Image: Hubble
In a picture from 2002, the space shuttle Columbia's robot arm moves the Hubble Space Telescope out of the payload bay, at the end of what may have been the observatory's last repair job.
By Senior Space Writer
updated 2/9/2004 5:41:33 PM ET 2004-02-09T22:41:33

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

Two spacecraft — the Hubble Space Telescope and the Tropical Rainfall
Measuring Mission, or TRMM — are not long for this world. And that’s a worry for NASA. The agency has not yet worked out the mechanics of crashing Hubble into the Pacific Ocean, even though it plans to do so.

Both satellites are sparking decision-making about how best to safely deorbit hunks of space machinery without knocking somebody in the noggin with falling flotsam.

Given our planet’s pervasive cover of oceans and remote landscape, pieces of satellite debris that withstand re-entry forces frequently fall harmlessly to Earth. But they can also hit in places too close for comfort.

Last month, a 154-pound (70-kilogram) titanium rocket-motor casing fell near San Roque in Argentina. It was later identified as debris from a third stage of a Delta 2 booster used to launch a Global Positioning System satellite back in 1993.

NASA has adopted a head’s-up mind-set about government property that comes steaming in from the heavens.

Stay of execution?
On Jan. 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 the 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, Ed Weiler, the associate administrator for NASA’s Office of Space Science, said that 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.

Out of sight, out of mind
But there are other ideas about Hubble’s 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 the 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.

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.

Blaze of glory
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 re-entry, 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 telescope’s precision control ability.

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

Last year, the HST Program Office at NASA’s Goddard Space Flight Center scripted an "End of Mission Options" plan.

"Although portions of HST will burn up in the atmosphere during re-entry, 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 there’s good reason to avoid a chancy uncontrolled re-entry of Hubble.

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

NASA’s own safety standard that considers impact risk, predicted total debris casualty area, orbit inclination and year of re-entry, estimates the chances at less than 1 in 10,000.

Permitting Hubble to nose-dive back to Earth willy-nilly is in violation of the agency’s self-imposed safety standard.

Safety risk
If dealing with Hubble trouble is front-and-center at NASA, so is a recent re-entry survivability analysis regarding the Tropical Rainfall Measuring Mission spacecraft.

Image: TRMM
The Tropical Rainfall Measuring Mission spacecraft, shown in this artist's conception, is a project of NASA and Japan's space agency.
That analysis shows that after plowing through the Earth’s 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. The spacecraft was lofted into orbit in November 1997 from Tanegashima, Japan by that country’s 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, TRMM’s orbit will decay, then wander into Earth’s 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).

Orbital decay
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.

That’s a finding from a team of space debris analysts: Ries Smith, Jose Dobarco-Otero, Jeremiah Marichalar and William Rochelle. They are all subcontractors to NASA's Johnson Space Center in Houston, 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 Re-entry Survival Analysis Tool, or ORSAT. It is the primary NASA computer code for predicting the re-entry 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 re-entry profile — a three-instrument rainfall package of microwave and infrared sensors and two related Earth-observing instruments.

Footprint on Earth
The researchers found a number of TRMM components would survive an uncontrolled re-entry. Seven of those items were made of titanium, beating high melt temperatures and then smacking into Earth’s surface.

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

Looking at a TRMM re-entry in the 2006-2009 timeframe, a risk of between 1 in 4,600 and 1 in 4,530 was computed and, like an uncontrolled Hubble Space Telescope reentry, not compliant with the NASA criterion of less than 1 in 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 re-entering debris has been confirmed.

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