NEOSSat

NEOSSat (Near-Earth Object Surveillance Satellite) mission background

1. Advantages of searching for asteroids from low-Earth orbit
In addition to a darker sky and continuous availability, the 15 cm-diameter telescope on NEOSSat has other advantages over its giant Earth-based cousins: it can observe near the Sun, and can determine distances to near-Earth asteroids using parallax.  Circling the Earth in a polar orbit almost always in sunlight, the telescope has a sunshade that allows searching the sky to within 45° of the Sun, a part of the sky difficult or impossible to observe from the ground, but where near-Earth asteroids are concentrated.  This near-Sun region is also the only part of the sky where asteroids that orbit entirely inside the Earth’s orbit (IEO’s) may be discovered.  NEOSSat will survey this little-known population and discover hundreds of other near-Earth asteroids.  Orbiting pole to pole will allow the spacecraft to take advantage of an optical shift called parallax – this effect causes nearby asteroids to telegraph their identity and distance by shifting against the fixed stars.  Only a planet-orbiting, space-based telescope can routinely use this effect to separate nearby asteroids from the thousand times more abundant and more distant Main Belt asteroids located between the orbits of Mars and Jupiter.

2.  Inventory of Near-Earth Asteroids and The Spaceguard Survey
An estimated population of ~100,000 asteroids >140 m in diameter orbit in near-Earth space, many of them on Earth-crossing orbits.  Although most will impact the Sun or be ejected from the Solar System, a few percent of these objects will eventually impact the Earth.  For several decades calls have been made for our civilization to discover this asteroid population to prevent or mitigate a devastating impact, but the recognition in 1990 of the Chicxulub crater as the impact site for the asteroid impact that drove Cretaceous life extinct (including the dinosaurs) finally shifted mainstream opinion sufficiently so that previous search efforts were substantially augmented (most significantly with a N.A.S.A. funding program) with a goal of discovering most of the larger (>1 km diameter) objects (articulated N.A.S.A. goal is discovering 90% of >1 km diameter objects by end of 2008).   The Spaceguard Survey refers to a loosely bound international effort to discover and understand the near-Earth asteroid population.  The United States through N.A.S.A. has funded the most productive asteroid search programs to date, although both amateur and professional planetary scientists in many countries around the globe conduct asteroid searching and follow-on studies.  A variety of more capable ground- and space-based asteroid search and exploration projects have been proposed with at least two ground-based projects and at least one space-based (NEOSSat) well into development.  The U.S. Congress also directed N.A.S.A., in 2005, to undertake a new search goal of discovering 90% of the >140 m diameter population by 2020, but N.A.S.A. has not yet begun construction of any new search telescopes due to budget constraints.

3.  NORAD’s satellite tracking mission
NORAD, as part of its aerospace warning mission, keeps track of the positions and orbits of thousands of objects every day.  The data collected by NORAD is used to ensure the safety of satellites that are routinely used by billions of people each day, whether it is for weather forecasting, watching television, navigating traffic using GPS, or transmitting banking information for household purchases.  The data collected by NORAD helps keep astronauts in the space station and space shuttle safe from collision with the ever-increasing amount of space debris.  NORAD also regularly watches for space debris that is re-entering the Earth’s atmosphere, making sure that this space junk is not mistaken for a ballistic missile attack.

4.  The NESS (Near-Earth Space Surveillance) project
The NESS project will use NEOSSat primarily to search for asteroids near the Sun.   This near-Sun region is relatively rich in some orbital classes of near-Earth asteroids, and is difficult to search using ground-based telescopes, so this is currently thought the greatest contribution that NEOSSat can make to the international effort to catalogue the near-Earth population of asteroids.  The spacecraft is, however, capable of searching for asteroids in any part of the sky.  The search plan involves the spacecraft taking a series of four images of the sky over 125 minutes; this timing is determined by the orbital period (~100 minutes) of the spacecraft’s Sun-synchronous orbit, and the need to re-image fields before any asteroids in a search field leave it.  The images are sent to the ground where every sequence of four will be searched for moving objects by computer programs at the data processing centre at the University of Calgary.  All tentatively identified new near-Earth asteroid discoveries will then be examined by eye; objects which pass this checking process will have their positions reported to the Minor Planet Center maintained by the International Astronomical Union, and follow up of the discovered asteroids will be made by NEOSSat or ground-based telescopes to better determine their orbits.

5.  The HEOSS (High Earth Orbit Space Surveillance) project
The aim of the HEOSS (High Earth Orbit Space Surveillance) project is to demonstrate that microsatellites can – in a effective, reliable, and efficient way – help keep track of objects in orbit around Earth.  HEOSS is dedicated to keeping track of objects in mid- to deep-space orbits, objects such as GPS satellites, communication satellites, and weather satellites.  HEOSS builds upon the success of the US “Space-Based Visible” sensor – which recently ended a 10 year mission keeping track of space objects from space - and the Canadian MOST mission – which showed that an inexpensive spacecraft can carry the type of telescope needed to keep track of space objects.  By using a small, focused, yet technologically advanced spacecraft to perform a mission that is typically done by much larger, more expensive, spacecraft, NEOSSat will demonstrate a typically Canadian solution to a problem – effective, dependable, advanced, yet only a modest draw on resources.

6.  Near-Earth asteroids (NEA’s) and Near-Earth objects (NEO’s)
The acronym NEO is a bit broader than NEA because it also includes comets, so the NEOSSat spacecraft is named to include both types of objects.  The proportion of extinct comets thought to be amongst the NEA population may be as high as ~10%, but this remains to be determined.  The NEA are grouped into four orbital classes:  Aten, Apollo, Amor, and IEO, based upon their current osculating orbital elements (a = distance from the Sun or semimajor axis; Q = aphelion distance; q = perihelion distance; AU = one astronomical unit or the average distance of the Earth from the Sun); Aten asteroids have a < 1 AU and Q > 0.983 AU, Apollo asteroids have a ≥ 1 AU and perihelion distances (q) ≤ 1.017 AU, while Amor asteroids have a > 1 AU, 1.017 < q ≤ 1.3 AU.  The “Interior to Earth’s Orbit” (IEO) class is one coined to describe the asteroid population with orbits entirely interior to that of the Earth’s orbit.  IEO asteroids have semi major axes (a) < 1 AU and aphelion distances (Q) < 0.983 AU, and so currently orbit completely interior to Earth's orbit; the IEOs have not been traditionally regarded as part of the NEAs, but this is in part a result that the first IEO was only discovered in 2003.  This latter orbital class is the least known, and is one of the foremost scientific targets of the NESS project, but NEOSSat will also be an efficient discoverer of Aten class asteroids compared to ground-based observatories.

7. Geosynchronous satellites and other high Earth orbits
In addition to the satellites in low Earth orbits and Sun-synchronous orbits at altitudes of ~300 to ~1,000 km above the Earth, satellites are deployed into high-Earth orbits of various types – the most noted of these is probably geosynchronous orbit.  Geosynchronous satellites are at an altitude of ~36,000 km and orbit the Earth once every 24 hours.  Since this matches the Earth’s rotation period, such a satellite appears to “hover” in the sky (often over the Earth’s equator), which is very useful for some types of communications or broadcast satellites.   Other high altitude orbits are also used to serve other parts of the Earth’s surface such as Molniya type orbits to serve far northern points.  Satellites used for navigation such as the Global Positioning System constellation of satellites are also in relatively high orbits of ~20,000 km elevation.