After Hubble: This Canadian telescope could fill coming 'ultraviolet gap'

Artist's impression of the Canadian CASTOR telescope, or Cosmological Advanced Survey Telescope for Optical and UV Research.
Artist's impression of the Canadian CASTOR telescope, or Cosmological Advanced Survey Telescope for Optical and UV Research. (Image credit: CASTOR team)

The ultraviolet universe is invisible from Earth's surface, and most of what we see comes from a single orbiting observatory that may retire in just a few years.

The Hubble Space Telescope remains in good health and may last until the 2030s, but it is gradually being dragged back into Earth's atmosphere. No spacecraft has been able to visit Hubble since 2009, two years before NASA retired its space shuttle fleet. There are early-stage plans to refurbish Hubble again, possibly using a SpaceX Dragon vehicle. But in the meantime, Hubble's eventual retirement could leave behind a big gap.

Hubble's sharp eyes include ultraviolet vision. Earth's atmosphere blocks most ultraviolet light, so it takes a space telescope to really bring into view the young, hot stars that astronomers want to look at via those wavelengths — or growing black holes, or explosions in deep space. (There are other ultraviolet telescopes on spacecraft now, but they tend to be on limited missions that do not span the scope of the universe as Hubble does.)

So the time is now, a team of 100 astronomers argues, to fund a new telescope to fly in 2029 with ultraviolet capabilities on board. The design is ready. The international and industrial partners are lined up. But what is needed is a commitment from the Canadian government to fund the next stages of the CASTOR telescope, which was deemed the top priority by the Canadian astronomy community in its last planning report, issued in December 2020.

"Hubble is almost 30 years old. It's been an amazing workhorse, but it's not going to last forever," said CASTOR lead team member Sarah Gallagher, the president of the Canadian Astronomical Society (CACSA) and an astrophysicist at Ontario's Western University.

"We're really in a beautiful position," Gallagher told, "based on all of the work that we've done, and choosing to tailor the requirements so that it really fits this niche. That's really exciting, scientifically."

Related: Jupiter's Great Red Spot turns blue in new ultraviolet view from Hubble Telescope (photo) 

CASTOR stands for Cosmological Advanced Survey Telescope for Optical and UV Research. The planned 3-foot-wide (1 meter) observatory is part of a new generation of telescopes that pack a lot of functionality into a small package. It would only cost something like $350 million USD ($480 million CAD) — roughly equivalent to Hubble's average yearly cost, absent the five space shuttle servicing missions. (Hubble's mission has cost a total of about $16 billion USD in constant dollars since its development began in 1977, according to NASA figures updated last month.)

Canadian astronomers would like to lead the Earth-orbiting telescope, slated to fly about 500 miles (800 kilometers) high — double the altitude of the International Space Station (ISS). Canada would contribute roughly 60% of the cost ($300 million CAD), with the balance coming from international partners. Big Canadian firms or branch companies like Honeywell, ABB, and Magellan are ready to start work when asked to do so. The UK Space Agency, NASA's Jet Propulsion Laboratory and space authorities in France, Spain, Israel all want to contribute, with launch pad possibilities in India or South Korea.

An image of the Hubble Space Telescope hovering in Earth's orbit. (Image credit: NASA)

But the funding decision is currently before Canada's Parliament, which is formulating the country's 2024 budget for release in the spring. A pre-budget submission from August from CASTOR's team argues that the mission would "solidify Canada's global leadership in astronomy" and allow the nation to at last lead an international astronomy project for the first time, after its decades of research and contributions to other major efforts. It's a big research team, too: representatives from the Canadian Space Agency (CSA), Canada's National Research Council, the Canadian Astronomical Society and many universities are on board in that country alone.

Canada, incidentally, is known for making smart space bets with limited tax dollars in a country hosting just 40 million people. The Canadarm robotic arm series has bought the nation astronaut seats since 1984; in 2023 alone, that commitment brought a promise to send CSA astronaut Jeremy Hansen to the moon on NASA's Artemis 2 mission, along with funding backup CSA astronaut Jenni Gibbons. Fellow CSA astronaut Josh Kutryk will also fly to the International Space Station in 2025.

Related: Canada soars into space with new moon and ISS astronaut missions

The Hubble Space Telescope studied two dramatic planetary nebulas, the Butterfly Nebula and a second that resembles a jewel-bug. (Image credit: NASA, ESA, and J. Kastner (RIT))

A flurry of other space announcements came this year, too. There are promises to update Canada's launching rules to support commercial rocket efforts. Canada also gave a funded commitment to extend and renew the longstanding Radarsat Earth observation series crucial for military and climate change observations. It even pledged a new "lunar utility vehicle" to support Artemis program moon astronauts, while continuing to fund a mini-rover that would launch in 2026. (The rover's science team is led by fellow Western University professor Gordon Osinski, who teaches geology to NASA and CSA astronauts on the regular, with some spaceflyers coming with him on expeditions to remote places here on Earth.)

While CASTOR has a smaller mirror than Hubble, the detector will look at much wider swaths of the sky in both optical and ultraviolet light. Seeing so much of the sky at once in ultraviolet, for example, will allow astronomers to focus on parts of the universe that change quickly: "Often those are very exciting things, like things exploding. We love cosmic explosions," Gallagher said. Detecting such outbursts quickly could allow powerful ground instruments like the gravitational wave-seeking LIGO to follow up on the finds.

Studying ultraviolet wavelengths at a wide scale, moreover, makes CASTOR complementary to two other wide-field missions astronomers are working on. The European Space Agency's Euclid telescope launched in July to deep space, and is already providing images in optical and near-infrared light. And, in 2027, NASA's infrared-seeking Nancy Grace Roman Space Telescope will follow Euclid to Lagrange Point 2. That's a gravitationally stable orbit a million miles (1.5 million km) away from us at the opposite side of the sun.

Gallagher says if all three telescopes could work together in real time, before Euclid's prime mission expires in 2029, that would be ideal — although CASTOR could always follow up on Euclid's work after the fact if needed.

"What's so beautiful about having CASTOR, plus Euclid, plus Roman, is they all have focus on different parts, different colors, and so having all three of them together makes them all better, because you get so much more information," she said. The ultraviolet view would provide extra information on the populations of galaxies (old versus young stars), or the sources of quasars that are powered by black holes.

"What I'm interested in, in particular, is these objects that change over time," Gallagher continued. "So if you go back, and you revisit them over and over again to see them get slightly brighter … you can look at how that changes as a function of color. And that can tell you about the whole structure of the system that's feeding the black hole."

Phase 0, the mission analysis and identification phase for the telescope, was completed in July, and the CASTOR team has been busy gathering support for their budget submission. Numerous Members of Parliament have been consulted. Letters of support came in from 13 different university vice presidents of research, along with the Canadian Association of Physicists. Gallagher herself met with representatives across Canada at the annual Space Borne conference this fall, which unites much of the scientific, government and industrial communities, to continue the funding discussions.

A diagram of the electromagnetic spectrum. (Image credit: Getty Images)

"This is a project that has been well-researched. A lot of the technology risk has been retired. The science case is just so exciting. That's why we've been able to get these partners on board," Gallagher said. But she acknowledged that securing funding for it is not guaranteed. 

"Everybody we talked to, honestly, is pretty excited about it. But it's a tough budget cycle, and we realize there's lots of important priorities as well. But I think this really is the sort of thing in which Canada can be a leader on an exciting, world-class mission. The potential for training and inspiring the next generation in all different fields is really fantastic."

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Elizabeth Howell
Staff Writer, Spaceflight

Elizabeth Howell (she/her), Ph.D., is a staff writer in the spaceflight channel since 2022 covering diversity, education and gaming as well. She was contributing writer for for 10 years before joining full-time. Elizabeth's reporting includes multiple exclusives with the White House and Office of the Vice-President of the United States, an exclusive conversation with aspiring space tourist (and NSYNC bassist) Lance Bass, speaking several times with the International Space Station, witnessing five human spaceflight launches on two continents, flying parabolic, working inside a spacesuit, and participating in a simulated Mars mission. Her latest book, "Why Am I Taller?", is co-written with astronaut Dave Williams. Elizabeth holds a Ph.D. and M.Sc. in Space Studies from the University of North Dakota, a Bachelor of Journalism from Canada's Carleton University and a Bachelor of History from Canada's Athabasca University. Elizabeth is also a post-secondary instructor in communications and science at several institutions since 2015; her experience includes developing and teaching an astronomy course at Canada's Algonquin College (with Indigenous content as well) to more than 1,000 students since 2020. Elizabeth first got interested in space after watching the movie Apollo 13 in 1996, and still wants to be an astronaut someday. Mastodon: