Upper limit on the radar cross section of the comet Kohoutek

An attempt to observe radar echoes from the comet Kohoutek was made at a radio frequency of 7840 MHz (λ ～- 3.8 cm) on 12 January 1974 using the Haystack Observatory radar in Massachusetts. A search for an echo over a range of band-widths covering 2Hz to 66kHz yielded no positive result. The upper limit on the radar cross section is therefore approximately $\text{10}{}^4\text{B}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{km}{}^2$, where B is the (unknown) bandwidth of the echo in Hertz. For $\text{B ? 100}\text{Hz}$, it follows that (i) the nucleus, if a perfect spherical reflector, must be less than 250 km in diameter, and (ii) the density of any millimeter-sized particles must be less than 1m^?3 for a coma of diameter 10⁴km.