The Near-Earth Asteroid (NEA) population orbits in a diffuse cloud encompassing Earth’s orbit. No single telescope, whether ground- or space-based, can quickly discover (nor determine orbits for) all the asteroids to a specified size limit. NEOSSat’s telescope, as used by the NESS project, will discover asteroids orbiting near the Sun that are difficult for ground-based surveys to find.
The Long Version (Or Why It Is Hard to Find All the Asteroids Quickly)
Millions of rocky objects orbit between the planets Mars and Jupiter in what researchers call the Main Asteroid Belt. These bodies are leftover from the accretion and formation of the planets 4.5 billion years ago. Their current orbital distribution is sculpted by effects of Jupiter’s gravity (and to a lesser extent those of other planets – primarily Saturn and Mars).
Getting kicked out of the Asteroid Belt. Some of these objects are always “leaking” out of the main belt due to various “perturbations” – a perturbing force is something that affects their orbits. Naturally once an asteroid is out of the belt it is subject to ever larger perturbing forces (the belt is the size it is because those are relatively stable orbits). These perturbations do essentially two things – they add energy to the orbit until the asteroid becomes Jupiter crossing, in which case Jupiter will throw it out of the Solar System (to wander alone for the remainder of foreseeable time), or they remove energy from the orbit so that the asteroid eventually plunges into the Sun. As asteroids on this evolutionary path have ever smaller orbits, they eventually cross the orbits of Mars, then Earth, Venus, and Mercury. When these asteroids are in the vicinity of the Earth’s orbit, we call them the near-Earth asteroid (NEA) population. This population is transient on geological/astronomical timescales; if we could look back ten or twenty million years, almost all the NEA’s would be different. On their journey to the Sun, a few percent of these asteroids will collide with the Earth, Earth’s Moon or the other inner planets.
Asteroids in the Earth’s Neighbourhood. So, at this moment asteroids of all sizes are passing by Earth in all directions. Whether north or south, east or west, towards the Sun or away, the asteroids have relatively the same concentration in near-Earth space. The good news is that space is so big these asteroids only rarely impact the Earth and the statistical rate of impacts is relatively well known. (The size distribution of the NEA’s is also relatively well known.) This has been happening since the Earth first formed and will continue for the life of the Solar System. It happened before the first asteroid was ever discovered and will happen once they are all discovered. The first Earth-crossing asteroid (ECA), Apollo, was only discovered in 1932. With the growth of our observing technologies, particularly the development of charge-coupled device (CCD) sensors, and computer programs that can search for asteroids in images of the night sky, the number of known asteroids has sky rocketed over the last twenty years.
What’s a near-Earth asteroid look like? NEA’s are relatively small (largest is typically ~10 km diameter), rocky bodies that don’t reflect much sunlight, (the percentage of light reflected ranges all the way from ~3% to ~60% depending upon their composition) making them dim and somewhat difficult to detect even with CCDs. Being relatively close to the Earth, they also move relatively quickly across the sky, which adds to the challenge of finding them. (Their movement across the sky means that one can’t simply expose longer to detect fainter objects – after a certain “dwell time” on a CCD pixel the asteroid has moved on so that exposing any longer lowers the signal to noise ratio rather than increasing it). Ground-based telescopes can only search for asteroids at night, so are in large part limited to looking only on one side of the Earth at any given time. Telescopes are also limited by how low to the horizon they can look by the Earth’s atmosphere which starts to “blur” images quite significantly below ~30 degrees above the horizon. So, a telescope in the northern hemisphere can’t search much of the southern sky and vice versa.
Taking their pictures. Asteroids appear as fuzzy little dots of light in CCD images that don’t look any different than stars except that if a series of images of the same patch of sky are taken, the asteroid can be seen to move with respect to the fixed stars. That is why asteroid searches are often referred to as “moving object” surveys. If one wants to find all the NEA’s then, one would like to image all the night sky that can be done from planet Earth as faint as one can image it; several telescopes are dedicated to this search at this time. Naturally, the available search intervals are constrained by weather and the phases of the Moon (it is bright enough to severely compromise moving object searches when it is up). The bigger the telescope, the fainter it can search, but it also needs to be able to cover a large area of sky quickly (observers would speak of a wide “field of view”). The entire sky doesn’t need to be searched every night, but depending upon the size of telescope, the sky “refreshes” with a characteristic period. In other words, the asteroids that were there leave and new ones may now occupy that part of sky. For typical ground-based asteroid search telescopes, the sky refreshes in five to seven days for opposition (opposite to the Sun) and near-opposition regions. So the current ground-based search effort adds up to a certain area of sky searched to a certain limiting magnitude a certain number of times per month. For example, when modelling an asteroid discovery rate, one would speak of 20,000 square degrees searched to limiting magnitude of V20 twice per month. Many researchers have written about search design and characteristics of asteroids to be found by searches and two references are listed below.
The limitations of a ground-based search. The telescopes are only searching space on one side of the Earth (the night side) when no Moon is present in the sky (two weeks of the month). What about the asteroids going by on the daylight side of the planet, or the north or south pole, or when the Moon is full? What about all the NEA’s that are far from the Earth – as far as on the other side of the Sun? What about the asteroids that are too far away when they go by to be seen because they are too faint? The current searches are doing what can be done with available resources, and the asteroids that are far from the Earth are found by continuing the search for years – i.e. an asteroid on the other side of the Sun at the current time will eventually come into the Earth’s night sky if its orbital period isn’t exactly the same as the Earth’s. (This interaction between the asteroid’s orbital period and the Earth’s orbital period is called the synodic period – asteroid–Earth synodic periods as long as ~fifty years are known.) Or what if an asteroid doesn’t have an orbital shape and orientation that places it inside Earth’s orbit when it passes near the Earth?
Good news of the hazard front. Over the last twenty years, ~80 – 90% of the impact hazard from NEA’s has been “retired.” The majority of large NEAs (~>1 km diameter that could cause global destruction of Earth’s ecosystems) have been found, had their orbits determined, and are now known to not be going to impact the Earth any time soon. This certainly isn’t to say that all hazard has been removed plus our civilisation is interested in discovering and knowing the orbits of all NEA’s for other reasons. They are the nearest solid bodies to us in the Solar System and are natural targets for exploration and eventual exploitation.
Where does NEOSSat fit into all of this? The NEA discovery rate can be substantially to dramatically increased if new search telescopes are constructed. Various serious and careful proposals have been made for both ground-based and space-based telescopes. Some of the proposed observing systems are very capable, but that usually leads to higher costs. Asteroid searches can also benefit significantly from data taken by telescopes that have been launched for astronomical studies, and good cooperation has occurred with the recent WISE mission and is expected with the upcoming GAIA mission. NEOSSat comes into this mix of current and planned search telescopes as a relatively modest priced space telescope that is nonetheless able to make a significant contribution to discovery of a fraction of the NEA population by searching for the asteroids that orbit near the Sun. These two orbital classes are known as the Atiras – asteroids that orbit entirely inside Earth’s orbit – and Atens – asteroids that orbit mostly inside Earth’s orbit but that sometimes cross it. The ability to search near the Sun continuously is one of the big advantages of a space-based platform so NEOSSat will be used by the NESS project to fill in this knowledge gap for the NEA population. Over its lifetime, NEOSSat’s observations are expected to reduce the current impact hazard by another few percent, but its main contribution will probably be constraining the Atira asteroid population size and orbital distribution. If the project is lucky it may discover a relatively large asteroid (or two) somewhat close to the Earth dynamically, that would be natural exploration targets.