Spazio. Asteroide si disintegrerà in atmosfera terrestre
Ingresso previsto per le 4.46 sui cieli del Sudan, nessun rischio
postato 5 ore fa

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Washington, 6 ott. (Ap) - Un asteroide del diametro massimo di 4,5 metri entrerà nell'atmosfera terrestre alle 4.46 ora italiana, disintegrandosi negli strati superiori: lo hanno annunciato i ricercatori dell'università di Harvard, che hanno escluso qualsiasi rischio di impatto al suolo.

L'ingresso nell'atmosfera dovrebbe prodursi sui cieli del Sudan e la scia luminosa lasciata dall'asteroide - troppo piccolo per avere un nome - dovrebbe essere visibile in tutta l'Africa settentrionale.

E' la prima volta che gli astronomi sono riusciti a seguire la traiettoria di un corpo celeste in orbita di collisione con la Terra prima che questa avesse luogo: a causa della bassa albedo (capacità di riflessione) della superficie gli asteroidi sono di norma virtualmente invisibili agli strumenti ottici a meno di non passare davanti a un corpo luminoso e troppo piccoli epr causare effetti gravitazionali osservabili.

A further question: Will travel between different planetary systems ever be possible? I am sorry to give an unpopular answer, but I believe this to be an uncompromising, no. Communication is a different matter. If living creatures at a high technological level exist on planets belonging to any of the nearest thousand stars it would be feasible to establish communication. A two-way interchange of information would take many centuries to develop. Even so, perhaps we should be starting now? (Hoyle, 1950)

Classification of Galactic Civilizations

Type I civilizations have a technological level close to the level presently attained on Earth. The energy consumption of Type I societies is approximately 4x10¹⁹ erg/s.

Type II civilizations have the capability of harnessing the energy radiated by their own star (4x10³³ erg/s). They may construct large Dyson Spheres to capture the energy from their primary star.

Type III civilizations are those who have an energy utilization equal to that of their galaxy (4x10⁴⁴ erg/s).

What Would a Sought-After Signal Look and Sound Like?

The narrower the band-width of a transmitted signal is of fixed total power, the greater the range of successful transmission. Thus, we can expect the signals to be of narrow band-width. This has the advantage of providing simple methods for distinguishing the signals from naturally occurring cosmic noise, which is extremely broad in band-width. For those using Microsoft's Internet Explorer you should hear what a narrow-band signal would sound like.

Detection Range of Cosmic Signals

Given an effective radiated power of the transmitter (in watts), the effective area of the receiving antenna (in square meters), the excess receiver noise temperature of the receiver used (in K), the averaging time of the receiver (in seconds), and the accepted band-width of the signal (in Hz), the range at which we can detect a signal transmitted by an intelligent civilization, is

where the constant is calculated from 1/[9.4608x10¹⁵(4pik) ^1/2]. Here the constant is the number of meters per one light year, and k is the boltzmann constant.

A Good SETI Receiver Should

1. Operate at a frequency greater than 1420 MHz, with performance near 
1420 MHz (21 cm) desirable. 2. Operate over a very large range of band-widths, 
including 10 MHz and less. 3. Have receiver frequency much less than the band-width 
during the averaging time of the receiver, specifically not more than about 1 
MHz in several minutes. 4. Have the highest sensitivity available. 

 
5. Eliminate receiver noise effects as much as possible. 6. Discriminate against 
broad-band cosmic noise, if possible.