any reasonable time—and targeting objects in space is far harder. And, though not a problem for orbital kinetic weapons, any directed energy weapon would need huge amounts of electricity. So far the most practical batteries are lithium, and the most practical means of generating electricity in space is photovoltaic modules, which are currently only up to 30% efficient, and fuel cells, which have limited fuel. Current technology might not be practical for powering effective lasers, particle beams, and railguns in space. In the context of the Strategic Defense Initiative, the Lawrence Livermore National Laboratory in the United States worked on a project for expandable space-based x-ray lasers powered by a nuclear explosion, Project Excalibur, a project canceled in 1992 for lack of results. General William L. Shelton has said that in order to protect against attacks, Space Situational Awareness is much more important than additional hardening or armoring of satellites. The Air Force Space Command has indicated that their defensive focus will be on "Disaggregated Space Architectures". Space debris Anti-satellite attacks, especially ones with kinetic kill vehicles, can form space debris which can stay in orbit for many years and could interfere with future space activity or in a worst case trigger Kessler syndrome. In January 2007 China did a satellite knock out whose detonation alone caused more than 40,000 new chunks of debris with a diameter > 1 cm and a sudden increase in the total amount of debris in orbit. The PRC is reported to be developing "soft-kill" techniques such as jamming and vision kills that do not generate much debris. Possible warfare over space Most of the world's communications systems rely heavily on the presence of satellites in orbit around Earth. Protecting these assets might seriously motivate nations dependent upon them to consider deploying more space-based weaponry, especially in conflicts involving advanced countries with access to space. Since 2017, the United States Air Force has run an annual military exercise called "Space Flag" at Peterson Space Force Base, which involves a red team simulating attacks on U.S. satellites. Robert Zubrin, aerospace engineer and advocate for human exploration of Mars, stated that anti-satellite weapons capabilities of nations increases, space infrastructures must be able to defend itself using other satellites that can destroy such weapons. Or else, he states, satellite-based navigation, communications and reconnaissance capabilities would be severely limited and easily influenced by adversaries. Science fictional space warfare Space