Space Earth's Orbit and Planetary Missions Maneuvers

To control and/or operate a satellite in an Earth orbit, various trim maneuvers may be necessary as an interplanetary mission requires various maneuvers along its different phases

Earth's Orbit Maneuvers

Planetary Missions Maneuvers

Earth's Orbit Maneuvers

To fly a satellite, or the Shuttle in orbit, in the sense that an airplane is flown, is impossible as there is no atmosphere in space. The main orbital characteristics like the speed, the altitude, the perigee and the apogee, or the inclination of the orbit, are first determined at the instant of the last booster engine cut off

Further into the flight, alterations of the orbit may be wanted like to reach a higher orbit, or to increase the apogee or perigee of the satellite (its farthest or nearest point to the Earth). Such maneuvers, called "Orbit Trim Maneuvers" (OTMs) are performed via the engines embarked aboard the satellite. Most of the time, it's about small rocket engines, and thrusters. Such devices allow three-axis stability and spin's control too. Such thrusters generally work with the hydrazine fuel, which is dangerous as nitrogen tetroxide serves as the oxidizer. Reaction wheels are features which spin to help too a spacecraft maintain attitude control as control moment gyroscopes for attitude control consist of spinning flywheels, tilted using gimbals, to produce torque that can turn a spacecraft. Reaction wheels are used to control a spacecraft attitude, or which way it is facing relative to the Sun, Earth or a planet. Increasing the rotation rate of a reaction wheel causes the spacecraft itself to rotate in the opposite direction. That is a more fuel-efficient ways to procede with than using thrusters for attitude control

OTMs all work on the same basis, that is the craft is oriented appropriately to the desired maneuver, and a magnitude and a time of application are determined

• generally a positive thrusting period tangential to the orbit raises every point of the orbit, except the thrusting point, as it increases the satellite's speed. The contrary for a negative one. Every point of the orbit is lowered, except the thrusting point, as the speed is decreased. A usual application of such thrusts is to decrease the apogee (farthest Earth), or to raise the perigee (nearest Earth), of a satellite. In the first case, a negative thrust is fired when the craft is nearest Earth (perigee). This reduces the satellite's speed. In the second case, the contrary. A positive thrust is given when the craft is farthest Earth (apogee). It increases the satellite's speed
• generally, an "out-of-plane thrusting period", performed outside the plane of the orbit, is used to modify the inclination of the orbit
• the thrusting forces, at last, may be used perpendicularly to the spacecraft path, in the orbital plane. Such a radial thrusting period may be exerted in direction of the Earth, or away from it

Time and fuel -or power- efficient maneuvers might help for swifter orbital moves to future orbital observatories or reconnaissance satellites which need to reorient themselves rapidly from a point to another. Engineers, until now, had the craft to follow a straight line. New procedures are derived from early 1700's Swiss mathematician Bernoulli, who had shown that straight line is not the most rapid path between two points as a bead, for example, which moves from a point to another is moving faster when following a curved path and allowing gravity to assist into the acceleration. Such procedures might also become common aboard the ISS. The satellite's reaction wheels are then used simultaneously as that is adding speed to the maneuver and the craft follows a 5-points star-like pattern. In case of a multicraft mission, probes usually are deployed one at a time into separate orbits. When aboard a Atlas, for example, the Centaur upper stage spins up, deploys the first probe, stops its spin, and then turns to aim the second probe toward its orbit. It spins back up again and separates the second probe. With 149 U.S. government-owned spacecraft and 275 commercial satellites currently in geosynchronous Earth orbit, or GEO, a technique by NASA is now aiming to provide to a remote-controlled robot can service and refuel a satellite in orbit as experimented aboard the ISS. A fully robotic, autonomous maintenance vehicle could service satellites, including those that were not originally intended to be serviced. Five 'R' capabilities would be: refueling, repositioning, remote survey, component replacement or repairing an ailing satellite. Such a new concept is shaking the accepted paradigm that a satellite must be decommissioned when out of fuel as the search is of the responsability, since 2009, of the Satellite Servicing Capabilities Office (SSCO) at NASA’s Goddard Space Flight Center

The way NASA handled the return into the Earth's atmosphere of a Earth science mission by the summer of 2010 is a good example of how such a re-entry is to be performed and how a mission may end. The ICESat mission had launched by January 2003 as a three-year mission with a goal of returning science data for five years. It was the first mission of its kind –specifically designed to study Earth's polar regions with a space-based laser altimeter. The failure of the primary instrument in February 2010 brought the mission to a end. Before any other decision, because the spacecraft remained in operating conditions, NASA's Science Mission Directorate accepted proposals for engineering tests to be performed using ICESat. In the perspective of the definitive disposal of the satellite, mission flight controllers began firing ICESat's propulsion system thrusters on June 23, 2010 to lower its orbit. Thruster firings ended on July 14, safely reducing the lowest point of the spacecraft's orbit to 125 miles (200 km) above Earth's surface. The orbit has since naturally decayed. ICESat was successfully decommissioned from operations on Aug. 14. All remaining fuel on the spacecraft is now depleted, and atmospheric drag is slowly lowering ICESat's orbit until the spacecraft re-enters the Earth's atmosphere by itself and burns there. The vast majority of ICESat will burn up in the atmosphere during re-entry. Of the spacecraft's total mass (about 2000 lbs.), only a small percent will reach the surface of Earth. Some pieces of the spacecraft, weighing collectively about 200 pounds, are expected to survive re-entry. The risk of harm coming to anyone on Earth from this debris is estimated to be very low. ICESat is a polar orbiting mission. Non-controlled reentry of a Earth-orbiting satellite may spread debris over a 500-mile (800-km) area or, in case of the German ROSAT science mission, a heat-resistant piece like a telescope mirror may cause damage similar to ones by a airliner dropping one engine. Since 1991, space agencies worldwide have adopted new procedures for satellites falling back Earth (along with space junk procedures). No NASA satellite, for example, should fall uncontrolled during the next 25 years. Reentry and when the satellite reaches 90 miles (150 km) of altitude may travel far, change its flight pattern or even direction. Usually most of a satellite parts burn up during reentry as fragments of weight may reach ground however, like a total of 1.87 tons (1,7 tonnes). Rules in that matter, from a international point of view is that uncontrolled reentries should have a less than 1 in 10,000 chance of injuring anyone on the ground. Typically main body of a craft breaks apart typically at 43.5-50 miles altitude, after which the insides are scattered. NASA, in the case of ICESat was to monitor the satellite down to its last orbits issuing the most updated predictions for the potential fall of debris. Decay of a orbit generally is often depending in large part on solar activity. Spacecraft in a geosynchronous orbit might not fall back Earth for at least 1,000 years as a garage parking spot is also used, generally, for decommissioned satellites. It is hard to predict the precise time and location of a satellite re-entry due to the instability of the space environment that directly causes premature orbital decay as the upper atmosphere and ionosphere react strongly to even small changes -quite small energy inputs- in near-Earth space whatever the location of such inputs. Numerous factors make that derelict satellites tumbled in unpredictable ways before uncontrolled reentry. International regulations state that minimal debris should be left to propagate within heavily trafficked orbits, especially the low orbits favoured by Earth-observing missions and some classes of communication satellite, not to mention manned spacecraft and the International Space Station. The requirement is that low-earth orbit satellites are removed within 25 years of ending their lives. Or they should end up at an altitude where atmospheric drag gradually induces reentry, or alternatively be dispatched up to quieter 'graveyard orbits.' Designing low Earth-orbit satellites for demise, could be also a possibility

In terms of returning a spacecraft Earth either manned or unmanned, a rotor system could be also used in place of parachutes on returning spacecraft Earth, giving a capsule stability and control and touch down anywhere in the world, whether it be a runway or the top of a building. In other words, wherever a helicopter could land, a spacecraft could too. The rotor concept also would fit nicely with spent rocket boosters. NASA researched the concept for the Apollo capsules but opted for the parachute return for the sake of shortening development time during Moon race

Planetary Missions Maneuvers

The logics of the planetary missions is about the same, at the mission's beginning, than those of a satellite in an Earth's orbit. Once the last launcher elements has burned, the interplanetary spacecraft is on its trajectory. From that point on, the craft is coasting towards its target(s). It's the cruise phase. During the cruise phase of a mission, navigators’ assessments of the spacecraft’s trajectory use three types of tracking information from ground antennas of NASA’s Deep Space Network (DSN). One method is 'ranging,' which measures the distance to the spacecraft by timing precisely how long it takes for a radio signal to travel to the spacecraft and back (or a 'two-way method'). A second is 'Doppler,' which measures the spacecraft’s speed relative to Earth by the amount of shift in the pitch of a radio signal from the craft. A newer method, called 'delta differential one-way range measurement,' adds information about the location of the spacecraft in directions perpendicular to the line of sight. For this method, pairs of antennas on different continents simultaneously receive signals from the spacecraft, and then the same antennas observe natural radio waves from a known celestial reference point, such as a quasar, which serves as a navigation reference point. Currently, most missions rely on ground-based antennas paired with atomic clocks for navigation. Ground antennas send narrowly focused signals to spacecraft, which, in turn, return the signal. NASA uses the difference in time between sending a signal and receiving a response to calculate the spacecraft’s location, velocity and path. Planetary missions are carrying one main solid rocket motor, smaller rocket engines (of the order of magnitude of 400 Newton), and thrusters (of the order of 1 to 20 N). Rocket engines may take the role of the main solid rocket engine. Thrusters are mostly used to keep the craft from rolling during the maneuvers as planetary missions are featuring too reaction wheels which spin to help a spacecraft maintain attitude control. Like a example of a behavior of a planetary vessel en route, just take the one of rover Curiosity which, during the winter 2011-2012, was on its journey to Mars and was spinning about itself twice a second. Such a revolution is deviced to improve stability

As far as ion engines are concerned, such engines blast a small stream of charges particles to propel a spacecraft. They start slow but can build up tremendous speeds over time because they produce levels of thrust relative to chemical thrusters, but does so at higher specific impulse (or higher exhaust velocities), which means that an ion thruster has a fuel efficiency of 10-12 times greater than a chemical thruster. A ion engine usually needs to operate in excess of 10,000 hours to slowly accelerate the spacecraft to speeds necessary to reach the asteroid belt or beyond. As of early 2013, a ion thruster ran for over 43,000 hours, which means that the thruster has processed over 770 kilograms of xenon propellant and can provide 30 million-newton-seconds of total impulse to the spacecraft, demonstrating performance which permits future science spacecraft to travel to varied destinations, such as extended tours of multi-asteroids, comets, and outer planets and their moons. Such ion engines thus are able to be a substitute to the initial thrust to interplanetary journeys which, until now, is provided by some stage of the launcher

The 'International Terrestrial Reference Frame,' or ITRF is used by Earth-orbiting satellites as also a fundamental reference for interplanetary navigation of spacecraft. The ITRF is managed by the International Earth Rotation and Reference Systems Service’s International Terrestrial Reference System Product Center at the Institut National de l’Information Géographique et Forestière, known as IGN, in Paris, France. The ITRF is made up of specific geographic positions around the world, along with information about how each one drifts over time. Along a trajectory, the engineers team have to perform some Trajectory Correction Maneuvers (TCMs) which are aiming at correcting any discrepancy between the projected trajectory, and the actual one. TCMs are also mandatory due to that the firing of the last stage has to aim to a point different from the probe's actual trajectory so the rockets, which are not treated to avoid bio-pollution at a planetary object like probes are, fly to those. To estimate the effects of a TCM about a probe's trajectory, flight engineers rely upon tracking measurements by Earth stations and models they build as the trajectory unfolds. 'Navigation is all about statistics, probability and uncertainty' however. As a probe may also fire some thrusters daily to keep its solar panels directed to the Sun and its antenna to the Earth, such small firings have also to be taken in account and counterbalanced. Some missions are so successfully launched that the first push given after launch precludes some of the first TCMs planned. The craft trajectory may be adjusted in velocity and/or in direction, hence the craft is generally rotated in its three-dimensional frame of reference as the rocket engines, or thrusters, are fired for a certain amount of time to reach the desired magnitude of changes. Engines involved may be bi-propellant, like with hydrazine and nitrogen tetroxide or mono-propellant. A velocity change is called a "delta-V". It's usually of the order of meters or tens of meters per second, that is a minute fraction of the craft's speed. The craft, after the burn, is returned to its regular cruise-phase attitude. The improvement in the modelling of trajectories is such that most recent missions often skip planned TCMs, like NASA's Twin Rovers mission to Mars. A TCM in early 2011, for example, which occurred at the extended Stardust mission to a comet, adjusted the craft flight path, and changed the spacecraft's speed by 2.6 yards per second (2.6 meters per second) as the mission was closing in on the comet at 24,236 miles per hour (39,000 kilometers per hour) and still two weeks and 8.37 million miles (13.5 million kilometers) away from its target. The Stardust spacecraft fired its rockets for 130 seconds, consuming about 10.6 ounces of fuel. As as a probe like the cometary Stardust one is concerned, its attitude and translational thrusters had fired almost half-a-million times each over 12 years in total. Every deep-space exploration spacecraft has a fuel supply customized to get the job done, with some held in reserve for contingency maneuvers and other uncertainties. A change of trajectory aiming a both changing the velocity of a craft and its axis (to the left, or right, for example) uses, on the one hand, a change of velocity in the direction of the axis of journey and a lateral change. First is generally performed through some thrusters during a seemingly long duration (like 19 minute for example to gain some 12.3 miles per hour) as the second one, on the other hand, is through the firing of thrusters each time that the rotation of the craft brings them to the appropriate location for the push (they can be fired, for example, 200 times 5 seconds during a 2-hour period, or a total of 40 minutes of push). A craft usually spins about itself for reasons of stabilization and that is generally done in the axis of motion. Any trajectory modification is checked from the Earth through spacecraft data and Doppler-effect changes in radio signal from the craft. As a probe heads to its planetary target, on a other hand, the team also checks the status of the mission's instruments as it power, calibrate those and other actions, like taking dark images for the imaging systems as the timing of such operations varies function of the mission. Another aspect of a interplanetary journey to be mentioned is that a spacecraft then is subjet to both energetic solar radiation and cosmic rays, effects of which are still ill-known until now and how a craft respond to them. Such solar events are striking orbiting vessels also. In 2012 thus, the orienting and stabilizing instrument of the ESA Venus Express mission at Venus was blinded by a solar storm as usually spacecraft recover in such a case. In terms of timekeeping, time for a mission, as critical for the moment, for example, of a planetary landing of fly-by, is nowadays computed through a two-way system in which information is sent to Earth, requiring a ground team to calculate timing and navigation and then transmitting it back to the spacecraft. One-way navigation technology is to be tested, increasing navigation and radio science data quantity by two to three times, improving data quality by up to 10 times as future human exploration of the solar system will demand more tracking from the Deep Space Network than can currently be delivered with the existing system. That's the way the ultra stable, highly accurate 'Deep Space Atomic Clock,' or DSAC, will enable a spacecraft to calculate its own timing and navigation data in real time, allowing pinpoint landings and alleviating the question of when signal delays are too great for the ground to interact with the spacecraft during the event. Albeit impacted by gravity, a embarked atomic clock will have a precision been refined to permit drift of no more than one nanosecond in 10 days, due to the work of NASA engineers at the Jet Propulsion Laboratory. NASA generally managed to dramatically improve the accuracy of the trajectory of its planetary missions, especially, for example, those landing at Mars, allowing for reduced landing ellipses. Spacecraft now may also be allowed to software updates while on its journey to a target. The sextant, in terms of manned navigation, a navigation tool since old which have a optical sight to take precise angle measurements from land or sea was considered by NASA -and still is for manned deep space missions. NASA’s Gemini missions conducted the first sextant sightings from a spacecraft as Apollo designers built a sextant into vehicles and Jim Lovell demonstrated on Apollo 8 that sextant navigation could return a space vehicle home. The ability to sight angles between the moon or planets and stars offers crews another option to find their way home if communications and main computers are compromised

Once at its target, the maneuvers of a planetary mission depend on what the mission precisely is. That is whether the mission is a flyby mission -the craft just fly by the target, an orbital one -the craft enters into the planet's orbit, or an atmospheric or landing mission -the craft makes it way down to the planet's surface either to collect data about the atmosphere or a lander eventually lands at the planet. In any case, these phases of the mission mark the end of the cruise phase. A planetary mission teams of scientists and flight engineers generally work together at a same institution -often the JPL- during the first few months after a landing, for example. At Mars further, teams first work according to Martian days, or sols, which are 40 minutes longer than Earth's day as they later pass back to Earth's time. Non-targeted flybys are flybys of opportunity which require no special maneuvers, but rather the celestial object happening to be relatively close to a spacecraft’s path. That mostly occurs with missions orbiting inside a gas giant system which its moons

• in case of a flyby mission, the craft is just grazing its target at a more or less large distance. Few, if not nothing, may be done in terms of maneuvers. Most of the work relies on the computation of the trajectory and the TCMs performed during the cruise phase both in terms of speed and lateral aiming. The craft has to reach the correct point, at the correct time. Most of the operations consist into the correct positionment of the craft. The parameters stored aboard the spacecraft have the engines/or thrusters actioned so the mission instruments are correctly aiming at the planet in the purpose of gathering the planned data. Last minute correction of such data may be done, due to last minute data beamed back Earth regarding the craft trajectory. First flyby ever of a planet occurred on Dec. 14, 1962 when NASA's Mariner 2 spacecraft sailed by Venus on Dec. 14, 1962 by 21,600 miles (34,762 km), scanning the planet for 42 minutes
  
• an orbital mission is when the craft enters into orbit around it target-planet, like the case for the orbiters at Mars or such a craft like Galileo in the Jupiter's system. Like for a flyby the trajectory has to bring the craft at the correct point at the correct time. Once the craft intercepted by the planet gravitational field, a maneuver is performed, which is called an "orbit insertion". This is done through the firing of the main engine(s). Such a burn aims at tremendously decrease the craft's speed so the gravity field captures it. Once done, the craft is in orbit around the planet. Whence, the mission may be a pure orbiter mission, that is the craft will orbit around the planet to gather data like a satellite would at Earth. In this latter case, the orbit adjustment may be obtained by aerobraking, the craft skimming the planet's atmosphere until the wanted orbit is acquired (aerobraking can be used also later in the mission to move, for example, from a elliptical to a more circular orbit). Aerobraking allows to achieve orbit at a cheaper use of fuel. Or it may be a "system" mission, that is a mission which is spending numerous, various, orbits around a gas giant, like Galileo did about Jupiter. In the first case, the craft performs the maneuver a terrestrial satellite is performing. In the second case, various gravity-assisted flybys and engine burns are used as "Orbit Trim Maneuvers" (OTMs) to bring the craft to multiple encounters with the planet and its moons. The aerocapture concept might be developed, on the other hand, like a technique of its own, and used for the whole of the orbital capture of a spacecraft, in the perspective of the current fuel conservation concerns. When a mission is looking towards the Sun from behind a planet, that is a geometry referred to by planetary scientists as a 'high solar phase.' Around Moon, a station-keeping maneuver is a periodic adjustment to the orbit to prevent the spacecraft from hitting the lunar surface as a phasing maneuver is firing the engines to slightly speed up or slow down the spacecraft on the orbit. A planetary craft, generally, may be oriented through reaction wheels, which are gyroscope-like devices, or thrusters, which are small engine using a fuel called hydrazine. In some cases a combination of both is used. When in orbit, maneuvers to modify the orbit for varied purposes may also be termed 'station keeping burns'
  
• an atmospheric or a landing mission needs the craft to enter the planet's atmosphere and/or to cross it down to the surface. NASA Langley researchers are experts in modeling and simulations for entry, descent and landing (EDL) at Mars since the Viking landers in 1976. Simulations first developed during the 1960s have even been validated and updated with experience as real EDLs are used then to improved models. In the case of an atmospheric craft or probe ultimate Trajectory Correction Maneuvers may be needed to have the craft/or probe reach the atmosphere. From there, the craft/or probe is heatshield-protected as it's braked with parachutes once an appropriate speed is reached. The heatshield is then discarded as the probe begins its data work. In case of a landing mission, the first part of the process is the same. The cruise phase must have brought the craft to the correct point of entry into the atmosphere. In terms of how flight engineers bring the craft to that point at a planet like Mars, for example, they aim for a point inside of a target box that is 1.7 by 7.15 miles (2.8 by 11.5 kilometers) only wide, situated along the trajectory of the mission should it consist into a flyby and miss the planet left or right. Mars' gravity then just pull the spacecraft into the Martian atmosphere. That box is located unto a so-called 'B-plane', a plane perpendicular to the one of the trajector of the mission. The entry is first slowed through a heatshield. Heatshield protection may come into two sorts, like a heat shield protecting the vehicle by absorbing heat and re-radiating it out into space as active systems keep heat away from the vehicle's structure by partially burning or charring it. The sequel may vary as the craft may be brought to surface through parachutes, parachutes and airbags, or parachutes and retro-rockets. As re-entry at Earth's atmosphere had been answered with the blunt body design, the Martian atmosphere, for example is very thin, at about 1/100th of the atmospheric density of Earth and does not create enough drag on a vehicle and more room is needed to slow, with probes typically entering Mars atmosphere at speeds over 15,000 mph, which are called 'hypersonic' speeds. A craft is going so fast the particles are actually breaking up and a lot of energy ends up imparting a heat pulse to your vehicle. Innovative techniques like parachutes, retro rockets, and even air bags were first developed as that was enough for mission which landed below Mars 'sea level' as probes had more of a distance to slow down, not taking in account the weight which could be put inside a mission. A way to improve landing accuracy was used by the Curiosity rover mission at Mars in 2012, which consisted into a 'guided entry', or manipulating the lift to entry into Martian atmosphere through small thrusters on the back shell to steer out unpredictable variations in the density of the Mars atmosphere and even performing 'S turns' or bank reversals. Most of supersonic deceleration technologies used by NASA for planetary missions date back to 1972 and the supersonic parachute tested for the Viking program. Larger payloads however are conducting to testing new ones, like supersonic inflatable decelerators, large pressure vessels that inflate around an entry vehicle and slow it from Mach 3.5 or faster to about Mach 2, at 20 or 26 feet in diameter (six meters, eight meters). A 110-foot (33.5-meter) parachute will further slow the entry vehicle from Mach 2 to subsonic speeds needed for a safe landing. Inflatable structures idea really isn't new as NASA engineers in the past have thought about making the system work. But its major obstacle was trying to find materials to make it happen. Such devices can be used at any planet with an atmosphere. In terms of a manned mission back Earth, for example, other searches are that heat shields might be made of soil of Moon, Mars or a asteroid, allowing that a future spacecraft leave Earth without carrying a heavy heat shield and instead make one on the surface of another world and ride it home safely. Such heat shield akin to the Apollo era ones, are ablative as they peel off during reentry. Pyrotechnic devices are often used during a entry phase to blow, cut or unlatch elements apart on the craft which avoids to use electrical motors for example. As far as the work of a rover on a planetary surface is concerned, a closest double of the rover exists on Earth as it is used to test maneuvers or to troubleshoot any problem. In terms of telecommunications, using relay via orbiters, compared with a rovers' capability to transmit directly to Earth from the planetary surface, greatly increases science data return
• as far as the question of landing manned missions on the surface is concerned, NASA is thinking about landing pads that could be robotically constructed in advance of future human expeditions to destinations such as the Moon or Mars, greatly reducing the potential for blowing debris. That is of importance especially for manned mission at Mars where spacecraft's rocket exhaust are to dig a deep hole under the lander and fluidize the soil due to Mars' atmospheric density that is 1 percent that of Earth and focusing the plume into a narrow jet. Robotic landers would go to a location on Mars and excavate a site, clearing rocks, leveling and then stabilizing the regolith to withstand impact forces of the rocket plume. Another option is to excavate down to bedrock to give a firm foundation. Fabric or other geo-textile material could also be used to stabilize the soil and ensure there is a good landing site. Robotic rovers would also place homing beacons around the site to help a spacecraft reach the specific spot. At the Moon, with no atmosphere, the ejected material would travel vast distances at high velocity

As far as to turn a planetary mission off is concerned, varied possibilities exist. A way may be to command transmitters off to preclude the remote chance that at some point in the future, craft's transmitter could turn on and broadcast on a frequency being used by other operational spacecraft and to let the craft orbit forever on a given orbit, the characteristics of which are also projected over the next 100 years to make sure no interference will occur. A way used with the Galileo mission at Jupiter, for example, is to make the mission serve in terms of science as the craft had been sent into the gas giant upper layers of gas and transmitting data until its destruction there. Of note that meteorids swarms may affect missions journeying into the solar system or in orbit around the Earth. Two examples include Mariner IV, a NASA planetary exploration spacecraft, which encountered a meteoroid stream between the orbits of Earth and Mars in September of 1967 with 17 hits in 15 minutes causing a temporary change in attitude but no loss of power and some thermal shield damage as the craft could return to normal operation within a week, and Olympus, a ESA communication satellite, which was struck by a Perseid meteoroid near the time of the shower peak in August 1993. It sent the spacecraft tumbling. By the time control was restored the onboard fuel had been exhausted, thus ending the mission

In terms of impact upon a comet or a asteroid, NASA developed a 'AutoNav' system, or 'Autonomous Navigation,' the orbit determination of it will detect the asteroid and compute its location in space relative to the impactor. That mode will also decide for some final rocket burns, called Impactor Targeting Maneuvers (ITMs) as no command could be sent from the Earth in time enough. The last ITM will occur about when the targeted object is on 1,500 miles (2,400 kilometers) away

Ground teams, in terms of ordering a craft to perform such or such task or maneuver, are uploading through the planetary telecoms networks, a 'command load' -- a set of commands -- to a spacecraft as the execution of such a set may last a few days, or longer. The load comes under the form of lines of informatic code. A list of commands sent to a probe to follow to carry out science observations, is termed a 'sequence.' Most often a planetary mission is featuring a simulator twin at Earth allowing to check commands or troubles

The debilitating effects of radiation, generally, has always been a challenge to spacecraft engineers for missions at Earth and elsewhere in the solar system. With a experiment in the 2010's concerning two twin brothers, 93 percent of a astronaut's genes were found returning to normal once back Earth as genes related to immune system, DNA repair, bone formation networks, hypoxia (tissues' lack of oxygen), and hypercapnia (carbone dioxide in blood) might endure long term changes. A stay in space also make telomeres of chromosomes longer, those elements related to aging when shortening. In terms of long-duration manned space journey like to Mars, on a other hand, a protracted sojourn in space is now known to bring astronauts to experience some unpleasant physical changes like losing bone and muscle mass, fewer red cells produced, heart to feel atrophy, and even eyesight deterioration. Diet and exercise are the ways to counteract those changes. Weightlessness also was found lately to make that a specific transmitter in the immune cells, called the Rel/NF-?B pathway, stops working, altering astronauts' immunity. The Radiation Assessment Detector (RAD) inside the Curiosity mission which shipped to Mars in 2012 was the first instrument to measure the radiation environment during a Mars cruise mission from inside a spacecraft that is similar to potential human exploration spacecraft. It put constraints upon the effectiveness of radiation shielding needed. Radiation in space are of several types. Gamma rays, X-rays and other charged particles, like neutrons -- high-energy particles from our Sun and outside our solar system interact with other particles or matter -- are harmful to astronauts. Radiation particles are a danger to astronauts as they can pass throughs skin, depositing energy and damaging cells or DNA. This damage can increase the risk for diseases later in life or cause radiation sickness during the mission. Galactic cosmic rays are emitted from supernovae events under the form of immense clouds of high-energy charged particles. Trapped radiation are spiraling inside the Earth's magnetosphere. Solar energetic particles result from solar energetic events. Ultraviolet radiation at last, the less energetic of all and impart energy unto atoms and molecules with which they are interacting. Such radiation have acute, like nausea or vomiting fatigue, etc., or chronic, like cancers or cataracts, etc., effects on astronauts. Radiation exposure is measured in units of Sievert (Sv) or milliSievert (one one-thousandth Sv). NASA has established a three percent increased risk of fatal cancer as an acceptable career limit for its astronauts currently operating in low-Earth orbit. Current spacecraft shield much more effectively against SEPs than GCRs. A 'albedo,' of galactic cosmic rays also reflects off the Moon's surface due to galactic cosmic ray protons penetrating as much as a yard (meter) into the lunar surface, bombarding the material within and creating a spray of secondary radiation and a mix of high-energy particles that flies back out into space. In terms of powering a mission, NASA first turned to solar power through the use of solar panels. The Vanguard 1 satellite, launched by 1958, was the first solar-powered explorer as today's solar technology can power spacecraft out to the orbit of Jupiter only. A next generation of solar panels is on the shelves to be lighter and more efficientto convert sunlight into power. A first step was accomplished with the NASA Juno mission to Jupiter with more progress like a larger size, avoiding any over, or underpower, or varied tints picking up different kinds of light. NASA is also considering using more radioisotope, or nuclear power source aboard its deep space mission as due to circumstances, it only possesses 35 kilograms of the plutonium-238 which is needed. First radioisotope power units were developed in the late 1950s and early 1960s by the US and Soviet space programmes as European ESA never developed nuclear power sources for missions. The United States has used radioisotope power units on 27 missions, from a Navy navigation satellite launched in 1961 to the Mars Curiosity rover in 2011. The isotope of choice is 238Pu, partly because it produces a high amount of power per gram of material, and partly because of worker safety: it emits only a-particles, which are relatively easy to shield against. With a half-life of 87.7 years, 238Pu can produce power for decades as it fades with time however. Through a contract with the US DOE, NASA now comes with a steady supply of the isotope, having enough to fuel about two missions a decade

Organisms hitching a ride on a spacecraft have the potential to contaminate other celestial bodies, making it difficult for scientists to determine whether a life form existed on another planet or was introduced there by explorers. Currently, spacecraft landing on planets or moons where life might exist must meet requirements for a maximum allowable level of microbial life, or 'bioburden.' Ultraviolet (UV) radiation and peroxide treatment are processes used to clean craft