<s>
Radar	B-Algorithm
astronomy	I-Algorithm
is	O
a	O
technique	O
of	O
observing	O
nearby	O
astronomical	O
objects	O
by	O
reflecting	O
radio	O
waves	O
or	O
microwaves	O
off	O
target	O
objects	O
and	O
analyzing	O
their	O
reflections	O
.	O
</s>
<s>
Radar	B-Algorithm
astronomy	I-Algorithm
differs	O
from	O
radio	O
astronomy	O
in	O
that	O
the	O
latter	O
is	O
a	O
passive	O
observation	O
(	O
i.e.	O
,	O
receiving	O
only	O
)	O
and	O
the	O
former	O
an	O
active	O
one	O
(	O
transmitting	O
and	O
receiving	O
)	O
.	O
</s>
<s>
Radar	B-Application
systems	I-Application
have	O
been	O
conducted	O
for	O
six	O
decades	O
applied	O
to	O
a	O
wide	O
range	O
of	O
Solar	O
System	O
studies	O
.	O
</s>
<s>
The	O
radar	B-Application
transmission	O
may	O
either	O
be	O
pulsed	O
or	O
continuous	O
.	O
</s>
<s>
The	O
strength	O
of	O
the	O
radar	B-Application
return	O
signal	O
is	O
proportional	O
to	O
the	O
inverse	O
fourth-power	O
of	O
the	O
distance	O
.	O
</s>
<s>
Radar	B-Application
techniques	O
provide	O
information	O
unavailable	O
by	O
other	O
means	O
,	O
such	O
as	O
testing	O
general	O
relativity	O
by	O
observing	O
Mercury	O
and	O
providing	O
a	O
refined	O
value	O
for	O
the	O
astronomical	O
unit	O
.	O
</s>
<s>
Radar	B-Algorithm
images	I-Algorithm
provide	O
information	O
about	O
the	O
shapes	O
and	O
surface	O
properties	O
of	O
solid	O
bodies	O
,	O
which	O
cannot	O
be	O
obtained	O
by	O
other	O
ground-based	O
techniques	O
.	O
</s>
<s>
Relying	O
upon	O
high-powered	O
terrestrial	O
radars	B-Application
(	O
of	O
up	O
to	O
one	O
megawatt	O
)	O
,	O
radar	B-Algorithm
astronomy	I-Algorithm
is	O
able	O
to	O
provide	O
extremely	O
accurate	O
astrometric	O
information	O
on	O
the	O
structure	O
,	O
composition	O
and	O
movement	O
of	O
Solar	O
System	O
objects	O
.	O
</s>
<s>
In	O
particular	O
,	O
optical	O
observations	O
measure	O
where	O
an	O
object	O
appears	O
in	O
the	O
sky	O
,	O
but	O
cannot	O
measure	O
the	O
distance	O
with	O
great	O
accuracy	O
(	O
relying	O
on	O
parallax	B-Device
becomes	O
more	O
difficult	O
when	O
objects	O
are	O
small	O
or	O
poorly	O
illuminated	O
)	O
.	O
</s>
<s>
Radar	B-Application
,	O
on	O
the	O
other	O
hand	O
,	O
directly	O
measures	O
the	O
distance	O
to	O
the	O
object	O
(	O
and	O
how	O
fast	O
it	O
is	O
changing	O
)	O
.	O
</s>
<s>
The	O
combination	O
of	O
optical	O
and	O
radar	B-Application
observations	O
normally	O
allows	O
the	O
prediction	O
of	O
orbits	O
at	O
least	O
decades	O
,	O
and	O
sometimes	O
centuries	O
,	O
into	O
the	O
future	O
.	O
</s>
<s>
In	O
August	O
2020	O
the	O
Arecibo	O
Observatory	O
(	O
Arecibo	O
Planetary	B-Algorithm
Radar	I-Algorithm
)	O
suffered	O
a	O
structural	O
cable	O
failure	O
,	O
leading	O
to	O
the	O
collapse	O
of	O
the	O
main	O
telescope	O
in	O
December	O
of	O
that	O
year	O
.	O
</s>
<s>
There	O
is	O
one	O
remaining	O
radar	B-Algorithm
astronomy	I-Algorithm
facility	O
in	O
regular	O
use	O
,	O
the	O
Goldstone	O
Solar	O
System	O
Radar	B-Application
.	O
</s>
<s>
The	O
maximum	O
range	O
of	O
astronomy	O
by	O
radar	B-Application
is	O
very	O
limited	O
,	O
and	O
is	O
confined	O
to	O
the	O
Solar	O
System	O
.	O
</s>
<s>
The	O
distance	O
to	O
which	O
the	O
radar	B-Application
can	O
detect	O
an	O
object	O
is	O
proportional	O
to	O
the	O
square	O
root	O
of	O
the	O
object	O
's	O
size	O
,	O
due	O
to	O
the	O
one-over-distance-to-the-fourth	O
dependence	O
of	O
echo	O
strength	O
.	O
</s>
<s>
Radar	B-Application
could	O
detect	O
something	O
~	O
1km	O
across	O
a	O
large	O
fraction	O
of	O
an	O
AU	O
away	O
,	O
but	O
at	O
8-10	O
AU	O
,	O
the	O
distance	O
to	O
Saturn	B-Device
,	O
we	O
need	O
targets	O
at	O
least	O
hundreds	O
of	O
kilometers	O
wide	O
.	O
</s>
<s>
The	O
Moon	O
is	O
comparatively	O
close	O
and	O
was	O
detected	O
by	O
radar	B-Application
soon	O
after	O
the	O
invention	O
of	O
the	O
technique	O
in	O
1946	O
.	O
</s>
<s>
The	O
next	O
easiest	O
target	O
is	O
Venus	B-Application
.	O
</s>
<s>
The	O
first	O
unambiguous	O
detection	O
of	O
Venus	B-Application
was	O
made	O
by	O
the	O
Jet	O
Propulsion	O
Laboratory	O
on	O
10	O
March	O
1961	O
.	O
</s>
<s>
JPL	O
established	O
contact	O
with	O
the	O
planet	O
Venus	B-Application
using	O
a	O
planetary	B-Algorithm
radar	I-Algorithm
system	O
from	O
10	O
March	O
to	O
10	O
May	O
1961	O
.	O
</s>
<s>
Venus	B-Application
-	O
first	O
radar	B-Application
detection	O
in	O
1961	O
.	O
</s>
<s>
The	O
Magellan	B-Architecture
mission	O
mapped	O
the	O
entire	O
planet	O
using	O
a	O
radar	B-Application
altimeter	O
.	O
</s>
<s>
Earth	O
-	O
numerous	O
airborne	O
and	O
spacecraft	O
radars	B-Application
have	O
mapped	O
the	O
entire	O
planet	O
,	O
for	O
various	O
purposes	O
.	O
</s>
<s>
One	O
example	O
is	O
the	O
Shuttle	B-Algorithm
Radar	I-Algorithm
Topography	I-Algorithm
Mission	I-Algorithm
,	O
which	O
mapped	O
large	O
parts	O
of	O
the	O
surface	O
of	O
Earth	O
at	O
30	O
m	O
resolution	O
.	O
</s>
<s>
Mars	B-Application
-	O
Mapping	O
of	O
surface	O
roughness	O
from	O
Arecibo	O
Observatory	O
.	O
</s>
<s>
The	O
Mars	B-Application
Express	O
mission	O
carries	O
a	O
ground-penetrating	O
radar	B-Application
.	O
</s>
<s>
Saturn	B-Device
System	O
-	O
Rings	O
and	O
Titan	O
from	O
Arecibo	O
Observatory	O
,	O
mapping	O
of	O
Titan	O
's	O
surface	O
and	O
observations	O
of	O
other	O
moons	O
from	O
the	O
Cassini	O
spacecraft	O
.	O
</s>
<s>
Radar	B-Application
provides	O
the	O
ability	O
to	O
study	O
the	O
shape	O
,	O
size	O
and	O
spin	O
state	O
of	O
asteroids	O
and	O
comets	O
from	O
the	O
ground	O
.	O
</s>
<s>
Radar	B-Algorithm
imaging	I-Algorithm
has	O
produced	O
images	O
with	O
up	O
to	O
7.5-meter	O
resolution	O
.	O
</s>
<s>
With	O
sufficient	O
data	O
,	O
the	O
size	O
,	O
shape	O
,	O
spin	O
and	O
radar	B-Application
albedo	O
of	O
the	O
target	O
asteroids	O
can	O
be	O
extracted	O
.	O
</s>
<s>
Only	O
19	O
comets	O
have	O
been	O
studied	O
by	O
radar	B-Application
,	O
including	O
73P/Schwassmann	O
-Wachmann	O
.	O
</s>
<s>
There	O
have	O
been	O
radar	B-Application
observations	O
of	O
612	O
Near-Earth	O
asteroids	O
and	O
138	O
Main	O
belt	O
asteroids	O
as	O
of	O
early	O
2016	O
.	O
</s>
<s>
Being	O
smaller	O
the	O
Goldstone	O
Solar	O
System	O
Radar	B-Application
is	O
less	O
sensitive	O
and	O
unable	O
to	O
provide	O
the	O
same	O
predictive	O
capacity	O
.	O
</s>
