followed by the discovery of other similar bodies, with the equipment of the time, appeared to be points of light, like stars, showing little or no planetary disc, though distinguishable from stars due to their apparent motions; this prompted the astronomer Sir William Herschel to propose the term "asteroid", coined in Greek as ἀστεροειδής, or asteroeidēs, meaning'star-like, star-shaped', derived from the Ancient Greek ἀστήρ astēr'star, planet'. In the early second half of the nineteenth century, the terms "asteroid" and "planet" were still used interchangeably. Overview of discovery timeline: 10 by 1849 1 Ceres, 1801 2 Pallas – 1802 3 Juno – 1804 4 Vesta – 1807 5 Astraea – 1845 in 1846, planet Neptune was discovered 6 Hebe – July 1847 7 Iris – August 1847 8 Flora – October 1847 9 Metis – 25 April 1848 10 Hygiea – 12 April 1849 tenth asteroid discovered 100 asteroids by 1868 1,000 by 1921 10,000 by 1989 100,000 by 2005 ~700,000 by 2015 Asteroid discovery methods have improved over the past two centuries.
In the last years of the 18th century, Baron Franz Xaver von Zach organized a group of 24 astronomers to search the sky for the missing planet predicted at about 2.8 AU from the Sun by the Titius-Bode law because of the discovery, by Sir William Herschel in 1781, of the planet Uranus at the distance predicted by the law. This task required that hand-drawn sky charts be prepared for all stars in the zodiacal band down to an agreed-upon limit of faintness. On subsequent nights, the sky would be charted again and any moving object would be spotted; the expected motion of the missing planet was about 30 seconds of arc per hour discernible by observers. The first object, was not discovered by a member of the group, but rather by accident in 1801 by Giuseppe Piazzi, director of the observatory of Palermo in Sicily, he discovered a new star-like object in Taurus and followed the displacement of this object during several nights. That year, Carl Friedrich Gauss used these observations to calculate the orbit of this unknown object, found to be between the planets Mars and Jupiter.
Piazzi named it after Ceres, the Roman goddess of agriculture. Three other a
Conn Cétchathach, son of Fedlimid Rechtmar, according to medieval Irish legendary and annalistic sources, a High King of Ireland, the ancestor of the Connachta, through his descendant Niall Noígiallach, the Uí Néill dynasties, which dominated Ireland in the early Middle Ages, their descendants. The Annals of the Four Masters says that five roads to Tara, which had never been seen before, were discovered on the night of Conn's birth. According to the Lebor Gabála Érenn, he took power after killing his predecessor Cathair Mór. In other sources his predecessor is Dáire Doimthech; the Lia Fáil, the coronation stone at Tara, said to roar when the rightful king stood on it, roared under Conn for the first time since Cúchulainn split it with his sword when it failed to roar for Lugaid Riab nDerg. In the saga Baile in Scáil, Conn treads on the stone by accident while walking the ramparts of Tara, implying that the stone had been lost and half-buried since Cúchulainn's time. A druid explains the meaning of the stone, says the number of cries the stone made is the number of kings who will follow Conn, but he is not the man to name them.
A magical mist arises, a horseman approaches who throws three spears towards Conn asks him and the druid to follow him to his house, which stands on a plain by a golden tree. They enter, are welcomed by a woman in a gold crown. First they see a silver vat, bound with gold hoops, full of red ale, a golden cup and serving spoon, they see a phantom, a tall beautiful man, on a throne, who introduces himself as Lugh. The woman is the sovereignty of Ireland, she serves Conn a meal consisting of an ox's rib 24 feet long, a boar's rib; when she serves drinks, she asks "To whom shall this cup be given?", Lugh recites a poem which tells Conn how many years he will reign, the names of the kings who will follow him. They enter Lugh's shadow, the house disappears, but the cup and serving spoon remain. An earlier text, Baile Chuinn Cétchathaigh gives a poetic list of kings, many of which are recognisable from the traditional List of High Kings of Ireland, but without narrative context. Conn had a long reign – twenty, twenty-five, thirty-five or fifty years according to different versions of the Lebor Gabála, spending much of it at war with Mug Nuadat, king of Munster.
According to the medieval text Cath Maige Leana, Mug Nuadat's father, Mug Neit son of Deirgtine, had expelled the kings of Munster, Conaire Coem and Mac Niad mac Lugdach. The two kings fled to Conn, married his daughters and Sadb respectively. Mug Neit was defeated and killed after two battles in County Offaly. Mug Nuadat led his father's forces in retreat through Munster, fighting Conn to a standstill before escaping by sea to Beare Island, thence to Spain. Conn withdrew. Nine years Mug Nuadat, who had married the daughter of the king of Spain, landed with an army near Bantry Bay and forced Conaire and Mac Niad to submit to his overlordship. With the kings of Ulster and Leinster, he marched north to Mag nAi and forced Conn to make a treaty with him, dividing Ireland between them: Conn controlling the north, or Leth Cuinn, Mug Nuadat the south, or Leth Moga, with the border lying between Galway in the west and Dublin in the east. After fifteen years of peace Mug Nuadat broke the treaty and declared war, along with the kings of Ulster and Leinster.
He led his army near Tullamore, County Offaly. Conn retreated to Connacht, gathered his forces, retook Meath from the king of Ulster, he marched south to Mag Leana and destroyed Mug Nuadat's army in a surprise night attack on his camp. Mug Nuadat was killed in the fighting, Conn became king of all of Ireland. Geoffrey Keating tells the story differently. In his account Mug Nuadat obtains an army from the king of Leinster and expels the kings of Munster, here Lugaid Allathach, Dáire Dornmhor and Aonghus. Aonghus flees to Conn, who gives him an army with which to reclaim his kingdom, but Mug Nuadat defeats this and a further nine attempts by Conn to drive him out of Munster, forcing Conn to divide Ireland with him; when hostilities break out again and Mug Nuadat's armies gather for battle at Mag Leana, but Conn kills Mug Nuadat in his bed in an early morning attack. According to a medieval source, the hero Fionn mac Cumhaill was born in Conn's time, his father, Cumhall, a warrior in Conn's service, was a suitor of Muirne, daughter of the druid Tadg mac Nuadat, but Tadg refused his suit, so Cumhall abducted her.
Conn went to war against him, Cumhall was killed by Goll mac Morna in the Battle of Cnucha. But Muirne was pregnant, Tadg rejected her, ordering her to be burned, she fled to Conn, Conn put her under the protection of Cumhall's brother-in-law Fiacal mac Conchinn. It was in Fiacal's house that she gave birth to a son, renamed Fionn; when he was ten, Fionn came to Tara put himself into Conn's service. He learned that every year at Samhain, the monster Aillen would put everyone at Tara to sleep with his music, burn down the palace with his fiery breath. Fionn killed Aillen, having kept himself awake by pressing the head of his spear to his forehead, warded off Aillen's flame with his magical cloak, Conn made him head of the fianna in place of Goll. Conn had two sons and Art. Connla fell in love with a fairy woman from Mag Mell, went with her to her otherworld home in her crystal boat, leaving Art alone. After that Art was known as
In computer networking, port knocking is a method of externally opening ports on a firewall by generating a connection attempt on a set of prespecified closed ports. Once a correct sequence of connection attempts is received, the firewall rules are dynamically modified to allow the host which sent the connection attempts to connect over specific port. A variant called single packet authorization exists, where only a single "knock" is needed, consisting of an encrypted packet; the primary purpose of port knocking is to prevent an attacker from scanning a system for exploitable services by doing a port scan, because unless the attacker sends the correct knock sequence, the protected ports will appear closed. Port knocking is implemented by configuring a daemon to watch the firewall log file for connection attempts to certain points, to modify the firewall configuration accordingly, it can be performed on the kernel level or by a userspace process examining packets at a higher level, allowing the use of "open" TCP ports to be used within the knock sequence.
The port "knock" itself is similar to a secret handshake and can consist of any number of TCP, UDP or sometimes ICMP and other protocol packets to numbered ports on the destination machine. The complexity of the knock can be anything from a simple ordered list to a complex time-dependent, source-IP-based and other-factor-based encrypted hash. A portknock daemon on the firewall machine listens for packets on certain ports; the client user would carry an extra utility, which could be as simple as netcat or a modified ping program or as complicated as a full hash-generator, use that before they attempted to connect to the machine in the usual way. Most portknocks are stateful systems in that if the first part of the "knock" has been received an incorrect second part would not allow the remote user to continue and, would give the remote user no clue as to how far through the sequence they failed; the only indication of failure is that, at the end of the knock sequence, the port expected to be open is not opened.
No packets are sent to the remote user at any time. While this technique for securing access to remote network daemons has not been adopted by the security community, it has been used in many rootkits before year 2000. Defeating port knocking protection requires large-scale brute force attacks in order to discover simple sequences. An anonymous brute force attack against a three-knock TCP sequence would require an attacker to test every three port combination in the 1–65535 range and scan each port between attacks to uncover any changes in port access on the target system. Since port knocking is by definition stateful, the requested port would not open until the correct three-port number sequence had been received in the correct order and without receiving any other intervening packets from the source; the average case scenario requires 141 trillion packets to determine a correct three-port number. This technique, in combination with knock attempt-limiting, longer or more complex sequences and cryptographic hashes, makes successful port access attempts difficult.
Once the successful port knock sequence is supplied to open a port, firewall rules only open the port to the IP address that supplied the correct knock, adding dynamic functionality to firewall behaviour. Instead of using a preconfigured static IP whitelist on the firewall, an authorised user situated anywhere in the world would be able to open any necessary port without assistance from the server administrator; the system could be configured to allow the authenticated user to manually close the port once the session is over or to have it close automatically using a timeout mechanism. To establish a new session, the remote user would be required to reauthenticate using the correct sequence; the stateful behaviour of port knocking allows several users from different source IP addresses to be at varying levels of port knock authentication allowing a legitimate user with the correct knock sequence through the firewall while the firewall itself is in the middle of a port attack from multiple IP addresses.
From any other attacking IP address, the ports on the firewall will still appear to be closed. Using cryptographic hashes inside the port knock sequence defends against packet sniffing between the source and target machines, preventing discovery of the port knock sequence or using the information to create traffic replay attacks to repeat prior port knock sequences. Port knocking is used as part of a defense in depth strategy. If the attacker were to gain port access, other port security mechanisms are still in place, along with the assigned service authentication mechanisms on the opened ports. Implementation of the technique is straightforward, using at the bare minimum a shell script on the server and a Windows batch file or command line utility on the client. Overhead on both the server and client in terms of traffic, CPU and memory consumption is minimal. Port knock daemons are not complex to code. A port knock system implemented on ports such as the SSH sidesteps the issue of brute force password attacks on logins.
In the case of SSH, the SSH daemon is not activated without the correct port knock, the attack is filtered by the TCP/IP stack rather than using SSH authentication resources. To the attacker