What is the wattage (ie power) of the probe mentioned here in this
article? http://news.bbc.co.uk/2/hi/science/nature/4587580.stm
I heard that the Apollo missions did some experiments with a 5 watt
laser shining it back to earth to measure distance/ speed.

Hello zulander and thank you for your question.

The article you refer to mentions that the name of the probe is
'Messenger' and that the laser used was the 'laser altimeter'  If we
look at the specifications of the Messenger probe (from Space Flight
Now) we see that the power of the laser altimeter is 38.6 watts.

Details:

"The US space agency (Nasa) craft is equipped with a laser altimeter
that will map the topography of Mercury by timing the return of laser
pulses fired at the planet.

The instrument and the ground station in Maryland transmitted laser
pulses back and forth in a test of the payload carried out in May."
http://news.bbc.co.uk/2/hi/science/nature/4587580.stm

--------------------------------------------------------------

Messenger Probe Specifications

"Mercury Laser Altimeter

Mass: 7.4 kilograms (16.3 pounds)
Peak Power: 38.6 watts
Development: NASA Goddard Space Flight Center"
http://www.spaceflightnow.com/delta/d307/040728messenger.html

Thank you for your question, and if you need any clarification of my
answer, do not hesitate to ask before rating my answer.

Very best regards,

THX1138

Search strategy included:
Messenger probe laser  watts
://www.google.com/search?hl=en&lr=&ie=ISO-8859-1&safe=off&as_qdr=all&q=Messenger+probe+laser++watts

Thanks, great answer, exactly what I wanted.

It's very doubtful that Apollo experimented with ranging lasers
pointed at Earth from the Moon. At just a few watts the signal would
be just about undetectable, either upon return to the spacecraft or
even on Earth. It's true that lasers are fired at the Apollo
corner-cube reflectors. But those beams are extremely powerful, and
even at such high wattage, the number of individual photons returned
on a good night of measuring is countable on the fingers & toes.

   I don't recall offhand, but it's not out of the question that
Apollo experimented from orbit with the same kind of laser ranging of
the lunar surface that you refer to for the Mercury probe.