NASA is learning the hard way the gist of an old axiom: What goes up must come down.

On January 16, NASA Administrator Sean O'Keefe made clear his verdict to cancel the next Hubble servicing mission. That shuttle visit would have maintained the telescope and plugged in new astronomical instruments to yield fresh outlooks on the surrounding universe.

O'Keefe cited new space shuttle safety guidelines set out following the Columbia tragedy as the primary basis for his judgment.

That decision has stirred up the ire of "Hubble huggers" around the globe. NASA is getting an earful from astronomical malcontents urging the agency to give the orbiting eye a stay of execution.

For the moment, NASA policy is to auger Hubble in that is, use a still-to-be-built rocket motor that autonomously attaches to the observatory. That motor then precisely propels the telescope into a pre-picked ocean crash zone.

Remains of the 24,500 pound (11,136 kilogram) telescope that outlive atmospheric heating would plop down far from populated areas and shipping lanes.

NASA still has to figure out exactly how to do this. Last week, the Associate Administrator for the NASAs Office of Space Science, Ed Weiler, said even if Hubble stops doing science, it should remain safely in orbit until 2013 without any servicing. The assumes the Sun plays along, and peak levels of solar activity remain as predicted, other analysts say.

NASA's 2005 budget request calls for $300 million over the next five years to plan how the mission-ending robotic mission would work.

But there are other ideas about Hubbles future being floated, both in and outside of NASA.

One thought is to boost the Hubble into a higher orbit. Even though the observatory is surely doomed by old age and eventual blindness, shoving it far above Earth might be an out-of-sight, out-of-mind strategy for NASA.

For one, it could serve as a long-term space environment testing ground for the materials Hubble is made of.

Some think the telescope might be useful as a hub to rehearse robotic servicing techniques for eventual use on the Moon, Mars, or at the L1 Lagrangian point between Earth and Sun. L1 is a gravitationally balanced spot where the James Webb Space Telescope is to be parked next decade.

And there are views that a still-orbiting Hubble would be an attractive addition to the list of flybys for ogling space tourists in the future.

Some engineers think the robotic booster that would bring Hubble down -- which still has to be engineered and then built -- could instead be used to push Hubble higher.

There is no doubt that bringing down Hubble in a blaze of glory is going to be dicey, assuming NASA stands firm on its decision to do that.

Without any intervention at all, Hubble would spiral into Earth and disintegrate in an uncontrolled manner. Roughly a year prior to an uncontrolled reentry, the observatory will start to encounter longer pointing times to focus in on select targets and degraded science due to the increase in atmospheric density. That situation will overwhelm the telescopes precision control ability.

What happens if Hubble Space Telescope (HST) came screaming in on its own?

Last year, the HST Program Office at NASAs Goddard Space Flight Center scripted an "End of Mission (EOM) Options" plan.

"Although portions of HST will burn up in the atmosphere during reentry, a significant debris field is expected to hit the surface of the Earth. In particular the massive primary mirror and its surrounding titanium main ring the structural backbone of HST will almost certainly impact the Earth," the plan explains.

So theres good reason to avoid a chancy uncontrolled reentry of Hubble.

Calculations that include projections of human population densities versus time show a 1:700 probability of human casualty resulting from an uncontrolled HST re-entry.

NASAs own safety standard that considers impact risk, predicted total debris casualty area, orbit inclination, and year of reentry, should be less than 1:10,000.

Permitting Hubble to nosedive back to Earth willy-nilly is in violation of the agencys self-imposed safety standard.

If dealing with Hubble trouble is front-and-center at NASA, so is a recent reentry survivability analysis regarding the Tropical Rainfall Measuring Mission (TRMM) spacecraft.

That analysis shows that after plowing through the Earths atmosphere, leftover chunks of TRMM possibly pose a safety risk to us ground dwellers.

TRMM is a project of NASA and the Japan Aerospace Exploration Agency (JAXA). The spacecraft was lofted into orbit in November 1997 from Tanegashima, Japan by that countrys H-2 booster.

The spacecraft is measuring the amount and distribution of rainfall in tropical and near-tropical areas of the Earth. Data gleaned is used to predict climatic changes on a global scale.

Within the next few years, TRMMs orbit will decay, then wander into Earths atmosphere. That will result in break-up and demise of most of the spacecraft.

TRMM is 17 feet (5.2 meters) tall and tips the scale at roughly 5,764 pounds (2,620 kilograms).

Due to the size, mass and the material properties of TRMM hardware there is a possibility that some of the craft will survive the atmospheric plunge and became a safety risk.

Thats a finding from a team of space debris analysts: Ries Smith, Jose Dobarco-Otero, Jeremiah Marichalar, and William Rochelle. They are all subcontractors to the NASA Johnson Space Center (JSC) in Houston, Texas working through Lockheed Martin Space Operations. Dobarco-Otero is with Hernandez Engineering, a subcontractor to Lockheed Martin on the study.

They reported their TRMM work in a specialized newsletter published last month by NASA's Orbital Debris Program Office at JSC.

The TRMM study made use of an Object Reentry Survival Analysis Tool (ORSAT). It is the primary NASA computer code for predicting the reentry survivability of satellite and launch vehicle upper stage components entering from orbital decay or from controlled entry.

By using the ORSAT code, more than 200 TRMM components, representing more than 91 percent of the total mass of the spacecraft were studied, such as its structural subsystem, electrical subsystem, and high-gain antenna. Also, all five instruments onboard TRMM were given a reentry profile - a three-instrument rainfall package of microwave and infrared sensors and two related Earth observing instruments.

The researchers found a number of TRMM components would survive an uncontrolled reentry. Seven of those items were made of titanium, beating high melt temperatures and then smacking into Earths surface.

The footprint on Earth for an estimated 246 pounds (112 kilograms) of leftover spacecraft pieces is projected to be some 295 miles (476 kilometers) in length.

Looking at a TRMM reentry in the 2006-2009 timeframe, a risk of between 1:4,600 and 1:4,530 was computed and, like an uncontrolled Hubble Space Telescope reentry, not compliant with the NASA criterion of less than 1:10,000.

Whether these odds run for or against NASA is yet to be seen. Putting on a "not to worry" face appears justified by history.

During the past 40 years an average of one cataloged piece of debris fell back to Earth each day. No serious injury or significant property damage caused by reentering debris has been confirmed.