Hubble at 15

As well as being our Society's 15th birthday, this year is also the 15th anniversary of the Hubble Space Telescope (HST), launched on the 24th April 1990 in the cargo bay of the shuttle Discovery. Over its lifetime, astronomers all over the world have used it to take spectacular images of an enormous variety of objects: from planets in our own solar system, to nearby star forming regions, to galaxies in the distant universe. Despite its track record, however, the future of this great instrument hangs in the balance.

Hubble's history has not been a smooth one, but when you consider the massive technical obstacles involved in building, launching and maintaining an instrument of this sort, you cannot fail to be impressed. No one can forget the initial difficulties when, on first light, the spherical aberration in the primary mirror was discovered. At the time, many people took the opportunity to criticise the project (and NASA as a whole), and it's future looked murky for a time. The first servicing mission, SM1, fixed this problem and full operations finally began. Now, safety concerns, budget cuts and changed priorities at NASA have meant that that the future operation of the HST is being threatened once again.

Servicing missions

The first servicing mission, SM1 (STS-61, Endeavour), was launched in December 1993. The goals of the mission were not just to fix the optics, but to install new equipment, and prove the in-orbit servicing concept was workable. Hubble was designed from the start to be easily serviced by astronauts. The body of the telescope has many handrails, and the instruments are modular in design: they can be pulled out and replaced with relative ease. As well as installing the COSTAR module (Corrective Optics Space Telescope Axial Replacement) to fix Hubble's defective vision, the astronauts on SM1 installed several new components, including WFPC2, an upgraded version of the original Wide Field and Planetary Camera which contained improved detectors and gave better performance in the ultra-violet part of the spectrum. They also replaced several other critical components such as solar panels, gyroscope control units and flight computer coprocessors.

The second servicing mission, SM2 (STS-82, Discovery), flew in February 1997. The astronauts on this mission installed two new instruments: the Space Telescope Imaging Spectrograph (STIS) and the Near Infrared Camera and Multi-Object Spectrometer (NICMOS). STIS provided a great improvement in Hubble's spectral capabilities, its two dimensional detectors provided 30 times more spectral data and 500 times more spatial data than the previous instruments. They also replaced several pieces of electronic and mechanical hardware such as the Fine Guidance Sensors, part of the vital electronics which keeps the telescope pointing in the right direction accurately during an observation.

The third servicing mission was split into two parts, SM3A and SM3B. The first of these, SM3A (STS-103, Discovery) was launched in December 1999. This mission was made more urgent when another of the telescope's gyroscopes failed, leaving only two of the six operational. The gyroscopes provide vital data on the orientation of the satellite, and three are required to provide sufficient information to keep it pointing in the right direction for each observation. Hubble was designed to operate with a minimum of three functional gyroscopes, so when the fourth one failed, a mission had to be put together quickly (which, for the space program, means seven months). As well as replacing all six gyroscopes, the crew installed new insulation and guidance sensors, and upgraded the flight computer and transmitter.

The second half of the third servicing mission, SM3B (STS-109, Columbia), flew in March 2002. This mission installed the new Advanced Camera for Surveys (ACS), a highly sensitive new camera working in the visible through to far ultra-violet parts of the spectrum. It also replaced the solar panels with two smaller, but more efficient, units, and installed a new cooling system to revive NICMOS which had depleted the supply of solid nitrogen ice keeping it cooled since being installed in 1997.

SM4 - Cancelled?

Since the shuttle accident of 2003, when Columbia suffered a catastrophic failure on re-entry, NASA's remaining shuttle fleet has been grounded. It was at this time that the fourth Hubble servicing mission, due to be launched in 2005/6, was cancelled by the then administrator, Sean O'Keefe. This decision, based largely on safety concerns, potentially shortened the lifetime of the instrument by several years. Since then there has been a huge response from astronomers around the planet in support of the continued operation of the observatory, and NASA agreed to re-examine options for a final servicing mission.

It was suggested that the safest way to perform the tasks due to be carried out on SM4 would be to fly a completely automated mission, with no human astronauts required. When this idea was initially proposed, many people were sceptical. The technology required to perform such a complicated operation is by no means trivial.

So what were the goals of SM4? Aside from the standard orbit boost, the crew would have replaced one of the fine guidance sensors, fixed damaged insulation and installed some new science instrumentation. One of these new instruments, the Wide Field Camera 3 (WF3), was to replace WFPC2, while the Cosmic Origins Spectrograph (COS) would have replaced COSTAR. Why replace the corrective optics? With the replacement of WFPC2, all the instruments on the observatory would have their own built-in corrections to compensate for the spherical aberration of the primary mirror, making COSTAR superfluous.

Gyroscopes

Hubble is currently still in operation, but how long for depends on many factors. Some of the instruments are still functioning normally, although STIS ceased operations in August 2004 due to a power supply failure. STIS is currently in safe mode and is powered down. Although engineers have been working on possible work-arounds, it is looking likely that the only way this instrument would be put back into operation is with some kind of servicing mission. More seriously, from a general operations point of view, two of the six gyroscopes on board have failed, leaving Hubble operating on three with one as backup.

The gyroscopes provide the data required to keep the telescope pointing in the right direction during an observation. Currently, three are required in order to accurately point the telescope so if another were to fail, there would be no backup. After the initial decision to cancel the fourth servicing mission, scientists and engineers started working on ways to enable Hubble to function using just two gyroscopes in order to prolong the life of the remaining components. They realised that data from the fine guidance sensors can provide the same information that would be provided by a third gyroscope. Simulations and tests have been carried out using mock-ups of the telescope on the ground and, once the new system has been approved, one of the gyroscopes will be switched off, leaving Hubble functioning on just two.

NASA's reasoning

A shuttle servicing mission to Hubble was looked at in depth by the Columbia accident investigation board. Although several successful servicing missions have been carried out before, it is now thought too risky to send people to the orbit of Hubble. If the shuttle has a problem while in orbit with the ISS then there is the possibility of at least having somewhere relatively safe for the astronauts to go until another craft can be sent up to bring them down. Hubble orbits at a higher altitude and, should there be a problem, there is no where to go. A rescue would be virtually impossible in that scenario.

One proposed solution to this dilemma is to send a robotic mission to service the telescope. This, although the safest from a human life point of view, is not the most practical method of servicing the telescope. The mission would have to be designed, built, tested and flown within two years in order to stand a reasonable chance of saving the HST. Of course, this has to be done on a budget too.

Although this sounds like an incredible amount of work on a very short timescale, the reality is that the basic technology already exists. Using the robotic arm model of the Remote Manipulator System (RMS) on the shuttle, the same company (MD Robotics) came up with the Special Purpose Dexterous Manipulator, Dextre for short, a robotic arm designed for operation on the ISS. After the suggestion that a robotic mission to Hubble was the only way to service it, a mock-up of the Hubble was taken up to the test facility at MD Robotics in Ontario and engineers were able to demonstrate that Dextre would be capable of removing WFPC2, installing WFPC3 and COS and connecting up new batteries.

NASA's priorities have often been dictated by politics. Currently, a large proportion of their budget (both monetary and time) is going into President Bush's 'Moon and Mars' program. His speech to NASA in January 2004 laid out the three goals for NASA over the coming years: completion of the ISS (including returning the shuttle to flight, and then retiring the fleet by 2010), developing the new Crew Exploration Vehicle (CEV) as a replacement for the shuttle (with the first manned mission no later than 2014), and a return to the Moon by 2020.

Cynics among you may notice that there is not much mention of science in there. Much of the estimated cost of sending manned missions back to the Moon will come from budget reallocations within NASA itself. This could mean cuts in some of the science programs, and it is possible that some missions currently in the design phase may be scrapped. This assumes it will actually happen of course. Bush Senior also proposed sending humans to Mars when he was in office in 1989, but the project was estimated at 500 billion dollars and was eventually abandoned.

JWST: Hubble's replacement?

Often referred to as Hubble's successor, the James Webb Space Telescope is due for launch in 2011. Named in honour of NASA's second administrator, the JWST is designed to answer specific scientific questions: how do galaxies form, what triggers star formation, and what are the characteristics of extra-solar planets? It is designed to study the optical and ultra-violet light from the early universe which has been redshifted to the infra-red part of the spectrum, hence the JWST is essentially an infra-red telescope. This differs from the HST which works from the infra-red, through optical and into the ultra-violet and is far more "general purpose".

JWST will have an impressive array of scientific instruments to help it achieve its goals: a near infra-red camera (NIRCam), a near infra-red spectrograph (NIRSpec) capable of taking the spectrum of up to 100 objects simultaneously, a mid infra-red camera and spectrograph (MIRI) and a tuneable filter camera (FGS-TF) which will be able to image in a narrow wavelength range.

It is not just the instrumentation that will be cutting edge. The segmented mirror design is making use of the most recent developments in materials science so as to be light and robust. The deployment of the mirror poses a technical challenge as the whole satellite will be launched on a standard rocket, possibly a European Ariane, so the mirror will have to be deployed in space. The mirror will be made up of 16 hexagonal panels and will be over six metres in diameter with a total area of 25 m². Also unlike Hubble, the JWST will be situated at L2, a point 1.5 million km from Earth. If there happens to be a problem with any of its components after launch there will be no possibility of a servicing mission, so let's hope the mirror is the right shape!

That is the current plan. In 2003 it was reported that the budget for the JWST was already in danger of seriously overrunning the original projections, it now appears that this has become the reality. The mirror has already been scaled back from the original 8-m design, and the current suggestions to reduce the cost could result in either a further reduction in collecting area down to 4-m, or scrapping one of the three science instruments. Neither of these solutions is attractive to the astronomers who will ultimately use the instrument.

There is hope for optical astronomy in space, however. A group of astronomers from Johns Hopkins University, Rochester Institute of Technology and the National Astronomical Observatory, Japan have proposed the Hubble Origins Probe (HOP). The early design is similar to Hubble with a 2.4 metre mirror, and able to make use of both WF3 and the COS, should they not end up on Hubble itself, as well as a new instrument known as the Wide Field Imager.

So, while HOP tries to get off the drawing board, and JWST tries to decrease its costs, Hubble continues to orbit at a height of roughly 600 km, taking 97 minutes to complete one orbit of the Earth. In the 15 years during which it has been in operation so far, the HST has completed more than 80,000 orbits, travelling a total of 3.5 billion km or 25 times the distance from the Earth to the Sun, and captured over 500,000 images and spectra. Here's hoping for many more.

Related Info

Hubble's future >
Sean O'Keefe's comments >
NASA's JWST >
Bush's vision (BBC) >
Bush's vision (Whitehouse) >
Servicing missions >
HOP >

Megan Argo, June 2005