OBSERVATIONAL 
             TECHNIQUES

In this chapter we outline the astronomical techniques needed to discover most Near Earth Objects above a given size. We then briefly describe the observations necessary to determine the orbit and physical characteristics of any detected object.

Most of the measurements can be made with ground-based telescopes and radars, rather than with expensive space-based missions.While some dedicated facilities are essential, much valuable work can be done by occasional or serendipitous use of telescopes or spacecraft with different prime scientific aims. In the past amateur astronomers around the world have contributed to the study of Near Earth Objects and this should continue. But for almost all activities dedicated professional work is essential.

Discovery/survey
Because we cannot predict where new objects may come from, we must observe the whole sky, frequently and systematically. This calls for specially designed wide-angle telescopes with advanced detector arrays coupled to very fast computers. They should operate automatically and remotely, and be dedicated to observations of Near Earth Objects. An example of such a facility is the US LINEAR system with two 1m telescopes, observing objects of diameter above about 1 kilometre. For complete coverage, survey telescopes are required on good sites in each hemisphere.

Bigger dedicated telescopes would allow surveys of smaller objects: a 3 metre instrument would cover objects down to a few hundred metres in diameter. In addition, larger ground-based telescopes primarily intended for extra-galactic surveys, such as the new British 4 metre VISTA instrument and the 6.5 metre survey telescope proposed in the United States, would inevitably detect many Near Earth Objects. Such large instruments could also help to discover long-period comets.

Some classes of objects are difficult, or perhaps impossible, to discover from the ground, for example asteroids with orbits inside that of the Earth’s (Inner Earth Asteroids).The European Space Agency has recently studied a space-telescope mission primarily to survey objects of this type. Missions including space telescopes, such as the Agency’s GAIA proposal and NASA’s SIRTF, could be used to discover such objects.

Follow-up observations to determine the orbit of a Near Earth Object
To determine the orbit of an object after its initial discovery, conventional narrow-angle ground-based telescopes are needed in each hemisphere. Because a newly discovered object rapidly becomes fainter as it moves away from the Earth, follow-up observations must be made within days of discovery. While some telescopes should be dedicated to this function, follow-up observations can be made with pre-arranged and rapid access to suitable telescopes used for other purposes. For very accurate and rapid orbit determinations of a known object which is near the Earth, radar is an exceptionally useful technique.

The mass and composition of an asteroid or comet
The destructive power of an asteroid depends primarily on its energy, which is proportional to its mass.The mass of a Near Earth Object is surprisingly difficult to measure using ground-based telescopes. It depends first on knowing the brightness of the object and the proportion of the Sun’s light which it reflects (called its albedo), from which its size may be deduced. But we also need to know its density to determine its mass: the density is deduced from its chemical composition (icy, carbonaceous, stony or metallic) using spectroscopic observations at visible and infra-red wavelengths. Because of the uncertainties in knowing the albedo (which can vary by a factor of five or more), the size and therefore the mass of an observed object can be very greatly in error. It is also subject to uncertainty in the density. NASA’s recent rendezvous mission, NEAR, has shown that the asteroid Mathilde has a density substantially smaller (perhaps by a factor of three) than expected from its chemical composition; this asteroid must be porous or consist of a loose aggregation of rocks.

Such uncertainties make it hard to predict whether a particular asteroid might cause a global or a regional catastrophe. To do better we can use ground based radar which – as well as measuring the position and velocity – can also determine the size, shape, gross structure and spin of an object when it is sufficiently near the Earth, but not its mass.There is no suitable radar facility in the southern hemisphere.

But for more accurate measurements of mass, composition and gross structure, space rendezvous missions are needed. In this way the mass of an object can be determined by measuring the pull of its gravitational field on the spacecraft; its shape measured photographically; and its chemical composition found using mass spectrometers. Approximately 20 sub-groups of asteroids and comets are thought to exist. A rendezvous mission to a member of each of these would enable direct information to be determined and linked to corresponding ground-based spectroscopic observations of an unvisited object. Relatively inexpensive microsatellites could fulfil this purpose.

So far, no mission has yet been able to determine an object’s internal composition or whether it is hollow. Such an observation will be attempted on a comet by NASA’s Deep Impact mission to be launched in 2004.

Crown Copyright © 2000. All rights reserved.
Conditions of Use