New Asteroids -- Discovering and Cataloging
Up until the 1980s, asteroids catalogued were discovered accidentally on telescopes' photographic plates, showing up as a small streak. Most astronomers viewed an asteroid streak as a nuisance blemishing an otherwise perfect photographic plate, especially if the asteroid was streaking across the other image of interest to the astronomer. Most such streaks were unreported. (Astronomical observations use time exposure photography, exposing the film to the object in view for long periods of time, which is why the asteroid shows up as a streak. It's actually moving fairly slowly as viewed from Earth.)
In the 1970s and 1980s, electronic sensors started to replace film in certain telescopes, which ushered in the era of automated computer searches for asteroids. First, defense applications started using vidicon cameras to look for the satellites of adversaries (which could easily change their orbit to elude tracking). In defense applications, poring over photographs was a laborious task that left a lot to be desired in terms of thoroughness, fatigue and timeliness. (In fact, in detecting the streaks of adversaries' satellites, many asteroids would show up as a small streak.) Instead of film, this revolutionary new system was an electronic camera using a sensor array combined with computer analysis. It was very simple. A picture was taken of the sky. A few minutes later, another picture was taken, and a few minutes later still another picture. The images were stored in the computer, and as each image additional was taken, the computer would look for anything that moved.
In the 1980s astronomers started using more sensitive "CCD" electronic sensors in cameras, whereby film stopped being used for this purpose. CCD sensors have proven to be a key to accelerating the discovery of asteroids because it is automated, thorough and very fast -- and CCD sensors are extremely sensitive.
However, viewing an asteroid in one night is not enough to chart its orbit. Also, because of the shoestring budgets for asteroid searches (non-defense work), it is important to understand the physical science and political limitations of the current system.
Determining an asteroid's orbit
In order for an asteroid to be catalogued, we must determine its orbit, and hence there must be at least three viewings to chart its orbital elements, one viewing within a few days after the initial sighting, and another between about two weeks and a month later. Using the data from the second observation to chart a rough orbit, we must find the asteroid a third time to get better precision and thus increase our chances of finding the asteroid many years into the future. If we wait too long, we lose the asteroid. This third viewing usually gives us sufficiently precise orbital data so that we have a better chance of relocating the asteroid at a later date using a reasonable amount of telescope search time.
Also, many asteroids are detected at their brightest time, and they may fade in brightness rapidly. Similar to how the Moon goes between "full moon" and its smaller phases, asteroids go thru phases according to the angle of view from the Earth. Also, the distance between asteroids and Earth varies a lot in one month.
Unfortunately, the best telescopes are booked months in advance, some of them years in advance, those telescopes which are looking at distant objects and just happen to accidentally see an asteroid, too. If we wait more than a few days for a second viewing, good chance we won't find the asteroid. This led to telescopes being dedicated to asteroids.
The Earth's atmosphere is a big problem, because asteroids usually move across the sky to the horizon in about a month and it's hard to see near the horizon due to the thickness of the atmosphere and the horizon glow from manmade sources. Most asteroids near Earth spend most of their viewable time near the horizon. When the Moon is out, its glare further reduces the detectability of faint asteroids by ground based telescopes, especially near the horizon. (Many asteroids are detected when they are near the middle of the sky, and near "full asteroid" illumination.)
If we are to discover asteroids using a telescope above Earth's orbit where we can look in any direction at any time, 24 hours a day, we would have to budget and launch up a dedicated telescope for searching for asteroids and studying them. (Scheduling time on the Hubble telescope in low Earth orbit would be extremely difficult, as it's booked years in advance by very powerful interests, looking at distant objects in the universe. Also, its field of view is narrow, and we need wider angle views to find asteroids.)
With dedicated asteroid telescopes, by current methods, an electronic camera automatically scans a section of the sky at different times, separated by around 30 minutes to 2.5 hours. The different star fields are recorded in a computer. The computer compares the images for points of light which have moved. Normally, it requires more than two exposures to distinguish a moving object from the effects of detector noise and cosmic rays. It has also been said that if the asteroid is very close to Earth, it could possibly travel a long enough distance in the sky in too short an amount of time so that there would be a chance it won't be detected, as the two points of light are too far apart on successive images for the computer to see them as related.
The best search time is when the moon is not out. Around full moon, the cameras often focus on following up on newly discovered objects. The atmosphere disperses a lot of light, which affects sensitive sensors underneath this "atmospheric ocean". (Clouds are also a downer, killing scheduled viewings.)
Amateur astronomers can help discover asteroids, as well as assist with followup observations of newly discovered asteroids to confirm them and help chart their orbits. Indeed, amateur astronomers are needed around the world. Japanese amateurs have been remarkably productive.
Indeed, one of the biggest limitations now seems to be on follow-ups.
All the results from all the survey groups, including discoveries of asteroids, are reported at the end of each night of observing to the Minor Planet Center (MPC) at the Smithsonian Astrophysical Observatory, which is responsible for the collection and dissemination of astrometric observations and orbits for minor planets and comets on behalf of the International Astronomical Union, as discussed elsewhere on this website. That information is public domain.
Infra-red vs. visible light telescopes
Asteroid searches are biased because asteroids are easiest to detect when they are lined up with the Earth and the Sun, and hence brightest -- similar to the "full moon", they are "full asteroid". If they don't happen to pass by in that way, but are seen in a phase (e.g., "half asteroid", analogous to half moon vs. full moon), they are less likely to be detected. This makes small asteroids as well as dark volatile rich asteroids even harder to detect.
One solution is an infra-red telescope, to detect asteroids by their heat. This method does not depend upon reflected sunlight, only heat radiated by the asteroid. The one big problem is that such a detector must be placed in orbit, above the atmosphere.
The NASA WISE Satellite Telescope, 2009
In 2007, when Congress held hearings with NASA regarding the status of its NEO detection program, NASA officials proposed the Wide-field Infrared Survey Explorer (WISE) satellite to look for NEOs, figuring it could detect about 300 per year. The WISE satellite was already under construction and designed to study the universe, so that near Earth objects were just an added secondary mission.
The satellite launched and went into operation in 2009, though it was at maximum sensitivity for only 10 months because it required a coolant for its sensors, and the hydrogen coolant eventually ran out. It came out discovering 136 near Earth asteroids and comets, out of a total of more than 33,000 asteroids and comets total. This included an Earth Trojan asteroid. However, the total number of near Earth asteroids and comets was less than half what was expected (about 300).
After the cryogenic fluid ran out, two of the four sensors could still operate well enough to detect near Earth asteroids. However, the NASA division responsible for funding, which was most interested in exploring the universe, decided to terminate the mission rather than try. Enough money was put together quickly from another division of NASA (Planetary) to allow an attempt at using the satellite without its cryogenic supplies. A one month trial period was allowed, and it worked, so the mission was extended, but the money scraped together ran out after a few months, whereby the satellite was put into hibernation in February 2011. The extended mission was called NEOWISE, and discovered "many" near Earth objects but I haven't seen exact numbers on that talley yet. (Can someone help out?)
The NEOWISE satellite could probably be taken out of hibernation if funding permitted.
Data is still being analyzed for all objects observed in the universe, but no great surprises are expected as regards near Earth asteroids and objects. The data set from the satellite was released in March 2012 (two months before this article was written).
Canada's NEOSSat, 2013
Canada launched its Near Earth Object Surveillance Satellite (NEOSSat) on February 25, 2013, on an Indian rocket, the Polar Satellite Launch Vehicle (PSLV).
NEOSSat is pointed to find asteroids closer to the Sun than Earth (mainly Atens) which are extremely difficult to detect by ground based telescopes due to the atmosphere and Sun. NEOSSat tracks asteroids it discovers, and also tracks space junk and unknown satellites.
However, as space.com pointed out, this satellite could not detect an asteroids of 15 meters width at long range, like the Russian meteor which exploded about 10 days before the launch of NEOSSat.
The satellite costs approximately $ 25 million. Funding comes from the Defence Research Development Canada (DRDC) and the Canadian Space Agency (CSA). The original launch date was 2010, but it eventually launched in 2013 with a cluster of satellites on Canada's first generic multi-mission microsatellite bus. It is the size of a suitcase and is in an 800 km high orbit.
Arkyd, by Planetary Resources, Inc.
Asteroid mining company Planetary Resources, Inc. announce in April 2012 that they were building multiple asteroid probes, called the Arkyd series, actually mass produced, to launch in the near future to do spectroscopic characterization of known near Earth asteroids and objects, as well as to find additional ones which are difficult to see from below Earth's atmosphere and on the night side of our viewing area.
B612 Foundation's Sentinel Satellite for Asteroid Discovery, 2016
The B612 Foundation, which is a private, nonprofit organization co-founded by astronauts in 2002 for the purpose of helping protect Earth from asteroid threats. The first 10 years were focused on research on methods to deflect asteroids, and advocacy. In 2012, however, the B612 Foundation announced new plans to launch their own satellite, called Sentinel, to search for asteroids.
The current plan is to launch The Sentinel on a Falcon 9 rocket in December 2016.
The satellite will be launched to escape Earth orbit inward towards the Sun and take up an approximately Venusian orbit, and then point the camera outward away from the Sun, in order to detect asteroids which travel between the Earth and Venus.
Using an infrared camera, it expects to detect approximately 90% of asteroids larger than 140 meters which travel near Earth, and to detect many asteroids down to a size of about 30 meters width, over its 6.5 year life.
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