Deep Space 2
The Deep Space 2 mission, which launched in January 1999 as part of NASA's New Millennium Program, consisted of two highly advanced miniature probes to Mars. They were intended to be the first spacecraft ever to penetrate below the surface of another planet. The Deep Space 2 probes were also the first landers to use only an aeroshell, lacking parachutes or rockets to reduce their impact velocity.
Each probe weighed just 2.4 kg (5.3 lb) and was encased in a protective aeroshell. They rode to Mars aboard another spacecraft, the Mars Polar Lander. Upon arrival just above the south polar region of Mars on December 3, 1999, the basketball-sized shells were to be released from the main spacecraft, plummeting through the atmosphere and hitting the planet's surface at over 644 km/h (400 mph). On impact, each shell was designed to shatter and its grapefruit-sized probe was to punch through the soil and separate into two parts. The lower part, called the forebody, would penetrate as far as 0.6 meters (about 2 feet) into the soil; the upper part of the probe, or the aftbody, would stay on the surface to radio data to the Mars Global Surveyor spacecraft in orbit around Mars, which will then send the data to Earth. The two sections of the probe would remain connected via a data cable.
The probes reached Mars apparently without incident, but communication was never established after landing. It is not known what the cause of failure was.